Evolution of Wheat Under Cultivation

Feldman, Moshe; Levy, Avraham A.

doi:10.1007/978-3-031-30175-9_13

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Abstract

The chapter deals with the various steps, periods, and processes that led to the domestication of the wheat as well as with the archaeological sites where domestication took place. Additionally, the chapter describes the ecogeographical characteristics of the area of wheat domestication, the selection of non-brittle rachis, large grain size, rapid and synchronous germination, free-threshing grains, and yield. The genetic basis of non-brittle rachis and free-threshing grains are delt with in details. The formation of hexaploid wheat, T. aestivum, and the spread of its free-threshing form to almost all parts of the globe to become the main cultivated wheat, are reviewed. The production of synthetic Triticum aestivum, and Triticale are also referred to in this chapter.

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13.1 Introduction

Based on the chronological system used in the east Mediterranean, the seven millennia between 13,000 and 6200 uncalibrated years BP are divided into 4 periods (Harris 1998; Table 1.1): the late Epipalaeolithic (the Natufian) (13,000–10,300 BP), the Pre-Pottery Neolithic A (PPNA) (10,300–9500 BP), the Pre-Pottery Neolithic B (PPNB) (9500–7500 BP), and the Ceramic or Pottery Neolithic (7500–6200 BP). The archaeological characterization of these periods was described by Harris (1998), and the events related to the wheat culture were summarized by Kislev (1984), Bar-Yosef (1998), and Harris (1998). In the Natufian period, the hunter-gatherers of the Levant collected grains of wild cereals (wheat, barley, oat, rye and Aegilops) as well as seeds, fruits and roots of other plants. No evidence of cultivation has been obtained from this period (Harris 1998). It is in the PPNA that one finds the first indications of cultivation of wild cereals in the western part of the Fertile Crescent, wild emmer and wild barley in the southern part and wild einkorn and, to some of extent, also wild barley and wild timopheevii in the northern part (Fig. 13.1). Domesticated forms of these wheats, characterized by non-brittle rachis, appeared at the beginning of the PPNB (Table 13.1). Somewhat later, wheat with free-threshing grains (most probably tetraploid) also appeared in the western part of the Fertile Crescent, and together with domesticated emmer, einkorn, and timopheevii spread to the eastern wing of the Fertile Crescent (eastern Turkey, northern Iraq, and south-western Iran), and neighboring regions, where they established contact with Ae. tauschii Coss. (=Ae. squarrosa L.) resulting in the formation of hexaploid wheat. In the Pottery Neolithic, the wheat culture spread to Europe, Asia, and Africa.

Table 13.1 Uncalibrated and calibrated dates before present (BP) on remains of wild emmer, domesticated emmer and free-threshing tetraploid wheat from sites of the western flank of the Fertile Crescent (the Levantine Corridor) in pre-pottery Neolithic periods

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The beginning of cultivation of wheat (and several other plant species) in the western part of the Fertile Crescent, i.e., the Levant, around 10,300 years ago, marked a dramatic turn in the development and evolution of human civilization, as it enabled the transition from a hunter-gatherer and nomadic pastoral society to a more sedentary agrarian one (Eckardt 2010). This change from hunting and gathering to cultivation of plants represents one of the most remarkable events that human society experienced. It triggered the development of human civilization, boosting sedentism, urbanization, and population growth. As such, the Neolithic shift, from an economy based on hunting and gathering to a system based on food production through the domestication of plants and, later also animals, was one of mankind's most dramatic transformations. It was a major socio-economic and cultural change that affected human evolution on the one hand and facilitated the development of human civilization on the other hand.

What brought hunter-gathers in the Levant, to start cultivation of wild plants during the early Neolithic period and become farmers? This question fascinated archeologists, anthropologists, plant geneticists and evolutionary biologists, starting with Darwin (1868). Several hypotheses have been raised concerning the reasons for the shift of early Neolithic man from hunting and gathering in a nomadic or semi-nomadic way of life to agricultural activities in sedentary village dwellings. A long-standing assumption on the origin of agriculture has been population pressure: the increase in population size towards the end of the Natufian period forced people to intensify food production. Moreover, considering (1) the reduction in food sources because of climatic changes that occurred during the end of the 11th millennium BP (the Younger Dryas during the end of the Pleistocene, 13,500–11,800 BP), (2) the over-exploitation of the immediate environment by the increasing human population, and (3) the development of relatively large communities with complex social organization brought about by accelerated sedentism, there was a great increase in food stress, partly also due to reduction in the number of big game (Dembitzer et al. 2022), and, thereby, a pressure to enhance food production. Despite various hypotheses, such as pressure on food resources due to increase in human population, and of climate change, the “why” of wheat cultivation remains a mystery. Climatic changes towards the end of the Pleistocene (Table 2.5) are regarded as important for their impact on the availability of the wild progenitors and human subsistence and are the favored component underpinning explanations for why cultivation began. Of particular importance is the Younger Dryas dry cold episode from approx. 13,500–11,800 years BP (Bar-Yosef 1998, 2003; Harris 1998; Hillman et al. 2001).

A reduction in food sources, by itself, is not a sufficient cause to account for plant domestication (the process whereby wild plants have been evolved into crop through artificial selection). Climatic changes causing thinning out of food supply had occurred in the past and pre-Neolithic man reacted to them, most probably, by migration. Why had man not started plant domestication earlier? Presumably, at the end of the Natufian period, humans reached a cognitive stage that enabled them to comprehend the life cycle of plants and, thereby, to assume control over plant production. In addition to the development of tools for planting, harvesting, and food preparation as well as storage facilities and other agrotechnological skills, this required the ability to be engaged in more intensive socio-economic relationships. Apparently, the late Natufian or early PPNA humans became well familiar with the useful plants in their immediate environment through the gathering and harvesting of wild plant stands (Ladizinsky 1985). This know-how prepared them to start cultivation.

With the increase in agricultural activities, the economy of the early Neolithic communities became largely dependent on the products of cultivation. Hunting, fishing, and gathering of wild fruits, seeds, and plants supplemented the diet, which became, however, increasingly dependent on cultivated wheat, barley, and pulses (Bar-Yosef and Kislev 1989). The domesticated wheat, barley, rye, oat, and legumes provided mankind with a highly nutritive food which was low in water content and, therefore, easy to store, transport and process. This was a prerequisite for the development of human civilization.

Prior to this event, during the Epipalaeolithic phase, the climate was favorable, and dense populations of hunter-gatherers were settled in territories that included some possible year-round settlements (Hillman 1996). Due to a limited amount of available data, little is known of this pre-domestication period when human were gatherers and brought grains near their dwellings. The earliest data relevant to wheat comes from archeological records from the upper paleolithic Last Glacial Maximum period ~23,000 years ago in the hunter-gatherer sedentary camp of Ohalo II on the shores of the lake of Galilee (Fig. 13.1), suggesting that there might have been a period of cultivation of wheat, that was not yet domesticated (Snir et al. 2015). The evidence is that extensive farming-related activity was detected at this site, as deduced from various flints, sickle blades and stone grinding tools, fauna remains, and large amounts of seeds (~10,000 seeds from cereals) including wild emmer wheat, wild barley and wild oats, in what was a human-disturbed environment containing seeds from presumed weedy species. If this interpretation is correct, this would be the earliest known site of wild wheat cultivation. Alternatively, it might be a site where wild emmer wheat, harvested from nearby wheat stands, was brought, and processed. Ohalo II is a singular case as there is a gap of ~10,000 years before other human sedentary settlements were found in the Late Natufian and early PPNA period (Fig. 13.1). The Younger Dryas brought an end to this, with most sites being abandoned, and it is argued that a few groups may have resorted to cultivation during this period. Village populations reappeared throughout the area during the Pre-Pottery Neolithic A (PPNA) period (12,000–10,800 calibrated years ~10,300–9500 uncalibrated years BP; Tables 1.1 and 13.1), and many of them appear to have been cultivators. Finds of domesticated plants are generally widespread in the subsequent Pre-Pottery Neolithic B (PPNB; 10,800–8300 calibrated years ~ 9500–7500 uncalibrated years BP; Tables 1.1 and 13.1), and it was by this period that they began to spread beyond the domestication zone into central Turkey, Cyprus, Crete, and southern Greece (Fuller 2007). There is a consensus among archaeobotanists (not considering Ohalo II) that cultivation of wild cereals predated morphological domestication by >1000 years.

For many years it has been assumed that the transition to agricultural activities was a revolutionary process, and, accordingly, it was referred to as the “Neolithic or Agricultural Revolution”. However, the accumulating archaeological data indicate that this shift was rather an evolutionary process (Kislev 1992; Hillman 1996; Harris 1998; Bar-Yosef 1998), a step-by-step domestication of different plants and animals. Each step, however, seems to have occurred during several hundred or even thousand years, and each crop may have been domesticated separately in time and place. Some PPNA villages were established in the absence of plant and animal domestication, which in turn occurred sometimes in the absence of village formation. Yet, on a prehistoric scale, sedentism and agriculture emerged virtually simultaneously. This is the concept of a diffuse beginning of agriculture. Only later, following the establishment of several domesticated crop plants, could these domesticated plants and animals have made such a powerful impact when spread from the western part of the Fertile Crescent (Kislev 1992). Although the recent accumulating archaeological data provide a clearer insight as to the origin and spread of wheat agriculture, still it is difficult to determine the exact date of the earliest agricultural activities, partly because only a few such operations were practiced in the early stages.

Several major events and phases are recognized in the development of wheat cultivation (Table 1.1): (i) harvesting from wild emmer and einkorn stands—a phase of agrotechnical development and preparation for cultivation; (ii) a pre-domestication cultivation period when wild emmer and einkorn were grown in small plots—the first phase of cultivation; (iii) appearance of non-brittle emmer and einkorn; cultivation of brittle and non-brittle types of emmer and einkorn wheats in mixture—the second phase of cultivation; (iv) appearance of free-threshing, naked tetraploid wheat; cultivation of wild and domesticated emmer and naked tetraploid wheat in mixture; expansion of wheat culture to all regions of the fertile crescent; appearance of non-brittle hexaploid wheat; significant increase in human population and site size—the third phase of cultivation; (v) spread of durum wheat culture to central Asia, southern Europe, and Egypt—expansion of durum agriculture; (vi) spread of bread wheat to many parts of the world and the accumulation of landraces—expansion of bread wheat agriculture; and (vii) modern breeding and the green revolution.

The agrotechnical phase was developed mainly during the Natufian period (15,000–12,000 calibrated years ~13,000–10,300 uncalibrated years BP; Table 1.1), although its beginning can be traced back several millennia earlier (Kislev 1984). Remnants found in Natufian sites in the Levant indicated that wheat as well as other plants were the important dietary constituents in this period. The unearthing of sickle blades, pestles, pounding stones, and querns as well as storage pits attest to the extensive collection of cereals by the Natufians. Several permanent or semi-permanent settlements that were based on hunting and gathering were formed during that period. One of these is the permanent settlement at Tell Abu Hureyra, in the Euphrates basin of northern Syria, first settled in 11,500 uncalibrated years BP. Remnants of many animals as well as of wild species of einkorn, barley, rye and legumes from late Natufian layers were identified at this site (Hillman 1975). It is therefore, assumed that the Natufian economy was based on hunting and on intensive collection and consumption of seeds. At that time, the flora and fauna on the border of the Mediterranean maquis were much richer than today's providing settlers with a large and reliable source of plant and animal food. Thus, hunters and gatherers could settle and increase in population, without engaging in agriculture. The invention of the sickle for harvesting and of other tools for grain-processing comprise major components of the agrotechnical revolution Kislev 1984. During this period the Natufians collected the large-grained wild grasses and legumes and became acquainted with their biology—a prerequisite for their cultivation. The initial attempts at plant cultivation, presumably mainly by women, might have taken place during this period.

The “domestication revolution” (12,000–8300 calibrated ~10,300–7500 uncalibrated years BP) can be divided into two sub-phases: cultivation of wild forms of cereals, and cultivation of domesticated forms Tables 1.1 and 13.1). Based on the archaeological evidence, the transition from the Natufian to the Neolithic culture occurred in the Jordan Valley, the Damascus basin, and the Middle Euphrates (Bar-Yosef and Kislev 1989; Bar-Yosef 1998; Harris 1998) and was rather rapid, although the early farmers were involved both in plant cultivation and in hunting. The earliest experiments with the cultivation of wild forms of emmer and barley in the southern part of the Fertile Crescent and of einkorn, wild timopheevii, and barley in the northern part of this region, took place at early PPNA; wild einkorn in the north Levant as soon as 13,000 Cal-years BP (Abu Hureyat I and Mureybit I-III) and 11–12,000 Cal-Y BP, with wild emmer in the south Levant in Netiv Hagdud and Zaharat adh-Dhra (see reviews by Nesbitt 2001; Willcox 2012) and wild barley and to a lesser extent also of wild emmer, in the eastern part of the fertile crescent (Riehl et al. 2013). There was no clear morphological evidence for the presence of domesticated wheat. Domestication of animals (sheep and goats) occurred later (about 9000–8500 uncal years BP). Cereal farming itself may have originated in areas adjacent to rather than within the regions of greatest abundance of these wild cereals. Such prehistoric settlements peripherally to, rather than within, the current distribution areas of these species are Jericho, Gilgal, Netiv Hagdud, and Gesher in the Jordan Valley, Tell Aswad and Tell Ghoraife near Damascus, and Tell Abu Hureyra and Tell Mureybit in northern Syria and Chogha Golan in the Zagros mountains (Fig. 13.1).

Of the various species of cereals that grew in the Fertile Crescent and were harvested by the Natufians, only wild emmer and barley in the south, and barley and einkorn, and possibly also wild timopheevii in the north, were cultivated by the early PPNA people (Van Zeist and Bakker-Heeres 1985; Kislev et al. 1986; Riehl et al. 2013). Assuming that the amount of grain collected per unit time was the most important criterion (Evans 1981), wild stands of wheat and barley were preferred over other cereals, because their large, heavy grains borne in spikes facilitated their harvesting. Indeed, Harlan (1967) in wild einkorn and Ladizinsky (1975) in wild emmer succeeded in gathering considerable amounts of grains per hour of harvesting. Wild emmer was a better candidate for domestication than barley, having larger grains and two grains in each dispersal unit compared to one in barley. Indeed, Ladizinsky (1975) harvested from wild stands twice as many grains of wild emmer as of wild barley. The taste of wheat, its high nutritional quality, and its large free grain might have also contributed to its preference over barley. In addition, judging from today’s pattern of distribution, barley was probably very common and grew in abundance within a short distance from the early Neolithic settlements, while the dense stands of wild emmer were somewhat more distant. This might have created additional pressure to cultivate wheat, as sufficient quantities of barley could have been harvested from nearby wild stands.

The early Neolithic settlements in the Jordan Valley were located on the edges of alluvial fans or on alluvial terraces (Bar-Yosef and Kislev 1989). It is assumed that cultivation was based on sowing in such alluvial fans and terraces, as well as on the edges of freshwater ponds where the water table was always high, and the soil was periodically fertilized by floods of mud. The exploitation of wetted soils is also evidenced in other early agricultural sites such as Tell Aswad and Tell Ghoraife in the Damascus basin (Van Zeist and Bakker-Heeres 1985).

The “invention” of cultivated fields by the early Neolithic people of the Levant and the development of agronomical practices associated with field preparation and sowing, as well as the selection of seeds to be sown, comprise the initial stages in agricultural technology. The conversion of alluvial terraces and, later on, also of grass-stands of annuals, into fields was undoubtedly a major change through which man affected not only cultivated plants but also plants that remained in the wild. The establishment of cultivated fields by early Neolithic man could have been inspired by the observation of natural herbaceous covers of annuals, such as wild emmer and barley, in open forest belts of deciduous oak. Man may have burnt off unwanted grass in the open forest and used the clear space for sowing wheat or barley and, later on, expanded this space to adjacent areas by clearing evergreen woods and shrubs. The transformation of the landscape from a high perennial evergreen vegetation to cultivated fields of annual grasses was perhaps the first and most important man-initiated change in the world of vegetation. The selection of grains for the next sowing season was the first event whereby man started to shape the genetic structure of the cultivated wheats. Whatever were the criteria for selection, these samples of early cultivated wheats became subjected to different selection pressures than those affecting their sibs that remained in the wild stands. The second sub-phase of the “domestication revolution”, the cultivation of domesticated forms, presumably started several hundred years, or more, after the beginning of cereal cultivation. In the cultivated forms, the ripe ear no longer disarticulates into single spikelets as easily as in the wild forms, thus allowing the harvesting of intact spikes without the need to collect single spikelets.

We know little of the predomestication period, when human were gatherers and brought grains near their dwellings, due to a limited amount of available data. The earliest data relevant to wheat comes from archeological records from the upper paleolithic Last Glacial Maximum period approximately 23,000 years ago in the hunter–gatherer sedentary camp of Ohalo II on the shores of the lake of Galilee (Fig. 13.1), suggesting that there might have been a period of cultivation of wheat that was not yet domesticated (Snir et al. 2015). The evidence is that extensive farming-related activity was detected at this site, as deduced from various flints, sickle blades and stone grinding tools, fauna remains, and large amounts of seeds (approximately 10,000 seeds from cereals) including wild emmer wheat, wild barley, and wild oats, in what was a human-disturbed environment containing seeds from weedy species (presumably a field). If this interpretation is correct, this would be the earliest known site of wild wheat cultivation. Alternatively, it might be a site where wild emmer wheat, harvested from nearby wheat stands, was brought and processed (Piperno et al. 2004). Ohalo II is a singular case as there is a gap of approximately 10,000 years before other human sedentary settlements were found during the Younger Dryas in the Late Natufian early Pre-pottery Neolithic-A period throughout the fertile crescent (Map). Yet, it is generally accepted (Kislev 1992; Hillman 1996; Harris 1998; Bar-Yosef 1998) that domestication occurred about 1000 years, or more, after the initial cultivation of wild cereals, namely, at the beginning of the PPNB (ca. 9500 uncal years BP) when types with non-brittle spikes and some also with naked grains were clearly identified (Kislev 1992). On the other hand, for several legumes and flaxes the archaeological data indicate that these were already domesticated in the PPNA.

Regardless of the exact time of domestication, the most characteristic feature of domesticated cereals is the dependency on man for sowing: once the seed-dispersal mechanism was lost, the genetic donor to the next generation became solely determined by man. Man's selection and raising of domesticated wheat could have occurred only after ploughing and sowing had been practiced (Kislev 1984). It seems that at the end of the PPNA all essential agricultural practices had been already established. From this stage onward, the transformation of some of the wild cultivated forms into domesticated wheats proceeded very rapidly. This transformation involved not only the loss of the self-propagation mechanism, but also that of self-protection (stiff glumes), resulting in a free-threshing, naked grains. Increase in seed size and the loss of seed dormancy leading to a uniform and rapid germination was also achieved at this stage. Traits of strong negative adaptive value in the wild that had positive value under cultivation were favored by the early farmers. As such, the farmer’s election imposed a new evolutionary pathway in the cultivated wheats, entirely distinct from that operating in wild wheats.

The replacement of the wild cultivated forms by domesticated ones was a slow process. According to Harris (1998), the establishment of domesticated populations of cereals was slowed down by constant gene flow from wild forms, which either grew in mixture or nearby the domesticated forms. Moreover, no severe selection in favor of domesticated forms was applied by the early farmers, possibly because of the techniques of harvest that were practiced at that time.

Hence, the establishment of agriculture in the Levant and neighboring regions was a very gradual process that took place over a period of 3000 years (from the beginning of the PPNA to the end of the PPNB). During these three millennia, agriculture became the main production system that supported man in southwest Asia.

The third phase of the agricultural revolution, the expansion of agriculture (in the 8th and 7th millennia BP; Kislev 1984; Harris 1998; Bar-Yosef 1998), was accompanied by a rapid and radical change in the economic organization of the Near East and the surrounding regions. Cereal culture spread from the western flank of the Fertile Crescent to other parts of this region (Civáň et al. 2013), and from there to central Asia through northern Iran, to Southeastern Europe through Transcaucasia, to Europe and North Africa through southwest Anatolia, and to Egypt through Israel and Jordan (Feldman 2001). According to Bar-Yosef (1998), expansion of agriculture was mainly due to the spread of farmers to new territories rather than the adoption of farming by the hunters–gatherers. Most wheats were the hulled domesticated emmer and einkorn; the more advanced naked tetraploid and hexaploid wheats were relatively rare.

Kislev (1994a) subdivided the “Agricultural Revolution” into three stages: the Agrotechnical Revolution, the Domestication Revolution, and the Expansion of Agriculture. Weiss et al. (2006) divided the process of domestication of plants into three stages: “gathering,” in which people gathered annual plants from wild stands, “cultivation,” in which wild plant genotypes were systematically sown in fields of choice, and “domestication,” in which mutant plants with desirable characteristics were raised. Similar divisions of the process of domestication were suggested by Harris (1989) and Fuller (2007). Hillman and Davis (1990) defined domestication as a process causing populations of cultivated plants to lose features, particularly reproductive features, necessary for their survival in the wild habitats, i.e., a process which ultimately renders crop populations dependent on human intervention for their reproduction. Such a process involves genotypic changes in entire populations. Doebley et al. (2006) defined domestication of a plant species as the various genetic modifications in its wild progenitor made to meet human needs.

The most characteristic feature of domesticated wheats is their dependence on humans for gathering and planting. This development involved genetic changes in several crucial traits, defined by Hammer (1984) as the “domestication syndrome.” In emmer wheat, this syndrome is characterized, sensu stricto, by suppression of seed dispersal, increased seed size, and free-threshing grains. While wheat domestication involves a limited number of chromosome regions (Table 13.2), though many relevant quantitative trait loci (QTLs) have also been detected (Peng et al. 2011).

Table 13.2 Domestication syndrome genes

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One can distinguish between these sensu stricto syndrome traits and traits that were selected during a “process of optimization” for cultivated fields. These latter changes involved gradual genetic modifications, mainly due to non-conscious and sometimes conscious selection by early farmers, to increase crop adaptation to cultivated fields and to human consumption and needs. In tetraploid wheat, such optimization led to changes that resulted in rapid and uniform germination, erect plants, increased plant height, reduced tillering, and larger spikes and grains. Meyer and Purugganan (2013) made a distinction between domestication and diversification, with the former referring to the onset of evolutionary divergence from the wild ancestral species, and the latter referring to the subsequent evolution of new varieties.

Although many genes in various crops have been proposed to be domestication genes, only a few have been shown to have been targeted by selection (Olsen and Wendel 2013). When exposed to selection, both the favored genetic variant and other, neutral genes linked to it, increase in frequency, a phenomenon called a selective sweep (Wang et al. 1999; Olsen et al. 2006). Consequently, the related genetic region shows reduced genetic diversity.

13.2 Domestication of Tetraploid Wheat

13.2.1 History

The origin of domesticated plants and the development of agriculture have always stimulated man’s curiosity and imagination. In the ancient world, where every phenomenon and every event were explained in a mythological manner, the philosophers and historians of many nations considered domesticated plants to be a generous gift from the gods. Thus, the ancient Egyptians were grateful to Isis and Osiris for bringing wild emmer wheat and wild barley from Mt. Tabor, Israel, to Egypt and teaching people how to cultivate them. Similarly, the ancient Greeks ascribed the gift of these important cereals to Demeter and the Romans to the goddess Ceres.

During the last few centuries, botanists and students of agricultural history have attempted to explain the origin of domesticated plants and their evolution under domestication on a scientific basis. Driven by the desire to find the historical and evolutionary truth, Link (1816), de La Malle (1826), and particularly de Candolle (1886), were the first to realize that historical, linguistic and folkloristic evidence is insufficient to trace the origin of domesticated plants. Botanical, geographical and archaeological studies are necessary to advance this field of science; the only definite demonstration of the origin of a domesticated plant is the discovery and identification of its wild prototype. In cases where the wild prototype is unknown or extinct, the origin and full history of the domesticated crop can never be ascertained. In such a situation, it is very difficult, if not impossible, to identify the site(s) of domestication, the genetic changes that led to the formation of the primitive domesticated form, and the evolution of the domesticated crop under cultivation.

One of the first attempts to identify the progenitor of domesticated bread wheat, was in the middle of the nineteenth century, when several botanists regarded the natural hybrid derivatives of bread wheat x Aegilops geniculata (formerly Ae. ovata) as the ancestral forms of common wheat. The hybrid was first discovered by Requine, in 1821, in southern France, and later on, it was also collected in northern Italy and North Africa. Because of its resemblance to common wheat, Requine (see Fabre 1852) named it Aegilops triticoides. Due to the fact that there were also many intermediate forms between Ae. triticoides and common wheat, Fabre (1852, 1855) concluded that all domesticated wheats originated from Ae. geniculata. Since he found, in several cases, that grains from Ae. geniculata ears growing near wheat fields, yielded Ae. triticoides plants, he assumed that under cultivated conditions, Ae. geniculata gradually transformed into bread wheat. This hypothesis, which was accepted by several botanists, was disproved by Godron (1854, 1856, 1858a, b, 1869, 1876), who demonstrated the hybrid nature of Ae. triticoides and all other intermediate forms between Ae. geniculata and bread wheat. He produced similar forms by crossing bread wheat with Ae. geniculata and backcrossing the hybrids to the two parental species.

These experiences with Ae. triticoides emphasize the need for a better definition of key features characterizing the wild prototype of the domesticated wheats. The wild prototype must be a valid species and, therefore, a self-propagating plant. It should contain a spike similar to that of domesticated wheats, but with a brittle rachis that disarticulates into single spikelets upon maturity. In addition, the wild prototype, like many other wild grasses, should have tightly closed glumes, resulting in a ‘hulled grain’ for protection against extreme climatic conditions and herbivores. Eco-geographically, the prototype should be characterized by a specific distribution and occupy well defined primary habitats.

In the second half of the nineteenth century, several botanists (e.g., Hausknecht) were of the opinion that all domesticated wheats derived from the single wild species, that was known to botanists at that time, T. monococcum ssp. aegilopoides. Yet, the majority of botanists interested in the origin of domesticated wheats, maintained the opinion that wheat origin is polyphyletic, and that at least two wild species serving as the progenitors.

The first evidence of wild wheat, presumably wild emmer, came from the Chaldean priest Berosus, who lived at about 2700 years before present (BP). He mentioned the occurrence of wild wheat in Mesopotamia (Syncellus, Frag. Hist. Graec., vol. 2, p. 416). In more recent times, Linnaeus (1753) cited Heintrelmann who found wild wheat in northwest Iran. Olivier (1807) found wheat, barley, and spelt in uncultivated areas northwest of Anah, on the right bank of the Euphrates, and mentioned that he had already seen such wild wheat several times in northern Mesopotamia. Since specimens of these cereals have not been preserved, it is not possible to identify the wheats to which Olivier referred, but it is highly probable that the spelt was wild emmer. In 1877, Andre Michoux saw spelt wheat growing wild north of Hamadan, western Iran, (see de Candolle 1886). Based on these reports, de Candolle (1886) assumed that the Euphrates basin was the distribution area of the two wild progenitors of the domesticated wheats.

Since wild tetraploid wheat was not known at the second half of the nineteenth century, most botanists and archaeologists agreed at that time with the theory of Solms-Laubach (1899), namely, that domesticated wheats, other than those derived from wild T. monococcum, originated in central Asia and that the wild progenitors of domesticated durum and bread wheats were lost as a result of drastic climatic changes. All previous evidence (mentioned above) were neglected. Moreover, no serious attention was given to the German botanist Friedrich August Körnicke, and several other botanists, who maintained, not only that a prototype of such a progenitor exists, but that they already had two spikelets of such wild wheat.

In 1855, the Austrian botanist Theodor Kotschy collected a plant of wild barley from Rashaya, the northwestern slopes of Mt. Hermon, Lebanon, and kept it in the herbarium of the National Museum of Vienna. In 1873, Körnicke, who analyzed barley plants in several European herbaria, found a segment of wheat spike among the culms of Kotschy’s barley. Since this spike had a brittle rachis and two grains in each spikelet, Körnicke believed that it belonged to wild wheat, the progenitor of most domesticated wheats, and named it T. vulgare Vill. var. dicoccoides Körn. [This wild wheat is currently known as T. turgidum ssp. dicoccoides (Körn. ex Asch. and Graebn.) Thell.]. Körnicke believed that it grew in the Mt. Hermon area and, since he could not obtain support for a scientific expedition to that area, he asked Aaron Aaronsohn, an agronomist and amateur botanist who lived in Israel (then Palestine), to search for this wild wheat in the Mt. Hermon area. Schweinfurth (1906) reports that Aaronsohn found this wild wheat first in the settlement of Rosh Pina, northern Israel, and later the slopes of Mt. Hermon.

One year later, Aaronsohn found that this wild wheat grew abundantly in the southern Levant, namely, in northeastern Palestine, northwestern Jordan, southwestern Syria and southeastern Lebanon, having a wide range of morphological forms across the distribution area (Aaronsohn 1909, 1910). Later, it was also found in northern Syria, southeastern Turkey, northern Iraq and southwestern Iran (Harlan and Zohary 1966; Kimber and Feldman 1987). Specimens were also collected in 1910 by Theodor Strauss in the mountainous region of western Iran near Kerind. Thus, Körnicke’s hypothesis that the wild progenitor of domesticated wheats, a two-grained wild wheat, still grows in the Near East, was fully confirmed. Aaronsohn’s collections of wild wheat specimen from the southern Levant served as the basis for a series of botanical and genetic studies by Cook (1913), Schulz (1913a, b), von Tschermak (1914), Flaksberger (1915), Percival (1921), Vavilov 1932), and others.

Understanding the “where”, “when”, and “how” of wheat domestication was significantly advanced by the discovery of wild emmer wheat, Triticum turgidum (L.) Thell. ssp. dicoccoides (Körn. ex Asch. and Graebn.) Thell. in nature by Aaron Aaronsohn in 1906 (Aaronsohn and Schweinfurth 1906). Following this discovery, Schultz (1913b) was able to assemble the first natural classification of the wheat species in which he recognized three series: einkorn (one-grained wheat), emmer (two-grained wheat), and dinkel (Table 10.1).

Soon after, von Tschermak (1914) crossed wild emmer with the domesticated forms of tetraploid wheat, T. turgidum ssp. dicoccon (Schrank) Thell. and ssp. durum (Desf.) Husn., and concluded from the high fertility of the hybrids that the wild and the domesticated forms are genetically very close. Cook (1913) and Percival (1921) supported this notion by reporting on the occurrence of natural hybrids between these taxa in Israel (then Palestine) and Syria, as well as their spontaneous hybridization wherever they were brought into contact with each other in experimental fields. The discovery of the chromosome number of the wheats (Sakamura 1918) showed that the three series of Schulz represent a polyploid series in which the einkorn are diploids (2n = 14), the emmer are tetraploids (2n = 28), and the dinkel are hexaploids (2n = 42). Not surprisingly, analysis of first meiotic metaphase configurations in hybrids between wild emmer and domesticated tetraploid wheat showed that the wild chromosomes were fully homologous to those of all the domesticated subspecies of T. turgidum (Kihara 1924, 1937, 1940; Sax 1921a, b, 1922; Percival 1921). These studies supported the hypothesis of Körnicke (1889), Schweinfurth (1906, 1908), Aaronsohn and Schweinfurth (1906), Aaronsohn (1909, 1910), Schulz (1913b), von Tschermak (1914), and others, that domesticated emmer, ssp. dicoccon, the hulled type of T. turgidum and therefore the primitive domesticated tetraploid wheat, derived from wild emmer, ssp. dicoccoides, by a series of mutations (Fig. 13.2). Genetic, cytogenetic, and more recently evidence from whole genome sequence (Avni et al. 2017) all indicate that ssp. dicoccoides is the direct progenitor of domesticated emmer, durum, and bread wheat.

Vavilov (1926) rejected the view that the domesticated forms of tetraploid wheat derived from wild emmer. His conclusion was based on sterility of hybrids between domesticated emmer or durum and what he thought was wild emmer. This latter species was actually another wild tetraploid wheat taxon, T. timopheevii (Zhuk.) Zhuk. ssp. armeniacum (Jakubz.) van Slageren, from Transcaucasia, which differs from wild emmer in its genomic composition (Lilienfeld and Kihara 1934). In Vavilov’s view (1926), Aaronsohn’s eco-geographical data were insufficient to regard wild emmer as the progenitor of domesticated tetraploid wheat. From the wealth of varieties and forms of domesticated emmer in Ethiopia, as compared with the relatively few variations in Israel and Syria, he considered Ethiopia, where wild emmer does not grow, to be the site of domestication of tetraploid wheat. Vavilov’s concept, however, was rebutted by morphological, eco-geographical, cytogenetic, and molecular data.

13.2.2 Ecogeographical Characteristics of the Area of Wheat Domestication

Following the discovery of wild einkorn, T. monococcum ssp. aegilopoides, during the second half of the nineteenth century, and of wild emmer, T. turgidum ssp. dicoccoides, in the beginning of the twentieth century, the putative progenitors of the domesticated einkorn and emmer wheats, respectively, their distribution area was determined. It was then relatively easy to establish the geographical region that was the cradle of agriculture, and the specific regions from which these two-wheat subspecies were taken into cultivation.

The wild progenitors of cultivated wheats are natural constituents of some of the open oak-park belts and the herbaceous plant formations in southwest Asia. Their assumed center of origin and current center of distribution and diversity is in the “Fertile Crescent”—a hilly and mountainous region extending from the foothills of the Zagros mountains in south-western Iran, through the Tigris and Euphrates basins in northern Iraq and southeastern Turkey, continuing southwestward over Syria to the Mediterranean, and extending to central Israel and Jordan. Wild taxa belonging to 4 species of wheat and 17 species of the closely related genus Aegilops are endogenous to this region, which most likely was the arena of wheat domestication. This assumption is supported by ample archaeological evidence.

The Fertile Crescent is bounded by the Mediterranean in the west, by chains of large and high mountain ranges in the north and east (the Amanos in north-western Syria, the Taurus in southern Turkey, Ararat in north-eastern Turkey and the Zagros in western Iran), and in the south by the Syrio-Arabian desert with its western extension (Paran desert) in the Sinai Peninsula. Situated between the sea, the mountains, and the desert, the Fertile Crescent is under the influence of several different climates: on the one hand, it enjoys the temperate Mediterranean climate with a short, mild and rainy winter and long, hot and dry summer, yet, on the other hand, it is influenced by the more extreme steppical climate of the Iranian and Anatolian plateaus in the east and north, and by the desert climate in the south. Consequently, the Fertile Crescent encompasses two different phytogeographical regions, the Mediterranean in the south-western part and the Irano-Turanian in the north-eastern part, and is also affected by two other regions, the Saharo-Arabian in the south and the Euro-Siberian in the North. The Mediterranean part of the Fertile Crescent includes Israel, Jordan, Lebanon Syria and the western part of south-eastern Turkey and it centers around the Syrio-African rift (the Jordan rift valley in the south, the Beqa Valley of Lebanon and the Orentos valley in Syria). The Irano-Turanian part includes the eastern part of south-eastern Turkey, northern Iraq, and southwestern Iran and is influenced by the continental climate of the Iranian and Central Asiatic steppes.

No wonder, therefore, that this region is ecologically very diversified, comprising of a wide array of different habitats. Its versatile ecological conditions are manifested by its wide array of plant formations, ranging from well-developed Mediterranean forests and maquis, through open parks, shrubs and herbaceous formations, to small shrub and steppical plant formations. The open parks and the herbaceous formations, containing many annual grasses and legumes, occupy the open habitats in the edges and openings of the Mediterranean maquis, which presumably served in the past as the main pasture area for wild sheep, goats and gazelles—the game of pre-agricultural man.

It is generally accepted that Near Eastern agriculture originated within the distribution area of the wild progenitors in the Fertile Crescent region. Unfortunately, no signs of earlier farming communities were found there (with the possible exception of Ohalo) probably because of climate conditions, wild plants were not available in the same area but rather, further west and south (Hillman 1996; Bar-Yosef 1998). Indeed, new geological, climatic, and archaeological data from the east Mediterranean region indicate that from 13,500 to 11,500 years ago there was the Younger Dryas climatic event, which was characterized by a cold and dry climate (for details see Hillman 1996; Bar-Yosef 1998). Hillman (1996), using palaeobotanical data, reconstructed the phytogeographical belts of this region during the Younger Dryas and concluded that the habitats of the annual cereals lie mainly in the open areas of the oak-park maquis in a relatively narrow strip of the east Mediterranean (Fig. 13.1). This narrow strip, called the “Levantine Corridor”, begins in the Taurus foothills (Diyarbakir area) in south-eastern Turkey and extends along the Mediterranean southward, incorporating the middle Euphrates through the Damascus basin, the Lebanese mountains, the two sides of the Jordan Rift Valley into the Sinai Peninsula (Bar-Yosef 1998). The current distribution of wild wheats in this area is as follows: wild emmer is the dominant wild wheat in the central-southern part of the corridor (from north of Damascus to Jericho), on terra-rosa and basalt soils, while wild einkorn (T. monococcum ssp. aegilopoides), T. urartu, and wild T. timopheevi and also more sporadically wild emmer, are distributed in the northern part of the corridor. The distribution of the diploid species, ssp. monococcum and T. urartu, extended southwards up to Mt. Hermon, in the central part of the corridor.

The accumulating archaeological data indicate that agriculture originated in the Levantine Corridor (Table 13.1) This is clearly apparent from the distribution of the earliest Neolithic sites (10,300–9500 BP) (Fig. 13.1): Tell Mureybit and Tell Abu Hureyra in the Middle Euphrates in the northern part of the corridor, Tell Aswad and Tell Ghoraife in the Damascus basin, and Netiv Hagdud, Gilgal, and Jericho in the Jordan Valley, between the Lake of Galilee and the Dead Sea (Bar-Yosef and Kislev 1989; Hillman 1996; Bar-Yosef 1998). All these settlements were established along the ecotone between the relatively temperate Mediterranean phytogeographical region and the steppical Irano-Turanian region. The predominant plant formations in this transitional zone are open parks and herbaceous covers containing a large number of annual grasses and legumes.

13.2.3 Domestication of Emmer Wheat, ssp. dicoccon

13.2.3.1 Opening Remarks

Hillman (1996), using palaeobotanical data, reconstructed the phytogeographical belts of the east Mediterranean region during the Younger Dryas, 13,000–11,700 uncalibrated years BP. He concluded that the habitats of the annual cereals lay mainly in the open areas of the oak-park forest, in a relatively narrow strip of the east Mediterranean. Wild emmer was a natural constituent of this corridor but was more widespread in its central-southern part than in its northern part. This pattern of distribution of wild emmer wheat was also in existence about 2000 years later, when human in this region started to cultivate wild emmer.

Being native to the marginal Mediterranean habitats of the Fertile Crescent, wild emmer T. turgidum ssp. dicoccoides was preadapted for cultivation. It is an annual plant, which grows in mild winters and endures the dry, hot summer as seeds. Wild emmer is predominantly self-pollinated and has relatively large grains that assist the safe and rapid re-establishment of the stand. Their large seed size rendered them very attractive to the ancient gatherer. Its annual habit made it also amenable for dry farming, while its self-pollination system could have aided in the fixation of desirable mutants and recombinants resulting from rare outcrossing events. While wild emmer occupies poor, thin, rocky soils in its natural habitats, it responds well when transferred to richer habitats.

Cultivation imposed a new evolutionary direction on the wheats, whereby traits that had the greatest adaptive value in the cultivated field were preferred. Consequently, selection pressures exerted by farmers have operated in a different, and sometimes, even contradictory manner in cultivation and in wild. During the 10,000 years of wheat cultivation, the criteria for selection varied from time to time and from place to place, as suggested by Evans (1981) in regard to yield criteria. The first farmers selected plants with large grains and with more grains per spike. Later on, farmers selected for a higher ratio of grain harvested to grain sown, i.e., indirectly selecting for profuse tillering, many grains per spike, and strong grain retention. As the amount of arable land became a limiting factor and the crop monotypic, selection preferred higher number of grains per unit area. In this case, emphasis was given to lines which were weak competitors, yielded well in dense planting, and responding well to fertilizers and various agrochemicals. Thus, during the process of adaptation to the various cultivated environments and to the different demands of man, wheat has responded with a number of significant morphological and physiological changes. The main genetic changes reflect the process of adaptability to agriculture, particularly the loss of the ability of the plants to propagate and protect the seeds, which left wheat became fully dependent on the farmer for its survival.

The earliest data relevant to wheat comes from archeological records from the upper paleolithic Last Glacial Maximum (LGM) period ~23,000 years ago found in the hunter-gatherer’s sedentary camp of Ohalo II on the shores of the lake of Galilee, suggesting that there might have been a period of cultivation of wheat, that was not yet domesticated (Snir et al. 2015). The evidence is that extensive farming-related activity was detected at this site, as deduced from various flints, sickle blades and stone grinding tools, fauna remains, and large amounts of seeds (~10,000 seeds from cereals) including wild emmer wheat, wild barley and wild oats, in what was a human-disturbed environment containing seeds from weedy species (presumably a field). If this interpretation is correct, this would be the earliest known site of wild wheat cultivation. Alternatively, it might be a site where wild emmer wheat, harvested from nearby wheat stands, was brought and processed. Ohalo II (Snir et al. 2015) is a singular case as there is a gap of ~10,000 years before other human sedentary settlements were found in the Late Natufian and early Pre-pottery Neolithic-A (PPNA) (Table 1.1). There is a consensus among several archaeobotanists (not considering Ohalo II) that cultivation of wild cereals predated morphological domestication by >1000 years with wild einkorn in the north Levant as soon as 13,000 Cal-Y BP (Abu Hureyat I and Mureybit I-III) and 11–12,000 Cal-Y BP, with wild emmer in the south Levant (Tell Aswad, Netiv Hagdud, Zaharat adh-Dhra (see reviews by Nesbitt 2002; Willcox 2012). During these periods, there was no clear morphological evidence for the presence of domesticated wheat.

In recent decades, cytogenetic, genetic, and evolutionary studies supplemented by archaeological investigations, contributed to an improved understanding of events occurring during the transition from hunting/gathering to farming (Willcox 1998). In recent years, two important advances have occurred in archaeobotany: the recognition of pre-domestication cultivation and evidence for different sub-centers of crop domestication within the Fertile Crescent (Fuller 2007). Evidence for pre-domestication cultivation has been recognized through the statistical composition of wild seed assemblage (Colledge 1998, 2001, 2002; Harris 1998; Willcox 1999, 2002; Hillman 2000; Hillman et al. 2001). As is well known from later agricultural periods, archaeobotanical assemblages are made up predominately of crops and weeds, together with some gathered fruits and nuts. This pattern was already recognized in the PPNA and in some Late Epipalaeolithic sites, by samples dominated by wild cereals. It was suggested that the primary domesticated crops of the Neolithic Revolution, namely, einkorn wheat, emmer wheat, barley, lentil, pea, chickpea, and flax, appeared initially in a core area, from which they spread throughout the Middle East (Zohary and Hopf 2000; Salamini et al. 2002; Lev Yadun et al. 2000; Abbo et al. 2010). Recent archaeobotanical data, however, indicate that pre-domestication cultivation of the wild progenitors of these species was carried out autonomously in very early sites of the Near-Eastern PPNA (Pre-Pottery Neolithic A; Weiss et al. 2006). In accordance, Willcox (1998) concluded that the wild progenitors of Old-World cereals and legumes were exploited for several millennia, before the appearance of domestic counterparts and in each site, the local wild prototype were taken into cultivation.

At first, wild emmer was grown for several hundred years or more, until forms with a tough rachis and non-brittle spike gradually appeared, and for one millennium or more, were grown in a mixture with brittle forms and gradually replaced them throughout the Levantine Corridor (Kislev 1984; Tanno and Willcox 2006; Feldman and Kislev 2007). An archeological record shows that in ancient sites where agriculture was practiced, a mixture of spikelets with fragile and non-fragile rachises was found, and it took several millennia until the non-fragile spikes became prominent in farming of emmer wheat (Kislev 1984). Also, in agreement with this archaeological finding, genetic data showed that the loss of spike fragility in wild emmer was a gradual process (Nave et al. 2019) and was presumably spread very slowly from field to field by farmers. The gradual and prolonged replacement of wild emmer by the domesticated form was presumably caused by the fact that farmers used to collect the spikelets from the ground rather than harvest the spikes (Kislev 1984; Kislev et al. 2004), applying only a weak selection in favor of the latter. If wild cereals were harvested simply by passing through stands and shaking or beating spikes to knock spikelets into a basket, then the shattering, wild-type genotypes would be the ones to predominate in the next year’s crop. Also, a good portion of wild spikelets that fell to the ground were not collected and germinated next season, contributing to the new generation of wild wheat.

Hole (1998) discussed the spread of agriculture from its apparent origin in the southern Levant, into the northeastern part of the Fertile Crescent. The first evidence of cultivation of wild emmer was found in PPNA sites in the southern Levant: Jericho, Netiv Hagdud, Gilgal, Gesher, all clustered within a 15 km radius on alluvial fans in the Jordan Valley, and sites in southern Syria (Kislev 1997; Colledge 2001; Willcox 2005; Van Zeist and Bakker-Heeres 1982; Hopf 1983). All these sites are dated to approximately calibrated 10,300–9500 BP, just following the Younger Dryas cold interval which terminated ca. 11,000 BP or a little later (Becker et al. 1991). Mixtures of wild and domesticated emmer were found in the southern Levant in the early PPNB (9500–9000 BP) (Hopf 1983; Kislev 1988; Rollefson et al. 1985; Colledge 2001; Van Zeist and Bakker-Heeres 1982). The first actual domesticates on the Euphrates appeared in Halula, north Syria, no earlier than 9000 BP, at least 500 years after their occurrence in the Jordan Valley. True agricultural villages appeared in the uplands of the Zagros at 8500 BP, about 1000 years later than in the Levant. Once agriculture began, it spread through diffusion to indigenous people (Harris 1996), or perhaps had been ‘invented’ repeatedly, or introduced by emigrant colonizers (Hole 1998).

Fuller (2007) suggested that the domestication syndrome in cereals usually meets the following six criteria: (1) Mutations forming a non-brittle rachis. This is often regarded as the single most important domestication trait, rendering a species dependent upon the farmer for survival; (2) Reduction in seed dispersal aids. Wild wheat and barley have a range of structures that aid seed dispersal, including hairs, barbs, awns and even the general shape of the spikelet. Domesticated wheat spikelets are less hairy, have shorter or no awns and are plump, whereas in the wild, they are heavily haired, barbed, and aerodynamic in shape (Hillman and Davies, 1990). This can be considered to have come about by the removal of natural selection for effective dispersal, and once removed, metabolic ‘expenditure’ on these structures is reduced. These traits might evolve under initial cultivation and can be regarded as part of ‘semi-domestication’; (3) Trends towards increasing grain size. This is likely to be selected for by open environments in which larger seedlings have advantages, surviving deeper burial within disturbed soils; thus, this trait is generally selected for by tillage and cultivation (Harlan et al., 1973). Larger seeds are strongly correlated with larger seedlings; (4) Loss of germination inhibition. Crops tend to germinate as soon as they are wet and planted: (5) Synchronous tillering and ripening. Planting at one time and harvesting at one time will favor plants that grow in synchronization; (6) More compact growth habit, erect versus prostrate plants.

13.2.3.2 Selection for Non-brittle Rachis

Several wild characters which had no pre-adaptive value for cultivation were selected against during domestication. The change from the wild emmer to the most primitive domesticated form domesticated emmer, T. turgidum ssp. dicoccon, involved the selection of phenotypic characters that suited the farmer rather than the wild environment. This group of traits represent the ‘domestication syndrome’ (Hammer 1984), and in wheat, mainly involved dramatic changes in seed dissemination, size and germination, and mode of seed protection (Zohary et al. 2012). Morphologically, domesticated emmer is similar to wild emmer, but differs from it by several of the domestication syndrome traits that affected the morphology and physiology of the evolving domesticated form.

Willcox (1998) stated that in the Late Natufian (11,000–10,300 BP), einkorn was dominant at Mureybit and Abu Hureyra, north Syria, while wild emmer and wild. barley was dominant at Ohalo II, Israel. In the Pre-Pottery Neolithic A (PPNA; 10,300–9500 BP), wild emmer was found in several sites of the southern Levantine Corridor in the western flank of the Fertile Crescent) (Table 13.1), namely, in Israel, Jordan, and southern Syria, while wild einkorn was found in the northern Levantine Corridor, e.g., Jerf el Ahmar and Nureybit, northern Syria (Willcox 1998). There was small-scale cultivation (Harris 1996), using of locally available wild cereals as seed stock. No evidence exists for domesticated cereals in this period. However, at early PPNB (9500–9000 BP), mixtures of wild and domesticated wheats were found in several sites (Kislev 1984; Tanno and Wilcox 2006).

Emmer domestication during early PPNB has been reported for several sites in the southern and northern parts of the Levantine corridor (Table 13.1). In many sites during this period, wild types remained at significant frequencies. The wheat crop consisted of a mixture of wild and domestic types. Naked wheat (free-threshing) was found from the middle to late PPNB (9000–7500 BP) at several sites of the Levantine corridor (Table 13.1). Emmer that was absent in the Euphrates Valley, northern Syria, during earlier periods, appears to have been introduced from elsewhere at Abu Hureyra and Halula, together with naked wheat (Willcox 1995, 1996). Taking the data from the PPNA and PPNB together, there is evidence for independent in situ cereal domestication at different sites, einkorn in the northern part of the Levantine corridor and emmer in the southern part (Willcox 1998).

Kislev (1989, 1992) and Nesbitt (2002) pointed out that it is extremely difficult to distinguish between brittle and non-brittle forms of emmer in archaeological material. This is because the ripe ear of both wild and domesticated forms of emmer disarticulates into single spikelets, spontaneously or after threshing, respectively, (Kislev et al. 2004). It is only after unearthing parts of spikes with a tough, non-fragile rachis that cultivation of non-brittle forms of emmer can be assumed. Thus, determining the exact time of origin of non-brittle forms of emmer is a central problem in the study of wheat domestication. Clear-cut evidence for domestication dates to the 9th millennium BP, with finds of the small-grain, free-threshing, naked, tetraploid wheat, ssp. parvicoccum, in Ramad, southwestern Syria, in 8300–8000 BP (Van Zeist and Bakker-Heeres 1982), in Tell Aswad, southwestern Syria, in 8900–8500 BP (Van Zeist and Bakker-Heeres 1982), and in Can Hassan, south Anatolia, in 8400–7700 BP (Hillman 1972, 1978; Kislev 1979/1980, 1992; see below). The free-threshing tetraploid wheat was presumably preceded by the more primitive, hulled, domesticated emmer wheat (Kislev 1979/1980).

Morphological and genetic evidence have indicated that the domesticated subspecies of T. turgidum, and particularly domesticated emmer, are closely related to wild emmer wheat T. turgidum ssp. dicoccoides (Feldman 2001; Feldman et al. 1995; Feldman and Kislev 2007). This closeness is apparent from the fact that hybrids between wild emmer and domesticated subspecies of T. turgidum exhibit regular chromosome pairing at meiosis, and are fully, or almost fully, fertile (von Tschermak 1914; Percival 1921). These close relationships are also apparent from the spontaneous hybridization and gene flow occurring occasionally when wild and domesticated T. turgidum grow side by side (Cook 1913; Percival 1921; Feldman and Kislev 2007; Luo et al. 2007). These data clearly imply that domesticated emmer derived from wild emmer and that all other domesticated subspecies of T. turgidum derived from domesticated emmer.

The most critical changes in the transition of a wild type to a domesticated form involves loss of wild-type seed dissemination method, seed size, and seed dormancy (Zohary et al. 2012). Among these changes, the most conspicuous trait differentiating between wild emmer and domesticated emmer, ssp. dicoccon, is rachis brittleness at maturity; in the wild subspecies, the rachis is brittle, while it is tough and non-brittle in the domesticated form. The brittle rachis in wild emmer is an essential trait, leading at maturity to disarticulation of the spike, from the top downwards, into individual arrowhead-shaped spikelets. Each spikelet, equipping with two long, strong, and straight awns above, a sharp rachis segment below, and with very stiff hairs that are bent backwards, is an effective seed-dispersal unit that is very valuable for seed dissemination and self-planting under wild conditions (Harlan et al. 1973; Zohary et al. 2012). This seed-dispersal unit facilitates self-burial in the soil through a lateral movement of the awns caused by daily changes in humidity (Elbaum et al. 2007). Buried spikelets are thus protected from birds, rodents and ants during the long, dry summer, and ensuring successful germination after the first rain. This must have proved a nuisance to the ancient farmer who had to collect most of the spikelets from the ground or cut the culms before the grains matured. No wonder, therefore, that types with brittle heads were selected against and, in contrast, plants with a tough rachis that did not disarticulate at maturity were favored. Consequently, in domesticated emmer the mature spike remains intact on the culm and breaks into individual spikelets only upon slight application of mechanical pressure at threshing (Dorofeev et al. 1980). This type of tough rachis was called a ‘semi-tough’ rachis, not to be confused with the fully tough rachis of the free-threshing tetraploid subspecies wheats which remains intact when threshed (Hillman and Davis 1990). This difference was caused by mutations for a semi-tough rachis, that may occasionally occur in the wild (Kamm 1974) and thus, prevents the dissemination of seeds. As such, in the wild, it has a negative adaptive value and it is soon selected against. The selective pressure is especially strong in grazed areas, where non-brittle mutants are presumably eliminated soon after appearance. If they survive the first summer, their grains may germinate on the culm after the first rain, and the seedlings then dry up. In contrast, under cultivation, it has a positive value, facilitating easy harvesting, and was therefore preferred by farmers. Thus, domestication transformed wheat to be human-dependent, that can survive only under cultivation in human agricultural niches to meet human needs (Peng et al. 2011). On the other hand, domestication of plants has prompted man to be dependent on food production through cultivation of plants rather than by gathering them from nature. Hence, the transition from seed dispersal through spike fragility to dispersal by farmers is a key event in the domestication of cereals (Konopatskaia et al. 2016), and presumably, was one of the first modifications during domestication.

Despite its strong negative adaptive value, mutants with non-brittle spikes were found repeatedly in Israeli populations of wild emmer (Kamm 1974), indicating that this mutation is not a rare event. von Tschermak (1914) showed that the main morphological difference between wild and domesticated emmer, i.e., the non-brittleness of the spike, is determined by a small number of genes. Indeed, spike non-brittleness in segregating F₂ populations between wild emmer and durum wheat was determined by two complementary recessive genes (Levy and Feldman, unpublished data). Studying a complete series of chromosome-arm substitution lines in which wild emmer chromosome arms substituted for their common wheat homologues, Rong (1999) and Millet et al. (2013) showed that spike non-brittleness is determined by two recessive genes, one on the domesticated chromosome arm 3AS and the second, with a somewhat stronger effect, on the domesticated arm 3BS. Nalam et al. (2006), using recombinant inbred line populations, obtained similar results, showing that the Br (brittle rachis) loci are located in wild emmer chromosome arms 3AS and 3BS, while the domesticated types possess the recessive br alleles. Genotypes homozygous for only one br allele (either in 3AS or in 3BS) exhibit semi-brittle spikes in which the upper part of the spike disarticulates at maturity and the lower part remains on the culm (Kamm 1974; Rong 1999; Millet et al. 2013).

The two loci, controlling the rachis character in tetraploid wheat, were first designated Br2 and Br3 (Watanabe and Ikebata 2000). However, in accordance with the rule for the symbolization of genes in homoeologous sets, Watanabe et al. (2002) proposed to designate the group 3 brittle rachis genes as follows: Br‐A1, (formerly Br2), and Br‐B1 (formerly Br3). The dominant alleles of these loci, located on the short arm of chromosome 3A and 3B, respectively, determine rachis brittleness in wild emmer wheat, which leads to spike disarticulation. Homozygosity for dominant alleles in one locus and for recessive alleles in the second locus, i.e., Br-A1/Br-A1/br-b1/br-b1 or br-A1/br-A1/ Br-B1/Br-B1, determines fragility only of the upper part of the spike (Rong 1999; Millet et al. 2013; Avni et al. 2017). Homozygosity for recessive alleles in both loci, br-A1 and br-B1, determines a non-brittle rachis (Watanabe and Ikebata 2000; Watanabe et al. 2002; Nalam et al. 2006; Millet et al. 2013; Konopatskaia et al. 2016).

Wedge-type spikelets are formed when disarticulation occurs immediately below the rachis internode (above the rachis node) leading to a dispersal unit of a spikelet and a rachis segment below it, while barrel-type spikelets are formed when the breakage occurs above the rachis internode (below the rachis node), resulting in the formation of a dispersal unit of a spikelet and a rachis segment beside it (Zohary and Hopf 2000). Sakuma et al. (2011) and Li and Gill (2006) provided an overview of the disarticulation systems and inflorescence characteristics, along with the genes underlying these traits, in the Triticeae tribe. Based on the observed phenotype of single chromosome additions into bread wheat, the orthologous brittle-rachis gene Br1, determining wedge-type disarticulation, has been located in various Triticeae species on the short arm of homoeologous group 3 chromosomes (Watanabe and Ikebata 2000; Li and Gill 2006), including Dasypyrum villosum chromosome 3 V (Urbano et al. 1988), Thinopyrum bessarabicum chromosome 3Eb (King et al. 1997), wild barley Hordeum spontaneum chromosome 3H (Takahashi and Hayashi 1964), Ae. speltoides on chromosome 3S (Friebe et al. 2000; Li and Gill 2006), Ae. bicornis chromosome 3S^b (Riley et al. 1966, cited in Urbano et al. 1988), and in Ae. sharonensis chromosome 3S^sh (reported by Miller and cited in Urbano et al. 1988). In Ae. longissima, the Br1 gene determines a spike-type of disarticulation (umbrella-type dispersal unit) is also located on the short arm of chromosome 3S^l (Friebe et al. 1993; Urbano et al. 1988), but it causes fragility only in one site at the lower part of the spike. Br1 genes determining umbrella-type disarticulation exist on the short arms of group 3 chromosomes in several other Aegilops species, namely, Ae. searsii chromosome 3S^s (Friebe et al. 1995), Ae. peregrina chromosome 3 Sv (Yang et al. 1996), on Ae. geniculata chromosome 3M^o (Friebe et al. 1999), Ae. uniaristata chromosome 3N (Miller et al. 1995; Iqbal et al. 2000b). Within Triticum itself, the above-rachis node disarticulation gene(s) have been located in wild T. timopheevii chromosome arms 3AS and 3GS (Li and Gill 2006; Nave et al. 2021), in wild emmer on chromosome arms 3AS and 3BS (Nalam et al. 2006; Millet et al. 2013, 2014), and in the feral hexaploid wheat, T. aestivum ssp. tibetanum on chromosome arm 3DS (Chen et al. 1998; Watanabe et al. 2002). All of these chromosomes carry a gene responsible for disarticulation below the rachis internode. Thus, there is ground for supposing that the disarticulation genes, as a whole form an orthologous set. Intriguingly, in Ae. tauschii, the above-rachis internode disarticulation trait leading to barrel-type disarticulation is controlled by a gene (Br2) mapping to the long arm of chromosome 3D (Li and Gill 2006) whereas the Br1 gene is located ssp. tibetanum on 3DS (Chen et al. 1998; Watanabe et al. 2002), indicating the existence of this gene on chromosome 3DS of T. aestivum and obviously also on 3DS of Ae. tauschii. The Br2 gene is paralogous to the orthologous Br1 genes, probably formed in Ae. tauschii by a duplication followed by an intra-chromosomal transposition to the long arm. Such translocation of genes is a far from rare event during evolution (Tarchini et al. 2000; Li and Gill 2002; Pourkheirandish et al. 2007; Faris et al. 2008; Sakuma et al. 2010).

With the fully assembled the genome of wild emmer wheat, Avni et al. (2017), using a population derived from a cross between wild emmer (line Zavitan) and domesticated durum wheat (cv. Svevo), identified the mutations in the brittle rachis genes Br-A1 and Br-B1 that lead to non-fragile spikes in domesticated emmer. They revealed, in agreement with previous studies (Rong 1999; Watanabe et al. 2002, 2006; Nalam et al. 2006; Millet et al. 2013, 2014), genomic regions regulating the brittle rachis phenotype in wild emmer, on the short arm of chromosomes 3A and 3B (15.5 and 32.5 Mb, respectively). Yet, Avni et al. (2017) found out that each Br1 gene is actually a compound locus, consists of duplicated genes, TdBtr1 and TdBtr2, that exhibit homology to the Btr1 and Btr2 genes controlling brittle rachis in wild barley and wild T. monococcum (Pourkheirandish et al. 2015, 2018). Thus, Avni et al. (2017) identified the orthologous genes in tetraploid wheat, i.e., TtBtr1-A and TtBtr2-A on chromosome arm 3AS, and TtBtr1-B and TtBtr2-B on chromosome arm 3BS. The homoeology of TtBtr1-A, TtBtr2-A, TtBtr1-B, and TtBtr2-B from wild emmer wheat with Btrl and Btr2 on chromosome arm 3HS of wild barley (Hordeum spontaneum) and with Btr1 and Btr2 on chromosome arm 3AS of wild T. monococcum, suggests that the location of the genes for the brittle rachis trait in these species has been conserved.

In diploid wheat, T. monococcum, Pourkheirandish et al. (2018) reported that a single non-synonymous amino acid substitution at position 119 (alanine in wild form to threonine in domesticated form) of the protein product of Btr1 is responsible for the loss of function mutation that leads to the non-brittle rachis trait. Substitution at position 10 in the protein product of Btr2, from an aspartic acid in wild to glutamic acid in domesticated einkorn, had no effect on rachis brittleness (Pourkheirandish et al. 2018). Their data supported the hypothesis that the substitution at position 119 of Btr1 causes a functional change implying that the brittle/non-brittle rachis trait in diploid wheat is only controlled by the allelic status at Btr1.

Similar to the situation in diploid wheat, also in durum wheat (i.e., cv. Svevo) the mutant alleles btr1-A and btr1-B presumably are likely loss-of-function alleles (Avni et al. 2017). The causative mutation in Btr1-A is a 2 bp deletion in the coding sequence which causes a loss-of-function frame shift, and in Btr1-B, the loss of function is due to a 4 kbp insertion, 50 bp upstream of the stop codon (Avni et al. 2017). Diversity analysis of 113 wild emmer, 85 domesticated emmer, and 9 durum accessions showed that all domesticated accessions carry the loss-of-function alleles for both brittle rachis genes btr-A1 and btr-B1 (Avni et al. 2017). On the other hand, the fact that no polymorphisms were detected between the coding regions of wild (line Zavitan) and domesticated (cv. Svevo) TtBtr2-A or TtBtr2-B alleles, led Avni et al. (2017) to assume that the combination of the mutations in the two TtBtr1 genes are complementary and sufficient to achieve the non-brittle rachis phenotype. These researchers developed a pair of near-isogenic lines (NILs), each carrying one functional allele (TtBtr1-A or TtBtr1-B) in the background of Svevo. Both NILs exhibited an intermediate brittle rachis phenotype, in which the upper part of the spike was brittle, and the lower part was non-brittle. Evidently, these two homozygous recessive mutations of the TtBtr1 gene (but not in TtBtr2) appear to be required for transforming the brittle-rachis of wild emmer to non-brittle one in domesticated tetraploid wheat. Diversity analysis of 113 wild emmer, 85 domesticated emmer, and 9 durum accessions showed that all accessions of domesticated emmer and durum carry the loss-of-function alleles for both btr1-A and btr1-B. The requirement for two homozygous recessive mutations, suggests that selection for non-brittle rachis in tetraploid wheat may have been a gradual long process, first selection for plants showing only partial spike brittleness, due to a recessive mutation in one of the brittle rachis l genes, and later, selection for plants showing complete non-brittleness, arising from possession of two recessive mutations in both loci. Indeed, archaeological findings suggest that rachis non-brittleness took several hundred years to become established (Kislev 1984; Tanno and Willcox 2006; Feldman and Kislev 2007; Purugganan and Fuller 2011). The loss of fragility gave rise to the first known domesticated wheat, T. turgidum ssp. dicoccon, which is grown to this day, albeit on a small scale (de Vita et al. 2006).

Several studies have shown that some QTLs affect the brittle-rachis trait, namely, on chromosomes 2A (Peng et al. 2003; Peleg et al. 2011; Tzarfati et al. 2014), 3A (Watanabe et al. 2006), and 1B (Tzarfati et al. 2014). Thanh et al. (2013), analyzing F₂ plants derived from a cross between domesticated emmer and wild emmer, detected seventeen QTLs on chromosomes 1B, 2A, 2B, 3A, 3B, 4A, 5B, and 7B that affected plant and spike characteristics. Two regions on chromosomes 2A and 3B had a large effect on rachis fragility, and nine regions on chromosomes 2A and 5B affected traits related to seed production. Their results indicated that selection for these QTLs occurred during the domestication of emmer wheat, prior to the appearance of free-threshing forms. It was suggested that superimposition of the effect of all of the QTLs on that of the major genes br-A1 and br-B1 and the subsequent improvement or strengthening of the non-brittle phenotypes, might have evolved later, under domestication (Abbo et al. 2012, 2014).

13.2.3.3 Selection for Large Grain Size

An additional requirement for the newly domesticated wheat was increased grain size. It is well known that wild and domesticated cereal grains differ in size, and this has been used to infer the domesticated status of cereals, already in the earliest PPNB, including sites from the Jordan Valley, the upper Euphrates in Syria, and the first settlements in Cyprus (Colledge 2001, 2004). Domesticated forms differ from the wild form in kernel morphology; in the domesticated forms, the grain tends to be wider, thicker and rounder in cross-section compared to the wild form (Willcox 1998, 2002, 2004). Current indications from available archaeobotanical evidence indicate that evolution of grain shape and size preceded the loss of wild-type seed dispersal mechanism (Willcox et al. 2008). It is possible that the early farmers tended to cultivate, and subsequently, to domesticate, large-seeded genotypes of wild emmer (Blumler 1992, 1994; Diamond 1997). In fact, Harlan and Zohary (1966) suggested that a large-seeded race of wild emmer, growing abundantly in the vicinity of the Upper Jordan Valley, is the likely progenitor of domesticated emmer.

Fuller (2007) explored the disjunction in wheat between seed size increase and loss of seed dispersal, and rates at which these features evolved were estimated from archaeobotanical data. He concluded that changes in grain size and shape evolved prior to non-brittleness of the rachis. Initial grain size increases may have evolved during the first centuries of cultivation of wild emmer, within perhaps 500–1000 years (Fuller 2007).

In accordance with the above, the growing morphometric database for wheat and barley from the Near East indicates that wheat and barley grains increased in size starting in the PPNA and earliest PPNB (Colledge 2001, 2004; Willcox 2004). This is before widespread evidence for the existence of tough, non-brittle rachises and loss of natural seed dispersal.

Willcox (1998) suggested that the evolution of large grains occurred over a few centuries. This explains why large domestic-type grains were already widespread and predominant on most Near Eastern sites by the start of the PPNB (approx. 9500 years BP). This evidence raises the question of how large-grained varieties of wild emmer evolved. One possibility is that methods of processing, such as use of sieves after threshing and winnowing, served to bias larger grains for stored cereals.

Gegas et al. (2010), analyzing morphometric and quantitative traits in several recombinant-doubled haploid populations of elite winter lines of common wheat and a comparison of grain material from primitive wheat species and modern elite varieties, concluded that grain shape and size are independent traits in both modern varieties and in primitive wheat species that are under the control of distinct genetic elements. They suggested that wheat domestication resulted in a switch from production of a relatively small grain with a long, thin shape to a more uniform, larger grain, with a short, wide shape. Their data illustrated the complex history of domesticated wheat evolution, suggesting that various traits arose independently at different stages. For example, these authors suggested that grain size increased early in domestication, through alterations both in grain width and length, followed, at later stages, by further modifications in grain shape, primarily, changes in grain length. In addition, the decrease in phenotypic diversity in grain morphology in modern commercial wheat is shown to be the result of a relatively recent and severe bottleneck that may have occurred either during the transition from hulled wheat to the free-threshing subspecies, or, more recently, through modern breeding programs.

13.2.3.4 Selection for Rapid and Synchronous Germination

Other requirements for the newly domesticated wheat were prevention of untimely germination, rapid and uniform germination after sowing, simultaneous ripening of grains, and erect rather than prostrate culms. Wild emmer wheat has two types of seed dormancy: a post-harvest type and a long-range type. The first type prevents premature and untimely germination—an important feature, pre-adapted to agriculture, particularly in view of the fact that seeds were often stored under unsuitable conditions. The second type of dormancy ensures a temporal distribution of germination in nature: it is invariably the larger grain of the second floret in each spikelet that germinates in the first autumn, while the smaller, darker grain of the first floret germinates the following year. Such temporal distribution of germination, referred to as differential dormancy (Nave et al. 2016), prevents, on the one hand, crowding of plants, and, on the other hand, ensures the occupation of sites for two years. Horovitz et al. (2013) reported that differential dormancy characterizing intact spikelets also exists in grains that were separated from the spikelet. This shows that differential dormancy is not the result of inhibitory factors extracted from the glumes and palea, but, rather, is caused by internal factors within the grains themselves.

Using phenotypic data from a wild emmer x durum wheat population and a high-density genetic map, Nave et al. (2016) exposed the genetic mechanism controlling differential grain dimensions and dormancy within wild tetraploid wheat spikelets. They showed that, in wild emmer, the lower grain within the spikelet is about 30% smaller and more dormant than the larger, upper grain. They also revealed a major locus on the long arm of chromosome 4B that explains >40% of the observed variation in grain dimensions and seed dormancy within spikelets. The domesticated variant of this locus, designated QGD-4BL, likely fixed during the domestication process, favors spikelets with seeds of uniform size and synchronous germination. In addition, a QTL for mean kernel weight was located on chromosome 5A (Tzarfati et al. 2014).

13.2.3.5 Monophyletic Versus Polyphyletic Origin of Domesticated Emmer

Harlan (1975) asserted that emmer wheat was domesticated many times in various parts of the Fertile Crescent. Yet, new evidence showed very clearly that in most regions where agriculture began, primary crops were domesticated only once or very few times. Concurrently, Blumler (1992, 1996) suggested that emmer was domesticated only once, and that diffusion of cultivation of emmer was far more important than independent invention. In agreement, the results of Haudry et al. (2007), who analyzed nucleotide diversity at 21 loci in wild emmer and domesticated wheats, are consistent with a monophyletic origin of all the domesticated lines studied, which is consistent with a single domestication event for emmer wheat (Zohary 1999). This is in agreement with recent archaeological data that support the view of emmer domestication as a geographically diffuse, gradual process, rather than an independent discovery and use of non-brittle types (Blumler 1996; Weiss et al. 2006

The genetic evidence reported by Salamini and coworkers (Salamini et al. 2002; Ozkan et al. 2002, 2005, 2011), identify the ancestral population of wild emmer from which domesticated emmer derived. Salamini and coworkers used amplified fragment length polymorphism (AFLP) fingerprinting of nuclear DNA to estimate the genetic similarity between wild and domesticated emmer populations (Ozkan et al. 2002; Salamini et al. 2002). They constructed neighbor-joining trees, from which they concluded that wild populations from the Karacadag Mountains in southeastern Turkey, are more similar to domesticated emmer than are other wild populations. This fact supports the hypothesis of a monophyletic origin of domesticated emmer (Zohary 1999). However, Allaby and Brown (2003, 2004) claim that a monophyletic origin might be erroneously inferred when populations are examined by AFLP genotyping and neighbor-joining analysis. Questioning the use of AFLP analysis for studies on the origin of crops, they argued, based on simulation data, that results resembling a monophyletic tree can be obtained when merging data from populations that were independently domesticated. In 172 out of 180 simulations, the domesticated hybrid formed a single clade in the resulting neighbor-joining tree (Allaby and Brown 2003). They argued that the combination of hybridization among populations, migration, and genetic drift may affect the shape of phylogenetic trees, which cannot be used to correctly reconstruct the history of the relationships between domesticated and wild populations (Allaby and Brown 2004).

All the domesticated wheats containing he BBAA or BBAADD genome have the same mutation, supporting a monophyletic origin. Later introgressions might have happened that change the phylogeny. But the seminal events are shared by all wheats. Yet, the study of Nave et al. (2019), does not rule out a series of events occurring at different places and at different times. Willcox (2002) and Tanno and Willcox (2006) report that archaeological data support the view that domestication occurred independently in several places. Recent work on domesticated emmer wheat has identified two different lineages of a gluten genes which are so different that they are estimated to have evolved apart hundreds of thousands of years ago, i.e., long before domestication of wild emmer wheat. Such evidence implies two separate domestications of emmer (Allaby et al. 1999; Brown 1999; reviewed in Jones and Brown 2000).

Some domestication events may not be on the genetic record because the early cultivars have disappeared, and present-day populations represent only a fraction of those grown in the past. For instance, a large fraction of the ancient gene pool of domesticated tetraploid wheat was lost in the Near East about 2500 years ago, or earlier, when ssp. durum replaced domesticated emmer and the small-grained, free-threshing tetraploid wheat, ssp. Parvicoccum (Nesbitt 2002). Nonetheless, it may be reasonable to assume that the technology to cultivate wild plants originated in one of the sites of the Levantine Corridor and its spread to other Levantine sites motivated the local PPNA people to start cultivating wild plants that grew in their vicinity. Indeed, archaeological evidence indicates that different Near Eastern communities cultivated various local species during the PPNA (Weiss et al. 2006).

Archaeobotanical studies showed that acquisition of the full set of traits observed in domesticated emmer was a prolonged process, intermediate stages being seen at early farming sites throughout the Fertile Crescent (Brown et al. 2009). New genetic data are confirming the multiregional nature of cereal domestication, correcting a previous view that each crop was domesticated by a rapid, unique and geographically localized process. Brown et al. (2009) reviewed the evidence that has prompted this reevaluation of the origins of domesticated crops in the Fertile Crescent. Taken together, the archaeobotanical morphotypes and genetics suggest the occurrence of several domestications of wheat and barley in the Near Eastern Fertile Crescent region, and there is no reason to attribute them all to a single micro-region or a single process of agricultural origins, but to at least two or perhaps three (Willcox 2005).

Civáň et al. (2013) used datasets of nuclear gene sequences and novel markers detecting retrotransposon insertions in ribosomal DNA loci, to reassess the evolutionary relationships among subspecies of tetraploid wheat, T. turgidum. They concluded that domesticated emmer has a reticulated genetic ancestry, sharing phylogenetic signals with wild populations from all parts of the wild emmer range. Assuming that the extent of the genetic reticulation cannot be explained by post-domestication gene flow between domesticated emmer and wild emmer, they suggested that domesticated emmer originated from a hybridized population of different wild lineages. Consequently, they claimed that the phylogenetic relationships among tetraploid wheats are incompatible with simple linear descent of the domesticated emmer from a single wild emmer population and proposed an alternative model for the emergence of domesticated emmer. During a pre-domestication period, diverse wild populations were collected from a large area west of the Euphrates and were cultivated in mixed stands. Within these cultivated stands, hybridization gave rise to lineages displaying reticulated genealogical relationships with their ancestral populations. Domesticated emmer was derived from such reticulated lines of wild emmer. Then again, the mixtures of wild and domesticated emmer that existed for one millennium or more, enabled gene flow between the newly formed genotype bearing a non-brittle rachis and the different genotypes of wild emmer. Moreover, during the long period of cultivation, introgression of various wild emmer genes into domesticated emmer, from plants that grew close to domesticated emmer, was not a rare event (Blumler 1998; Luo et al. 2007).

Similar conclusion was reached by Oliveira et al. (2020). These researchers used genotyping-by-sequencing (GBS) to investigate the evolutionary history of domesticated tetraploid wheats and identified 1,172,469 single nucleotide polymorphisms (SNPs) in 189 wild and domesticated wheats. Principal component analyses separated wild emmer from e domesticated emmers and the naked wheats, showing that SNP typing by GBS is capable of providing robust information on the genetic relationships between subspecies of tetraploid wheat. Their data suggest that domesticated tetraploid wheats have closest affinity with wild emmers from the northern Fertile Crescent, consistent with the results of previous genetic studies on the origins of domesticated wheat. However, a more detailed examination of admixture and allele sharing between domesticates and different wild populations, along with genome-wide association studies (GWAS), showed that the domesticated tetraploid wheats have also received a substantial genetic input from wild emmer types from the southern Levant. Taking account of archaeological evidence that tetraploid wheats were first cultivated in the southern Levant, Oliveira et al. (2020) suggest that a pre-domesticated crop spread from the southern Levant to southeast Turkey and became mixed with a wild emmer population from the northern Fertile Crescent. Fixation of the domestication traits in this mixed population would account for the allele sharing and GWAS results that we report. Oliveira et al. (2020) also propose that feralization of the component of the pre-domesticated population that did not acquire domestication traits has resulted in the modern wild population from southeast Turkey displaying features of both the domesticated and wild emmer from the southern Levant, and hence appearing to be the sole progenitor of domesticated tetraploids when the phylogenetic relationships are studied by methods that assume a treelike pattern of evolution.

13.2.3.6 Site(s) of Origin of Domesticated Emmer

Domestication may not have taken place where the wild cereals were most abundant (Harlan and Zohary 1966). Why should anyone cultivate a cereal where natural stands are as dense as a cultivated field? If wild cereal grasses can be harvested in almost unlimited quantities, why should anyone bother to till the soil and plant the seed? Harlan and Zohary (1966) suspected that harvesting of wild cereals lingered on long after some people had learned to farm, and that farming itself may have originated in areas adjacent to, rather than within, the regions of greatest abundance of wild cereals.

Already Harlan and Zohary (1966) assumed that the thin, sporadic stands of wild emmer in the Taurus-Zagros arc would hardly have been very attractive to the food-collecting cultures of the region. In contrast, the massive stands around the Lake of Galilee would surely have been more useful to a harvester of wild grass seeds. The archaeological and genetic evidence tends to point in the same direction (Table 13.1). While there is currently more knowhow on the genetic changes that led to the formation of domesticated emmer, the site(s) in which it was formed still require more clarification. Harlan and Zohary (1996) assumed that most of the domesticated subspecies of T. turgidum derived from the race of wild emmer now found in the upper Jordan watershed and thus, concluded that emmer was likely domesticated in that region.

Harlan and Zohary (1966) were not the first to propose the southern Levant as the place where wheat farming began. Vavilov, travelling the Levant in 1926, noticed a peculiar form of wild wheat that accompanied domesticated durum wheat in Israel (then Palestine) (Vavilov 1957), and concluded that it must be the wild progenitor of domesticated wheats because of its similarity to domesticated ssp. durum. This form was very distinct from other forms of wild emmer, which led Vavilov to classify it as a subspecies of wild emmer. Its spikes were large, with rough spikelets and large grains, undoubtedly representing the closest wild source of cultivated wheat, especially of durum wheat (Vavilov 1962). This type of wild emmer is currently considered as a derivative from hybridization of wild x domesticated durum wheat.

Archaeological data showed that the cultivation of wild emmer took place within or near the current geographical distribution area of wild emmer, i.e., in the Levantine Corridor (Hillman, 1996; Harris, 1998; Bar-Yosef, 1998; Bar-Yosef and Kislev, 1989; Bar-Yosef and Belfer-Cohen, 1992). The Levantine Corridor is the western part of the Fertile Crescent, encompassing a relatively narrow strip east of the Mediterranean Sea, from southeast Turkey in the north, to the Sinai Peninsula in the south (Bar-Yosef 1998). This corridor was divided into northern part including southeastern Turkey and northern Syria, and southern part including southwestern Syria, Southeastern Lebanon, eastern Israel and western Jordan., Bar-Yosef and Kislev (1989) and Kislev (1989, 1992) stated that wild emmer is the earliest wheat in archaeological material from early Neolithic sites in the southern Levantine Corridor (Table 13.1).

It is reasonable to assume that during the long cultivation of wild emmer (10,300–9500 BP) non-brittle types of emmer were derived from mutations of brittle types in various sites in the Levantine Corridor. Indeed, remnants of apparently non-brittle emmer wheat, mixed with remnants of brittle types, were found in several sites of the Near East, dating to the middle of the 9th millennium BP (Table 13.1).

Unfortunately, it is not known when man adopted the mutations that transformed wild emmer into domesticated emmer. In spite of being hulled and therefore, difficult to thresh, domesticated emmer was one of the most prominent crops for almost 6000 years, from the PPNB to the Iron Age, in farming villages throughout the Near East. In the 8th millennium BP, it was taken from the hilly and mountainous regions of the Fertile Crescent, in mixtures with naked tetraploid wheat, to the lowlands of Mesopotamia, from where it spread further to Central Asia and India, and westward to Anatolia, then spreading to the Mediterranean basin and Europe. During the 6th millennium BP, domesticated emmer was taken to Egypt, and later to Ethiopia, some 5000 years ago (Feldman 2001).

Domesticated emmer could have evolved from wild emmer by a monophyletic, diphyletic, or polyphyletic manner. Salamini and coworkers, using amplified fragment length polymorphism (AFLP) fingerprinting of nuclear DNA to estimate the genetic similarity between wild and domesticated emmer populations (Ozkan et al. 2002; Salamini et al. 2002), concluded that wild populations from the Karacadag Mountains in southeastern Turkey, are more similar to domesticated emmer than are other wild populations. On the other hand, Mori et al. (2003), using large-scale chloroplast DNA fingerprinting, analyzed larger sample of wild emmer from southeastern Turkey and Iraq. They concluded that two distinct maternal lineages were involved in the domestication of emmer, and that the domestication of emmer occurred independently in at least two locales; one site was in the Kartal Dagi Mountains, southern Turkey, about 280 km west of Karacadag, and the second could not be identified, but several closely related haplotypes of wild emmer were found in four geographically distant regions. After analyzing these same accessions by AFLP, Ozkan et al. (2005) concluded that wild emmer accessions from the Karacadag region and from the Sulaimanyia region in northern Iraq, are equally closely related to domesticated emmer. To revisit the question of emmer domestication, Luo et al. (2007) pointed out that populations of wild emmer from the Kartal Dagi Mountains and Urfa plateau in southeastern Turkey, and from Iraq and Iran, were either inadequately sampled or not sampled at all by Ozkan et al. (2002), and consequently, the site at which emmer was domesticated remains inconclusive. They performed restriction fragment length polymorphism (RFLP) analysis at 131 loci and found that gene flow between wild and domesticated emmer occurred massively across the entire area of wild emmer distribution and concluded that emmer was likely domesticated in the Karacadag area (Diyarbakir region) in southeastern Turkey, which was followed by subsequent hybridization and introgression from wild to domesticated emmer in the southern Levant. Alternatively, although less likely from their point of view, emmer was domesticated independently in the Karacadag area and in the southern Levant (Luo et al. 2007). Thus, the data of Mori et al. (2003), Ozkan et al. (2002, 2005, 2011), and Luo et al. (2007) suggest a diphyletic or polyphyletic origin of domesticated emmer.

Nave et al. (2019) investigated, via haplotype analysis of a large collection of wild and domesticated emmer accessions, the geographical birthplace of the recessive mutations in the brittle rachis genes, Br-A1 and Br-B1 (=Btr1-A and Btr1-B), mutations that determine spike non-brittleness. The precursor of the domesticated haplotype of br-A1 was detected in 32% of the wild accessions gathered throughout the Levant, from central Israel to eastern Turkey. In contrast, the precursor of the domesticated haplotype of br-B1 was found in only 10% of the tested wild accessions, all from the southern Levant. Moreover, this haplotype is shared by all domesticated tetraploid and hexaploid wheats tested so far (Naveh et al. 2019). The phytogeographical results challenge the above thinking regarding the birthplace of domesticated emmer as well as regarding the concept of di- or poly-phyletic origins of domesticated emmer. Specifically, the precursor of br-B1 presents a direct evolutionary link between domesticated wheat and wild emmer from the southern Levant, contrary to the widely held view that the northern Levant was the center of domestication of wild emmer. On the basis of the evidence presented by Nave et al. (2019), it is hypothesized that humans may have spread certain wild emmer genotypes from across the Fertile Crescent prior to domestication. Such a ‘pre-domestication circulation of wild crop progenitors’ theory could reconcile the wide distribution of the precursor of the domesticated br–A1 haplotype and also aligns well with archaeological evidence of wild crop harvesting and utilization in the southern Levant prior to the Neolithic era. However, the identification of a single southern Levant wild emmer genotype that carried the progenitor haplotypes for the alleles br-A1 and br-B1 suggests that the first domesticated emmer appeared there. Later, domesticated emmer spread out to northern Levant, capturing local genetic diversity along the way, while maintaining the br-A1 and br-B1 mutations.

Recently, Gornicki et al. (2014) sequenced whole chloroplast genomes isolated from twenty-five chloroplast genomes, and genotyped 1127 plant accessions representing 13 Triticum and Aegilops species. They detected a higher diversity of the chloroplast genome in the southern Levant populations of wild emmer than in the northern population and suggested that wild emmer originated there. They further suggested that the major chloroplast haplotype H1 of wild emmer was the founder of all domesticated species of the lineage, most of which also carry H1. This haplotype exists in the southern Levant as well as in southeastern Turkey. The geographic distribution of the chloroplast haplotypes of the wild tetraploid wheats, wild emmer and wild timopheevii, and those of Ae. speltoides, the assumed donor of the cytoplasm to the tetraploid wheats, demonstrates the possible geographic origin of the emmer lineage in the southern Levant and of the timopheevii lineage in northern Iraq.

13.2.4 Origin of Tetraploid Wheat with Free-Threshing Grains

13.2.4.1 Genetic Control of the Free-Threshing Trait

The loss of self-seed dissemination and the temporal control of germination was followed in the more advanced domesticated subspecies of T. turgidum, by the loss of self-seed protection. Both the wild form and the more primitive domesticated forms of T. turgidum, i.e., ssp. dicoccon and ssp. paleocolchicum, have hulled grains. Their grains are enclosed in the spikelet by tough glumes that do not break during threshing. In hulled wheats, the products of threshing are spikelets, rather than grains. Usually, the spikes of hulled domesticated wheats break during threshing at the same place at which their wild counterpart disarticulates spontaneously. In other words, threshing these still primitive wheats ‘mimics’ the shattering pattern of their wild progenitor; the individual spikelet with the internode segment at the base of the product of threshing. However, instead of the smooth abscission scars, which characterize the wild form, the surface of the breakage scars in the domestic hulled wheats is rough.

Domestication of tetraploid wheats went one step further, changing the hulled ssp. dicoccon to a free-threshing form. Hence, the free-threshing (or naked) tetraploid wheats emerged from already domesticated crops, i.e., domesticated emmer wheat, and are therefore termed secondary crops (Hillman and Davis 1990). All more advanced domesticated subspecies of T. turgidum are free threshing. Their glumes are thinner and do not invest the grains tightly, and their rachis is fully tough. Consequently, threshing releases naked kernels. Because of this difference in threshing product, hulled wheats handling by the farmer is different from handling of the free threshing. In the hulled wheat, the grains have to be freed from the spikelets (usually by pounding) before they can be used. The utilization of naked wheat is simpler since threshing yields free grains. Because of the different appearances of the marketed products, hulled and free-threshing wheats were often regarded in antiquity as different cereals, and they were even called different names. For instance, in the bible (Exodus 9:32) free-threshing type was called ‘wheat’ (chitta in Hebrew) whereas hulled type was called ‘rie’ (=spelt; kosemeth in Hebrew).

The appearance of naked kernels was the second most important step in domestication of tetraploid wheat, after development of the non-brittle rachis. This trait could have been derived from a mutation that reduces the toughness of the glumes and increases the rigidity of the rachis (McFadden and Sears 1946; Morris and Sears 1967). The hulled, non-free-threshing emmer wheats contain the q gene, which determines a speltoid spike, characterized by an elongated spear-shaped spike, easily broken rachis and rigid, thick glumes. A clear distinction of glume shape has long been known between free-threshing ssp. carthlicum, which bears round glumes without keels, like free-threshing hexaploid wheats, and other free-threshing tetraploid wheats, which bear boat-shaped, keeled glumes (Muramatsu 1986). Watkins (1928, 1940) reported that, with the exception of ssp. carthlicum, all tetraploid wheats, including the free-threshing forms, had keeled glumes and therefore, possess the gene K, which was later determined to be the same as q (Mac Key 1954a). Mac Key (1954a, b, 1966, 1968, 1975) suggested that the free-threshing tetraploids contain a polygenic genetic system dictating this trait. However, Muramatsu (1978, 1979, 1985, 1986) showed that all the free-threshing forms of tetraploid wheat carry the dominant allele Q, but its expression may be modified, to some extent, by the genetic background (Muramatsu 1986). The indication is that the keeled glume in tetraploid wheat is due to genes of the A and B genomes and not to genes of the D genome (Fans et al. 2006). This is implied from the fact that glumes do not show a clear keel in Ae. tauschii, the D genome donor to the hexaploid wheats (Muramatsu 1986). Muramatsu assessed the effects of chromosome 5A of various tetraploids in the background of hexaploid wheat, cv. Chinese Spring, and found that not only ssp. carthlicum has the QQ genotype, but also ssp. polonicum and ssp. durum. There are some varieties of tetraploid wheat that have square-head spikes. Because squareheadedness is one of the pleiotropic effects of Q, it is unlikely in a plant with genotype qq. Indeed, Muramatsu (1979) demonstrated that ssp. dicoccon var. liguliforme, which has a semi-tough rachis with keeled glumes and square-headed spikes, contains Q. Based, on this, Muramatsu (1986) concluded that there is a wide phenotypic variation of characteristics in different QQ lines, and suggested that this range of variation is very narrow in the absence of Q, but when Q is present, they express an obvious phenotype.

Thus, all the extant, free-threshing tetraploids contain the dominant Q factor, determining the free-threshing trait (Muramatsu 1986; Simons et al. 2006). The Q factor, located on the long arm of chromosome 5A (Sears 1954), is one of the most significant domestication loci, as it controls the free-threshing characteristic and several other domestication-related traits. While the 5A homoeoallele has the most significant contribution, other homoeoalleles (on 5B in tetraploid and 5B and 5D in hexaploid wheat) were also shown to be involved in the domestication traits (Zhang et al. 2011). The q gene homoeoallele on chromosome 5B became a pseudogene after allotetraploidization. Expression analysis indicated that, whereas Q plays a major role in conferring domestication-related traits, the q homoeoallele on 5D contributes directly and q on 5B indirectly to suppression of the speltoid phenotype. Hence, according to Zhang et al. (2011), the evolution of the Q/q loci in polyploid wheat resulted in the hyper-functionalization of Q on 5A, pseudogenization of q on 5B, and sub-functionalization of q on 5D, all contributing to the domestication traits.

Q is thought to be a major regulatory gene for floral development (Muramatsu 1986). It encodes an AP2-like transcription factor that played an important role in the domestication of polyploid wheat (Simons et al. 2006; Zhang et al. 2011). Actually, Q has pleiotropic influences on many other domestication-related traits, such as spike density and length (square-head spike), fertility of the basal spikelets, glume shape and tenacity, glume keel formation, rachis fragility, plant height, spike emergence time, and chlorophyll pattern along nerves as well as grain size and shape, in a complicated interaction pattern (Mac Key 2005; Simons et al. 2006). This pleotropic effect stems from the fact that Q is a transcription factor that activates many different genes (Simons et al. 2006; Zhang et al. 2011). Actually, the Q gene has a high degree of similarity to members of the AP2 family of transcription factors.

The mutation from q to Q occurred at the tetraploid level, from where it was transferred to hexaploid wheat (Simons et al. 2006). Muramatsu (1963) proposed that Q is a triplication of q, since five doses of q conferred the same phenotype as two doses of Q. However, the Q gene was recently isolated and characterized (Faris et al. 2003; Jantasuriyarat et al. 2004; Simons et al. 2006) and the data of the latter, involving Southern analysis and sequencing of a large bacterial artificial chromosomes (BACs) spanning Q, indicated that Q is not a duplication of q, but most likely arose through a gain-of-function mutation. The two alleles differ in a single nucleotide (GAG in q and GCG in Q), leading to a change in one amino acid, namely, all q-containing forms have valine in position 329, whereas all Q-containing forms possess an isoleucine at this position (Simons et al. 2006). The mutation that gave rise to Q occurred only once leading to the world’s cultivated wheats (Simons et al. 2006). Although Simons et al. (2006) considered a SNP, leading to the substitution of a valine by an isoleucine at position 329, as a possible cause for the Q mutation, they also noted a conserved SNP in the miRNA 172 binding site. Subsequent work by Debernardi et al. (2017) showed that in fact the SNP within the miRNA binding site is the causal polymorphism for the functional difference between the Q and q alleles. Moreover, Q is more abundantly transcribed than q (Simons et al. 2006), which lies in accord with the finding of Muramatsu (1963) who showed that extra doses (five or six) of q mimics the effect of Q in common wheat. Increased transcription of Q was most obviously associated with spike compactness and reduced plant height, as in plants tetrasomic for chromosome 5A (Sears 1954). The higher level of transcription of Q is also consistent with its dominant nature, as well as with the disruption of the miRNA 172 suppressive effect (Debernardi et al. 2017). Zhang et al. (2020) reported on new advances on Q’s mode of action using transcriptomics and phenotypic analyses. They show that modification of cell wall thickness and composition of glumes, e.g., lignin versus cellulose ratio, correlates with the expression of genes involved in secondary cell wall biosynthesis.

The discovery of a gene on the short arm of chromosome 2D of Ae. tauschii, designated Tg, that affects threshability by conferring tenacious glumes in synthetic 6 × amphiploids T. turgidum-Ae. tauschii (Kerber and Dyck 1969; Kerber and Rowland 1974; see Sect. 13.3.2), promoted search for orthologous Tg genes in subgenomes A and B of T. turgidum and in its parental diploids. Consequently, Simonetti et al. (1999), analyzing a set of recombinant inbred lines, derived from a cross between the cv. Messapia of durum wheat and accession MG4343 of wild emmer, found that chromosome arm 2BS of wild emmer carries a gene that suppresses the free-threshing trait by determining tenacious glumes. Evidently, chromosome arm 2BS of wild emmer contains a Tg allele that determines tough glumes and 2BS of the free-threshing form ssp. durum contains the tg allele determining soft glumes (Simonetti et al. 1999). That the Tg allele exists on chromosome 2M^o of Ae. geniculata was shown by conferring tenacious glumes thru these chromosomes in addition and substation line in bread wheat (Friebe et al. 1999). Moreover, these authors reported that the spike morphology of disomic addition line 2M^o is similar to those of all of the homoeologous group 2 chromosomes of the Triticeae that have been added to CS wheat, in having tenacious glumes. Faris et al. (2014) and Sharma et al. (2019) reported that domesticated emmer, like wild emmer, also carries Tg alleles on chromosomes 2A and 2B. In accordance with the rule for the symbolization of genes in homoeologous sets, the Tg genes in emmer wheat should be designated Tg-A1 and Tg-B1. The gene Tg-B1 was found homoeoallelic to Tg-D1 on chromosome arm 2DS of Aegilops tauschii (Faris et al. 2014).

Genetic analysis indicated that the effects of the three genes, Q, tg-A1, and tg-B1, are additive, with Q having the most profound effect on threshability, and that free-threshing alleles are necessary at all three loci to attain a complete free-threshing phenotype (Sharma et al. 2019). Thus, the free-threshing trait in tetraploid wheat is determined by the three complementary genes, Q, tg-A1 and tg-B1, and therefore, three mutations were required to produce the free-threshing character in ssp. parvicoccum, the primitive free-threshing tetraploid wheat, changing the genotype qqTgTgTgTg to QQtgtgtgtg (Jantasuriyarat et al. 2004; Faris et al. 2014; Sharma et al. 2019). These mutations must have occurred within a relatively short period (Sharma et al. 2019). Domesticated emmer, ssp. dicoccon, first appeared in the Levant in the early PPNB period, i.e., from 9500 to 9000 uncalibrated years ago, and early free-threshing tetraploid wheat, presumably spp. parvicoccum, appeared in the middle to late PPNB, i.e., 9000–7500 uncalibrated years ago (Table 13.1). Muramatsu (1979) showed that a variety of domesticated emmer, ssp. dicoccon var. liguliforme, which has a semi-tough rachis with keeled glumes, contains the Q allele. This taxon is hulled presumably because it carries the Tg alleles on chromosome 2A and/or 2B. The presence of Q in some lines of domesticated emmer may indicate that the mutation from q to Q may have already occurred in this subspecies.

Another major gene affecting the free-threshing trait was identified and allocated to the short arm of chromosome 2A^m of diploid wheat, T. monococcum (Taenzler et al. 2002; Sood et al. 2009). A recessive mutation of this gene, called sog (soft glume), determines the soft glume trait in var. sinskajae of domesticated T. monococcum [according to van Slageren (1994) this taxon should receive the rank of a variety rather than of a species]. Whereas Simonetti et al. (1999), Taenzler et al. (2002), and Jantasuriyarat et al. (2004) considered tg and sog to be orthologoues, Sood et al. (2009), comparing the map positions of sog and Tg using homoeologous group-2-specific RFLP markers, found these genes to be non-orthologous.

Several minor genes and modifiers also are involved in determining the threshability trait (Tzarfati et al. 2014). Similar to the finding of Jantasuriyarat et al. (2004) in hexaploid wheat, QTLs for threshing time and threshing efficiency were found on the long arm of chromosome 5A of tetraploid wheat, overlapping with the position of the Q gene, and on chromosomes 2A and 2B which, according to their locations, may correspond to the sog gene (Sood et al. 2009). Peleg et al. (2011) and Tzarfati et al. (2014) detected additional QTLs affecting threshability on chromosome 4B and chromosome 3A. QTLs affecting glume toughness were also found on chromosomes 4A, 6A and 7B (Simonetti et al. 1999; Peleg et al. 2011). Taken together, the threshability trait in tetraploid wheat seems to be under the control of several major and minor genes (Tzarfati et al. 2014).

Interestingly, the number of domestication-related QTLs mapped to the A subgenome was two-fold higher than those found on the B subgenome, i.e., 24 QTL effects for domestication and domestication-related traits in the A subgenome versus only 11 such QTLs in the B subgenome (Tzarfati et al. (2014). This is in accordance with the concept of ‘genome asymmetry’, implying that the A subgenome is dedicated to the control of morphological traits, house-keeping metabolic reactions and yield components (Peng et al. 2003; Feldman et al. 2012).

In addition to the above-mentioned classical domestication traits that were selected in the process of domestication of ssp. dicoccon and evolution of ssp. parvicoccum, durum and other free-threshing subspecies of T. turgidum, several other domestication traits that were advantageous to the farmer were selected over time. These include plant erectness versus the prostrate grassy types, simultaneous ripening of grains, increased number of seeds per spikelet, increased grain size, and reduced seed dormancy (Feldman 2001). Golan et al. (2015), compared grain weight, embryo weight, and the interaction between these two traits in ssp. durum and wild emmer wheat. They found that grain weight was increased under cultivation without any parallel change in embryo weight, resulting in a significantly reduced (30%) embryo weight/grain weight ratio in durum wheat. Using a population of recombinant inbred substitution lines, they found that a cluster of loci affecting grain weight and shape was located on the long arm of chromosome 2A, whereas a locus controlling embryo weight was mapped to the short arm of chromosome 2A. Their results suggest a differential selection of grain and embryo weight during the evolution of domesticated wheat.

At the end of the Pre-Pottery Neolithic B period in the Near East, about 7500 uncalibrated years BP, all the major evolutionary processes required to produce domesticated tetraploid wheat had already been completed (Kislev 1984). Notably, the origin and establishment of the main domesticated wheat, ssp. dicoccon and ssp. parvicoccum, occurred within the first millennium of cultivation, i.e., in the middle of the 9th millennium BP. The establishment of these new forms within such a short span of time must have been associated with an extremely high rate of evolution. This burst of evolutionary changes presumably resulted from the conditions prevailing in the cultivated field, which were entirely different from those in the wild. The selection pressures imposed by early farmers were understandably different from those operating in the wild. Characters with a negative advantage in nature were preferred under cultivation, thus establishing new evolutionary trajectories. This might explain the appearance of types with a non-brittle rachis, naked grains, and an erect stature, which are characterized by uniform, rapid germination. The spread of wheat culture to neighboring regions exposed the plant to new climatic, edaphic, and biotic conditions, and, hence, to new sets of selection pressures. The wheat genotypes might have reacted by increased mutability, possibly due to the temporal activation of various transposable elements, as a result of the new environmental stresses. In addition, the spread of domesticated tetraploid wheat to new regions facilitated contact between different domesticated related species, such as different types of tetraploid and hexaploid wheat, or even with wild species of related genera, with which they could exchange genes.

Several puzzling questions still remain regarding the evolution of the various domesticated tetraploid wheats: the time and site(s) of origin of domesticated emmer, the mode of origin of the first naked tetraploid wheat, ssp. parvicoccum, and the reasons for its extinction about 1900 years ago [Kislev (1986) reported this date as the latest cropping of ssp. parvicoccum in southern Judea, Israel], and the reasons for the late establishment of ssp. durum as a major crop in the Near East, despite its sporadic occurrence in older archaeological material from 7500 to 6500 years BP (Nesbitt and Samuel 1996).

13.2.4.2 Time of Complete Replacement of Cultivated Wild Emmer by Domesticated Tetraploid Wheats

At the dawn of cultivation, wild emmer wheat must have been sown from seeds gathered from wild stands, i.e., domestication occurred during the course of cultivation (Hillman and Davis 1990). Measured domestication rates in crops of wild cereals indicated that emmer and barley domestication would have occurred only if they were harvested in a partially ripe (or near‐ripe) state, using specific harvesting methods (Hillman and Davis 1990; Zohary 1996). Hillman and Davies (1990, 1999) concluded that under certain conditions, namely, non-conscious selection of plants with non-brittle spikes, wild einkorn, emmer, and barley could become completely domesticated within 200 years, and perhaps even 20–30 years. The mutation rates in plants are commonly between 10⁵ and 10⁶ per base pair, per year (Rédei 1998). Assuming a mutation rate of 10⁵ for genes that affect spike brittleness, and since non-brittleness in domesticated emmer is determined by two recessive homoeoalleles, the rate of spontaneous mutation in both genes is 10¹⁰. Supposing a sowing of 200 grains per m², which is observed in traditional cropping systems (Gepts 2004), both of these mutations would be expected to appear in a total planting area of 5000 ha. Hillman and Davies (1999) estimated that areas sown in the PPNA for a family of five, ranged between approximately 0.5 and 2.8 ha. Assuming that the average family field was about 1.5 ha and that there were several families (five) in each of the approximately 30 PPNA sites, then the area of cultivated wild emmer at that period was about 225 ha. It would then take about 20–25 years for a plant containing the two mutant alleles to be formed. Due to the self-pollinating system that is predominant in emmer wheat, homozygous plants for the two mutant homoeoalleles would be fixed within a few generations. If people harvested wheat with a sickle and cut the entire spike, or plucked individual ears, or pulled plants up from the roots, this would tend to disperse shattering spikelets and retain all non-fragile mutants. In this manner, genotypes with a non-brittle rachis could be replanted the following year and over time, would dominate the population at the expense of wild, brittle types (Hillman and Davies 1990).

Yet, Fuller (2007) claimed a much slower replacement under cultivation of wild emmer plants by those with non-brittle rachis, i.e., 1000–2000 years later. The PPNA period lasted 800 years, during which several plants with non-fragile spikes could have independently formed in different fields of the Levant. Moreover, during the first millennium of the PPNB, wild and domesticated emmer were grown in mixed stands (Kislev 1984), and several additional mutants with non-brittle spikes could have independently formed. Mutation rates may, therefore, have not been a limiting factor in the multiple formations of domesticated emmer (Gepts 2004). Moreover, the mixture of a few genotypes of domesticated emmer with many genotypes of wild emmer in many fields, facilitated inter-genotypic hybridization, resulting in countless transfers of alleles for non-brittle spikes to other genotypes of wild emmer.

Archaeobotanical evidence clearly shows that the process of domestication was very slow (Willcox 1995) and finds indicate that domestic and wild cereals occurred as mixtures on several early Neolithic sites over a period of at least one millennium (Willcox 1998). Remains of wild emmer are identified by the smooth scar on the rachis segment, indicating normal abscission, whereas, in domesticated emmer, the scar is rough because the rachis had been broken apart by threshing. Archaeobotanical finds clearly show that late Epipalaeolithic and early Neolithic distributions of wild cereals were much more extensive than in earlier periods (Hillman 1996), and that the collected cereals differed on the various sites. But once cultivation of several preferred crops had been chosen, they became widespread on the account of others. For example, emmer, became more widespread at the expense of einkorn in southern part of the Fertile Crescent.

Gross and Olsen (2010) pointed out that while artificial domestication experiments conducted in cereals showed that classical domestication traits, such as the loss of rachis fragility and seed dormancy, can arise and increase in frequency over a short time period when subjected to strong selection (Hillman and Davis 1990), archaeological data indicate that the appearance of plants with non-brittle rachis was gradual, at least in wheat and barley (Kislev 1984; Tanno and Willcox 2006; Fuller 2007). In these crops, the non-brittle phenotype appeared only after an increase in grain size, a trait that, itself, reflects selection for germination under active cultivation conditions. Thus, although the loss of rachis fragility would be expected to greatly facilitate the harvesting of grains in planted fields, the phenotype did not actually appear in the initial stages of active cultivation and selection (Fuller 2007; Gross and Olsen 2010).

The archaeological evidence does not support the view that harvesting with a sickle was the selective force that led to rapid domestication of plants with a non-brittle rachis. Preserved sickles, or lithic sickle blades, are known from the Natufian period (13,000–10,300 years BP), in a period for which there is no evidence for domesticated wheats. Namely, in the Near East, sickles were in use prior to agriculture and were only applied to harvest wheat relatively late, after domestication (Fuller 2007). It is possible that sickles were used for harvesting of sedges (Cyperaceae) and reeds (Phragmites), as materials for basketry or thatching (Kislev 1984; Fuller 2007). As indicated by the archaeobotanical evidence (Fuller 2007), the rate of evolution of tough rachis einkorn and barley was far too slow to be accounted for by a model of conscious strong selective pressure that would be expected if sickling was used for cereal harvest, as modelled by Hillman and Davies (1990). Thus, it appears that early cultivators continued to employ the time-efficient harvesting methods associated with hunter-gatherers. Once cultivated populations had noticeably large proportions (majorities) of non-shattering types, then the transfer of the sickle technology to agriculture may have been seen as an obvious enhancement. As others have noted, the harvesting of cereals when green, i.e., immature, by plucking or beating, regardless of technique, will not select for domesticated types (Hillman and Davies, 1990; Willcox, 1999).

The gradual and prolonged replacement of wild by domesticated forms presumably was caused by the fact that farmers used to harvest immature spikes or collect the spikelets from the ground rather than harvested mature spikes (Kislev 1984; Kislev et al. 2004) applying only a weak selection in favor of the latter. If wild cereals were harvested simply by passing through stands and shaking or beating spikes to knock spikelets into a basket then the shattering, wild-type genotypes would be the ones to predominate in the next year’s crop. Also, a good portion of wild spikelets that fell to the ground were not collected and germinated next season, contributing to the new generation of wild wheat. All these practices delayed the selection for domesticated emmer.

The complete replacement of wild emmer by genotypes with non-brittle rachis was a gradual process (Fuller 2007). Actually, quantitative assessment of diploid wheat rachis remains from several sites suggested a gradual increase in the proportion of the domesticated-type spikes over the course of the PPNB (Tanno and Willcox 2006). Similar estimates were made for emmer wheat (Kislev 1984). In general, there is contrast between early sites, which largely or entirely contain wild-type chaff remains, while later sites are dominated by domesticated-type remains, with some intermediate proportions for sites chronologically in the middle. The rates of evolution do not come anywhere close to the 20–100 years estimated by Hillman and Davies (1990), who assumed sickle harvesting of morphologically wild near-mature plants or uprooting of whole plants. This vast difference in domestication rates raises questions about how to explain the absence of selection for domesticated non-shattering genotypes. From their data, Tanno and Willcox (2006) suggested that domestication occurred somewhat more quickly in wheat, perhaps around 1500 years, as opposed to barley, which shifted over a period of 2000 years or slightly more. In accord, Fuller (2007) drew attention to the fact that the shift to non-brittle rachis (full domestication) appears to have started about 500–100 years after large grain size had already evolved (semi-domestication), and this might therefore suggest a minimum estimate of 2000 years for the evolution of both aspects of the domestication syndrome.

The first free-threshing tetraploid wheat, ssp. parvicoccum, appeared several hundred years after the appearance of domesticated emmer, ssp. dicoccon (Kislev 1979/1980). These two tetraploid forms were the most important crops in the Mediterranean basin and Near East until the Hellenistic period, (ca. 2300 years ago), when they were gradually replaced by the more advanced, free threshing, with large grains, durum wheat, T. turgidum ssp. durum (Nesbitt 2002). This subspecies was already found by Hillman (1978) in layers of Pottery Neolithic Can Hassan III (7500–6200 years BP). It is perplexing that in spite of its early origin, ssp. durum was established as a major crop in the Mediterranean basin and the Near East only during the Hellenistic period. Perhaps the early types were less adapted to the conditions in these regions than were ssp. parvicoccum and ssp. dicoccon. Adapted genotypes of ssp. durum only evolved at a later period by a series of mutations, or through introgression of genes from the other two tetraploid subspecies. The cultivation of ssp. durum as an admixture with ssp. parvicoccum in the Near East may explain the morphology of some peculiar taxa of ssp. durum, such as Horan wheat in the Levant, characterized by a short compact spike and plump grains, reminiscent the spike of ssp. parvicoccum (Kislev 1979/1980). Crosses between ssp. durum and ssp. parvicoccum might have resulted in such intermediate forms.

Today, ssp. durum is the principal tetraploid wheat. Its yield is relatively high under moderately dry conditions, and it grows as a major crop in the Mediterranean Basin, the Near East, India, the USSR, and in low rainfall areas of the great plains of the United States and Canada. Its large, hard-textured grains yield low-gluten flour suitable for macaroni and semolina products. Most other free-threshing subspecies of tetraploid wheats (turgidum, polonicum, turanicum, and carthlicum) are probably of a relatively recent origin and deviate from ssp. durum in only a few characters (Mac Key 1966; Morris and Sears 1967).

Maccaferri et al. (2019) genotyped the Global Tetraploid Wheat Collection consisting of 1856 accessions that represent the four main germplasm groups historically involved in tetraploid wheat domestication and breeding: wild emmer, domesticated emmer, durum landraces, and durum modern cultivars. The results show that two domesticated emmer populations from southern Levant displayed the closest relationship to all durum landrace populations, while the modern durum cultivars germplasm was mostly related to the two durum landrace populations from North Africa and Transcaucasia.

13.2.5 Domestication of Timopheevii Wheat

Compared with the available information concerning the domestication of T. turgidum, little is known about the domestication of the second allotetraploid wheat, T. timopheevii. Domesticated timopheevii wheat is hulled with stiff glumes which its current cultivation is restricted to a few localities in Western Georgia, Transcaucasia (Zhukovsky 1928). It is generally accepted that domesticated timopheevii, i.e., ssp. timopheevii, derived from wild timopheevii, ssp. armeniacum (Jakubziner 1932; Dorofeev et al. 1980). Nesbitt and Samuel (1996) alleged that the restricted cultivation of ssp. timopheevii to western Georgia may indicate that this crop was a secondary domesticate: when emmer cultivation spread to Transcaucasia, local populations of ssp. armeniacum could have grown as a weed in emmer fields and eventually became domesticated. Badaeva et al. (1994) suggest that the high karyotypic stability of domesticated timopheevii, as compared to the high degree of polymorphism of wild timopheevii, may be the result of its recent domestication or to its domestication in restricted area. Also, Mori et al. (2009), thought that ssp. timopheevii was domesticated later than emmer. They evaluated molecular variation at 23 microsatellite loci in the chloroplast genome and found no variations among the analyzed six accessions of domesticated timopheevii, suggesting a monophyletic origin of this crop. Moreover, none of the wild timopheevii plastotypes collected in Transcaucasia were closely related to the plastotype of domesticated timopheevii. On the other hand, the plastotypes found in northern Syria and southern Turkey showed closer relationships with domesticated timopheevi suggesting that the cultivation of wild form of T. timopheevii and subsequent its domestication might have occurred in southern Turkey and northern Syria. Interestingly, Vavilov (1935) already suggested that T. timopheevii of western Georgia was probably originally introduced from northeastern Turkey. Likewise, Menabde and Ericzjan (1942; cited by Dorofeev et al. 1980) assumed that domesticated T. timopheevii was in the ancient kingdom of Urartu, eastern Turkey, and northwestern Iran. Immigrants from Urartu introduced it into western Georgia. This scenario of introduction of domesticated timopheevii into Georgia from the south should not be rejected (Dorofeev et al. 1980).

Furthermore, judging from the present geographical distribution of its wild progenitor, ssp. armeniacum, (Table 9.2), the carbonized grains, spikelets, and clay impressions found at Jarmo, northeastern Iraq, (ca. 8750 noncal years BP) and Cayonu Tepesi, southeastern Turkey, (ca. 9000 noncal years BP), that could belong to this taxon rather than to wild emmer, evidence that may indicate that during the down of agriculture both wild ssp. dicoccoides and wild ssp. armeniacum could have been taken into cultivation in southeastern Turkey and northern Iraq. If such assumed cultivation of ssp. armeniacum lasted sufficient time mutations for non-brittle timopheevii could have been happened. Then, this raises the following question: if domesticated timopheevii were indeed produced in the northern Fertile Crescent why were they replaced by domesticated emmer?

New archaeobotanical data (Jones et al. 2000), show that at three Neolithic sites and one Bronze Age site in northern Greece, spikelet bases of a “new” type of glume wheat (NGW) have been recovered. These spikelet bases are morphologically distinct from those of the typical domesticated einkorn, emmer, and spelt wheats. NGW were also recorded from Neolithic and Bronze Age sites in Turkey, Hungary, Austria, and Germany (Jones et al. 2000). Ulaş and Fiorentino (2021) analyzed morphologically remains of NGW spikelet bases from two Turkish settlements. attestating to its large-scale presence in Anatolia. It seems likely that the NGWs are tetraploids and have morphological features in common with T. timopheevi. Such domesticated timopheevii could have formed independently from the Georgian one by a separate domestication process(s) of wild timopheevii in the northern Fertile Crescent and its cultivation can spread westwards to Europe as a mixture with other wheats or as a pure crop. According to Jones et al. (2000), it is difficult to establish if the new type was cultivated as a pure crop or as a part of a mixture with other domesticated wheats. In any case, its cultivation has ceased over large geographical areas since the Bronze Age.

Czajkowska et al. (2020), using PCR primers specific for the wheat B and G subgenomes, detected DNA sequences from the G subgenome in two NGW accessions, the first comprising grain from the mid 9th millennium BP at Çatalhöyük in Turkey, and the second made up of chaff from the later 7th millennium BP site of Miechowice 4 in Poland. The Miechowice chaff also yielded a B genome sequence, which they ascribe to an admixture of emmer and the NGW. Their result therefore, support the conclusion of Jones et al. (2000) that the NGW is a member of the T. timopheevii group. Hence, domesticated timopheevii can no longer be looked upon as a minor crop, restricted to western Georgia, but instead must be viewed as a significant component of prehistoric Eurasian agriculture. It is therefore an important question why the cultivation of domesticated timopheevii come to an end in West Asia and Europe.

Nave et al. (2021) hypothesized that T. timopheevii, like T. turgidum, was also domesticated through mutations in the Bt1 genes, but should carry distinct, novel bt-A1 and bt-G1 mutated alleles. To examine this hypothesis, they analyzed the sequence variation associated with the brittle rachis trait in various wild and domesticated T. timopheevii and T. turgidum accessions. Their analysis revealed a novel, recessive, loss-of-function br-A1 allele in domesticated T. timopheevii, affecting a partially brittle rachis phenotype. This allele exists in all the studied accessions of domesticated T. timopheevii and was also found in one wild timopheevii accession that exhibits partial rachis brittleness. The mutation in this timopheevii allele is different from the mutation br-A1 of all studied accessions of domesticated T. turgidum. This mutation, found exclusively in the coding sequence of the bt-A1 haplotype of T. timopheevii, is responsible for changing seven amino acids in the C-terminal end of the protein coded by this mutation. Such a modification is expected to alter the function of the protein, contributing to the nonbrittle rachis phenotype. Using T. turgidum primers, the promoter region for Bt-B1 could not be amplified in any T. timopheevii accessions, exemplifying the gene-level distance between the two species (Nave et al. 2021). Their results support the concept of independent domestication processes for the two allotetraploid wheat species. The structure at the bt-A1 locus in each lineage supports the diphyletic origin of the two allotetraploid wheats (Mori et al. 2009; Jiang and Gill 1994).

A free-threshing mutant, designated T. militanae Zhuk. and Migush, was selected from a single specimen of ssp. timopheevii. According to van Slageren (1994), species described only on the basis of a mutation but never released as a commercial cultivar, should not be regarded as new species but rather, should be made synonym under the cultivated subspecies from which they were isolated. The genetic basis of the free-threshing trait in this mutant was not studied. It would be interesting to see if it controlled by the same genes system (Q and tg) as the free-threshing subspecies of T. turgidum.

A unique non-brittle cytotype of the timopheevii lineage is T. zhukovskyi, an auto-allohexaploid carrying genome GGAAA^mA^m, that was discovered in 1957, in Western Georgia, by Menabde and Ericzjan (Jakubziner 1959). This species is isolated from the Zanduri wheat (admixture of ssp. monococcum, ssp. timopheevii, and T. zhukovskyi) (Jakubziner 1959; Dorofeev 1966). It therefore must have been derived in the cultivated fields from hybridization between domesticated timopheevii (genome GGAA) and domesticated monococcum (genome A^mA^m). T. zhukovskyi is currently grown in a limited area in Transcaucasia and has never been cultivated alone.

13.3 Hexaploid Wheat

13.3.1 Introduction

Since no wild prototype of the hexaploid group is known to exist, many theories have been proposed as to the time, place, and way of origin of the various subspecies of T. aestivum. The fact that ssp. spelta is hulled wheat that disarticulates into spikelets when a slight mechanical pressure is appliedled as in thrashing, led de Candolle (1886), Hackel (1890), Schulz (1913b), and Carleton (1916) to consider it more primitive than the non-brittle, free-threshing hexaploid forms, and thus, as the oldest form of T. aestivum. This conclusion is further supported by the fact that all crosses of either hulled or free-threshing tetraploid wheats with all used lines of Ae. tauschii yielded only hulled forms resembling ssp. spelta, indicating that this subspecies is the prototype of hexaploid wheat (McFadden and Sears 1946; Kerber and Rowland 1974), and therefore, the predecessor of the more advanced, free-threshing forms. Already Schroder (1931), based on anatomical evidence, proposed that ssp. aestivum arose from ssp. spelta. With the understanding that ssp. spelta is the most primitive subspecies of T. aestivum, it was assumed that the free-threshing forms of T. aestivum derived from it as a result of mutations (McFadden and Sears 1946). Indeed, the principal differences between the major hexaploid taxa are due to one or two genes that affect gross morphology (Mac Key 1954b) (Table 10.11).

Yet, the genetic data suggesting that the first hexaploid wheats were hulled, spelt-type, and more primitive than the free-threshing forms, do not agree with the archaeological chronology. While free-threshing forms of T. aestivum, i.e., ssp. aestivum, were found at the middle of the 9th millennium BP and were abundant in the pre-historic Near East from the 8th millennium onwards, thus far there is archaeological evidence for ssp. spelta only a thousand years later (Kislev 1984). Neolithic, Near Eastern ssp. spelta is very rare and earlier evidence for the existence of ssp. spelta is still missing. There is evidence of spelta grains from Yarim Tepe II, northern Iraq, dating back to the 7th millennium BP and probably also from Yarim Tepe I, about one thousand years earlier (Kislev 1984). These discrepancies between the genetic and archaeological data pose some difficulties in tracing the early history of the hexaploids. Indeed, several researchers (see Tsunewaki 1968) postulated that spelt wheat could not be the progenitor of bread wheat, but rather, its derivative. On the other hand, assuming that the first hexaploids were hulled, their absence from the prehistoric remains of the Near East may indicate their lack of advantage over domesticated emmer and free-threshing forms of T. turgidum in that area. Ssp. spelta is grown today in extreme environments of the Near East, such as the high plateau of west-central Iran, eastern Turkey, and Transcaucasia. This cultivation is possibly of an ancient origin. The free-threshing ssp. aestivum was preferred by the early farmers of the region and quickly replaced the hulled forms. As man migrated to new areas, cultivated wheats encountered new environments, to which they responded with bursts of variation, resulting in many endemic forms.

13.3.2 Genetic Control of Non-Brittle Rachis in Hexaploid Wheat

McFadden and Sears (1946) were first to synthetize an allohexaploid wheat by crossing domesticated emmer, T. turgidum ssp. dicoccon (genome BBAA), with the wild diploid species, Ae. tauschii (genome DD) and doubled the chromosome number of the F₁ hybrid by colchicine treatment. The allohexaploid thus produced resembled morphologically T. aestivum ssp. spelta, exhibited full chromosome pairing and had high pollen and seed fertility. Also, its hybrids with several natural subspecies of T. aestivum had high pairing. McFadden and Sears (1946) thus demonstrated the origin of ssp. spelta, the primitive form of T. aestivum. The parents and the synthetic allohexaploid had different types of rachis fragility. The spike of ssp. dicoccon does not disarticulate at maturity but threshing breaks it upon the rachis node into wedge-type spikelets, each has the rachis internode below it, those of Ae. tauschii disarticulate at maturity below the rachis node into barrel-type spikelets, each carries a rachis internode besides it, and the synthetic spelta similar to the natural ssp. spelta, breaks after threshing into spikelets with rachis internode below, besides, below and besides, and without any rachis internode at all (McFadden and Sears 1946), indicating that the fragility gene(s) of the two parents are codominant. Codominance of the fragility genes was also reported by Kihara and Lilienfeld (1949) in synthetic hexaploid wheats, produced by crossing wild emmer ssp. dicoccoides with Ae. tauschii. They found that the spike is fragile; the wedge-type disarticulation is seen in the main part of the spikes and the barrel-type in the upper spikelets.

Similar codominant effect was noted by Sears (1941a, b) in the amphiploid wild einkorn T. monococcum ssp. Aegilopoides—Ae. tauschii. This synthetic allotetraploid had a relatively semi-tough rachis, that at maturity breaks in different places, above the rachis node as in ssp. aegilopoides, and below the rachis node as in Ae. tauschii. The codominance effect of these fragility genes was noted also in hybrid between wild emmer and Ae. tauschii (Matsumoto et al. 1963) and in an accession of the semi wild wheat, T. aestivum ssp. tibetanum (Tsunewaki et al. 1990).

The tetraploid parent ssp. dicoccon has non-fragile rachis that breaks after threshing into wedge-type spikelets, i.e., each spikelet with the rachis internode below it, while the diploid parent Ae. tauschii, being a wild species, disarticulates at maturity into barrel-type dispersal units, i.e., each spikelet with the rachis internode beside it. Li and Gill (2006) mapped rachis disarticulation genes in wheat and its wild relatives. The Br1 gene for wedge-type disarticulation was mapped to a region delimited by the DNA markers Xpsr598 and Xpsr1196 on the short arm of chromosomes 3A in T. timopheevii. The barrel-type disarticulation gene, designated by Li and Gill (2006) as Br2, was mapped in Ae. tauschii to an interval of 4.4 cm between Xmwg2013 and Xpsr170 on the long arm of chromosome 3D. Avni et al. (2017) found that the non-fragile rachis of the tetraploid parent is controlled by two compound recessive genes, br-A1 that contains the genes btr1-A and btr2-A and br-B1 containing the genes btr1-B and btr2-B (Avni et al. 2017). The btr1-A and btr1-B genes are complementary in bringing about loss-of-function of the dominant genes resulting in non-fragile rachis so that at maturity the spike remains intact on the culm and breaks into individual spikelets only upon slight application of mechanical pressure at threshing (Dorofeev et al. 1979). This type of tough rachis was called a ‘semi-tough’ rachis, not to be confused with the fully tough rachis of the free-threshing tetraploid subspecies wheats which remains intact when threshed (Hillman and Davis 1990).

In wild barley, Hordeum spontaneum, the formation of wedge-type dispersal units is genetically determined by the genes Btr1 and Btr2, a pair of dominant, complementary, linked genes mapping to the short arm of chromosome 3H. A 1 bp deletion in the Btr1 coding sequence, and one of 11 bp in the Btr2 coding sequence are sufficient to convert a brittle rachis to a non-brittle one (Pourkheirandish et al. 2015). In wild T. monococcum, i.e., ssp. aegilopoides, the substitution of a single residue in the Btr1 product converts a brittle to a non-brittle rachis (Pourkheirandish et al. 2018; Zhao et al. 2019). In the tetraploid wheat, mutations at both the A and B genome copies of Btr1 are required for the formation of a non-brittle rachis (Avni et al., 2017).Hence, Btr1 orthologs are required for disarticulation above the rachis nodes, since the loss-of-function btr1 mutant forms a non-brittle rachis in Hordeum, and in diploid and tetraploid wheat (Pourkheirandish et al. 2015, 2018; Avni et al. 2017). Ae. tauschii lacks an intact copy of Btr1 and disarticulates below the rachis nodes; the inference is that Btr1is not required to effect disarticulation below the rachis nodes.

All the Triticeae species that disarticulate at maturity into wedge-type spikelets have the Br1 compound locus that is located on the short arm of group 3 chromosomes (Watanabe et al. 2002; Li and Gill 2006). Hence, the Br1 genes are orthologous. Ae. tauschii exceptionally produces barrel-type dispersal units (Kihara 1954). Rather than mapping to the short arm of chromosome 3D, the locus responsible for this trait in Ae. tauschii maps to the long arm of chromosome 3D (Li and Gill 2006; Katkout et al. 2015; Amagai et al. 2015). Consequently, it was designated Br2 (Li and Gill 2006). The gene Br-D2 is paralogous to the genes Br1 on 3AS, 3BS, and 3DS. It presumably resulted from a duplication of Br-D1, followed by an intra-chromosomal transposition to the long arm of 3D (Li and Gill 2006).

Zeng et al. (2020b) found that the Br-D2 locus of Ae. tauschii lacks an intact copy of Btr1 and disarticulation in this species occurs below, rather than above, the rachis node, resulting in barrel-type spikelets. Thus, the product of Btr1 appears to be required for disarticulation to occur above the rachis node resulting in wedge-type spikelets. Zeng et al. (2020a) reason that Ae. tauschii could be an evolutionary intermediate between the Poeae/Aveneae and the Triticeae tribes, since members of the former two tribes also lack Btr1. However, unlike members of the Poeae/Aveneae, Ae. tauschii does harbor an intact copy of Btr2. The above reasoning would require that Btr2, and later Btr1, were acquired independently. An alternative evolutionary pathway assumed that the truncated Btr1 sequences present in Ae. tauschiihare a common origin with other diploid Aegilops species of the D- lineage (Marcussen et al. 2014), but it diverged from the D-lineage 5.37 MYA (Li et al. 2022). After this divergence Ae. tauschii lost its intact copy of Btr1, but retained the truncated one (Zeng et al. 2020a). Disarticulation below the rachis nodes could have evolved in Ae. tauschii following the de novo recruitment (or perhaps neofunctionalization) of a co-operating gene(s). The latter may include orthologs of genes known to be responsible for shattering in rice (see list of genes in Zeng et al. 2020a) since orthologs of these genes are present in Ae. tauschii and are transcribed in the immature spike. Especially, the sh4 and OsCPL1 orthologs showed higher expression than the other ones in the immature spikes of Ae. tauschii. However, the genetic basis of the barrel type dispersal unit is not wholly unmistakable and has yet to be determined (Zeng et al. 2020b).

The presence of a Btr2 gene in each of the Triticeae species examined, suggest that its product is involved in the determination of the brittle rachis trait above the rachis node. In barley, the finding that Btr2 expression occurs in a thin cell layer above the rachis node has been taken to imply that this gene contributes to the formation of the disarticulation zone (Pourkheirandish et al. 2015). Whether Btr2 in Ae. tauschii is involved in the same way below the rachis node remains an open question. However, it is clear that Btr2 transcript is generated in immature Ae. tauschii spikes, although at a rather low abundance (Zeng et al. 2020b).

The fragility trait relies on the development of a disarticulation layer, in most species above the rachis node, resulting in wedge type dispersal units, but in some species below the rachis node, resulting in barrel type dispersal units (Fig. 2.3). Zeng et al. (2020b) showed that in Ae. tauschii Btr2 transcript is present in a region below the rachis node where the abscission zone forms. The implication is that in this species, the Btr2 product is involved in the formation of barrel type.

There are two forms of ssp. spelta Iranian and European. The Iranian spelt has a wedge-type disarticulation whereas the European spelt has a barrel-type disarticulation. It is speculated that the Iranian spelt originated from a cross between a tetraploid line(s) of T. turgidum having tough rachis (genotype br-A1, br-B1) and Ae. tauschii having brittle rachis (genotype br-D1, Br-D2) while the European spelt originated from a cross between common wheat carrying a loss-of-function mutation (see below; genotype br-A1, br-B1, br-D1, br-D2) and domesticated emmer wheat (genotype br-A1, br-B1). Consequently, the Iranian spelt has the rachis fragility genotype br-A1, br-B1, br-D1, Br-D2 exhibiting codominance of the rachis brittle genes, whereas the European spelt as the genotype br-A1, br-B1, br-D1, br-D2 exhibiting tough rachis.

Kuckuck (1964) suggested that the hexaploid wheat with a fragile rachis found by Dekaprelevich (1961) in Georgia, Transcaucasia, may have originated as an amphiploid between wild emmer, ssp. dicoccoides, and Ae. tauschii, independently of the origin of domesticated hexaploids, which are believed to have involved free-threshing tetraploid wheat as their tetraploid parent. Sears (1976) crossed wild emmer, ssp. dicoccoides, with Ae. tauschii and obtained a F₁ hybrid with a brittle rachis. Therefore, B- and W-type disarticulations are governed by two different paralogous loci on group-3 chromosomes.

After its formation, ssp. spelta, was not cultivated in southwest Asia on a large scale, presumably it could not compete with the free-threshing forms of tetraploid T. turgidum. After a relatively short time, about several tens of years, ssp. spelta was replaced by the free threshing type of T. aestivum, ssp. aestivum. In addition to a mutation in the Tg-D1 gene that suppressed the free-threshing trait (see below), additional mutation should have occurred in hexaploid wheat that converted the codominant Br-D2 gene to a recessive loss-of function allele, br-D2, that led to the formation of hexaploid wheat with fully tough rachis that remains intact when threshed, as that of the free-threshing subspecies of T. turgidum. These mutations have been critical for the establishment of bread wheat, ssp. aestivum as the most important wheat form. Ssp. spelta is currently grown in several locations in Iran and Transcaucasia. The transformation from the hulled form with semi-tough rachis ssp. spelta to a free threshing with fully tough rachis ssp. aestivum was relatively a fast process.

The feral hexaploid wheat ssp. tibetanum has fragile rachis and tenacious glumes. The rachis disarticulates at maturity to yield wedge-type spikelets. Study of the genetic control of the fragile rachis and glume tenacity in the feral hexaploid wheat ssp. tibetanum, using progenies of crosses and backcrosses of ssp. tibetanum with T. aestivum ssp. aestivum cv. Columbus, indicated that the fragile rachis and the hulled character of ssp. tibetanum were dominant over the tough rachis and free-threshing character of bread wheat (Cao et al. 1997). However, rachis fragility and glume tenacity of ssp. tibetanum were each controlled by a single gene. In the cross between ssp. tibetanum and spp. spelta, the F₂ and F₃ populations did not segregate by glume tenacity but did segregate by rachis fragility. Cao et al. (1997) concluded that ssp. tibetanum differs from ssp. aestivum in rachis fragility and glume tenacity and from the hulled subspecies of T. aestivum (ssp. spelta, and ssp. macha) in the pattern and degree of rachis fragility.

Chen et al. (1998), using monosomic and ditelosomic lines of bread wheat, found that the gene controlling the brittle rachis of ssp. tibetanum is dominant and located on the short arm of chromosome 3D. Consequently, it is orthologous to the Br1 gene of many Triticeae species and consequently, was designated Br-D1. As was found earlier (Tsunewaki et al. 1990), Br-D1 is different from the gene determining barrel-type brittle rachis in Ae. tauschii and ssp. spelta. This gene, designated Br-D2, is located on the long arm of chromosome 3D and it is paralogous to the Br1 genes (Li and Gill 2006). In the progenies of the cross spelta x tibetanum, F₁ plants exhibited the wedge and barrel types of dis-articulation, indicating that wedge-type disarticulation in ssp. tibetanum is codominant with the barrel type in spelt wheat. Similarly, mono-telosomic analysis indicated the rachis-fragilit genes were located at approximately 20 cM from the centromere on the short arms of chromosomes 3A and 3B of wild emmer and on 3D of ssp. tibetanum, representing the orthologous locus Br1 (Watanabe et al. 2002).

13.3.3 Genetic Control of Free-Threshing Grains in Hexaploid Wheat

The synthetic hexaploid wheat, involving hulled T. turgidum ssp. dicoccon-Ae. tauschii, produced by McFadden and Sears (1946), was a hulled form, resembling T. aestivum ssp. spelta. Yet, Kerber and Rowland (1974) reported that all 15 hexaploid wheats synthesized from various combinations of nine tetraploid wheats and seven forms of Ae. tauschii, were non-free threshing, regardless of the presence or absence of the Q allele in the tetraploid parent. Obviously, the genome of Ae. tauschii, genome D, carries a gene that suppresses in synthetic hexaploids the activity of the free-threshing gene Q that derived from the tetraploid parent. Indeed, Kerber and Dyck (1969) identified a single gene on the D genome that affected threshability in synthetic hexaploid wheat formed from hybridization of free threshing tetraploid wheat and Ae. tauschii. Monosomic and mono-telosomic analysis of this hexaploid line revealed the presence of a partially dominant gene for tenacious glumes, designated Tg, on the short arm of chromosome 2D (Kerber and Rowland 1974). This gene, should be designated Tg-D1, derived from the Ae. tauschii parent, inhibited the expression of Q in hexaploid wheat. These researchers concluded that primitive hexaploid wheat that carried the Tg allele on subgenome D, were hulled types. This finding reinforced the concept that the hulled forms of hexaploid wheat, ssp. spelta and ssp. macha, having the genotype QQTgTg, are the primitive forms of T. aestivum. A mutation from Tg-D1 to tg-D1 on chromosome arm 2DS is presumed to have occurred at the hexaploid level, resulting in the production of the free-threshing types.

As in tetraploid wheat, also in ssp. aestivum it is reasonable to assume that the effects of all the genes affecting free-threshing are additive, with Q having the most profound effect on threshability, and that the recessive alleles tg-A1, tg-B1 and tg-D1 are necessary at all three loci to attain a completely free-threshing phenotype. The mutations from q to Q, from Tg-A1 to tg-A1, and from Tg-B1 to tg-B1, occurred at the tetraploid level within a relatively short period (Sharma et al. 2019). The mutation to Tg-D1 to tg-D1 occurred at the hexaploid level.

While the Q gene on chromosome arm 5AL, has the most significant effct on free-threshing, its homoeoalleles on chromosome arms 5BL and 5DL, were also shown to be involved in this domestication trait (Zhang et al. 2011). The q homoeoallele on chromosome 5B became a pseudogene after allotetraploidization. Expression analysis indicated that, whereas Q plays a major role in conferring domestication-related traits, the q homoeoallele on 5D contributes directly and q on 5B indirectly to suppression of the speltoid phenotype. Hence, according to Zhang et al. (2011), the evolution of the Q/q loci in polyploid wheat resulted in the hyperfunctionalization of Q on 5A, pseudogenization of q on 5B, and subfunctionalization of q on 5D, all contributing to the domestication traits.

The Asiatic ssp. spelta has the genotype QQtg-A1tg-A1tg-B1tg-B1Tg-D1Tg-D1, whereas the European ssp. spelta, being derived in Europe from spontaneous hybridizations of hexaploid wheat, ssp. aestivum (genotype QQtg-A1tg-A1tg-B1tg-B1tg-D1tg-D1) and T. turgidum ssp. dicoccon (genotype qqtg-A1tg-A1tg-B1tg-B1) is hulled qqtg-A1tg-A1tg-B1tg-B1tg-D1tg-D1 having the genotype qqtg-A1tg-A1tg-B1tg-B1tg-D1tg-D1 or qqTg-A1Tg-A1Tg-B1Tg-B1tg-D1tg-D1.

Jantasuriyarat et al. (2004), using recombinant inbred lines of the International Triticeae Mapping Initiative (ITMI) mapping population, derived from a cross between a common wheat cultivar and the synthetic hexaploid wheat ssp. Durum-Ae. tauschii, revealed two QTLs, one located on the short arm of chromosome 2D and the second on the long arm of chromosome 5A, that consistently affected threshability-associated traits. The QTL on 2DS, presumably representing the effect of the Tg allele that derived from Ae. tauschii, explained 44% of the variation in threshability, 17% of the variation in glume tenacity, and 42% of the variation in rachis fragility, whereas the QTL on 5AL, believed to represent the effect of Q, explained 21 and 10% of the variation in glume tenacity and rachis fragility, respectively. Overall, Jantasuriyarat et al. (2004) found that free-threshing-related characteristics were predominantly affected by Tg and, to a lesser extent, by Q.

Other QTLs, on chromosomes 2A, 2B, 6A, 6D, and 7B, were significantly associated with threshability-related traits. Four other QTLs on chromosomes 1B, 4A, 6A, and 7A consistently affected spike characteristics (Jantasuriyarat et al. 2004). The QTL on the short arm of chromosome 1B explained 18% and 7% of the variation in spike length and spike compactness, respectively. The QTL on the long arm of 4A explained 11%, 14%, and 12% of the variation in spike length, spike compactness, and spikelet number, respectively. The QTL on the short arm of 6A explained 27% of the phenotypic variance for spike compactness, while the QTL on the long arm of 7A explained 18% of the variation in spikelet number. QTLs on chromosomes 1B and 6A appeared to affect spike dimensions by modulating rachis internode length, while QTLs on chromosomes 4A and 7A did so by affecting the formation of spikelets. Other QTLs that were significantly associated with spike morphology-related traits, in at least one environment, were localized on chromosomes 2B, 3A, 3D, 4D, and 5A (Jantasuriyarat et al. 2004).

13.3.4 Selection for Yield in Hexaploid Wheat

Mac Key (1986) assumed that in stands of wild wheat, characters that contribute to the successful annual re-establishment of the stand are of the highest selective value. There is no selective advantage for the overproduction of seeds in wild dense stands, particularly when the seed dispersal mechanism is such that most seeds fall near the mother plant. On the other hand, the ability to compete well immediately upon germination, driven by large, energy- and protein-rich seeds, ensures the re-establishment of the stand. Hence, in wild wheat, efficiency per seed is more important than prolificacy, i.e., the ability to produce more viable seeds. Yet, from the dawn of agriculture, prolificacy became more important than seed efficiency, as successful seedling establishment in the cultivated field was achieved by providing the seedlings with optimal growth conditions.

According to Evans (1981), the improvement in grain productivity was achieved through increased leaf size and flag-leaf area, increased size of the vascular system in the spike, advanced flowering time, delayed flag-leaf senescence, increased duration of grain-filling period, increased rate and duration of assimilates translocation to the grains, increased grain size and grain number per spikelet, and increased spike number per plant or per unit area (Table 13.3). Early farmers tended to select for a higher ratio of grain harvested to grain sown, i.e., indirectly for profuse tillering, many grains per spike, and strong grain retention. As the amount of arable land became a limiting factor and the crop became monotypic, selection for higher number of grains per unit area was the preferred criterion. Emphasis was placed on lines that were weak intra-line competitors, yielded well in dense planting, and responded well to fertilizers and various agrochemicals. One of the most important changes during cultivation was an increase in the proportion of the dry matter allocated to harvested grains (harvest index). Continuous selection for increased grain production and size enhanced synthesis and translocation of carbohydrates to the developing grains, with no corresponding improvement in the translocation of amino acids. This obviously resulted in increased grain size, as well as in grain number, and in low grain-protein percentage (Feldman et al. 1990)—a trait common to all cultivated cereals compared to their wild progenitors. On the other hand, there were few, if any, significant changes during wheat cultivation in photosynthetic capacity per unit leaf area, growth rate, and accumulated dry crop weight (biomass) (Table 13.3).

Table 13.3 Modified (A) and conserved (B) yield-related traits in cultivated wheat (after Evans 1981)

Full size table

The increased yield in domesticated wheat, stems, to a large extent, from delayed flag-leaf senescence, which prolongs the duration of post-anthesis photosynthesis, resulting in higher production of carbohydrates for the developing grains. This trait can also be achieved by accelerating spike development, which advances anthesis by a few days. In semi-arid regions, where the available amount of water is often a limiting factor during grain development, senescence is delayed for only a short period, while in mesic regions, senescence and, consequently, maturity in general, are delayed for longer periods.

Compared to domesticated wheat, wild emmer, is characterized by later anthesis and earlier grain ripening, which avoids the heat of the late spring. This shorter grain-filling period is apparently the consequence of relatively early and rapid flag-leaf senescence occurring about 3 weeks after anthesis. The degradation of most leaf proteins at this stage reduces and eventually fully eliminates carbohydrate assimilation capacity in the leaves (mainly the flag leaf), resulting in a higher N/C ratio in the assimilates translocated to the grains. Indeed, all wild emmer lines analyzed had high grain protein percentage (GPP) (Avivi 1977, 1979a, 1979b; Feldman et al. 1990), that may contribute to seedling vigor (Millet and Zaccai 1991). Under the semi-arid conditions that prevail in the natural habitats of wild emmer stands, a small number of medium-sized grains with high grain protein content suffice to ensure rapid germination and successful establishment of the next stand.

Grain protein content (GPC) is much lower in domesticated durum and bread wheat than in wild emmer (Avivi 1977, 1979a, 1979b; Feldman et al. 1990). Joppa and colleagues (Joppa and Cantrell 1990; Joppa et al. 1991; Joppa 1993), using substitution lines of wild emmer chromosomes in the background of T. turgidum ssp. durum cultivar Langdon (LDN), found that chromosomes 2A, 5B, 6A, and 6B of wild emmer increased GPC in the durum cultivar. Genotypes with wild emmer chromosomes 6B had the highest GPC (Cantrell and Joppa 1991). Using a population of recombinant inbred lines derived from cultivar LDN and the chromosome substitution line LDN (DIC6B), Joppa et al. (1997) mapped the higher GPC gene to chromosome arm 6BS. This gene also increased grain protein in several studied lines of tetraploid and hexaploid wheats (Joppa et al. 1997; Mesfin et al. 1999; Chee et al. 2001). Olmos et al. (2003) found that this factor for high-grain protein behaves as a simple Mendelian gene and designated it Gpc-B1, and mapped it proximal to the Nor2 locus.

Recombinant substitution lines (RSLs) carrying the Gpc-B1 allele of wild emmer exhibited on average, 12% higher concentration of Zn, 18% higher concentration of Fe, 29% higher concentration of Mn and 38% higher concentration of protein in their grains as compared to RSLs carrying the allele from durum wheat (Distelfeld et al. 2007). These authors also confirmed the effect of the wild emmer Gpc-B1 allele on earlier senescence of the flag leaveand suggested that the Gpc-B1 locus is involved in more efficient remobilization of protein, zinc, iron and manganese from the leaves to the grains, in addition to its effect on earlier senescence of the green tissues.

Brevis and Dubcovsky (2010) studied the grain yield, grain weight, protein yield (grain yield multiplied by grain GPC), and N harvest index (NHI) of bread and durum wheats using BC₆F₃ near-isogenic lines with and without the wild emmer Gpc-B1 allele. All the studied lines in three California locations during the years 2005–2007, showed higher GPC and grain protein yield when the wild Gpc-B1 allele was present. Bread and durum wheat lines having the wild emmer Gpc-B1 allele showed a significant decrease in grain weight whereas the decrease in grain weight in the durum lines was not significant.

Brevis et al. (2010), evaluated the effects of Gpc-B1 on bread-making and pasta quality in isogenic lines for the Gpc-B1 introgression in six hexaploid and two tetraploid wheat genotypes. In bread wheat, the wild emmer Gpc-B1 introgression was associated with significantly higher GPC, water absorption, mixing time and loaf volume, whereas in durum wheat, the introgression resulted in significant increases in GPC, wet gluten, mixing time, and spaghetti firmness, as well as a decrease in cooking loss. On the negative side, the wild emmer Gpc-B1 introgression was associated, in some varieties, with a significant reduction in grain weight, test weight and flour yield, and significant increases in ash concentration.

Uauy et al. (2006a) reported that flag leaf chlorophyll degradation, and change in peduncle color, and spike water content were fully linked to the wild emmer Gpc-B1 allele. The high GPC allele conferred a shorter duration of grain fill due to earlier flag leaf senescence and increased GPC in all four studied genetic backgrounds. The effect on grain size was more variable, depending on the genotype–environment combinations. These results are consistent with a model in which the wild-type Gpc-B1 allele accelerated senescence in flag leaves, leading to pleiotropic effects, such as on nitrogen remobilization, total GPC, and grain size. At this point, the possibility of multiple genes that are present within the region governing different aspects of these responses cannot be ruled out. However, the authors assumed that it is more likely that these multiple traits are pleiotropic effects of a single gene rather than the result of multiple independent genes.

The wild-type Gpc-B1 allele was isolated and found active in all studied wild emmer accessions, whereas in domesticated durum and bread wheat, it was mutated to a non-functional allele (Uauy et al. 2006a). In the free-threshing allotetraploid and allohexaploid wheats, the grain-filling period is longer than in wild emmer (about 3 weeks or more) and consequently, a greater amount of carbohydrates is assimilated and transported to the developing grains, thereby diluting the percentage of proteins and minerals in the grains (Uauy et al. 2006a). Since the wild type allele exists in several analyzed domesticated emmer (ssp. dicoccon) lines, it is assumed that the mutation to a loss-of-function allele occurred either in domesticated emmer or in durum wheat (Uauy et al. 2006a).

In a positional cloning of Gpc-B1, Uauy et al. (2006a) located the gene in a 7.4 kb region of chromosome arm 6BS, with complete linkage with the different phenotypes affected by Gpc-B1, verifying the assumption that Gpc-B1 is indeed a single gene with multiple pleiotropic effects. The annotation of this 7.4 kb region identified a single gene encoding a NAC protein, characteristic of the plant-specific family of NAC (NAM, ATAF and CUC) transcription factors (Ooka et al. 2003). NAC proteins play important roles in developmental processes, auxin signaling, defense and abiotic stress responses, and leaf senescence (Olsen et al. 2005). Phylogenetic analyses revealed that the closest plant NAC proteins were the NAC transcription factor ONAC01 in rice, and a clade of three Arabidopsis proteins, including No Apical Meristem (NAM) protein. On the basis of these similarities, Uauy et al. (2006b) designated the gene in T. turgidum and T. aestivum TtNAM-B1 and TaNAM-B1, respectively.

The NAC transcription factor coded by the wild-type NAM-B1 allele accelerates senescence and increases nutrient remobilization from leaves to the developing grains, whereas free-threshing wheat cultivars carry a non-functional NAM-B1 allele. Reduction in RNA levels of the multiple NAM homologs by RNA interference delayed senescence by more than 3 weeks and reduced wheat grain protein, zinc, and iron content by more than 30% (Uauy et al. 2006b).

Comparison of the wild-type and the LDN TtNAM-B1 sequences revealed a 1-bp substitution within the first intron and a thymine residue insertion at position 11, generating a frame-shift mutation in the durum LDN allele (Uauy et al. 2006b). Since the wild type TtNAM-B1 allele was found in all the 42 wild emmer accessions and in 17 of the 19 domesticated emmer accessions, while 57 durum studied lines lacked the functional allele, it can be assumed that the 1-bp frame-shift insertion was fixed during the domestication of durum wheat. The wild-type TaNAM-B1 allele was also absent from a collection of 34 varieties of hexaploid wheat representing different cultivars of geographic locations. Twenty-nine of these showed no polymerase chain reaction (PCR) amplification products of the TaNAM-B1 gene, which suggests that it was deleted, whereas the remaining five lines have the same 1-bp insertion observed in the durum lines.

In addition to the mutant TtNAM-B1 on chromosome arm 6BS, the durum wheat genome includes an orthologous copy (TtNAM-A1) on chromosome arm 6AS and a paralogous one (TtNAM-B2), 91% identical at the DNA level to TtNAM-B1, on chromosome arm 2BS. These two copies have no apparent mutations. The studies of Uauy et al. (2006b) suggested that the reduced grain protein, Zn, and Fe concentrations were the result of reduced translocation from leaves, rather than a dilution effect caused by larger grains. This hypothesis was confirmed by analyzing the residual nitrogen, Zn, and Fe content in flag leaves.

There are three known alleles in the NAM-B1 gene: the wild type (WT allele), a null allele, consisting of a deletion that covers 100 kb (deletion allele), and another null allele, bearing a 1 bp frame-shift insertion (insertion allele) (Uauy et al. 2006b). Lundström et al. (2017) studied the distribution of these alleles in wild and domesticated forms of T. turgidum. Out of 19 wild emmer accessions studied, 18 carried the wild-type allele and one carried the insertion allele. Among the 16 durum cultivars studied, 15 had the insertion allele and one carried the deletion allele. Among the 61 domesticated emmer accessions studied, 25 carried the deletion, 17 the insertion, and 19 the WT allele. Four landraces of hexaploid wheat were found to carry the wild-type allele (Asplund et al. 2013). The presence of the wild-type allele was also confirmed in several landraces of spring hexaploid wheat from Scandinavia (Hagenblad et al. 2012). Neither of the NAM-B1 alleles appeared to be limited to specific geographic areas. These findings show that both null alleles exist in domesticated emmer and durum wheats. Moreover, based on the finding that the insertion allele exists in several accessions of wild emmer, Lundström et al. (2017) suggested that this allele arose in wild emmer.

Seed size is influenced by the NAM-B1 gene, which has consequently been suggested to be a domestication gene (Dubcovsky and Dvorak 2007). Indeed, the NAM-B1 gene fulfils some criteria for being a domestication gene, by encoding traits of domestication relevance (seed size and increased carbohydrate production due to delay in leaf senescence) and, as such, has been under positive selection (Dubcovsky and Dvorak 2007; Hu et al. 2012, 2013; Hebelstrup 2017; Lundström et al. 2017).

The flag leaf is the main source of protein and micronutrients in the wheat grain (Pearce et al. 2014). During senescence of the flag leaf, photosynthesis ceases, and the enzymes involved, mainly ribulose-1,5-bisphosphate carboxylase/oxygenase (rubisco), are degraded into amino acids that are transported, together with other nutrients and minerals, to the developing grains (Barneix 2007). The delay in the onset of senescence facilitates the production of more carbohydrates that are transferred to the grains. Since the uptake of nitrogen and minerals from the soil ceases at anthesis, and assimilation of carbohydrates continue for several more weeks, the levels of protein, Zn, and Fe are diluted in the mature grain. The constant selection for increased yield in domesticated durum and bread wheat, presumably unintentionally led to preference of mutants with a delayed flag-leaf senescence, which prolongs the duration of post-anthesis photosynthesis.

Coupling delayed flag-leaf senescence with post-anthesis nitrogen absorption might improve both grain yield as well as GPC (Feldman et al. 1990). In this respect, some variation in the duration of post-anthesis nitrogen uptake, up to several days after anthesis, was observed among accessions of wild emmer (Zaccai 1992). This trait should be exploited to increase GPC without affecting grain yield.

13.3.5 Summary of the Main Events in Wheat Cultivation and Domestication

Wild diploid wheat, T. monococcum ssp. aegilopoides, was apparently first taken into cultivation in the northern Levantine Corridor, i.e., in the Karacadag area, southern Turkey (Heun et al. 1997), while wild emmer, T. turgidum ssp. dicoccoides, was first cultivated in the watershed of the Jordan River in the southern Levantine Corridor (Bar-Yosef 1998). Based on Kislev (1981), Bar-Yosef and Kislev (1989), Bar-Yosef (1998) and Harris (1998), the domestication and evolution of the various wheat species can be summarized as follows: (a) The earliest evidence of cultivation of wild emmer is from the first half of the 10th millennium BP in the Jordan Valley and the Damascus basin (Van Zeist and Bakker-Heeres 1985; Bar-Yosef and Kislev 1989). These sites are very close to the natural area of distribution of wild emmer today and 10 millennia ago. Later evidence of cultivation of wild emmer comes from the second half of the 10th millennium at Cayonu, east Anatolia, and from Ali Kosh, southwestern Iran. Around the middle of the 9th millennium BP, a non-brittle type of emmer, ssp. dicoccon, was derived from a brittle type by mutations. This domesticated wheat was grown for millennium or more in mixture with brittle forms and gradually replaced it throughout the Fertile Crescent. (b) Around the middle of the 9th millennium BP, small grains and rachis fragments of a dense ear of naked wheat were found in Tell Aswad, near Damascus, Syria (Van Zeist and Bakker-Heeres 1985). These are assumed to be remnants of tetraploid naked wheat, T. turgidum ssp. parvicoccum, a taxon that presumably evolved from ssp. dicoccon by a series of mutations (Kislev 1979/1980). Because all existing naked tetraploid wheats apparently possess on chromosome-arm 5AL the Q factor, which confers a tough rachis, soft glumes, and free-threshing grains (Muramatsu 1986), it is assumed that the extinct ssp. parvicoccum also possessed this gene. Hence, mutation of the q gene to Q led to the main character change distinguishing parvicoccum from dicoccon. (c) In the ancient fields of ssp. parvicoccum in northwestern Iran, where Ae. tauschii grew as a weed, a hexaploid hulled wheat, containing Q from ssp. parvicoccum and Tg from Ae. tauschii, viz. TgQ spelta, could have arisen by hybridization. Neolithic Near Eastern ssp. spelta is very rare, but there is evidence of spelta grains from Yarim Tepe II, northern Iraq, back to the 7th millennium BP, and probably also from Yarim Tepe I, about one thousand years earlier (Kislev 1984). Earlier evidence for the existence of ssp. spelta is still missing. (d) The next evolutionary step was apparently the formation of the free-threshing bread wheat from TgQ spelta by mutation of the Tg gene to its recessive allele tg (Kerber and Rowland 1974). The earliest remnants of bread wheat, ssp. aestivum, are from Can Hassan III, southern Anatolia, from about the middle of the 9^th millennium BP (Hillman 1996). These remnants indicate that free-threshing hexaploid wheat was formed a short time after the appearance of naked tetraploid wheat and its spread to northwestern Iran. (e) The naked tetraploid wheat ssp. durum might have arisen from ssp. parvicoccum. Ssp. durum appears somewhat later (Feldman and Kislev 2007). It is also possible that ssp. durum was derived directly from ssp. dicoccon or, more likely from hybridization between dicoccon and parvicoccum. Other free-threshing subspecies of tetraploid wheat, turgidum, polonicum. turanicum, and carthlicum, presumably derived from ssp. durum. (f) After bread wheat and domesticated emmer had been established in Europe, a second ssp. spelta appeared, mostly north of the Alps. It is therefore assumed that the European spelta, having genotype tgtgqq, arose from hybridization of bread wheat (tgtgQQ) with ssp. dicoccon. (qq). Alternatively, reverse mutation of Q back to q restored the character of hulledness to hexaploid wheat. (g) As a completely independent evolutionary development, cultivated einkorn, ssp. monococcum, arose from ssp. aegilopoides. This involved a series of events similar to those which occurred in the emmer group. The first records of domesticated einkorn are from ca. 9000 BP in Syria and Anatolia (review in Harris 1998; Bar-Yosef 1998). (h) Another side branch is a group of hulled, eastern wheat species, comprising of the wild ssp. armeniacum and its domesticated descendants ssp. timopheevii and T. zhukovskyi.

Vavilov (1926) coined the terms “center of origin” and “center of variation”. The first is the site at which a given taxon evolved and the second is the site at which it responded to the environment with a burst of variation. In most cases the center of variation overlaps the center of origin. Accordingly, domesticated diploid wheat presumably originated in the northern Levantine Corridor, whereas domesticated tetraploid wheat originated at the watershed of the Jordan River in the southern Levantine Corridor. Naked tetraploid wheat presumably also evolved in this region. Hexaploid wheat originated southwest of the Caspian Sea. With their migration into new areas, cultivated wheats encountered new environments, towards some of which they responded with increased variation and formation of many endemic forms. Several such secondary centers of variation were described by Vavilov (1926), e.g., the Ethiopian plateau and the Mediterranean basin for tetraploids, and Afganistan (Hindu-Kush area) for hexaploids. Transcaucasia is a secondary center for tetraploid as well as for hexaploid types. Such secondary centers of diversity are valuable to wheat breeders as additional gene pools to those existing in the primary centers of variation.

There are still several puzzling questions concerning the evolution of the various domesticated wheats: the absence of non-brittle forms in the second diploid wheat, T. urartu (even if the mutations for non-brittleness occurred only in ssp. aegilopoides, the mutant alleles could have been transferred to T. urartu through hybridization); the mode of origin of the first naked tetraploid wheat and the reasons for its extinction about 1900 years ago (grains of the latest cropping of ssp. parvicoccum were found by Kislev in southern Judea, Israel, from the Bar- Kokhba period, 135 AD); the reasons for the late establishment of ssp. durum as a major crop in the Near East, in spite of its sporadic occurrence in archaeological material already from 7500 to 6200 BP; the reason for the relatively small contribution of T. timopheevii ssp. armeniacum to wheat cultivation, either as a tetraploid crop or as the donor of the AG genomes for an hexaploid crop; and the appearance of ssp. aestivum in archaeological excavations about 1000–2000 years before its presumed predecessor, ssp. spelta.

13.4 Molecular Changes in Domesticated Wheat

The last two decades witnessed remarkable progress in the development of a new arsenal of genomic tools, which have facilitated more in-depth study of various aspects of wheat genetics, genomics and evolution. One of the most significant tools was whole-genome sequencing of the A genome of diploid wheat, T. urartu (Ling et al. 2013), the D genome of Ae. tauschii (Jia et al. 2013; Luo et al. 2017), the genomes of wild emmer (Avni et al. 2017), ssp. durum (Maccaferri et al. 2019), and bread wheat (The International Wheat Genome Sequencing Consortium 2018). Single-nucleotide polymorphism (SNP) mapping of a broad collection of bread wheat landraces and modern varieties has indicated the genomic regions that underwent selective sweep (also known as genetic hitchhiking), which refers to a process that reduces variation in regions of neutral sites linked to a recently fixed beneficial mutation as it increases in frequency in the population (Nielsen et al. 2005; Cavanagh et al. 2013). A major recent advance in durum transcriptome analysis was the development of tools for the discrimination of A and B genome homeologues from expression sequence data such as RNA-Seq (Krasileva et al. 2013), and the first step towards a wheat pan-genome sequence has been made (Walkowiak et al. 2020). Small RNAs datasets are also becoming available (Kenan-Eichler et al. 2011; Yao and Sun 2012). These recent genomic tool developments have facilitated the identification of additional loci that control domestication-related traits in wheat.

The assembly of the genome of the modern durum cultivar Svevo (Maccaferri et al. 2019) and the availability of the assembly of the wild-emmer genome (line Zavitan; Avni et al. 2017) enabled Maccaferri et al. (2019) to analyze genome-wide genetic diversity, and to reveal changes between the domesticated and the wild genomes that were imposed by domestication processes and by thousands of years of selection and breeding (=diversification according to Meyer and Purugganan 2013). Processes leading to modern cultivars of ssp. durum were revealed by the four main germplasm groups of the Global Tetraploid Wheat Collection, namely, wild emmer, domesticated emmer, durum landraces, and durum cultivars. Combined genetic diversity and selection signature analyses yielded a dynamic description of the modifications imposed on the genome by domestication and diversification. More specifically, the comparison revealed strong overall synteny, with high similarity in total high-confidence (HC) gene number (durum 66,559; wild emmer 67,182), chromosome structure and transposable element composition. Yet, a number of HC genes displayed differences between Svevo and Zavitan. At least two-thirds of the varied genes displayed variation in intact gene number, whereas the complete gene loss, caused by large structural variations, was responsible for asymmetric gene distribution in only one-third of the cases. In Svevo, variation leading to intact gene number variation included 4811 genes, which represent 7.2% of all HC genes, a value similar to the 5% found after the comparison between two cultivars in a recent pangenome study of hexaploid wheat (Montenegro et al. 2017). When the Svevo-specific genes were mapped to the Zavitan genome, 1493 genes (31%) were not found on the Zavitan sequence, 1225 (26%) corresponded to shorter counterparts of annotated Zavitan HC genes and 1095 (23%) were annotated as low-confidence (LC) genes or pseudogenes. The remaining 965 genes (20%, that is, 1.4% of all Svevo HC genes) mapped to unannotated regions. The presumed HC gene losses were predominantly located in the more distal chromosomal regions. The distal highly recombinogenic regions of chromosomes were enriched in gene displaying variation of intact gene number. contain most of the known QTLs and the HC genes displayed a reduced expression breadth (that is, average expression value) across all tissue/treatment conditions.

Ayal et al. (2006), using cDNA microarrays, studied the alterations in gene expression that occurred during wheat domestication, and focused on the following aspects: (i) the extent of variation in gene expression that can be attributed to domestication, (ii) the specific genes whose expression was altered (up or down regulated) during domestication and (iii) the range of variation in the wheat transcriptome of wild (T. turgidum ssp. dicoccoides) versus domestic tetraploid wheats (T. turgidum ssp. dicoccon and ssp. durum). The group reported that the expression levels of 63 genes, which represent 2.53% of the total number (2490) of tested genes, were up- or downregulated; 24 genes (0.96%) were downregulated (Table 13.4A) and 38 genes (1.53%) were upregulated (Table 13.4B) in the wild compared to the domesticated lines. Assuming that the genome of tetraploid wheat contains approximately 67,000 HC genes (Maccaferri et al. 2019), this means that approximately 1700 genes are differentially expressed between wild and domestic wheat and, out of these, the expression of approximately 650 genes was reduced (i.e., lost, mutated, or silenced) by the domestication process. A high proportion of the genes that were upregulated in the domesticated wheats were related to carbon metabolism, such as the Rubisco large and small sub-unit and the sucrose-synthase (SuSy).

Table 13.4 Changes in expression level of selected transcripts in domesticated wheat (After Ayal et al. 2006)

Full size table

The intra-specific variation in gene expression within ssp. dicoccoides was about four times higher than within the domesticated species. This reduction in variation during domestication was in the same range found at the genomic level, for nucleotide polymorphism. Nevertheless, Ayal et al. (2006) found that certain genes were more variable in the domestic varieties than in the wild, suggesting that selection under domestication led to the fixation of new mutations that did not previously exist in the wild.

Ben-Abu et al. (2014) focused on genomic changes that correlated with the process of domestication and evolution of modern durum by comparing gene expression and copy number variation of genes and transposons in four genetic groups: wild emmer, domestic emmer, durum landraces and modern durum varieties. Genes were clustered based on their pattern of change in expression during durum evolution, e.g. gradual increase, or decrease, or increase at the onset of domestication and plateauing later on. Few genes changed > twofold in copy number. However, interestingly, the copy number of transposons increased with domestication, possibly reflecting the genomic plasticity that was required for adaptation under cultivation. Extensive changes in gene expression were seen in developing grains. For example, there was an enrichment for certain functions, e.g., genes involved in vesicle trafficking in the endosperm showed a gradual increase in expression during durum evolution and genes related to germination and germination inhibition increased in expression in the embryo in the more recent stages of durum evolution (Ben-Abu et al. 2014).

Yuan et al. (2015) described differential expression of the CENH3 genes in wild versus domesticated tetraploid wheat. T. timopheevii ssp. armeniacum, the wild progenitor of domesticated T. timopheevii, had a higher transcript level of cxCENH3 while the domesticated forms had a lower expression of cxCENH3 and increased expression of PCENH3. Similar changes in the CENH3 expression model were found in wild and in domesticated types of T. turgidum; the wild subspecies of T. turgidum exhibited a higher expression level of cxCENH3 whereas in the domesticated forms, the differences in expression between cxCENH3 and PCENH3 were not so obvious. In contrast to the markedly higher expression level of cxCENH3 in wild tetraploids, expression of PCENH3 was enhanced to a level near that of cxCENH3 in the domesticated tetraploids (Yuan et al. 2015). These genome-wide analyses provide insights into the molecular basis of plant domestication, in particular for the non-obvious cellular functions that were selected during evolution under cultivation.

13.5 Founder Effect and Processes Contributing to Increased Genetic Variability of Domesticated Wheats

During their evolutionary history, wheats underwent several genetic bottlenecks: allotetraploidization, allohexaploidization, cultivation of wild emmer, domestication of emmer, selection of free-threshing tetraploids and hexaploids and replacement of the polymorphic fields of land races by high yielding cultivars (Table 13.5). It is reasonable to assume that the number of mutants that founded domesticated einkorn and emmer was not large and that the number of hexaploid plants produced by independent hybridization events between tetraploid wheat and Ae. tauschii was small. As pointed out by Ladizinsky (1985), one of the consequences of this ''founder effect”, namely, the establishment of a new taxon by a few individuals that necessarily represent only a small fraction of the genetic basis of the parental taxon, are narrow and very restricted genetic variability, a phenomenon described as a genetic bottleneck.

Table 13.5 Genetic bottlenecks during the evolution of domesticated wheat

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At the diploid level, Dhaliwal (1977), Zeven (1980) noted that domesticated einkorn is quite uniform while its wild progenitor, ssp. aegilopoides, exhibits a relatively wider variation. He ascribed this limited variability of domesticated einkorn to the presumed small sample of wild einkorn genotypes that had been domesticated. Moreover, within a short period after its selection by man, domesticated einkorn was spread westward from the area of wild einkorn, thereby largely preventing the introgression of genes from its wild counterparts. Concurring with Dhaliwal, Zeven (1980) assumed that further erosion in the gene pool of cultivated einkorn resulted from intergenotypic competition in the polymorphic fields of ancient farmers. Moreover, because of higher yield, the more extensive cultivation of domesticated emmer and free-threshing hexaploid wheat in Europe reduced very much the cultivation of einkorn.

The pattern of diversity for tetraploid each germplasm group was recently assessed by Maccaferri et al. (2019). They used the whole genome of ssp. durum cv. Svevo as a reference to carry out a phylogenetic analysis of a collection of 1856 accessions, including wild and domestic emmer, durum landraces and modern durum cultivars. Data analysis could clearly distinguish between all four genetic groups. Wild emmer exhibits the highest diversity, while there was a strong reduction in diversity through domestication, thus providing a valuable reference for assessing the reduction of diversity associated with domestication. Compared to wild emmer, each of the subsequently domesticated germplasm groups showed several strong diversity decrements that arose independently and were progressively consolidated through domestication and breeding. With few exceptions, the diversity depletions losses that occurred in the early transition of wild emmer to domesticated emmer, or domesticated emmer to free-threshing durum landraces, are confirmed. Domestic emmer showed a broad genetic variation compared to modern durum cultivars which was rather limited. Landraces of durum had also a rather broad genetic basis compared to modern durum cultivars, which were genetically related to a small group of landraces.

The genome of durum cultivars was characterized by a high number of regions that showed a strong reduction in diversity with near fixation of allelic diversity (genetic swift) presumably due to selection. This included 104 pericentric regions and 350 non-pericentric regions that overlapped significantly with genes known for their role in domestication, such as genes controlling spike fragility and threshability. This also included disease resistance loci, yellow pigment and some unexpected loci, such as a cadmium transporter, absent in wild emmer but widespread in durum cultivars.

Most of the strongest pericentromeric diversity depletions (chromosomes 2A, 4A, 4B, 5A, 5B, 6A and 6B) occurred during emmer domestication. Furthermore, one of the two brittle rachis regions marking the early domestication process (on 3BS) showed a sharp, localized reduction in diversity. The same region then underwent an extreme diversity reduction in the domesticated emmer-to-durum landrace transition. An additional 14 pericentromeric and 90 non-pericentromeric diversity depletions, including one harboring the major tough glume QTL governing threshability (Tg-2B), occurred during the domesticated emmer-to-durum landrace transition. Finally, several reductions in diversity were specifically associated with breeding of modern durum cultivars, including some associated with disease resistance and grain yellow pigment content loci.

Introgression from wild emmer into domesticated wheat is partly restricted by the mode of self-pollination. Moreover, the genes for rachis brittleness are dominant over those controlling non-brittleness, and therefore, the F₁ spikes disarticulate at maturity and only few F₂ spikes bear a non-brittle rachis. Accordingly, only a few of the segregants might fit the selection criteria of man and enrich the gene pool of the domesticated forms.

Although domesticated tetraploid wheat, T. turgidum, started with very restricted genetic variability, the original non-brittle emmer plants grew for 2–3 millennia, in mixed populations with wild forms and occasionally hybridized and exchanged genes with them. Moreover, even after the complete replacement of wild emmer by domesticated emmer, the latter and other subspecies of allotetraploid wheat continued to introgress with wild genotypes that grew in their vicinity and thereby, broadened their genetic basis (Huang et al. 1999; Dvorak et al. 2006; Luo et al. 2007). The F₁ hybrids between wild emmer and domesticated tetraploid wheat are fully fertile, and since the chromosomes of wild emmer are homologous to those of allotetraploid wheat many genes can be transferred from the wild into the domesticated chromosomes through crossover. As a result, over the 10,000 years of cultivation, there was an almost continuous flow of wild genes into domesticated tetraploid background, diversifying, to some extent, the domesticated tetraploid gene pool.

In contrast to allotetraploid wheat, allohexaploid wheat, T. aestivum, is partially isolated from its allotetraploid progenitor and wholly isolated from its diploid progenitor. Therefore, its primary genetic basis was more restricted. However, despite the partial sterility of the F₁ pentaploid hybrids between allohexaploid and allotetraploid wheats, few hybrid swarms resulting from spontaneous hybridization between hexaploid wheat and wild emmer were found in Israel and descried by Zohary and Brick (1962). Moreover, He et al. (2019), who performed targeted re-sequencing of 890 diverse accessions of hexaploid and tetraploid wheat to identify introgression from wild relatives, found that historic gene flow from wild relatives led to genome-wide increases in diversity and made a substantial contribution to the adaptive diversity of modern cultivars of hexaploid wheat, the principal domesticated subspecies of T. aestivum.

For several millennia, massive intra- and inter-specific gene flow has been facilitated by farming of mixtures of different genotypes and even different cytotypes, including representatives of two or even three different species of domesticated wheat, namely, T. monococcum, T. turgidum (emmer and free threshing) and T. aestivum (spelt and bread wheat), each represented by numerous different genotypes (Zeven 1980). These endless hybridizations during the long period of cultivation also enriched the domesticated genetic basis of both species. Walkowiak et al. (2020) used ten chromosome pseudomolecules and five scaffold assemblies of allohexaploid wheat to explore the genomic diversity among allohexaploid wheat lines from global breeding programs. Comparative analysis revealed extensive differences in gene content resulting from complex breeding histories aimed at improving adaptation to diverse environments, grain yield and quality, and resistance to stresses.

Spontaneous hybridization between domesticated wheat and the wild form of T. timopheevii, ssp. armeniacum, which shares with domesticated wheat the A subgenome but differs in the other subgenome(s), may have also occurred in the northeastern part of the Fertile Crescent and in Transcaucasia. The hybrids between these species are highly sterile, although a few seeds are produced upon backcrossing. Introgression of genes from various Aegilops species also contributed to the wide variation of domesticated allopolyploid wheats. Although intergeneric F₁ hybrids between domesticated wheat and more distant diploid and tetraploid species of Aegilops, as well as species of Secale, Agropyron, Haynaldia and other related genera, which grew within or near Mediterranean and Near Eastern wheat fields, are highly sterile, the few seeds produced upon backcrossing yield more-fertile plants. Such intergeneric hybridizations may often result in successful introgression, maintaining a weak but constant flow of genes into the domesticated background. Such introgression was recently well evidenced by Zhou et al. (2020), who identified composite introgression from wild populations contributing to a substantial portion (4–32%) of the bread wheat genome, which undoubtedly increased the genetic diversity of bread wheat and enabled its adaptation to new territories. Similarly, Walkowiak et al. (2020) presented evidence for introgressions from wild relatives to allohexaploid wheat.

Keilwagen et al. (2022) pointed out that intentional introgressions from crop wild relatives have been used to introduce valuable traits into domesticated plants. They demonstrated the utility of single nucleotide polymorphism-based methods to detect introgressions and predict the putative donor species. Analyzing ten publicly available wheat genome sequences with these methods they identified nine major introgressions from wild diploid and allotetraploid Triticum species, from diploid and allotetraploid Aegilops species and from Elymus elongatus ssp. ponticum. Keilwagen et al. (2022) traced introgressions to early wheat cultivars and show that natural introgressions, mainly those that harbour resistance genes, were utilized in early breeding history and still influence elite lines today.

Mutations have also played an important role in increasing genetic variability of domesticated wheats. The genetic structure of the allopolyploid species of wheat, i.e., four (in tetraploids) or six (in hexaploid s) doses of gene loci, reinforced by a diploid-like cytological behavior and predominantly self-pollination, has proven a very successful genetic system for facilitating rapid buildup of genetic diversity through genetic changes. In these wheat species, the accumulation of genetic variation through mutations or hybridizations is tolerated more readily than in diploid species. Moreover, polyploidy facilitates genetic diploidization, the process whereby existing genes in multiple doses can be diverted to new functions. Thus, tetraploid and hexaploid wheat can accumulate a significant amount of genetic variability through gene alterations. Mutations exerting a lethal or semi-lethal effect at the diploid level, such as Q, s, (the sphaerococcum gene) C, (the compactum gene) and Ph1, are viable at the polyploid level. Induction of mutations may have been accelerated by the activity of transposable elements (TEs) (e.g., Fedoroff 2000, 2012) and genome-restructuring genes (Feldman and Strauss 1983). Indications for the activity of such genes have been reported in wheat and in several of its wild relatives. The activation of TEs, mainly retrotransposons by various environmental and climatic stresses has an important evolutionary significance (e.g., Schrader and Schmitz 2019). In addition to the generation of genetic variability due to epigenetic changes, e.g., DNA methylation, chromatin acetylation and activity of various small RNA molecules, genome restructuring by TEs may lead to the formation of new linkage groups. Activity of genome -restructuring genes as well as TEs during wheat cultivation may explain the wide occurrence of chromosomal rearrangements among domesticated wheat taxa. Thus, domesticated allopolyploid wheat can accumulate a significant amount of genetic variation also through mutations.

The genetic diversity of the D subgenome of bread wheat is relatively limited, being 2–5 times lower than for the A and B subgenomes (Caldwell et al. 2004). Recently, there has been renewed interest in exploring the genetic diversity within the Ae. tauschii genome for the purpose of breeding. A study of a collection of 101 synthetic hexaploid wheat lines, made from crosses between tetraploid wheat and Ae. tauschii showed a high diversity, similar to that of the A and B subgenomes of hexaploid wheat (Bhatta et al. 2018). Whole genome sequencing was performed for 278 accessions of Ae. tauschii, including de novo sequencing for four accessions (Zhou et al. 2021). A core collection of 85 accessions, representing 99% of the species variation was defined and crossed with elite bread wheat cultivars to generate a new collection of Ae. tauschi—wheat synthetic octoploids (Zhou et al. 2021). This collection showed a promising phenotypic variation of potential for wheat breeding for trait such as grain weight and pre-harvest sprouting. Nyine et al. (2021) tested introgressions of 21 Ae. tauschii accessions into hard winter wheat and, also found that some introgressions were positively associated with yield traits.

13.6 The Three Phases of Selection Under Cultivation

During the 10,000 years of wheat cultivation, the criteria for selection varied from time to time and from place to place, as suggested by Evans (1981) in regard to yield criteria. Selection pressures exerted by farmers in different locales have operated in different, and sometimes even in contradictory, manners in different cultivated fields. Thus, during the process of adaptation to the various cultivated environments and to the different demands of man, wheat responded with a number of significant genetic changes. The main changes reflect the process of adaptability to agriculture, particularly of the plants to propagate and protect its seeds, through which wheat became completely dependent on the farmer for its survival.

The modern gene pool of the domesticated wheats has developed through three main phases of selection: occasional and sometimes non-intentional selection, exerted by the earliest farmers, simply by the processes of harvesting and planting; alongside more deliberate selection by traditional farmers in polymorphic fields; and selection as part of scientifically planned modern breeding (Feldman 1976, 2001; Feldman et al. 1995).

During the first phase, a very significant sequence of changes occurred in the transition from the wild to the domesticated forms (Table 13.6). Grain size, an important domestication trait that has been associated with successful germination and seedling growth under cultivated conditions, was among the first traits selected. It resulted in a change from production of small grains with a long, thin shape in wild wheat to more uniform, larger and wider grains in domesticated wheat. Grain size increased early in domestication, whereas changes in grain shape occurred at somewhat later stages (Gegas et al. 2010). These authors found that grain size and shape are primarily independently controlled traits, by a limited number of QTLs in both wild and domesticated wheat.

Table 13.6 Modifications that occurred in wheat during its three phases of cultivation (After Feldman 2001)

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Domestication of emmer, ssp. dicoccon, through the loss of rachis fragility was probably a gradual process, as suggested from both the archeological and genetic evidence. The archeological record (Kislev 1984; Tanno and Willcox 2006) showed a mixture of fragile and nonfragile types in ancient sites where agriculture was practiced, with nonfragile rachis becoming prominent in farming units only after 2–3 thousand years [Kislev (1984) in emmer wheat; Tanno and Willcox (2006) in einkorn]. Genetically, since the rachis-brittleness trait is controlled in wild emmer by two major dominant genes (Levy and Feldman 1989a, b; Watanabe and Ikebata 2000; Watanabe et al. 2002, 2005; Nalam et al. 2006; Millet et al. 2013), it probably took some time to obtain a homozygous recessive combination of these two genes.

According to Kislev (1984), the non-brittle types were probably selected by women, who were usually in charge of threshing. The non-brittle spikes were harder to thresh and were left aside as seeds for the next spring. These non-brittle types had been established in the cultivated field and spread gradually from farmer to farmer and from village to village throughout the Levant. The loss of rachis fragility gave rise to domesticated emmer one of the earliest forms of domesticated wheat, which is still grown today, albeit on a small scale (de Vita et al. 2006).

An additional requirement for the newly domesticated wheat was uniform and rapid germination. Wild wheat exhibits two types of dormancy: a post-harvest type and a next-year type. The first type prevents premature and untimely germination—an important feature, pre-adapted to agriculture, particularly in view of the fact that seeds were often stored under unsuitable storage conditions. The second type of dormancy ensures in wild wheat temporal distribution of germination; it is invariably the larger grain of the second floret in each spikelet that germinates in the first year, while the smaller grain of the first floret germinates in the second year. Induction of germination of all grains in the first year was of great advantage.

The loss of self-propagation and of the temporal control of germination was followed by the loss of self-protection of the grains. The wild forms have tightly closed tenacious glumes, resulting in a “hulled” post-threshing grain. Several primitive forms of domesticated wheat species, e.g., T. monococcum subsp. monococcum, T. turgidum subsp. dicoccon and paleocolchicum, T. timopheevii ssp. timopheevii, T. aestivum subsp. spelta and macha, retain this feature. The appearance of the free-threshing trait (naked kernels) in T. turgidum and T. aestivum, was the second-most important step in domestication of wheat after rachis non-brittleness. T. turgidum ssp. parvicoccum, a “fossil” subspecies of allotetraploid wheat was the first free-threshing form (Kislev 1979/1980). Ssp. durum may thus derive from hybridization between ssp. parvicoccum and ssp. dicoccon, receiving the free-threshing trait from parvicoccum and the large grains from dicoccon. The large grain of durum was probably preferred over the small grains of parvicoccum, leading to the prominence of durum as an allotetraploid wheat and to the extinction of parvicoccum. Similarly, it can be assumed that bread wheat, T. aestivum subsp. aestivum, received these mutations from tetraploid wheat but required additional mutation in the Tg gene of genome D to become free threshing (Kerber and Rowland 1974).

In addition to the above-mentioned classical domestication traits, several other domestication traits were selected, including increased number of seeds per spikelet, simultaneous ripening of grains, and plant erectness versus the prostrate grassy types (Table 13.6).

During the second phase of selection under cultivation, wheat culture spread into new areas, an event that required the adaptation of domesticated wheat to new climatic, edaphic, and biotic conditions that were not encountered by wheat before. This created environmental stress that induced a variety of genetic and epigenetic changes. The wheat genotypes might have reacted by increased mutability, possibly due to the temporal activation of various transposable elements. During the spread to Europe, for example, photoperiodic and thermoperiodic responses were modified to achieve an optimum balance between vegetative and reproductive phases: the vegetative period was extended to benefit from the longer summer days and rainy season, while the need for high temperatures for maturation gradually disappeared. In addition, the grain-filling period became longer, and flag-leaf senescence was delayed. These latter adaptations, allowing for larger amounts of carbohydrates to be assimilated and translocated to the developing grains, greatly contributed to higher yields through further increase in grain size and spike counts.

The presence of diverse genotypes within a single polymorphic field, largely prevented the outbreak of epidemics and severe damage by ecological hazards. Hence, growing various genotypes, and even species, as a mixture in a single field heightens yield stability—an economic consideration ranked much more important than occasional high yield (Zeven 1980). The traditional farmers preferred a “safe”, average yield each year, rather than a high yield for several years which might have been followed by crop failure. The lack of suitable means for long-term storage and for large-scale wheat import, translated to drastic consequences of crop failure in any given year. Hence, yield stability was of utmost importance.

The second phase of evolution under cultivation involved long and continuous selection for various agronomic and technological characters in the polymorphic fields of the traditional farmers. The main evolutionary advantage in such fields was the possibility for occasional hybridization between genotypes and even species. Because of the self-pollinating system, every hybridization resulted in a significant number of homozygous recombinants, thereby constantly providing the farmer with new genotypes for selection. Yet, in such fields, as numerous genotypes were grown in mixtures, the unit of selection was a combination of genotypes rather than a single one.

In such polymorphic fields, inter-genotypic competition played a decisive role. Since the contribution of every genotype to the next year’s seeds largely depended on the productivity of single plants, high tillering and vigorous vegetative growth were traits of high adaptive value. Plants with horizontal leaves, which shade weeds and competitors, had an advantage over genotypes with erect leaves.

The population in polymorphic fields had a genetic structure similar to that found in populations of wild wheats: a mixture of many genotypes partially isolated by the predominant self-pollination. According to Sewall Wright's model (Wright 1931), occasional gene exchange between partially isolated genotypes is one of the most effective processes in evolution. The genetic variation thus achieved was maintained in the traditional farming system; because the low yield of the landraces required the use of a greater proportion of the harvested seeds for yearly sowing, a large proportion of the genetic variation was transferred to the next generation (Zeven 1980).

During the second phase of evolution under cultivation, selection efforts resulted in increased plant height, increased tillering, development of canopy with wide horizontal leaves, larger seed size, increased grain number per spikelet, better flour quality, improved seed retention (non-shattering), increased competitiveness with other wheat genotypes and weeds, and better adaptation to a wider range or climatic and farming regimes.

In the third phase of selection under cultivation, driven by scientifically planned modern breeding, starting at the second half of the nineteenth century, the wheat field became genetically uniform and no longer conducive to spontaneous intra-genotypic gene exchange. On the other hand, large-scale gene migration was promoted by worldwide-introduction services. Massive scientific screening aided in revealing desirable genes, and modern methods for manipulating and transferring these genes from one genetic background to another became available. Hybridizations became confined mainly to intra-specific crosses. Lately, however, some inter-specific and inter-generic crosses have also been performed. Individual genotypes, rather than mixtures, became the unit of selection in experimental-station fields. Selection was made mostly for traits that improved wheat performance in dense stands, such as minimum intra-genotypic competition, upright leaves to improve light penetration and prevent shadowing of neighboring plants, low tillering, a higher number of seminal roots whose development is independent of tillering. High-yielding cultivars were produced that owe their performance to an increase in the number of fertile florets per spikelet and, sometimes, to the length or density of the spike, reduction in shattering, and resistance to fungal, bacterial, and viral diseases as well as to pests.

One of the great achievements of this phase is the the Green Revolution that took place in the 1960s. During the course of this revolution, use of genes for reduced height (Rht), originally from the Japanese cultivar Norin 10, facilitated the production of semi-dwarf (90–120 cm) or dwarf (60–90 cm) high-yielding cultivars that replaced the conventional tall (120–140 cm) ones. These cultivars introduced to India, Pakistan, Iran, the Mediterranean basin, and other wheat-producing regions, replacing the numerous landraces in every locale. The Green Revolution succeed to improve yield, increase resistance to various diseases and tolerance to agrochemicals. These high-yielding cultivars respond well to new agrotechnical practices particularly to high fertilizer application rates, without lodging, and improved harvest index. Currently, the dwarfing genes have been widely incorporated into most existing cultivars, and are responsible for a very significant increase in wheat yield.

The main achievements in breeding for grain quality have been improvements in milling and baking characteristics. Certain modern cultivars are easily milled because the pericarp and seed coat are only loosely attached to the endosperm. Flour yield is particularly high in cultivars with short, almost spherical grains. To date, less progress has been made in improving the nutritional value of the grain, and further efforts are needed, particularly toward increasing protein content and remedying deficiencies in amino acid composition and some minerals. Among the traits affected by domestication are those affecting the storage proteins, in particular the high-molecular-weight glutenins whose subunit number is lower in domesticated allotetraploid wheat than in wild emmer (Levy and Feldman 1988, 1989b; Laido et al. 2013). Selection for product taste and quality have led to reduced numbers of subunits of high-molecular-weight glutenins in the A and D subgenomes of domesticated allotetraploid and allohexaploid wheats (Feldman et al. 1987).

Today’s breeding techniques may achieve the objectives of the primitive and traditional farmers with greater prediction accuracy. The general goal was to achieve highest possible yields per area. Yet, the main limiting factor (globally) in achieving this goal was water availability, and plants have also been subsequently selected to address this goal and constraint.

13.7 Man-Made Allohexaploids

13.7.1 Synthetic T. aestivum Lines

Bread wheat, T. aestivum ssp. aestivum, has passed through two cycles of genetic bottlenecks, allohexaploidization that led to the formation of ssp. spelta from a relatively small number of individuals of tetraploid wheat and Ae. tauschii, and the evolvement of bread wheat, ssp. aestivum, by mutations from ssp. spelta (Table 13.5). However, during 9000 years of cultivation this narrow genetic basis became wider due to mutations and introgressions from domesticated and wild relatives. Yet, an additional reduction in genetic diversity of bread wheat occurred during the Green Revolution. This revolution achieved success in yield increase, better resistance to diseases and improved tolerance to various agrochemicals in many wheat growing countries. However, at the same time, this success was bought at the cost of an overall reduction in genetic diversity in bread wheat (Warburton et al. 2006). The loss in genetic diversity was mainly due to replacement of numerous land races by a relatively small number of elite cultivars and the lack of awareness to maintain the replaced land races in gene banks. The abandon of a large number of land races led to a sizeable genetic erosion in the genepool of bread wheat. The dangers of a narrow genetic base of domesticated wheats, mainly of bread wheat, have become a great concern in recent decades, and, consequently, increased the need to widen the genetic basis of bread wheat. In this endeavor, attempts have been made to transfer desirable genes from wild and other domesticated wheats and from various Triticeae species to bread wheat, and during the years, several successful gene transfers, mainly of disease resistant genes, were accomplished from various related species (Wulff and Moscou 2014). Yet, there are several obstacles in this endeavor that curtail transfer of desirable genetic material. As already pointed out by McFadden and Sears (1947), T. turgidum (genome BBAA), that has two homologous subgenomes to the A and B subgenomes of T. aestivum, its pentaploid hybrids with hexaploid wheat are only partially fertile. Moreover, the tetraploid F₁ hybrids between bread wheat and Ae. tauschii is extremely sterile.

To overcome the difficulties of partial sterility in crosses of T. turgidum x T. aestivum and the high sterility in T. aestivum x Ae. tauschii hybrids, McFadden and Sears (1947) suggested the construction and use of synthetic T. aestivum lines that combines the BBAA genome of T. turgidum with the D genomes of Ae. tauschii. Hybrids between the synthetic lines and bread wheat are fully fertile, chromosome pairing is complete, and high rate of recombination between the synthetic and the domesticated chromosomes consistently occurs.

The pioneering production by McFadden and Sears (1946) of synthetic T. aestivum (STA), that resembled ssp. spelta, by crossing domesticated emmer (ssp. dicoccon; genome BBAA) with Ae. tauschii (formerly Ae. squarrosa; genome DD) and doubling the chromosome number of the F₁ hybrid with colchicine, indisputably indicated the two parental species of T. aestivum and the mode of its origin. As a direct consequence of this important discovery, McFadden and Sears (1947) proposed to produce synthetic T. aestivum lines to overcome the partial sterility in hybrids between tetraploid and hexaploid wheat, and the high sterility in T. aestivum x Ae. tauschii hybrids, and thus, to be able to transfer with ease desirable genes from the synthetic to the domesticated hexaploid wheat. Soon after, Kihara and Lilienfeld (1949) produced amphidiploids from a cross of ssp. dicoccoides var. spontnneo-nigrum with Ae. tauschii through the union of two unreduced gametes. Like the STA produced by McFadden and Sears (1946), also these amphiploids were fertile, exhibited regular chromosome pairing, and resembled hulled form of hexaploid wheat. The hybrids between the synthetic hexaploids with ssp. Spelta and ssp. aestivum were fertile and exhibited full or almost full pairing (McFadden and Sears 1946; Kihara and Lilienfeld 1949). These studies paved the way to produce a large number of STAs using elite cultivars of ssp. durum and diverse genotypes of Ae. tauschii (Fig. 13.3). Such genetic resource will facilitate transfer of desirable genes from the A and B subgenomes of ssp. durum to the corresponding subgenomes of bread wheat and from the D genome of Ae. tauschii the D subgenome of bread wheat (Mujeeb-Kazi et al. 1996; Trethowan and Mujeeb-Kazi 2008).

The use of ssp. durum as the maternal parent in the production of STA lines guarantees success in the cross with Ae. tauschii and also that the resultant STA will have the cytoplasm of T. turgidum that is identical to that of T. aestivum. Chromosome doubling of the triploid F₁ hybrids occurs either naturally due to production of unreduced meiotic cells or after colchicine treatment. The STAs thus produced comprise an important genetic resource for transferring novel genetic variation to bread wheat including desirable genes from wild and domesticated subspecies of T. turgidum and from Ae. tauschii. With no reproduction barrier and almost complete chromosomal homology, crossing these STAs with modern elite bread wheat cultivars facilitates the transfer economically important genes to the domesticated background that may increase yield, widen resistance to fungal, bacterial and pests, improve tolerance to various abiotic stresses, and enhance nutritional and backing quality. No wonder, therefore, that STA has become an important component in wheat breeding (Mujeeb-Kazi et al. 2013; Ogbonnaya et al. 2013; Li et al. 2018; Rosyara et al. 2019; Aberkane et al. 2020).

New molecular technologies revealed a lower genetic diversity in the D subgenome than in the A and B subgenomes of bread wheat (e.g., Walkowiak et al. 2020; Gaurav et al. 2021). Nucleotide diversity in the A, B, and D subgenomes of bread wheat is considerably a lesser amount than that of the genetic variation in these genomes in the parental species (Zhao et al. 2020). It is important therefore, to introduce novel variation for desirable traits from the parental species of bread wheat. STAs can boost the genetic diversity in all three subgenomes of bread wheat. Ae. tauschii harbors substantial variation for many biotic and abiotic stress tolerance traits that are relevant in wheat breeding (Dudnikov and Kawahara 2006). In STA lines, the presence of the D subgenome, that derived from many liferent genotypes of Ae. tauschii, harbor unparalleled genetic diversity for addressing global wheat production constraints through genetic improvement (Mujeeb-Kazi et al. 2008).

From hundreds Ae. tauschii accessions hybridization efforts produced more than thousands STA combinations resulting from chromosome doubling of the F₁ hybrids with elite ssp. durum cultivars. During the last 50 years, more than several thousand different lines of STA were developed in the International Maize and Wheat Improvement Center (CIMMYT) (Warburton et al. 2006; Mujeeb-Kazi et al. 2008; Ogbonnaya et al. 2013; Das et al. 2016; Rosyara et al. 2019; Aberkane et al. 2020), in China (Li et al. 2018), and several other countries. This extensive production of STA represents a valuable resource of user-friendly genetic diversity (Warburton et al. 2006). Indeed, analysis of these STA lines provided encouraging diversity data for key abiotic constraints such as drought, salinity, and heat, ae well as for several biotic stresses (Mujeeb-Kazi et al. 2008). Likewise, Rosyara et al. (2019) estimated the contribution of the D subgenome of STA lines to derivative lines resulted from crosses with elite cultivars of bread wheat. Their results underline the importance of STA lines in maintaining and enhancing genetic diversity and genetic gain over years. STAs are a good source for novel resistance genes to fungal diseases and for pests most of which derived from Ae. tauschii (see review by Li et al. 2018).

This is an example of success utilizing wild relatives in mainstream breeding at large scale worldwide (Rosyara et al. 2019). However, STAs are hulled forms and cannot be used as cultivars because of the presence undesirable characters that derived from the wild parent Ae. tauschii, e.g., tenacious glumes, barrel-type rachis disarticulation, and other wild traits. Thus, it is required to remove these wild undesirable characters through crossing and backcrossing to T. aestivum elite cultivars and selection in the segregating progeny. Several derivative cultivars having higher concentrations of both micronutrients and macronutrients and higher yield than their parental bread wheat cultivars, have been developed at CIMMYT (Guzman et al. 2014). By now, about several tens of lines that derived from such crosses have been registered as cultivars around the world, particularly in China (Yang et al. 2009; Li et al. 2018; Hao et al. 2019).

13.7.2 Triticale

13.7.2.1 General Description

Triticale [Triticosecale (Wittm. ex Camus) MK], is a man-made cereal crop, resembles wheat, whose name derived from the scientific name of its two parents, Triticum and Secale. The name Triticale Tsch. (von Tschermak 1937) was given to wheat-rye amphiploids, and since then has been used more and more as a common name (Villareal et al. 1990). Triticale obtained from hybridization of wheat as female, either tetraploid ssp. durum or hexaploid ssp. aestivum, with rye, Secale cereale, as male, and double the chromosomes of the sterile F₁ hybrid by colchicine treatment to produce a fertile amphiploid. The idea was to combine the high yield potential, good grain quality, and disease resistance of wheat with the vigor and hardiness of rye and its resistance to various diseases. Both 4 × and 6 × triticales are produced with wheat as well as with rye cytoplasmic background, named Triticosecale and Secalotriticum, respectively.

Triticales have been synthesized at four different ploidy levels (4x, 6x, 8x, and 10x). Decaploid triticale (2n = 10x = 70; genome BBAADDRRRR) was obtained by Müntzing (1955) via crossing 8 × triticale with 2 × rye, or 6 × wheat with 4 × rye. It had cytological instability as well as poor seed set and could not be maintained and are thus lost (Müntzing 1955). Triticales at the other three ploidy levels are being used for a variety of studies. At early stages, octoploid triticale was the choice of geneticists and plant breeders but, with time, hexaploid triticale had greater potential to become a successful crop. Tetraploid triticale shows little promise to become a crop. Thus, most of the currently available triticales are hexaploids due to their superior vigor and reproductive stability compared to the octoploid type (Mergoum et al. 2009). Hexaploid triticales are being used for extensive cultivation in several countries, while octoploid triticales have been used for cultivation in a more limited scale in China (for review see Müntzing 1979; Gupta and Priyadarshan 1982). Tetraploid triticales, are produced by crossing 6 × triticale with 2 × or 4 × rye and backcrossed to rye (Krolow 1973). Some 4 × triticales that have mixogenomes A/B show considerable stability. Tetraploid triticales are still in different stages of development and may not be used for cultivation in the foreseeable future (Mergoum et al. 2009).

Detailed description of the history of triticale is given by Villareal et al. (1990) and Oettler (2005). Towards the end of the nineteenth century, breeders begun to cross wheat with rye, and the first triticale, which was octoploid, was produced in Germany by Rimpau in 1888, from crosses of ssp. aestivum and rye, followed by spontaneous chromosome doubling (Rimpau 1891). It was not until the 1960s that the first commercial releases became available for producers (Mergoum et al. 2009).

Triticale is a facultative autogamous allopolyploid species. As such, most lines are homozygous. The cytoplasm of triticale derived from the wheat maternal parent and is similar to that of allopolyploid wheats, T. turgidum and T. aestivum. Consequently, many Secale genes are not expressed in the background of wheat cytoplasm and the nuclear Triticum genome is predominant. Moreover, there is a sizeable reduction of the DNA amount of the rye subgenome in triticale, either 6x or 8x (Boiko et al. 1988; Ma and Gustafson 2005, 2006; Ma et al. 2004).

Taxonomical status of triticale

Chapter H of the international code of botanical nomenclature 2000 (ICBN 2000), dealing with names of hybrids, concluded in section H.6.2. that the nothogeneric name of a bigeneric hybrid is a condensed formula in which the names adopted for the parental genera are combined into a single word, using the first part or the whole of one and the last part or the whole of the other (but not the whole of both) and, optionally, a connecting vowel.

The problems concerning the taxonomy and nomenclature of triticale were discussed by Baum (1971), Gupta and Priyadarshan (1982), Gupta and Baum (1986) and Gupta (1986). In contrast to the current recommended usage of the nothogeneric name x Triticosecale, Gupta and Baum (1986) and Gupta (1986) advocated that triticale be not treated any longer as a nothogenus x Triticosecale, but rather, as a monotypic genus instead, with a new name, proper circumscription, and designation of a type specimen. Stace (1987) regarded that the main reason for not treating triticale as a genus stem from the inability to find characters that would distinguish the new genus from Triticum and the fact that the distinction between the two "is becoming increasingly blurred" (Stace 1987), presumably due to continued introgression from hexaploid wheats used in triticale improvement programs. Stace (1987) proposed that the correct nothogeneric name for plants derived from Triticum x Secale crosses is x Triticosecale Wittmack ex A. Camus. No correct name at species level is available for the commonest crop triticales. These triticales are, however, still described under the nothogenus x Triticosecale Wittmack and its continued usage has recently been recommended by Stace (1987). However, a detailed study of inflorescence, glume, lemma, and lodicule characters, conducted by Baum and Gupta (1990) in 108 accessions of hexaploid and octoploid triticales, in 102 herbarium specimens representing 21 species of Triticum and Aegilops, and in 30 herbarium specimens representing 12 species of Secale, justify in their opinion a generic status for triticales. These authors provided, a key for distinguishing the genera Secale, Triticum, Aegilops, and the nothogenus x Triticosecale.

Mac Key (2005) disagree with Baum and Gupta (1990) that triticale deserves a separate genus rank and instead, incorporated the three different triticale species in the genus Triticum. He argued that triticale, like all Triticum species, has the typical growth habit and inflorescence of diploid wheat, and are all to be considered as hybrids carrying the pivotal AA genome as an essential part and thus, designed of it. From this aspect, they can be defined as forming a natural hybrid genus. The inclusion of triticale into Triticum is based both on commercial and scientific considerations (Mac Key (2005). According to him, the inclusion of triticale into Triticum is based both on commercial and scientific considerations. Mac Key (2005) knows that “such a delimitation of the genus Triticum is not strictly following the International Code of Botanical Nomenclature 2000 (ICBN 2000) but has at least the advantage of preserving the basic frame set by the old traditional taxonomy”.

The basic principle of including triticale as a section in the genus Triticum has consistently (Mac Key 1954b, 1966, 1975, 1981, 1988) been to combine this genealogical aspect with trying to foresee a steadily ongoing, dynamic evolution. This idea appears now to have been supported also by the International Code of Nomenclature for Cultivated Plants 1995 (ICNCP). Mac Key (2005) included triticale as a separate section, Triticosecale (Wittm. ex Camas} Mac Key, sectio nov. comb. into the genus Triticum. Triticale is grouped into three different species, representing tetra- hexa-, and octoploid constitution, respectively. The 4 × triticale was not taxonomically treated before and is proposed to be named Triticum semisecale Mac Key, comb. nov. According to ICBN 2000, Art. 51, Triticum neoblaringhemii (Wittm. ex Camas} Mac Key, comb. Nov.for 6 × and Triticum rimpaui (Wittm.) Mac Key, comb. nov. for 8 × must be used.

Mac key’s (2005) inclusion of triticale as a section in Triticum and its three species are as follows: Section Triticosecale (Wittm. Ex Camus) Mac Key, Sectio. nov. 2n = 4x/6x/8x = 28/42/56. Triticum semisecale Mac Key, comb. nov. 2n = 4x = 28 (AARR or A/BRR) (Subtriticale). Triticum. neoblaringhemii (Wittm. Ex Camus) Mac key, comb. nov. 2n = 6x = 42 (BBAARR) (Triticale). Triticum rimpaui (Wittm.) Mac Key, comb. nov. 2n = 8x = 56 (BBAADDRR) (Eutriticale).

The above treatment follows the taxonomic treatment of the genus Triticum by Mac Key (2005) using the biological species concept based on genome composition. The titicales resemble morphologically wheat more than rye (Baum and Gupta 1990).

T. semisecale is a less-stable species whereas T. neoblaringhemii is stable and very successful in cultivation. On the other hand, T. rimpaui is not completely stable and mainly has historical importance. It has glumes almost as small as in rye, i.e., much smaller than in wheat and in the 6 × and 8 × triticales, but the spike shape is wheat. The glumes nerves converge at the tip like in rye. This species has larger lodicules than on wheat and even on rye (Baum and Gupta 1990).

Triticum neoblaringhemii has a characteristic ear feature with more elongated spikelets than in wheat and capacity of a larger seed size than in rye. Glumes are clearly different from rye and have, like wheat, nerves not converging at the tip and with the midrib placed asymmetrically. This species has lemmas more in texture and appearance like wheat and different from rye. Larger lodicules than on wheat and even on rye (Baum and Gupta 1990).

Triticum rimpaui has more wheat-like glumes, which are less elongate, slender, and more tough than on Triticum semisecale and T. neoblaringhemii but still separable from wheat. Lemmas are in texture and appearance much like in wheat but the nerves converge in contrast to wheat at the tip. Since T. rimpaui is more consistently produced through T. aestivum, ssp. aestivum, as wheat parent, its basal part of glume nerves may be less sharply marked. Hairiness below ear is proof of rye dependence but less reliable (Baum and Gupta 1990). Shriveled seeds are more common in T. rimpaui than in T. neoblaringhemii.

13.7.2.2 Cultivation

In difference from the other cereal crops, triticale is a young crop whose evolution happened only during the last 140 years and its most dramatic evolutionary events were almost all directed by humans (Mergoum et al. 2009). Most triticale cultivars are hexaploids (Villareal et al. 1990). The first hexaploid triticales synthesized from tetraploid wheat, mainly from ssp. durum (genome BBAA) and rye, Secale cereale (genome RR), are called primary hexaploids, while hexaploid triticales synthesized from crosses between primary hexaploid triticales and/or between primary hexaploid triticale and hexaploid wheats or octoploid triticale are called secondary hexaploid triticales (Lukaszewski and Gustafson 1987). One advantage of the secondary hexaploid triticale is the increased genomic diversity, including the insertion of portions of the D subgenome from hexaploid wheats /into the R subgenome of triticale.

Triticale has either winter or spring growth habit, vary significantly in plant height, tend to tiller less, and generally have larger inflorescence in comparison to wheat. The majority of triticale cultivars have prominent awns, but a limited number of current both spring and winter types exhibit awnless traits (less than 5 mm). These types have increased potential for use as a hay forage for livestock (Villareal et al. 1990).

The first release of a commercial triticale cultivar occurred in Europe, whereas `Rosner' a Canadian release was the first triticale cultivar developed in North America. Europe is the major triticale producing region; Poland, Germany, Belarus, and France are the major producers of triticale in the European region (Table 13.7).

Table 13.7 World total and top triticale producing countries in 2020 (From FAOSTAT 2022)

Full size table

Triticale is gaining popularity among the livestock growers across the globe as it can be used in the animal feed. Triticale has the digestibility and water-soluble sugars similar to oats and cereal rye. Additionally, triticale can also thrive in drought, low-fertile soil which attracts more forage growers and livestock growers. This increasing adoption of triticale in animal feed is one of the major driving factors for the global triticale market.

13.7.2.3 Breeding

The history of triticale breeding for cultivar development has been an agronomic success story (Villareal et al. 1990). The first triticale cultivars were characterized by low yields, tall and weak straw, shrunken and shriveled kernels, high susceptibility to ergot [Claviceps purpurea (Fr.) Tul.], high protein, and high levels of the amino acid lysine, and preharvest sprouting (Oettler 2005). The advantage of high protein and high lysine in livestock food was nullified by the poor yield performance and the high incidence of ergot. Modern triticale has overcome most of these problems, some of which caused by cytological differences of the wheat and rye genomes such as different length of meiotic duration, difficulties in chromosome segregation and aneuploidy, and different in amount of constitutive telomeric heterochromatin that affects meiotic pairing and grain shriveling (Lukaszewski and Gustafson 1987; Gupta and Reddy 1991). After decades of additional breeding and gene transfer from wheat and rye, the more advanced triticale cultivars, released in the 70th–80th of the previous century, have improved agronomic traits including high yields, resistance to lodging and ergot, plump kernels, but at the expense of protein content, which is now comparable to wheat (Skovmand et al. 1984; Oettler 2005).

The first hexaploid triticale was reported in 1938, and breeding efforts soon after concentrated on the production of hexaploids from various cultivars of tetraploid wheat, mainly ssp. durum, and Secale cereale, as well as intercrossing of both octoploid and primary triticales (Villareal et al. 1990). More intensive breeding programs with the explicit objective of developing triticale into a commercial crop were initiated during the 1950s in Spain (Sanchez-Monge 1974), Canada (Shebeski 1974), and Hungary (Kiss 1974). In 1953, the University of Manitoba, Winnipeg, Canada, began the first North American triticale breeding program working mostly with durum wheat–rye crosses. Since Canada's program, other public and private programs have initiated both durum wheat–rye and common wheat–rye crosses. These early breeding efforts concentrated on developing a high yielding and drought-tolerant human food crop species suitable for marginal wheat-producing areas. Both winter and spring types were developed, with emphasis on spring types. The major triticale development program in North America is now at CIMMYT in Mexico, with some private companies continuing triticale breeding. The CIMMYT Triticale Improvement Program started in 1964 under the leadership of N.E. Borlaug, followed by F.J. Zillinsky in 1968 (Zillinsky and Borlaug 1971). This program, in cooperation with the University of Manitoba, led to the release of the first cultivars, triticale numbers 57 and 64 in Hungary in 1968, followed by “Cachirulo” in Spain, and “Rosner” in Canada, both in 1969. Towards the 20th of the current century, triticale production was concentrated in Europe with nearly 90% of the world production; more than 3 million hectares were planted, and more than 14 million ton harvested in 2020 (Table 13.7). Production trends do show steady growth over the last 40 years. The leading producers of triticale worldwide are Poland, Germany, Belarus, and France.

A first major breakthrough came by chance when a triticale plant resulting from a natural outcrossed to unknown Mexican semi-dwarf bread wheat was selected in 1967. The selected line designated “Armadillo,” made a major contribution to triticale improvement worldwide since it was the first triticale identified to carry a chromosome substitution wherein a D-subgenome chromosome was substituted for the respective R-subgenome homeologue. Because of this drastic improvement in triticale germplasm, numerous cultivars were released, and the crop was promoted to farmers as a “miracle crop.” However, by the late 1980s, data from international yield trials revealed that complete hexaploid triticale (AABBRR) was agronomically much superior to some D-chromosomal substitutions, particularly under marginal growing conditions. Thereafter, triticale germplasm at CIMMYT was gradually shifted towards complete R-genome types to better serve these marginal environments.

The last several decades of research on triticale initiated by CIMMYT in association with National Agricultural Research Systems around the world, have resulted in significant improvements of triticale crop. Triticale today is an international crop grown in more than 41 countries with the number of countries and the acreage under triticale production increasing. Accordingly, the Food and Agriculture Organization, 15.5 million tons of triticale were harvested in 2018 in 41 countries across the world (FAOSTAT 2018).

Triticale cultivars are classified into three basic types: spring, winter, and intermediate (facultative) (Villareal et al. 1990). Spring types are generally insensitive to photoperiod and have limited tillering. Yet, most of the world triticale acreage is under winter types (Mergoum et al. 2009). Winter types have prostrate type of growth in the early stages of development, and they require vernalization to initiate heading. In general, winter types yield more forage than spring types mainly due to their long growth period. Intermediate (facultative) types are in-between spring and winter types (Mergoum et al. 2004; Salmon et al., 2004).

Current triticale breeding programs center on the improvement of grain yield, nutritional quality, plant height, biomass, and early maturity. These efforts make triticale a potential candidate for increasing global food production, particularly, for marginal and stress-prone growing conditions. Modern cultivars of triticale can be used for ethanol production (McKenzie et al. 2014).

Triticale breeding programs worldwide have emphasized improving the product quality and developing triticale cultivars for specific end-uses such as milling and baking purposes (Villareal et al. 1990). Emphasis has been also given to developing triticale for dual purpose (forage and feed grain), and grazing types (Villareal et al. 1990). Variability present in the triticale germplasm for preharvest sprouting and gluten quality has been exploited by breeders to develop cultivars with enhanced quality and sprouting resistance which has improved the bread-making qualities of triticale grain.

Lodging resistance in triticale has been successfully improved using the dwarfing genes from both Triticum and Secale species. This has resulted in a decrease of up to 20 cm in plant height and increasing yield as the semi-dwarf cultivars are high yielding and more responsive to inputs. Modern triticale cultivars are resistant to a wide range of biotic and abiotic stresses, resulted in increasing the acreage under triticale worldwide. Under marginal land conditions, where abiotic stresses related to environment (drought or temperature extremes) and soil conditions (extreme pH levels, salinity, toxicity, or deficiency of elements) are the limiting factors grain production, modern triticale cultivars have consistently shown its advantages and has outperformed the existing cultivated cereal crops (Mergoum et al. 2004; Estrada-Campuzano et al. 2012; Ayalew et al. 2018). Early maturity, a typical characteristic of modern triticale, allows escape from terminal developmental stresses, such as heat or frost, in highly productive environments, such as the irrigated subtropics and Mediterranean climates, which has contributed to triticale acceptance by farmers. Many triticale cultivars show tolerance to periods of drought. Substantial progress has continued to improve grain weight (Mergoum et al. 2004). Research reveals an increase in the adaptation and successful production of triticale to stressed environments, particularly to water stress (Barary et al., 2002). Both successful breeding and management have resulted in acceptance of triticale as a major alternate crop to traditional cereal crops.

In general, winter triticale produces higher forage biomass than spring types. Therefore, their use for forage (grazing), cut forage, silage, and grain or hay has been improved through the release of several forage-specific cultivars. In addition, in many countries cereal straw is a major feed source for animals and in some years can have greater value than grain. Under arid and semiarid conditions, triticale has been shown consistently to produce higher straw yields than wheat and barley (Mergoum et al. 1992).

In comparison with wheat, triticale appears to have good resistance to several common wheat diseases and pests including rusts (Puccinia sp.), Septoria complex, smuts (Ustilago and Urocystis sp.), bunts (Tilletia sp.), powdery mildew (Blumeria graminis), cereal cyst nematode (Heterodera avenae), and Hessian fly (Mayetiola destructor) (Skovmand et al. 1984). It also resists virus diseases, such as barley yellow dwarf, wheat-streak mosaic, barley-stripe mosaic, and brome mosaic (Skovmand et al. 1984). On the other hand, triticale has relatively greater susceptibility than wheat to diseases such as spot blotch (Bipolaris sorokiniana), scab (Fusarium sp.), and ergot (Claviceps purpurea) and bacterial diseases caused by Xanthomonas sp. and Pseudomonas sp. (Skovmand et al. 1984). In the past, susceptibility to ergot was a major limitation to triticales expansion since it was linked to floret sterility, but ergot is not seen as a major problem in current varieties.

In triticale, genetic diversity is increased through direct interspecific (bread wheat x triticale) and intraspecific (winter triticale x spring triticale) as well as octoploid x hexaploid triticale crosses. Such crosses have led to the formation of mixogenomes where some chromosomes from the B, A, R subgenomes of triticale have been replaced by some from the D subgenome. Hence, many modern triticale lines developed from such crosses carry D(A), D(B), and D(R) whole chromosome substitutions or chromosome translocations which add valuable traits to triticale.

The use of hybrid triticales as a strategy for enhancing yield in favorable as well as marginal environments has proven successful over time. Yield improvements of up to 20% have been observed (Oettler et al. 2001, 2003; Oettler 2005). Hybrid btriticale makes the optimum exploitation of heterosis possible and, with the aid of molecular markers, triticale germplasm is presently being investigated to establish genetically diverse heterotic groups (Oettler 2005).

13.7.2.4 Uses

Triticale is used for human food, animal feed, grazed or stored forage and fodder, silage, green-feed, and hay (Oelke et al. 1989; Mergoum et al. (2009). Quality evaluations of triticale grain for milling and baking show that it is inferior to bread-making wheat and to durum wheat for macaroni, but it is often considered superior to rye. Although, triticale contains gluten, it may play a role in the rising healthy food market due to its health benefits with its good essential amino acid balance, minerals and vitamins (Zhu 2018). Triticale is tested for possible use in breakfast cereals and for distilling or brewing (Oelke et al. 1989). During the last decades triticale has received attention as a potential energy crop, and research is currently also including the use of this crop biomass in bio-energy production (Villareal et al. 1990; McKenzie et al. 2014). As animal feed triticale is a good source of protein, amino acids, and vitamin B. The protein content of several triticale lines is somewhat higher than that of wheat and the amino acid composition of the protein is similar to wheat but may be slightly higher in lysine (Mergoum et al. 2009). Triticale has been and is increasingly grown for livestock grazing, cut forage (green chop), whole- plant silage, hay, and forage/grain dual purpose (Myer and Lozano del Rio, 2004). Straw is an important by-product of triticale grain production and is often overlooked (Myer and Lozano del Rio 2004). Triticale produces more straw than other small-grain cereals. Straw is frequently the only source of livestock feed in developing countries (Mergoum et al. 2004).

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Moshe Feldman & Avraham A. Levy

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Feldman, M., Levy, A.A. (2023). Evolution of Wheat Under Cultivation. In: Wheat Evolution and Domestication. Springer, Cham. https://doi.org/10.1007/978-3-031-30175-9_13

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DOI: https://doi.org/10.1007/978-3-031-30175-9_13
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Evolution of Wheat Under Cultivation

Abstract

13.1 Introduction

13.2 Domestication of Tetraploid Wheat

13.2.1 History

13.2.2 Ecogeographical Characteristics of the Area of Wheat Domestication

13.2.3 Domestication of Emmer Wheat, ssp. dicoccon

13.2.3.1 Opening Remarks

13.2.3.2 Selection for Non-brittle Rachis

13.2.3.3 Selection for Large Grain Size

13.2.3.4 Selection for Rapid and Synchronous Germination

13.2.3.5 Monophyletic Versus Polyphyletic Origin of Domesticated Emmer

13.2.3.6 Site(s) of Origin of Domesticated Emmer

13.2.4 Origin of Tetraploid Wheat with Free-Threshing Grains

13.2.4.1 Genetic Control of the Free-Threshing Trait

13.2.4.2 Time of Complete Replacement of Cultivated Wild Emmer by Domesticated Tetraploid Wheats

13.2.5 Domestication of Timopheevii Wheat

13.3 Hexaploid Wheat

13.3.1 Introduction

13.3.2 Genetic Control of Non-Brittle Rachis in Hexaploid Wheat

13.3.3 Genetic Control of Free-Threshing Grains in Hexaploid Wheat

13.3.4 Selection for Yield in Hexaploid Wheat

13.3.5 Summary of the Main Events in Wheat Cultivation and Domestication

13.4 Molecular Changes in Domesticated Wheat

13.5 Founder Effect and Processes Contributing to Increased Genetic Variability of Domesticated Wheats

13.6 The Three Phases of Selection Under Cultivation

13.7 Man-Made Allohexaploids

13.7.1 Synthetic T. aestivum Lines

13.7.2 Triticale

13.7.2.1 General Description

13.7.2.2 Cultivation

13.7.2.3 Breeding

13.7.2.4 Uses

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