1.1 The Importance of Wheat as a Staple Food

As one of the first cereals to be domesticated, the history of domesticated wheat and that of human civilization have been interwoven since the dawn of agriculture. In the course of its domestication, the wheat plant lost its ability to disseminate its seeds effectively and became completely dependent on human for seed dispersal. Man, in return, fostered this cereal to such an extent that it is now one of the world’s foremost crops. The domestication of wheat, and that of other edible plants, have provided humankind with the ability to produce sufficient food, leading to population increase and spread to almost all parts of the globe. It also enabled the colossal development of human civilization. As a result, humans became completely dependent on wheat and other domesticated plants for their survival.

Since the beginning of the cultivation of diploid and tetraploid wheat (first cultivation of wild forms, about 12,000 C14—calibrated years ago, and later, appearance of domesticated einkorn and emmer 10,800 calibrated years ago, and naked tetraploid wheat 10,200 years ago, and of hexaploid wheat about 10,000 years ago (Table 1.1), wheat has become one of the most important staple food nourishing mankind. Domestication of wheat has led to the formation of new wheat species, such as hexaploid wheat, Triticum aestivum, many subspecies and numerous cultivars. Most modern cultivars belong to bread wheat, since it is high-yielding, grows well in a variety of climates and soils, and very suitable for bread making because of the high gluten content of its endosperm. (The elastic gluten protein entraps the carbon dioxide formed during yeast fermentation enabling the leavened dough to rise). Most of the remaining modern cultivars belong to durum wheat, which is mainly grown in relatively dry regions, particularly in the Mediterranean basin, Australia, India, Russia and in the low-rainfall areas of the great plains of the USA and Canada. Its relatively large grains yield a low-gluten flour, suitable for pasta, flat bread (pita) and semolina products.

Table 1.1 Chronology in uncalibrated and 14C calibrated radiocarbon years BP (before present) for the Late Epipalaeolithic and the Neolithic periods in the Levant, the western flank of the Fertile Crescent
Full size table

Throughout 10,000 years of cultivation, bread wheat and, to a lesser extent, durum wheat, have been of supreme importance in facilitating and sustaining the development of human civilization in southwestern and central Asia, Europe, North and South Africa, North and South America, and Australia. From a plant that grew in a relatively small region in the Fertile Crescent, South-West Asia, wheat is now grown on more area than any other crop (219 Mha in 2021) and with global production of 766 million metric tons (FAOSTAT 2021). This high production makes wheat the second most-most produced cereal after maize (1018 Mt), that is extensively used for animal feed, and more than rice (745 Mt), the main human food crop in Eastern Asia (FAOSTAT 2021).

Bread wheat is high yielding in a wide range of environments, ranging from 67°N in northern Europe to 45°S in Argentina; however, in the subtropics and tropics, its cultivation is restricted to higher elevations (Feldman et al. 1995). It provides food to one-third of the global human population, about 20% of the global caloric requirements for human consumption and 20% of the protein consumed (Feldman et al. 1995; Shewry and Hey 2015). The world’s highest wheat-producing regions are the EU (160 Mt), China (125 Mt), India (100 Mt), Russia (60 Mt), the USA (60 Mt), Canada (34 Mt), Pakistan (24 Mt), Ukraine (24 Mt), Australia (23 Mt), and Turkey (20 Mt). Wheat makes up a significant portion of the calories consumed by mankind, where its grain contains most of the nutrients essential to man. These are carbohydrates (70–80%), proteins (8–15%), fats (1.5–2.0%), minerals (1.5–2.0%), and vitamins, such as the B complex and vitamin E. Globally, wheat is the leading source of vegetable protein in human food, bearing a higher protein content than maize and rice. In addition to the relatively high yield and good nutritive value of the wheat grains, their low water content, ease of processing and transport and good storage qualities have made wheat an important food staple for more than 35% of the world’s population. During the last 70 years, the global wheat area has increased by more than 50% and average yields have increased from 1.0 to 2.5 t/ha, reaching levels as high as 12 t/ha, mainly due to improved cultivars, wider use of fertilizers and improved-agronomic practices.

1.2 Interest in the Origin and Evolution of Domesticated Wheat

It is a reasonable assumption that in a pre-agricultural society, each gender had distinct roles relating to food provision, with men hunting, and women collecting and gathering seeds, fruits, and other plant materials. While collecting seeds of wild wheat and barley, they noticed that fallen seeds were later responsible for the growth of new plants. Consequently, women became aware of the profitability of sowing their surplus seeds for next year’s food (Kislev et al. 2004), and deliberately decided to plant seeds of these cereals in more desirable fields close to their dwelling (Kislev 1984). It is not clear when this brilliant discovery was made. One of the most ancient sites bearing signs of collecting and processing wild emmer wheat, Triticum turgidum ssp. dicoccoides, was found in Ohalo II, a 23,000-year-old hunter-gatherers camp on the south-western shore of the Sea of Galilee, Israel (Snir et al. 2015). About 10,500 years ago, wild wheat begun to be cultivated, and their spike remnants appeared in the archeological records discovered in several sites of the Levant. Few hundred years later on, archaeological remnants indicate replacement of wild wheat by domesticated emmer which, characterized by non-fragile spikes, and consequently, were not capable of dispersing their seeds and were dependent on human for their propagation. The beginning of cultivation of wheat, as well as other cereals and pulses, marked the Neolithic (or Agricultural) Revolution, was one the most important revolution in human history, laying the foundation for the development of human civilization.

The view that women were the first to cultivate plants is reflected in the biblical story: “And when the woman saw that the tree was good for food, and that it was pleasant to the eyes, and a tree to be desired to make one wise, she took of the fruit thereof, and did eat, and gave also unto her husband with her; and he did eat” (Genesis 3:6). This event describes the transition of man from a hunter and gatherer to a farmer; “… And unto Adam he said: Because thou hast hearkened unto the voice of thy wife, and hast eaten of the tree, of which I commanded thee, saying: Thou shalt not eat of it; cursed is the ground for thy sake; in sorrow shalt thou eat of it all the days of thy life. Thorns also and thistles shall it bring forth to thee; and thou shalt eat the herb of the field. In the sweat of thy face shalt thou eat bread … Therefore, the LORD God sent him forth from the garden of Eden, to till the ground…” (Genesis 3:17–23). This biblical myth, describing the expulsion of Adam and Eve from the Garden of Eden, may be considered a reflection of the Neolithic Revolution, during which humankind assumed control over its own food production. Prior to this period, the pre-agricultural hunter-gatherers gradually became acquainted with nature’s periodicity and with the life cycle of plants that produced edible seeds or fruits in their environment. During the Neolithic Revolution, man put his observations to practice and succeeded to domesticate a number of the local edible plants. Several early Jewish scholars of the second and third centuries AD assumed that the “tree of knowledge” was the wheat plant (Talmud Babylonian, Berakhot, 40: A; Bereshit Rabba, 15:8) and interpreted the ancient biblical story as an expression of man’s wish to fulfill his creation “in the image of God” by mastering his own food production.

The rather traumatic narrative of the biblical version of the beginning of agriculture, may indicate that man started to cultivate plants against the God’s command. By contrast to it, in many ancient nations, where mythology prevailed, cultivated plants were considered a generous gift given to man by gods. For this reason, the ancient Egyptians bestowed gratitude to Isis and Osiris for introducing wheat and barley into Egypt from Mt. Tabor in Israel, and for teaching people the secrets of their cultivation. Similarly, the ancient Greeks ascribed the gift of these important cereals to Demeter, and the Romans to the goddess Ceres. The Mayans in Mexico considered maize as the gift of God, and in Shaanxi, China, one of the cradles of Chinese civilization, there is a statue of a godlike personage that brought plants to people and taught them how to cultivate them.

Human interest in the origin of domesticated plants, the geographical sites of their origin, identification of their wild progenitors, and their evolution under cultivation, dates back to the beginning of historic time. These subjects have always attracted great interest, and have excited and stimulated man’s curiosity and imagination. Botanists, geneticists, agronomists, breeders, ethno-botanists and students of agricultural history have grappled with these mysteries by conducting extensive botanical, cytogenetic, molecular, and evolutionary studies on the genetic and genomic structure of the Triticeae tribe, in general, and of various species of the wheat group (the genera Aegilops and Triticum), in particular. The scientific study of the Triticeae tribe began with Linnaeus (1753), who classified the species of wheat in a separate genus Triticum, and has since been subjected to many taxonomic, morphological, eco-geographical, cytogenetic, molecular, and evolutionary studies.

One of the first attempts to identify the progenitor of domesticated wheat, specifically of common wheat, was made in the middle of the nineteenth century, by several botanists who concluded that the natural inter-generic hybrid and hybrid-derivatives of Aegilops geniculata Roth (=Ae. ovata L.) x common wheat, were the ancestral forms of this wheat [the nomenclature of the species of Triticum and Aegilops in this book is as defined by van Slageren (1994). In 1821, Requien discovered such a hybrid in southern France, that grew from a spike of Ae. geniculata. Later, similar hybrids were also collected in northern Italy and in North Africa. Because of its resemblance to common wheat, Requien (see Fabre 1855) named it Aegilops triticoides. Since backcrossed progeny of these hybrids to common wheat exhibited an intermediate morphology between Ae. triticoides and some lines of common wheat, Fabre concluded, in 1855, that bread wheat had originated from Ae. geniculata. He assumed that under cultivated conditions, Ae. geniculata gradually transforms into bread wheat. This hypothesis was disproved by Godron (1876), who confirmed that, Ae. triticoides and all other intermediate forms between Ae. geniculata and bread wheat, are hybrids and hybrid derivatives. Godron produced similar forms by crossing bread wheat with Ae. geniculata and backcrossing the hybrids to the wheat parent. Of note, because Ae. triticoides is a natural inter-generic hybrid, where Ae. geniculata is the female parent, and not a true species, Ae. triticoides should be referred to as x Aegilotriticum triticoides (Req. ex Bertal.) van Slageren and not Ae. triticoides (van Slageren 1994).

Revelation of the origins of x Aegilotriticum triticoides led de Candolle (1886) to understand that historical, linguistic, and folkloristic types of evidence, alone, are insufficient to reveal the origins of domesticated plants; botanical, genetic, and archaeological studies are essential. The origin and evolution of a domesticated plant can be best studied following the identification and analysis of the current and past distribution of its wild progenitor. This may indicate the changes that led to domestication, as well as the site of the initial cultivation. However, when such a wild progenitor is not found, or is extinct, understanding the complete history of the domesticated plant is greatly impaired.

Consequently, the x Aegilotriticum triticoides saga emphasized the need for a clear definition of the key features characterizing the wild progenitors of domesticated wheat. These progenitors must have been valid species and therefore self-propagating. They are assumed to have had a spike similar in its basic features to that of domesticated wheat, but with a brittle rachis that upon maturation, disarticulates into single spikelets (dispersal units), thereby facilitating self-dispersal. In addition, the wild progenitors, like all other wild grasses, seemingly had tightly closed glumes, resulting in “hulled” grains protected against extreme climatic conditions and herbivores. Eco-geographically, they should have occupied specific geographic regions and well-defined primary habitats. However, most of today’s domesticated wheat has undergone considerable morphological changes and only a few have retained ancestral morphological features, such as hulled grains, as in domesticated einkorn, emmer, and spelt wheat (see Chap. 10). Also, most domesticated wheat grow nowadays much larger areas as their progenitors. This renders the identification of the wild progenitors and the site of domestication more complex.

Domesticated wheats are classified into three main groups: diploids (einkorn), tetraploids (emmer, durum, rivet, Polish and Persian wheat), and hexaploids (spelt, bread, club and Indian shot wheat) (see Chap. 10). At the end of the nineteenth century, most botanists assumed that the domesticated wheat taxa had a polyphyletic origin and that at least two species of wild wheat progenitors, namely single-grained (einkorn) and double-grained (emmer) wheat, were taken into cultivation. The wild progenitor of domesticated einkorn was discovered in the middle of the nineteenth century, and that of domesticated emmer and durum at the beginning of the twentieth century. These discoveries enabled the use of the gene pool of both the progenitors and of other related species for wheat improvement. Only in the second half of the twentieth century, it became clear that there are no wild hexaploid progenitors and that all types of domesticated hexaploid wheat were formed in farmers’ fields by hybridization between domesticated tetraploid wheat and a wild diploid species of Aegilops.

1.3 The Need to Exploit Wild Wheat Relatives for Wheat Improvement

Given that only a small number of wild genotypes were selected for domestication, the genetic basis of domesticated wheat in the early stages of agriculture was relatively narrow, representing only a fraction of the large variation that existed in the wild progenitors. Yet, during the 10,000 years of wheat cultivation, the genetic basis of domesticated wheat has been broadened, to some extent due to mutations and sporadic hybridizations with their wild progenitors and other closely related species in southwest Asia. Moreover, the tendency of traditional farmers in many parts of the world, to grow a mixture of genotypes in one field (polymorphic fields) or even a mixture of species of different ploidy levels (Zeven 1980), enabled hybridization and introgression of genes among the various genotypes. This, coupled with wheat’s ability to self-pollinate, greatly facilitated the formation and selection of many distinct genotypes. Traditional farmers selected and planted grains of the lines most desirable for their specific needs and consequently, selection pressures were thus consistently exerted, albeit, in different directions, by farmers in different localities. These efforts resulted in numerous landraces that demonstrated better adaptation to a wider range of climatic and edaphic conditions and to diverse farming regimes. But, under modern plant breeding practices, which began towards the end of the nineteenth century, the wheat fields have become genetically uniform (one elite cultivar is grown not only in one field but in a whole region), so that spontaneous gene exchange between different cultivars in the farmer’s fields has become less likely. On the other hand, gene migration has been greatly increased by worldwide introduction and exchange of cultivars. Crosses between these cultivars have been restricted to the breeder’s experimental fields. In the breeder experimental stations, hybridizations have been mainly confined to intraspecific crosses and little use has been made of neither the gene pools of other wheat species nor of those of wild relatives, toward improvement of bread wheat. Such breeding practices, particularly the replacement in many countries of traditional varieties (landraces), suitable for local climatic and agronomic conditions, by a small number of elite, high-yielding cultivars (mega varieties), have greatly eroded the genetic basis of bread wheat. The Green Revolution, which started in the 60s of the previous century is responsible for the replacement of numerous landraces in India, Pakistan Turkey and North Africa, by a relatively small number of high-yielding cultivars and failure to conserve the replaced traditional landraces (Feldman and Sears 1981). Such erroneous practice still continues and contributes to the loss of genetic diversity in bread wheat, consequently reducing its adaptability to abiotic stresses, increasing its susceptibility to biotic pressures, and considerably limiting the ability of breeders to improve further its yield and quality. This outcome of the erosion in the genetic diversity of wheat has become a more serious problem in light of the current climate changes, combined with continuous growth of the world human population, which requires elevated wheat yields, to improve its resistance to biotic stresses and tolerance to abiotic ones, and to produce genotypes that can provide reasonable yields in new habitats.

Attempts to increase desirable genetic variation via irradiation or chemical treatment, yielded poor results. Moreover, transgenesis, which could broaden the gene pool for wheat improvement, is not yet commonly practiced in wheat. Therefore, utilizing the germplasm of the various Triticeae species remains one of the best options for addressing the challenges of genetic erosion and of yield demands (Feldman and Sears 1981). Consequently, advanced efforts have been made to support more efficient exploitation of the wild gene resources for wheat improvement. The vast genetic resource of the Triticeae contains numerous economically important genes that can be exploited to create a potentially new variation of domesticated wheat. Many of the species of the two sub-tribes of the Triticeae, the Triticineae and the Hordeineae, can be crossed with bread and durum wheat, and economically important genes can be transferred to the domesticated background, via the use of various cytogenetic manipulations.

Although this option is not new, during recent decades, only a small number of genes were transferred from wild species to domesticated wheat. Breeders preferred to use domesticated sources instead of wild ones because they did not know how to overcome the challenges in using them. Nevertheless, a number of inter-generic and inter-specific hybrids have been produced between several Triticeae species and bread or durum wheat. These hybrids, most of which were viable, have been used for a variety of purposes, including genomic analysis, studies of speciation, phylogeny and evolution, and as the starting point in efforts to introduce alien variation into domesticated wheat.

Already in the beginning of the previous century, Aaronsohn (1910), while discovering wild emmer (Triticum turgidum subsp. dicoccoides) in nature, was impressed by its wide range of adaptation. He noticed that certain forms of this wild taxon possess several valuable traits, namely, large grains, ability to grow in relatively dry habitats, and resistance to rust. Consequently, he recommended utilizing wild emmer in breeding programs, especially to improve the resistance of domesticated wheat to drought, extreme climatic and soil conditions, and rusts, and to increase grain size and yield. Aaronsohn believed that “the cultivation of wheat might be revolutionized by the utilization of wild wheat. Such utilization might facilitate the formation of many new varieties, some of which will be hardy and able to grow in dry and warm habitats or in areas with poor soil and can thus expand the wheat growing area” (Aaronsohn 1910, p. 52).

Aaronsohn’s belief that wild emmer can be utilized in the improvement of domesticated wheat was shared by Schweinfurth (1908), von Tschermak (1914), and several other early wheat geneticists. The fertile or partially fertile F1 hybrids between bread wheat and wild emmer, produced by von Tschermak (1914), indicated that gene transfer from wild tetraploid wheat into hexaploid domesticated types is possible by simple breeding procedures. It was hoped that wild emmer could be used for production of domesticated varieties that would be adapted to arid regions (von Tschermak 1914).

Vavilov (his work and ideas reviewed in Vavilov 1951) further advanced the notion of exploitation of wild species to improve wheat and other crops. He identified regions where the world’s major crops were first domesticated but still contained the greatest diversity of their wild relatives. McFadden (1930) was one of the first breeders that transferred a gene for stem rust resistance, later designated Sr2, to a variety of bread wheat from a domesticated emmer (a tetraploid wheat), thus producing the cultivars Hope and H-44, that was resistant to rust that severely infected wheat fields in the USA. Production of synthetic allopolyploids with new genomic combinations was one approach to evaluate and utilize wild germplasms. Broad hybrids in the Triticeae tribe have been attempted and studied for over 100 years. The first such hybrid was between wheat and rye (Wilson 1876). Rimpau (1891) described 12 plants recovered from seed of a wheat-rye hybrid that represented the first triticale. The idea was that such an allopolyploid would combine the cold tolerance of rye and the grain quality of wheat. But Triticale does not possess the grain quality of bread wheat and it is used mainly as an animal feed (Oettler 2005). In 1947, McFadden and Sears (1947) produced synthetic allopolyploids between different species of Triticum and Aegilops as a starting material for evaluation and transfer of desirable genes to a domesticated background.

Over the years, cytogenetic methods for proper genetic analysis and interspecific transfer of desirable characters have been developed, making the use of the wild gene resources more efficient. Sears (1972) described these methods, including induction of homoeologous pairing and recombination between wild and domesticated chromosomes by genetic means and use of ionizing radiation to translocate alien chromosome segments into a domesticated one. The induction of homoeologous pairing is, by far, the simplest gene transfer method (Sears 1972), but induced translocations have also produced some favorable results (e.g., Sears 1956; Knott 1971).

The production of aneuploid lines of bread wheat (Sears 1954), as well as several alien addition, substitution, and translocation lines (Riley 1965; Sears 1969, 1975; Feldman and Sears 1981; Feldman 1988; Millet et al. 2013, 2014), enabled the genetic analysis of individual alien chromosomes or even chromosome arm on the genetic background of domesticated wheat and facilitated the transfer of selected chromosomal segments without affecting the rest of the domesticated genome. Using these techniques, several key genes, primarily those affecting qualitative traits, namely, genes improving disease and virus resistance, have been transferred from species of Triticum, Agropyron, Aegilops, Amblyopyrum, Secale, and other Triticeae, into bread and durum wheat (Sharma and Gill 1983; Feuillet et al. 2008; Millet et al. 2014). Quantitative traits that are controlled by several or many genes that are widely distributed throughout the genome, are much more difficult to manipulate.

Harlan and de Wet (1971) classified primary, secondary, and tertiary gene pools of wild relatives of a crop, as determined by their genetic distance from the crop. A primary gene pool consists of species with genome(s) homologous to that of the crop, including land races of the crop as well as other related domesticated crops and wild species. Among members of this gene pool crossing is easy, hybrids are generally fertile, and exhibit good chromosome pairing, approximately normal gene segregation and generally simple gene transfer by conventional breeding methods. Transfer of genes from species with different ploidy levels required some manipulations, such as production of synthetic allopolyploids. A secondary gene pool consists of species with closely related homoeologous (partially homologous) genome(s), that are crossed relatively easily with the crop, but whose chromosomes do not pair regularly with those of the crop and therefore, transfer of genes from these species required cytogenetic manipulations such as the use of genes inducing homoeologous pairing or the use of genotypes lacking homoeologous pairing suppressors. The tertiary gene pool consists of more distantly related taxa, and exploitation of this gene pool required special cytogenetic and molecular manipulations (e.g., embryo rescue in the F1 hybrids, induction of chromosome pairing and recombination at meiosis of the hybrid, or induction of translocation with ionizing irradiation, transformation with selected genes affecting desired characteristics) in order to overcome inter-generic genetic barriers.

In the Triticeae, the primary gene pool of bread wheat, consists of hexaploid landraces and domesticated and wild tetraploid wheats (T. turgidum and T. timopheevii), diploid wheat (T. urartu and T. monococcum) and Ae. tauschii, all of which have a homologous genome(s) with that of bread wheat. The gene pool of tetraploid wheat has been exploited, to some extent, for wheat improvement via direct crosses (Millet et al. 2013), whereas that of Ae. tauschii has been subjected to allopolyploid bridging (crosses between bread wheat and synthetic allopolyploid of tetraploid wheat x Ae. tauschii). The secondary gene pool contains species of Aegilops and several species of Agropyron. Their genome(s) is homoeologous to that of common wheat and consequently, there is little pairing and recombination at meiosis of the F1 hybrids. However, pairing can be induced relatively easily by the use of mutants of genes that prevent homoeologous pairing (e.g., ph1b, ph1c, and 10/13) and genes from the diploid species Aegilops speltoides or Amblyopyrum muticum, that promote homoeologous pairing in hybrids with wheat. The tertiary gene pool of bread wheat contains all other Triticeae species. The wide morphological and ecological variation of the various Triticeae species may indicate that this tribe contains a very rich gene pool that can be exploited to widen the genetic basis of wheat. Most, if not all, of the Triticeae species, can be crossed with wheat and produce viable hybrids. However, in most cases, the hybrids are sterile, due to the lack of chromosomal pairing at meiosis, resulting in the production of imbalanced gametes; this limitation can be overcome by more radical cytogenetic manipulations (Feldman and Sears 1981; Feldman 1988). Thus, the gene pool of the entire tribe may serve as an important source of useful traits for the improvement of wheat.

During the twentieth century, studies in areas of taxonomy, eco-geography, cytogenetics, and evolution, have mainly focused on the genera Triticum, Hordeum, and Secale, as well as on the closely related genera Aegilops and several species of Agropyron (now Elymus). These studies have provided important information on the genetic structure of members of the primary and secondary gene pools, as well as on phylogenetic relationships between these gene pools and bread wheat. Consequently, several genes, mainly those conferring resistance to biotic stresses, that were not found in the domesticated gene pool of wheat, were successfully transferred to bread wheat (Fedak 2015; Zhang et al. 2015). It is estimated that genes were transferred from at least 52 species belonging to 11 genera (Aegilops, Agropyron, Amblyopyrum, Dasypyrum, Elymus (=Thinopyrum, Lophopyrum, Pseudoroegneria), Agropyron, Hordeum, Leymus, Psathyrostachys, Secale, and Triticum) (Wulff and Moscou 2014). However, currently, information on many of the wild relatives, mainly on the tertiary gene pool, is scanty and fragmentary. Although molecular studies in recent decades have improved, to some extent, the ability to identify, allocate and isolate useful genes in several Triticeae genomes (Paux and Sourdille 2009; Stein 2009; Krattinger et al. 2009; Hein et al. 2009; Eversole et al. 2009), their transfer to the domesticated wheat background has still encountered many obstacles. Further studies of these gene pools, in which information from the fields of cytogenetics and genomics should be combined, are essential.