Description of the seed supply
The composition of the seed supply in La Frailesca (Fig. 2) can be summarized as follows: 48% of all planted seed lots were commercial hybrids; 15% were open-pollinated varieties (OPVs); 27% were creolized seed; and 10% were traditional landraces. The formal sector (hybrids and OPVs), thus, provides 63% of all seed and the informal sector 37%. Within the informal sector, 73% of seed lots are creolized varieties while only 27% are traditional landraces. These numbers, together with the sales volumes of individual commercial varieties, were used to calculate group specific weights for each seed type. All 30 collected seed lots, together with within-group frequencies for each seed type, are listed in Table 1.
Within the formal seed system, we collected seven hybrid seed lots. Hybrids have been introduced only recently in La Frailesca, the first varieties dating back to the late 1990s. Open-pollinated varieties have a much longer history in the region. They mainly originate from public breeding programs and have been released in central Chiapas since the late 1970s by the National Agricultural Research Service (INIFAP). Five seed lots were obtained: V-524 (OV3), a variety created by CIMMYT and released by INIFAP in 1975. It was very popular until its removal from the market in 2001. Previous studies suggest that many creolized seed lots are derived from this variety (Bellon and Risopoulos 2001). V-424 (OV1-2), a variety selected for earliness by CIMMYT from the same population and released in 1981 by INIFAP. Both, sample of commercially sold seed (OV1) as well as seed from CIMMYT’s gene bank (OV2) were included. V-526 (OV4) and V-534 (OV5), these varieties were released by INIFAP in 1982 and 1989, respectively.
Within the informal seed system, three named landrace varieties were collected. The two main varieties Olotillo (CO) and Jarocho (CT) belong to two distinct races, Olotillo and Tuxpeño, respectively (Wellhausen et al. 1952). The name Jarocho suggests that it is an introduced variety, since the same word is commonly used to indicate the inhabitants of the neighboring state of Veracruz. It has a long history in the area, however, and there was no evidence of it being a creolized variety.
The name Olotillo, meaning thin cob, refers to the most obvious trait that distinguishes this race from most other races that have been described (Wellhausen et al. 1952). A single seed lot called Conejo (CC) was collected: based on its ear and plant traits, it probably belongs to the zapalote chico race.
Creolized varieties had very diverse nomenclature, reflecting their introduction history or population of origin. The varieties named V424 (RV1), Precoz (RV2) and Tuxpeño precoz (RV3), were most probably derived from V-424 which was sold under the popular name Tuxpeño Precoz. Seed lots called Tuxpeño (RV7) are likely to originate from V-524 (OV3) and to a lesser extent from V-526 (OV4) both of which went under this name. Pronase (RV5) refers to the former state-owned seed company PRONASE. This company sold different OPVs, so the name sheds little light on the seed’s identity. Similarly, the name San Gregoreo (RV4) reflects the label on seed bags containing unknown varieties that were distributed by the government around 1989. The variety Sardina (RV8), allegedly owes its name to the man who introduced it in the early 1980s and promoted it because its ears produced grain like “sardines in a can”. This variety was said to derive from V-424 (OV1-2), but this could not be verified. We found no evidence of creolized seed lots that were derived from hybrid maize. Most seed lots were planted in quantities of around 30 kg (equivalent to about 1.5 ha) regardless of whether they were formal or informal varieties. Olotillo (CO) and Conejo (CC) were planted in much smaller volumes (1–20 kg, 6 kg on average) and were apparently used for special purposes.
Phenotypic description of collected seed lots
Figure 3 presents the biplot based on the principal component analysis of the scaled trait values for each seed lot. Traits with the highest loadings on the first and second principal components are shown as gray arrows. Five clusters of seed lots could be distinguished: Three landrace clusters (CC, CO and CT), a cluster of improved varieties including all hybrids, creolized varieties and most OPVs (M) and an outlier pair formed by the two seed lots of V-424 (OV1-2). Conejo (CC) and V-424 are both early maturing varieties with a short plant height and small, sturdy ears. Conejo can be distinguished from V424 by its somewhat taller plants, narrower leaves and stems, shorter ears with fewer kernel rows and lower total grain weight. The two main landraces, Olotillo (CO) and Jarocho (CT), were separated from the other seed lots by being tall and late, presenting longer tassel branches as well as a lower number of kernel rows with slightly wider kernels and longer slimmer ears. Compared to hybrids and OPVs these two landraces had relatively narrow ear-leaves and lower ear and grain weight. CO and CT differed mainly in cob diameter and weight and relative ear diameter. Total grain weight differed quite substantially between the different seed lots (Fig. 4). Hybrids had the highest total grain weight as expected. With the exception of the early maturing V-424 (OV1-2) and the creolized variety RV6, all improved and creolized material had a higher individual grain yield than the traditional landrace seed lots. Their grain weight was generally equivalent to that of the classic V-524 variety (OV3). There was differentiation within the CT, CO and M clusters as well as within the set of creolized varieties for most traits. In the biplot, the creolized varieties RV3 and RV6 fell in between OV1-2 and the taller, later flowering OPVs (Fig. 3). When tested against the most probable ancestors, OV3 and OV1-2, significant differences were found for several traits. Surprisingly, one seed lot of the Jarocho landrace (CT4) grouped together with the improved and creolized varieties. It is relatively early flowering and short and has 14 kernel rows instead of the typical 10–12. This seed lot is likely to be a creolized variety in spite of its name.
The dendrogram based on pairwise distances for ear traits (Fig. 5a) confirmed the clear separation between landraces and improved and creolized varieties that was observed in the biplot. Hybrids (HB), formed a single cluster that included OV4, probably due to higher grain yield. The two V-424 seed lots (OV1-2) formed a separate cluster reflecting shorter cobs and lower grain weight. CT4 grouped closely with the latter cluster, just as RV3, RV6 and RV8. This again suggests a close relation of CT4 to improved maize. Among the landraces, all Olotillos (CO) grouped together, in accordance to their racial classification. Distances within this cluster were quite large, however, reflecting considerable variability in ear traits. Similar heterogeneity was present among the Jarocho (CT) seed lots.
Vegetative and flowering distances also produced separate clusters for improved (HB, OV and RV) and traditional varieties (CO and CT), with the exception of Conejo (CC) (Fig. 5b). Contrary to the results based on ear traits, Olotillo (CO) and Jarocho (CT) did not form separate clusters, suggesting that there is little difference in vegetative and flowering traits between the two races. Branch-lengths within the CT/CO cluster were short, revealing that these landraces are relatively homogeneous for these traits. In contrast, hybrids showed high diversity, with long branches separating the different varieties. Most of the OV and RV populations formed a poorly differentiated cluster that again included CT4. Only V-424 (OV1-2) was clearly different and clustered together with Conejo (CC) due to its earliness and short plant stature.
Clustering based on molecular distances showed extremely long branch-lengths between the different hybrids compared to those observed between landrace, creolized and OPV seed lots (not shown). This was not unexpected, given the fact that hybrids are produced by crossing two inbred lines. Each inbred line is fixed for a single allele at each locus, so the variance in allele frequency between different hybrids is expected to be large. We therefore excluded hybrids for a better appreciation of the relationships between the remaining seed lots (Fig. 5c). Modern varieties (OV and RV) were again clustered separately from traditional landraces (CO, CT and CC). The identity of CT4 as a creolized variety was confirmed by it falling within the cluster of creolized seed lots and OPVs.
Genetic differences between clusters were small. Pairwise θ between the OV and the CO seed lots was only 0.027 for example. The two main landraces Olotillo (CO) and Jarocho (CT) did not form separate clusters. Pairwise θ between the two races was only 0.015. This in contrast to average θ between seed lots within races which was 0.050 and 0.092 for Jarocho and Olotillo, respectively. The single seed lot of Conejo (CC) had a pairwise θ of 0.090 with respect to the other landraces. This relatively high differentiation of the CO and CC seed lots was probably caused by stronger drift due to small population sizes.
Analysis of within-group diversity
Average weighted distance between seed lots was calculated for different groups, representing subsets of the seed supply. The following comparisons were made: (I) The two main traditional landraces Jarocho (CT) and Olotillo (CO), creolized seed lots (RV), open-pollinated varieties (OV) and hybrids (HB). (II) Landraces (C), creolized varieties (RV) and seed lots from the formal system (F). (III) Formal seed (F), versus informal seed (I), containing all landraces and creolized varieties.
Comparison I showed that ear traits were more diverse within the Olotillo (CO) and Jarocho (CT) landraces compared to the other groups (Fig. 6); Hybrids (HB), OPVs (OV) and creolized (RV) seed lots were both more homogeneous than the two landraces (P < 0.025, estimated from the bootstrap distributions), in spite of the fact that they contained distinct varieties. Results were different for vegetative/flowering traits and molecular markers. As was suggested by the cluster analysis, hybrids were the most diverse group for these traits (although the difference in vegetative/flowering traits was not significant when compared to the CT group). OPVs and creolized varieties contained less vegetative/flowering diversity than both the Olotillo, Jarocho and Hybrid groups (P < 0.025). Molecular distance was similar within the OPV-, creolized- and Jarocho groups. Olotillo showed higher molecular differentiation than the other non-hybrid groups (P < 0.01). This probably reflects drift due to the smaller field sizes of Olotillo compared to CT, RV, OV and HB seed lots.
The traditional landraces as a whole were phenotypically more diverse than both creolized- and formal sector seed (comparison II, Fig. 6). The difference between landraces and formal seed being less pronounced for vegetative/flowering traits. The high diversity of the landrace group was in part due to the inclusion of the Conejo (CC) seed lot. Molecular differentiation was highest for formal seed, owing to the high differentiation of Hybrids. Creolized seed was the least diverse group both for phenotypic traits and molecular markers.
Formal seed was significantly less diverse than informal seed for phenotypic traits but not for molecular differentiation (comparison III, Fig. 6). Contrasting the results of comparisons II and III showed that the inclusion of creolized seed into the informal seed sector decreases diversity for all traits as compared to that found within the traditional landraces proper. The reduction, although significant for all traits, was minor relative to the diversity loss that would result from a complete replacement of landraces with creolized varieties (comparison II, Fig. 6). Moreover, the level of diversity observed for informal seed was higher than expected based on within-type distances only. For ear traits for example, we expected an average weighted distance of 0.17 instead of the observed value of 0.23. This higher diversity resulted from the phenotypic distance between creolized and landrace seed lots, which increased the distance between seed lots within the informal seed group.