Reticulate evolution and root crops yams

Yams, genus Dioscorea, evolved in three subhumid/humid tropical regions—Southeast Asia, West Africa, and the New World Tropics (Tamiru et al. 2007). Sub-Saharan Africa accounts for > 95% of yam production, and Dioscorea is the second most important African tuber crop after cassava (Scarcelli et al. 2006a; Tamiru et al. 2007). Indeed, in the densely populated areas of countries such as Ethiopia, this crop, like cassava in other areas, is a crucial factor in the livelihood of the resident human populations (Tamiru et al. 2007). Though produced throughout Sub-Saharan Africa, more than 90% of world yam production occurs in West Africa (Scarcelli et al. 2006a). The West African cultivars are recognized as belonging to the "Dioscorea cayenensis-Dioscorea rotundata complex," and are found to be in geographic proximity with wild species such as Dioscorea abyssinica, Dioscorea praehensilis, and Dioscorea burkilliana (Mignouna and Dansi 2003; Scarcelli et al. 2006a).

Two factors have facilitated a significant role for introgressive hybridization between cultivated and wild Dioscorea species in the evolution of the domesticated yam. The first, mentioned above, is the sympatry between the cultivars and numerous native taxa. Such sympatry has provided the opportunities for wild x domesticate hybrid formation

(Scarcelli et al. 2006a, b). However, the importance of these hybrids in affecting reticulate evolution of yams has been enhanced by particular agricultural practices by local farming communities. This practice, termed "ennoblement" by Mignouna and Dansi (2003) involves the incorporation of tubers from wild yams into the cultivated fields. The tubers selected from the wild and planted into the cultivated fields are referred to as "pre-ennobled" (Figure 7.2; Scarcelli et al. 2006b). One example of this process involves the Fon and Nago agriculturalists in Benin. These farmers collected tubers, either close to their villages or on hunting trips to more distant areas, which they transplanted into their fields. While these pre-ennobled plants were sometimes genetically and morphologically indistinguishable from known landraces (Mignouna and Dansi 2003), in some cases they were novel with regard to both genotype and phenotype (Mignouna and Dansi 2003).

Scarcelli et al. (2006a,b) tested the hypothesis that a proportion of the Dioscorea pre-ennobled plants utilized by indigenous farmers in Benin were actually crop x wild hybrids. Both of their studies did indeed provide evidence for the utilization by farmers of natural hybrids between the cultivated and wild species (Figure 7.2). In the first study, this was reflected by the nuclear genotypes of the wild-collected plants being equally closely related to the cultivar lineage and to wild D. praehensilis (Scarcelli et al. 2006a). The second, more detailed analysis (Scarcelli et al. 2006b), involved genotyp-ing of wild and cultivated plants within villages and across different geographic regions. At the village level, >50% of the pre-ennobled yam samples were found to be of hybrid origin (Scarcelli et al. 2006b). In addition, a paternity analysis estimated that 77.4% of seeds collected from cultivated plants had interspecific (i.e., wild species) fathers, indicating the high frequency occurrence of spontaneous interspecific hybridization onto cultivated plants (Scarcelli et al. 2006b).

Scarcelli et al. (2006b) detected an equally large effect from introgression when all samples of the various wild and cultivated (both pre-ennobled and previously domesticated plants) lineages were analyzed. Figure 7.2 illustrates the genetic assignment of the Dioscorea samples identified before genotyping as belonging to D. abyssinica, D. praehensilis, the cultivar (D. rotundata), or the class of pre-ennobled yams. The genotypic constitution of these plants indicated that 7.5% of all individuals assigned morphologically to one of the three species had conflicting genotypes (i.e., hybrid

Figure 7.2 Genetic assignments of wild/previously domesticated yams (left-hand diagram) and pre-ennobled yams (right-hand diagram) using nuclear microsatellite loci. D. abyssinica and D. praehensilis are wild species and D. rotundata is the cultivated lineage. The dashed lines indicate the cutoff for assignment to one of the three categories. The samples that fall outside of the corners demarcated by these lines (in both the left- and right-hand diagrams) are inferred to be various hybrid genotypes (Scarcelli et al. 2006b).

Figure 7.2 Genetic assignments of wild/previously domesticated yams (left-hand diagram) and pre-ennobled yams (right-hand diagram) using nuclear microsatellite loci. D. abyssinica and D. praehensilis are wild species and D. rotundata is the cultivated lineage. The dashed lines indicate the cutoff for assignment to one of the three categories. The samples that fall outside of the corners demarcated by these lines (in both the left- and right-hand diagrams) are inferred to be various hybrid genotypes (Scarcelli et al. 2006b).

or assigned to an alternate species) and that 37% of the pre-ennobled samples were hybrids (Figure 7.2; Scarcelli et al. 2006b). Both the sym-patry of the cultivated and wild Dioscorea species and the West African indigenous farming practices have allowed an enrichment of a key foodstuff. However, this type of process is not isolated to yam cultivation. Jarvis and Hodgkin (1999), in a review of numerous examples of domestication, indicated not only the prevalence, but also the fundamental importance of wild > crop introgres-sion when they stated:

The information reviewed here has shown that introgression between crop cultivars and their wild relatives is an ongoing process affecting the genetic diversity of crops today . . . The availability of large gene pools becomes even more essential as farmers need to adapt over time to changing conditions that result from new population pressures, land degradation, and environmental change.

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