Population Structure and Evolutionary Dynamics of Wild–Weedy–Domesticated Complexes of Common Bean in a Mesoamerican Region

yield is estimated between 4000 and 5000 kg/ha (Gepts, 1993). Low yields can be attributed in part to the poor The diversity, structure, genetic flow, and evolutive relationships knowledge and limited use of the genetic diversity of within and among three wild–weedy–domesticated complexes and a wild populations of this species (Gepts and Debouck, wild population isolated from crops of common bean (Phaseolus vulgaris L.) were analyzed under traditional agriculture, within a 1991; Gepts et al., 1999). Several studies have suggested region of the Mesoamerican center of domestication. Their diversity that wild bean populations present greater genetic variwas compared with the diversity of a local commercial variety and a ability than domesticated populations, which, because breeding line. A morphological marker and 37 polymorphic ISSR of the founder effect during the domestication process, loci were used. Total genetic diversity within the wild, weedy, and excluded valuable genetic variability for adaptive and domesticated populations across the complexes was very similar (0.24, productive characteristics (Romero and Bliss, 1985; Ro0.22, and 0.26, respectively). Weedy populations within each complex mero et al., 1986; Debouck and Tohme, 1989; Koenig were more closely related to the domesticated than to the wild populaet al., 1990; Acosta-Gallegos et al., 1998; Gepts et al., tions, suggesting they originated from introgression of wild-type alleles 1999). During domestication, humans selected a set of into the domesticated populations or predominant gene flow from morphological and physiological characters related to domesticated to wild populations. The wild population in closest proximity to the crop within its complex was more similar to the domestithe so-called domestication syndrome, including seed cated and weedy populations of its complex than to the rest of the dispersal (pod suture fibers, pod wall fibers), growth wild populations, suggesting displacement of the wild genetic diversity habit (determinacy, twining, number of nodes on the by gene flow from the domesticated population within its complex. main stem, number of pods, internode length), pod The high values of differentiation among wild, weedy, and domestilength and seed weight, number of days to flowering, cated populations within each complex suggest high autogamy or photoperiod sensitivity, harvest index, and seed pigmengenetic drift. However, the values of gene flow among populations tation (Gepts and Debouck 1991; Koinange et al., 1996). within the complexes were close to one, theoretically sufficient to Genetic compatibility between wild and domesticated counteract genetic drift and/or autogamy. We therefore assume that populations leads to wild–weedy–domesticated hybrid human selection is the most important evolutionary mechanism for complexes in sites with sympatric distribution by intromaintaining the high wild-domesticated differentiation by negative farmer selection of cultivated plants with morphological characters gression of genes from wild populations to domesticated that suggest introgression. Farmers may influence the magnitude and ones or vice versa. Weedy populations are defined here characteristics of gene flow among populations within each complex as wild populations growing in crop fields that were not by the management of the distance between the crops and the wild planted by farmers. Because they may be the result populations, the diversity within the landraces sown, and the tolerance of this introgression, they usually show morphological and harvesting of weedy populations. The high geographic differentiatraits reminiscent of one or the other parent, such as tion of the wild populations, together with the local differences in larger seeds than the wild parent or seed color or color human selection practices and agronomic management, could have patterns similar to those observed in wild beans. These generated multiple evolutionary lineages after domestication. Domeshybrid complexes constitute a valuable source of genes ticated populations within complexes were between two and four to the farmer or to the plant breeder (Debouck and times more diverse than the local commercial variety and four and nine times more diverse than the breeding line. New conservation Smartt, 1995; Beebe et al., 1997); however, at the same and breeding strategies are suggested to maintain and use the gene time they also represent a risk for the massive introducpools from these complexes. tion of genes from the domesticated to wild populations (Gepts et al., 1999; Papa and Gepts, 2003). Comparisons of wild and domesticated populations I mexico, the common bean is the main source of in their whole area of distribution indicate two main protein for the human population. Its present yield gene pools in which domestication took place (Becerrais calculated at around 650 kg/ha, while the potential Velásquez and Gepts, 1994; Debouck and Smartt, 1995; Gepts and Debouck, 1991; Koenig and Gepts, 1989; Tohme et al., 1996). One center of domestication is D. Zizumbo-Villarreal, P. Colunga-Garcı́aMarı́n, and P. Delgadolocated in Mesoamerica and the other in the Andes. Valerio, Unidad de Recursos Naturales, Centro de Investigación Cientı́fica de Yucatán, Mérida, México; E. Payró de la Cruz, Facultad de Studies on gene dynamics of wild–weedy–domesticated Ciencias Biológicas y Agropecuarias, Universidad de Colima; P. complexes within the Mesoamerican area of domesticaGepts, Dep. of Agronomy and Range Science, Univ. of California, tion are important, since four of the five domesticated Davis, CA. Received 2 June 2004. Genomics, Molecular Genetics & species and 45 of the 50 species of the genus Phaseolus Biotechnology. *Corresponding author ([email protected]). grow in this area, and their natural reproductive relaPublished in Crop Sci. 45:1073–1083 (2005). doi:10.2135/cropsci2004.0340 © Crop Science Society of America Abbreviations: ISSR, inter simple sequence repeat; QTLs, quantitative trait loci. 677 S. Segoe Rd., Madison, WI 53711 USA 1073 Published online May 6, 2005