Asymmetric gene flow and introgression between domesticated and wild populations.

Gene flow may have very different consequences for the structure of the genetic diversity of populations depending on its direction (symmetric versus asymmetric). If gene flow is symmetric between two or more populations, its effect will be the reduction of genetic diversity between the populations and an increase in the differences between individuals within each of the populations considered. Alternatively, if gene flow is much higher from one population (source) to another (recipient) than in the reverse direction, the long-term consequence will be the displacement of alleles of the recipient population with the replacement by alleles of the source population, unless there is strong selection against source alleles. Recently, we have used amplified fragment length polymorphism (AFLP) markers to study introgression between wild and domesticated common beans (Phaseolus vulgaris L.) in Mesoamerica. We have shown by both phenetic and admixture population analysis that gene flow is about three- to fourfold higher from domesticated to wild populations than in the reverse direction. In this work, we review the results obtained in P. vulgaris and we compare them with studies on other species, to understand to what extent asymmetric gene flow is a general phenomenon in the wild-domesticated context, and to investigate the main factors involved. Even if the information available on other crops is insufficient to determine whether asymmetric gene flow and introgression from crops into their wild relatives is a general phenomenon, and whether it may also displace the genetic diversity of the wild population in specific genomic regions, as seems to be the case in the common bean, the data available on genetic differentiation between wild and domesticated populations, the inheritance of the domestication syndrome and the demography of the two types of populations generally suggest that asymmetric gene flow and introgression is the most likely hypothesis to explain the observed patterns of genetic diversity in wild and domesticated populations also in the other crops considered.