Genetic structure and mating system of wild cowpea populations in West Africa

Outcrossing rates ranged from 1% to 9.5% (mean 3.4%), which classifies the wild cowpea breeding system as primarily selfing, though rare outcrossing events were detected in each population studied. Furthermore, the analyses of both the genetic structure of populations and the relationships between the wild and domesticated groups suggest possibilities of gene flow that are corroborated by field observations.As expected in a predominantly inbred breeding system, wild cowpea shows high levels of genetic differentiation and low levels of genetic diversity within populations. Gene flow from domesticated to wild cowpea does occur, although the lack of strong genetic swamping and modified seed morphology in the wild populations suggest that these introgressions should be rare.Agricultural systems present spectacular and well studied examples of evolutionary changes [1]. Indeed, crops and their wild relatives represent interesting systems from both agricultural and evolutionary points of view. Wild relatives represent larger amounts of genetic variability than their domesticated descendants and the maintenance of this variability is of central importance in crop conservation and improvement programs [2-4]. Wild relatives may also represent actual or potential weeds and they often constitute crop-weed complexes with the domesticated plants [1,5]. On the other hand, wild relatives are critical for understanding the process of domestication [6-8] as they illustrate how evolutionary forces operate with or without strong artificial selection [9-11].Cowpea, Vigna unguiculata (L.) Walp., plays an important role in the livelihood of millions of poor people in the developping countries of the tropical region where it is used as food, animal feed or as a cash crop [12]. Genetic variability of domesticated cowpea is low [13,14] and most of the genetic variation of this species remains in the wild gene pool [15].As compared to wild cowpea, domesticated cowpea is characterized by large