Genetic characterization of Malawian cowpea (Vigna unguiculata (L.) Walp) landraces: diversity and gene flow among accessions

Cowpea has been cultured from ancient times in Africa and Asia and is now widespread throughout the tropics and subtropics. Cowpea accessions can be distinguished phenotypically from one another by their growth habit, time to maturity, yield, and seed size and colour. Data on genetic diversity of farmer-developed accessions are lacking. The main objective of this study was to determine the pattern and extend of RAPD marker variation within and among cowpea populations from different agroecological zones and to determine the degree of genetic relationships and gene flow among different landraces used by local farmers. Twenty of the 30 RAPD primers tested allowed amplifications of random polymorphic (RAPD) loci. Overall, 80% of the scored loci were polymorphic. The degree of band sharing was used to evaluate genetic distance between accessions and to construct a phylogenetic tree. The genetic distance values among accessions varied between 0.09 to 0.57. Although some small clusters grouped accessions of the same growth habits, a general lack of agreement between clustering and morphological features was observed. The analysis of molecular variance revealed that within-region or types (among accessions) variation accounted for 96% of the total molecular variance. This high within-accession variability is being sustained by an uncontrolled gene flow among populations.