DArT-seq based SNP analysis of diversity, population structure and linkage disequilibrium among 274 cowpea (Vigna unguiculata L. Walp.) accessions

Abstract Background: Genetic diversity in a germplasm is crucial for continuous improvement of crop varieties. A panel of 274 cowpea (Vigna unguiculata L.) accessions of unknown genetic diversity was assembled from diverse sources. This study used 3127 SNP markers, generated with the diversity array technology (DArT), to assess genetic diversity, population structure and linkage disequilibrium (LD) in the assembled germplasm.Results: The structure analysis inferred three subpopulations within the germplasm, which was confirmed by Neighbour-Joining (NJ) clustering and principal component analysis (PCA). Low genetic distances (0.005 to 0.44) were observed between accessions. Accessions from Africa; West and Central Africa (113 accessions), East and Central Africa (93 accessions), and Asia (53 accessions) were predominant in the germplasm; and distributed across all subpopulations. High fixation indexes (0.48 ≤ FST≤0.56) were obtained for the inferred subpopulations. AMOVA revealed a very large contribution (99%) of within subpopulations variation to the observed genetic variation in the germplasm. However, the expected heterozygosity (He) was higher than the observed heterozygosity (Ho), indicating high proportion of inbred lines in the germplasm. Linkage Disequilibrium (LD) was observed in the germplasm, particularly on chromosome 6, which showed a low decay along the physical genetic distance between markers in the genome.Conclusions: Significant genetic structuration exists in the assembled cowpea germplasm which shows there is a potential for improvement of the crop. The subgroups consisted mainly of inbred lines which, although from different geographical regions shared alleles in common reflecting high movement of seeds and exchange of germplasm between regions. The presence of linkage disequilibrium in the germplasm paves a way for prospective whole genome-wide association studies in cowpea for quality attributes and important agronomic traits.