Title : Structure of cowpea [Vigna unguiculata (L.) Walp.] populations from Europe and Africa using SSRs markers
Cowpea (V. unguiculata L. Walp. ssp. unguiculata) is a self pollinating diploid (2n=2x=22) species with an estimated genome size of 620 Mb. It is an important crop for people in sub-Saharan Africa where it was domesticated. Many landraces are still cultivated and its grain is the main crop product used for human consumption. In Southern Europe, cowpea is restricted to a limited acreage and usually cultivated in drought-prone and marginal soils. Cultivation depends on a small number of locally adapted populations selected by farmers over time rather than on the use of modern varieties. These local populations survive on farm and there is a real risk of genetic erosion. In this work, using 12 Simple Sequence Repeats (SSR) markers, we present new insights into the genetic diversity, differentiation and structure of 97 common cowpea landraces from Europe and Africa. A total of 183 different alleles were identified with average Polymorphic Information Content (PIC) value of 0.67, indicating that the used loci are useful diversity indicators. The observed heterozygosity (Ho, mean of 0.31) and the expected heterozygosity (He, mean of 0.46) were both low, as expected for a self pollinating species. The model-based Bayesian cluster analysis in STRUCTURE visualized the genetic structure of the populations under examination. The SSR analysis clearly separated genotypes of distinct origins, such as those from Portugal and from Mozambique. In both regions, intermediate accessions were identified, with admixed genetic origin. The principal co-ordinate analysis (PCoA) was in accordance with the STRUCTURE results. SSRs were useful tools for studying genetic diversity and allowed the discrimination of cowpea local populations. The importance of some of these populations justifies the concern about their conservation.