Abstract
Cowpea [Vigna unguiculata (L.) Walp] is one of the important climate-resilient legume crops for food and nutrition security in sub-Saharan Africa. Ethiopia is believed to harbor high cowpea genetic diversity, but this has not yet been efficiently characterized and exploited in breeding. The objective of this study was to evaluate the extent and pattern of genetic diversity in 357 cowpea accestions comprising landraces (87%), breeding lines (11%) and released varieties (2%), using single nucleotide polymorphism markers. The overall gene diversity and heterozygosity were 0.28 and 0.12, respectively. The genetic diversity indices indicated substantial diversity in Ethiopian cowpea landraces. Analysis of molecular variance showed that most of the variation was within in the population (46%) and 44% between individuals, with only 10% of the variation being among populations. Model-based ancestry analysis, the phylogenetic tree, discriminant analysis of principal components and principal coordinate analysis classified the 357 genotypes into three well-differentiated genetic populations. Genotypes from the same region grouped into different clusters, while others from different regions fell into the same cluster. This indicates that differences in regions of origin may not be the main driver determining the genetic diversity in cowpea in Ethiopia. Therefore, differences in sources of origin, as currently distributed in Ethiopia, should not necessarily be used as indices of genetic diversity. Choice of parental lines should rather be based on a systematic assessment of genetic diversity in a specific population. The study also suggested 94 accesstions as core collection which retained 100% of the genetic diversity from the entire collection. This core set represents 26% of the entire collection pinpointing a wide distribution of the diversity within the ethiopian landraces. The outcome of this study provided new insights into the genetic diversity and population structure in Ethiopian cowpea genetic resources for designing effective collection and conservation strategies for efficient utilization in breeding.
Highlights
Cowpea [Vigna unguiculata (L.) Walp., 2n = 2x = 22] originated and was domesticated in Africa though the exact location of origin of domestication is still a matter of speculation and different authors suggest different areas in Africa, Northeastern Africa including Ethiopia [1,2,3,4], Central Africa [5], Southern Africa [6], and West Africa [5,7,8]
Three well-differentiated genetic populations or clusters were postulated from this study in the 310 Ethiopian cowpea landraces and 47 improved cultivars based on genome-wide single nucleotide polymorphism (SNP) scanning
Though this collection is only limited to Ethiopia, the result of this study shed light on the existence of genetic diversity in the landraces more than the cowpea collection used worldwide and it’s expected that these landraces might have unharnessed potential for future breeding owing different traits for cowpea improvement
Summary
There are five known subspecies of cowpea, of which three are cultivated (unguiculata, cylindrical and sesquipedalis) and two are wild (dekindtiana and mensensis) [9,10]. In Ethiopia, all five subspecies are known to exist, and are of particular significance, being landraces of subspecies unguiculata and cylindrical, in the drought-prone areas of eastern Ethiopia [11,12]. These subspecies are grown in the northern, southwestern and southern parts of Ethiopia [12]. Cowpea is grown for different purposes, mainly as a source of staple food and nutritional security for farmers in sub-Saharan Africa. In terms of nutrition security, cowpea is an affordable source of carbohydrate, protein, essential minerals, vitamins and folates, to poor people who cannot afford animal-based diets [19,20]
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