Assessment of Diversity among Cowpea Accessions from Semi-Arid Areas of Kenya

Abstract

Cowpea is an important legume crop adapted and widely grown in marginal areas. The crop is grown mainly from landraces and only a handful of improved varieties have been developed. Although breeding and identification of superior lines is dependent upon existence of crop diversity, there is limited information on diversity among the Kenyan cowpea. The objective of this study was to determine variation among cowpea accessions from semi-arid areas of Kenya at morphological and molecular levels. One hundred and ten cowpea accessions obtained mainly from semi arid region of Kenya were planted in a Randomized Complete Block Design with three replicates. Quantitative and qualitative morphological data were collected over the growing period and on harvested seed. For molecular characterization, DNA was extracted from leaves obtained from two week old seedlings grown in pots. Variation among the genotypes was determined through amplifying the DNA using twenty pairs of selected SSR markers. Even distribution of accessions across traits of the characters was recorded for immature pod color, leaf color, seed shape and testa texture, whereas uneven distribution was recorded for terminal leaflet shape, raceme position, pod attachment, pod curvature, mature pod color, flower color and eye color. ANOVA revealed significant differences (p=0.05) among accessions for number of days to 50% emergence, pod length, number of pods per plant and number of seeds per pod. The first five principal components accounted for 19.8, 18, 15.9, 12.4 and 11.22 of the total variation respectively amounting to 77%. Correlation analysis revealed significant (p=0.05) relationship for 50% emergence to 50% flowering (r= -0.2131), 50 % emergence to number of pods per plant (r= -0.5258), emergence to terminal leaflet length (r= -0.1881) and emergence to terminal leaflet width (r= 0.2042); terminal leaflet length to terminal leaflet width (r= 0.5230) and pod length to number of pods per plant (r= 0.5470). Based on morphological characteristics, the accessions were grouped into two main clusters, with one cluster having 103 accessions that included all registered varieties while the other cluster had seven accessions. Molecular characterization of one hundred and ten accessions (110) was done using eight pairs of the SSR markers that were polymorphic. Analysis of the molecular variance showed that close to 100% of the variation was within accessions. Heterozygosity ranged from 0 to 0.5 with a mean value of 0.19. The molecular data subjected to cluster analysis grouped the accessions into three groups. Therefore, cowpeas grown in semi rid areas of Kenya are variable and closely related to the registered cowpea varieties evaluated. The set of accessions could be used for identification of preferred lines for this region. The morphological data gave significant variation among the characteristics while molecular characterization showed no significant variations among the populations.