Population genetics of wild cowpea (Vigna unguiculata (L.) walp) from Coastal Kenya

Abstract

Cowpea, Vigna unguiculata (L.) Walp.is an essential crop in less-developed countries of the tropics. Insect pests seriously affect its production. As a result, there are endeavors towards transformation of the crop with Bacillus thuringiensis (Bt) insecticidal genes. The introduction of Bt-cowpea in Africa is viewed with concern because of the possibility of transfer of their engineered genes by pollen to the wild relative, subspecies unguiculata variety spontanea. Population genetic structure, indirect estimate of inter-population gene flow (Nm) and outcrossing rates of var. spontanea from coastal Kenya were studied using isoenzymes. Direction of crosses (pollen source) was also determined on naturally occurring hybrids. A sample of 545 individuals from 25 populations of var. spontanea revealed a total of 37 alleles at 16 loci representing 8 enzyme systems, namely aspartate aminotransferase (AAT), isocitrate dehydrogenase IDH phosphoglucomutase (PGM), glucose-6-phosphate isomerase (PGI), fluorescent esterase (FLE), shikimate dehydrogenase (SDH), phosphogluconate dehydrogenase (PGD) and malate dehydrogenase (MDH). Inspection of deviations from random mating (Hardy-Weinberg Equilibrium expectations) suggested a mixed mating system that varies from population to population in the degree of outcrossing. As such, a high level of genetic structure was detected (mean FTT = 0.619, Mean FIs = 0.493). From Wright's genetic differentiation coefficient, (mean FST = 0.349) the number of migrants exchanged between populations each generation (Nm) was 0.47 and similar to the value obtained from Slatkin's private allele method (Nm of 0.42). Estimation of natural outcrossing rates from one of the populations (Milalani) based on four enzyme systems i.e. endopeptidase (END), leucine aminoeptidase (LAP), FLE and PGD showed a substantial variation in magnitude between rainy (90% outcrossing in May) and dry (12% outcrossing in March) seasons. Naturally occurring hybrids collected from Milalani in coastal Kenya revealed cultivated haplotypes when screened using Single Nucleotide Amplified Polymorphism and confirmed by PCR-RFLP and suggests that cultivated cowpea seeds were deposited among the wild cowpea populations and subsequently fertilized by wild cowpea pollen to give rise to the hybrids. Implications of the study in the light of releasing transgenic cowpea are discussed.