Genetic diversity, candidate genes and gene expression in relation to drought tolerance in Ghanaian cowpeas (vigna unguiculata)

Cowpea (Vigna unguiculata) is grown mainly for its protein-rich grains, which are consumed in various forms in sub-Saharan Africa. The initial phase of the study was the screening of the germplasm for drought tolerance. One hundred and six cowpea accessions from Ghana were evaluated for seedling drought tolerance on an individual plant basis under greenhouse conditions using the pot screening method. The results suggested that there were more drought tolerance accessions in the germplasm than the susceptible ones. They cluster results from the morphological analysis were informative but inadequate to effectively determine variability of germplasm. DNA-based techniques are essential to assay the variation present in wild and cultivated populations of crops both to ensure breeders have the full range of diversity for assessment and application, and to prevent loss of diversity for future farmers. Three multi-locus PCR based molecular markers; SSR (simple, sequence, repeat), IRAP (inter-retrotransposon amplified polymorphism) and REMAP (retrotransposon---microsatellite amplified polymorphisms) were used for the diversity analysis. The results indicated the highly polymorphic nature of the DNA markers used; small groups could be used to identify each of the accessions and the diversity recorded by each marker showed good correlation coefficient values. The results from the cluster analysis could be used as basis for planning crosses of the lines for further analysis. The fragment candidate gene cloning and sequencing analysis indicated the enormous diversity amongst the accessions. Numerous SNPs were identified in both exonic and intronic regions as well as some deletions and insertions. The SNPs were largely nucleotide transitional in character and comparative analysis with other plant species indicated that some correlated with previously identified gene and polymorphisms related to drought stress. The study also identified many abiotic or drought inducible genes in cowpea from the differential display PCR (DD-PCR) analysis. Different conditions induced different stress-inducible genes and thus there must be several different signalling mechanisms that are important for future identification and analysis.