Development of a genetic linkage map and quantitative trait loci (QTL) analysis in cowpea (vigna unguiculata (L.) walp)
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Cowpea is a diploid plant species which contributes significantly to food security in developing countries, especially in Africa. This research project was carried out in view of the upcoming introduction of Bt cowpea in Africa which is likely to alter the equilibrium existing within the cowpea taxa. The objectives of this study were to develop viable microsatellite markers and construct the SSR based linkage map, identify quantitative trait loci that regulate yield, domestication related traits as well as flower scent and identify the volatile compounds that attract pollinators to cowpea flowers. In order to achieve these goals 159 F7 recombinant inbred lines including the two parents and 206 markers (202 SSRs and 4 morphological) were used. The first SSR based linkage map of cowpea was constructed that spans a genetic distance of2991cM. QTL for seed weight (SW), domestication related traits (DRT), flower scent/aroma were mapped in all 159 F7 plants and the two parents 524B x 219-01. Six QTL associated with 74 % of the phenotypic variance were detected for SW on chromosomes 1, 2, 3 and 10. Both the 524B and 219-01 alleles increased SWat six of the QTL on chromosomes 1, 2, 3 and 10. For domestication related traits, nine QTL (four for testa size and five for pod fiber thickness layer) explaining 54.5 and 47.9 % of the phenotypic variance, respectively were on chromosomes 1,2, 4, 6, 7 and 10. The 524B allele increased DRTs at three-fourth of all QTL. QTL for SW and DRTs were clustered on chromosomes 1 and 10. Association of SW and DRTs QTL may be the cause of the significant phenotype and genotypic correlation detected between the two traits. The test of linkage vs pleiotropy for SW and DRT QTL on chromosomes 1 and 10 suggested pleiotropy. For flower scent/aroma, 63 QTL were detected on chromosomes 1,2,3,4, 5, 6, 7, 8, and 10. In addition, a total of twenty-two different volatiles were identified by the GS-MS technique. Clustering ofQTL were observed on chromosomes 1,2, and 4 mainly, suggesting that it can occur either due to the presence of a single locus with pleiotropic effects on several volatiles or as a result of tightly linked different loci. Such loci may encode transcription factors that co-ordinately regulate genes, or they may encode enzymes that catalyse limiting steps in single pathways. It is anticipated that this resource will have an important impact towards the development of marker assisted selection systems for the cowpea breeding community, and for future genetic studies in cowpea.