Genetics of Cowpea Resistance to Bruchid (Callosobruchus maculatus Fab.)

Cowpea Vigna unguiculata (L.) is an important indigenous legume crop providing dietary protein, minerals, carbohydrates, fats, vitamins and income to many people in Africa, Asia, and Central and South America. However, its production is limited by insect pests in general and bruchids attack in particular. Bruchids particularly Cowpea weevil (Callosobruchus maculatus) is the most destructive pest causing complete storage loses in quality and quantity. This makes the grain unfit for consumption, marketing and planting. To minimize losses due to bruchids infestations, the majority of farmers in Sub-Saharan Africa are using chemical pesticides. The use of pesticides however is expensive, pose health hazards to farmers and consumers, and their continuous use can lead to development of insecticide resistant bruchids. The use of resistant genotypes is, therefore, a promising alternative control method to the hazardous pesticides for the management of C. maculatus. However there is a paucity of information on genetics and sources of cowpea resistance to bruchids. Thus the objectives of this study were to: (i) Identify new sources of cowpea germplasm that are resistant to bruchids; (ii) determine the biochemical traits involved in the resistance to bruchids; (iii) determine the mode of inherence of resistance to cowpea bruchids and combining ability and (iv) identify candidate genes that control bruchids resistance traits in the cowpea association mapping panel using individual SNP markers. The study was conducted at Makerere University Agricultural Research Institute - Kabanyolo (MUARIK) between May 2015 and March 2018. One hundred and forty five (145) cowpea genotypes were evaluated, in a completely randomized design (CRD), for their reaction to C. maculatus. As a result 18 genotypes (ACC23 × 3B, NE39 × SEC4, ALEGI×5T, ACC2 × ACC12, 3B × 2W, SEC1 × SEC4, IT84s-2246, TVu-2027, IT97K-499-35, IT95K-207-15 and IT90K-76) were identified as being resistant, suggesting that they could serve as donor source/parent to breed for cowpea resistance to bruchids. To study the mechanism of cowpea resistance to bruchids, different biochemicals were extracted from seed coat and cotyledons of seven cowpea genotypes, four susceptible and three resistant to bruchids. The results indicated that none of the studied seed coat biochemicals but α-amylase inhibitor and carbohydrate content extracted from the cotyledon was responsible for cowpea resistance to C. maculatus. Nine selected parents, comprising of four susceptible and five resistant ones, were crossed using a full diallel mating design. F2 plants and the parents were evaluated at MUARIK to study the heritability and gene action controlling resistance to C. maculatus. Additive gene effects were more important for all of the resistance traits (Baker’s ratio > 0.5). Parents 2419, TVu-2027 and IT84s-2246 showed significant negative GCA effects for number of eggs, and insect emergence and holes, and positive effect for median development period suggesting that the parents could be selected for breeding of cowpea resistance to bruchids. Likewise, crosses IT84s-2246 × 2419, 2419 × MU9, TVu-2027 × SECOW2W, 2419 × IT90K-76, 2419 × WC69, 2419 × SECOW5T and 2419 × SECOW2W showed negative SCA effects for number of eggs, insect emergence and holes and positive values for median development period indicating that the crosses were the best and could be used for further breeding. To dissect the genetic basis of resistance to bruchids at molecular level, 217 mini-core accessions were genotyped at the university of California using 51,128 SNPs and also phenotyped for their reaction to bruchids at Kabanyolo (MUARIK). Genome wide association studies identified 11 genomic regions associated to bruchids resistance. Further analysis of gene prediction using Phytozome, identified six candidate genes associated with the resistance traits which included (i) gene Vigun08g132300 control number of eggs, holes and insect emergence; (ii) Vigun08g158000 and (iii) Vigun06g053700 for number of eggs; (iv) Vigun02g131000, (v) Vigun01g234900 and (vi) Vigun01g201900 for median development period. The identified candidate genes are influencing resistance through their involvement in carbohydrate and protein biosynthesis, and their being regulatory element. The negative allelic effects of their corresponding SNP for number of eggs, holes and insect emergence, and positive effect on median development period was also another evidence for their involvement in resistance. Therefore, the information generated from genome wide association study could be used as a tool for analyzing the inheritance of the resistant genes, for monitoring the transmission of the resistance genes or genomic regions from parents to progeny, and for map-based cloning of those genes