GENETIC STUDIES OF COWPEA [Vigna unguiculata (L.) Walp.] FOR RESISTANCE TO THRIPS (Megalurothrips sjostedti Trybom) IN CAMEROON
Studies were conducted in the Sudano-sahelian zone of northern Cameroon to determine cowpea farmers‟ production constraints and preferences, identify sources of resistance to cowpea flower bud thrips, determine the mode of inheritance of resistance to cowpea flower bud thrips and identify Quantitative Trait Loci (QTL) associated with resistance to flower bud thrips. Participatory Rural Appraisal (PRA) conducted in four villages in northern Cameroon revealed insect pests as the most important cowpea production constraints, followed by the lack of improved varieties, drought and Striga gesnerioides. Cowpea flower bud thrips was reported as the most damaging, followed by the flower beetles, maruca, aphids and pod-sucking bugs. Farmers‟ most preferred traits were high grain yield, tolerance/resistance to insects and large seed size. One hundred and sixty cowpea accessions were screened under natural thrips infestation and the results revealed wide variability among the accessions. The Principal Component (PC) Analysis based on thirteen morphological descriptors explained 75.12% of the variation with score of thrips damage (-0.437), days to first flower appearance (0.450), days to 50% flowering (0.481) and days to 95% pod maturity (0.445) accounting for 30% of the total variation in PC1. Based on thrips damage rating, three classes were identified: tolerant/resistant (1.1-2.5), susceptible (2.6-5.0) and highly susceptible (5.1-9.0). Based on grain yield, seventeen accessions from tolerant/resistant class were selected and added to two farmers‟ preferred varieties and one thrips susceptible check. The accessions were screened under artificial infestation in the screen house. In addition to SANZI and TVx3236, nine accessions (NGT115, TVU889 and Tvx945 from Nigeria and 14BN-006, 14BN-027, 14MS-005, 14MT-016, 14MS-025 and 14BN-026 from Cameroon) were found to be tolerant/resistant to thrips. These genotypes could serve as donor parents to improve farmers‟ preferred and adapted cowpea varieties in Cameroon. Two sets of six generations (P1, P2, F1, F2, BC1P1, BC1P2) developed from cross of VYA (susceptible) x SANZI (resistant) and LORI (susceptible) x SANZI (resistant) were evaluated under artificial and natural thrips infestation. The generation mean analysis revealed that both additive and non-additive gene action were important with dominance x dominance being the most predominant. Broad sense and narrow sense heritability estimated varied from 0.58 to 0.74 and 0.61 to 0.65 for number of thrips per flower and rating of thrips damage, respectively, indicating the influence of environment on thrips rating. The number of genes controlling thrips resistance was three for number of thrips per flower and between 3 and 4 for score of thrips damage. One hundred and fifty (150) F2 mapping populations from the cross of SANZI (female) x VYA (male) were used to construct a genetic map with two hundred and thirty two (232) polymorphic SNP markers. Three significant QLTs Fthp129, Fthp28 and Fthp87 for thrips resistance were detected on Linkage Group (LG) 2, 4 and 6 accounting for 43.2% of the phenotypic variation among the F2 population. In Fthp129 (24.5%) and Fthp87 (6.5%), the high value allele was in the direction of the resistant parent, while for Fthp28 (12.2%) the high value allele was contributed by the susceptible parent indicating the presence of resistant genes in both parents. The resistant gene may be recessive in the susceptible parent. F2 individuals that lacked the three QTLs expressed more symptoms of thrips damage than the susceptible parent did as indicated by transgressive susceptible phenotypes. Further studies are needed to validate these QTLs for their useful exploitation in a molecular breeding programme.