Multi-phasic screening of stored cowpea genotypes (Vigna unguiculata L. Walp) for cowpea-weevil (Callosobruchus maculatus F.) tolerance and antixenosis resistance: in vitro and ex vivo approaches

Abstract

Cowpeas (Vigna unguiculata L. Walp) are an important economic food and nutritional security grain crop in southern Africa for both humans and livestock. Harvested grains suffer enormous damage from pernicious storage insect pests, mainly, the cowpea bruchid (Callosobruchus maculatus F.). The objective of multi-phasic screening of cowpea genotypes in germplasm collections is an initial and critical step needed to identify divergent parents when breeding for pest resistance. This study used highly informative and selective DArTseq (genotyping by sequencing) derived SNP markers for a genetic diversity analysis of 92 selected genotypes. Thirty cowpea genotypes were selected from the 92 DArTseq screened genotypes. Ten seeds from each genotype were placed in a permissive, two-compartment petri dish with 2:2 bruchids for no-choice oviposition preference. A tray of 30 wells accommodated 10 seeds from each genotype with 60:60 bruchids for free-choice oviposition preference. Thirteen cowpea genotypes were randomly selected from the previously screened 30 genotypes to be further assessed under a no-choice test. The seeds were then milled and transferred into test tubes for extraction and assaying for bioactive polyphenols. Data was analysed using SAS v9.4m7 software, R studio v1.2.5001-3 software, and XLSTAT 2016 software. Mean values for polymorphic information content, observed heterozygosity, expected heterozygosity, major allele frequency and the inbreeding coefficient were 0.345, 0.386, 0.345, 0.729, and 0.113, respectively, and positively validated genetic diversity among the 92 cowpea genotypes. The susceptibility index demonstrated that 25 out of 30 genotypes were resistant to cowpea bruchids during the 35-day infestation period. Most genotypes were pest tolerant under the no-choice test in comparison to free-choice. RV 343, PAN 311, and IT95K-207-15 genotypes showed high levels of tolerance. The 13 infested cowpea genotypes had higher phenolic and flavonoid concentrations than the non-infested genotypes. The free radical scavenging activities of infested genotypes produced the lowest antioxidant concentrations, while non-infested genotypes produced higher concentrations. The genotype 98K-503-1 produced the highest phenolic concentration, followed by Encore, which scored high on flavonoid concentrations, and Oloyin, which had the highest antioxidant activities. The DArTseq derived SNPs of the 92 cowpea genotypes were heterogeneous, and 30 genotypes exhibited high pest tolerance under the no-choice test. Bruchids had a significant influence on the bioactive polyphenol profile of thirteen cowpea genotypes.