Confirmation de QTL et validation de marqueurs SNPs associés à la résistance du niébé à Colletotrichum capsici, agent responsable de la maladie des taches brunes

Le niébé (Vigna unguiculata (L.) Walp.) est une légumineuse à graine très importante et constitue la principale source de protéines végétales pour l’alimentation des populations d’Afrique Subsaharienne. Sa production au Burkina Faso est entravée par la maladie des taches brunes provoquée par un champignon, Colletotrichum capsici (Syd.) Butler et Bisby. C’est dans la perspective d’accroître la productivité du niébé que nous avons entrepris de renforcer la lutte variétale contre cet agent pathogène. L’identification de marqueurs SNPs (Single Nucleotide Polymorphism) et QTL liés à la résistance à la maladie des taches brunes a été entrepris à partir d’une population biparentale F2 issus du croisement entre la variété sensible Tiligré et celle résistante KN-1. L’analyse QTL de la résistance du niébé à C. capsici à partir de la méthode ICIM add. a permis de confirmer et de valider respectivement un QTL majeur dénommé qBBDR2.1 et 9 marqueurs SNPs convertis, lesquels ont été cartographiés sur le chromosome Vu02 du niébé. Ce QTL dominant a présenté des effets additifs élevés liés aux allèles favorables de KN-1 et des valeurs de PVE de l’ordre de 51,50% et 55,33%, respectivement aux 21ème et 28ème JAI. English title: Confirmation of QTL mapping and validation of SNPs markers associated to cowpea resistance to Colletotrichum capsici, causal agent of brown blotch disease Cowpea (Vigna unguiculata (L.)Walp.) is one of the most important grain legume crops and constitutes the main source of plant protein for people food in sub-Saharan Africa. Cowpea production in Burkina Faso is constrained by brown blotch disease caused by a fungal, Colletotrichum capsici (Syd.) Butler and Bisby. In order to increase cowpea productivity we initiated a project to enhance host plant resistance to control the pathogen. The identification of SNP (Single Nucleotide Polymorphism) markers and QTL associated with brown blotch disease resistance was undertaken from a bi-parental F2 population resulting from a cross between the sensitive variety Tiligre and the resistant KN-1 to the disease. QTL analysis of cowpea resistance to C. capsici using the ICIM add method. Allowed to confirm and validate respectively a major QTL named qBBDR2.1 and 9 converted SNP markers, which were mapped on cowpea chromosome Vu02. This dominant QTL showed higher additive effects associated to alleles from KN-1 and PVE values of 51.50% and 55.33% respectively at 21 and 28 days after inoculation

Cowpea Aphid Resistance In Cowpea Line CB77 Functions Primarily Through Antibiosis And Eliminates Phytotoxic Symptoms of Aphid Feeding

Cowpea (Vigna unguiculata) is one of the most important crops in semiarid areas of the world, where it thrives in hot, dry conditions. While cowpea is able to withstand abiotic stresses, it suffers serious losses from biotic antagonists, including infestation by the cowpea aphid (Aphis craccivora). Cowpea aphid infestations are highly destructive, especially on young plants. However, it is unclear whether cowpea aphid damage is the result of aphids having phytotoxic effects on their hosts, or simple density effects. To better understand cowpea aphid damage and the potential for resistance traits to mitigate aphid impacts, we evaluated phenotypic changes in cowpea in response to variable aphid densities and systemic versus local infestations. Low aphid densities induced leaf distortions and pseudogalling, suggesting that cowpea aphids are phytotoxic to cowpea. Resistance to the cowpea aphid has been previously identified in an African cowpea germplasm, and near isogenic lines (NILs) containing resistance quantitative trait loci (QTL) were generated in the California blackeye cultivar background. Using a series of performance assays, we determined that resistance conferred by the two QTL counteracts aphid phytotoxicity and severely limits aphid growth and fecundity. Using choice assays, a preference by cowpea aphids for the susceptible NIL was observed. Electrical penetration graph analysis revealed that the resistance phenotype includes weak surface level deterrence and strong phloem based resistance that manifests during the sap ingestion phase. Our study provides evidence of phytotoxic traits in A. craccivora while identifying a viable means of counteracting aphid damage and reproductive potential through resistance.

Disparate Genetic Variants Associated With Distinct Components of Cowpea Resistance to the Seed Beetle Callosobruchus Maculatus

Cowpea ( Vigna unguiculata ) is an important grain and fodder crop in arid and semi-arid regions of Africa, Asia, and South America, where the cowpea seed beetle, Callosobruchus maculatus , is a serious post-harvest pest. Development of cultivars resistant to C. maculatus population growth in storage could increase grain yield and quality and reduce reliance on insecticides. Here, we use a MAGIC (multi-parent, advanced-generation intercross) population of cowpea consisting of 305 recombinant inbred lines (RILs) to identify genetic variants associated with resistance to seed beetles. Because inferences regarding the genetic basis of resistance may depend on the source of the pest or the assay protocol, we used two divergent geographic populations of C. maculatus and two complementary assays to measure several aspects of resistance. Using polygenic genome-wide association mapping models, we found that the cowpea RILs harbor substantial additive-genetic variation for most resistance measures. Variation in several components of resistance, including larval development time and survival, was largely explained by one or several linked loci on chromosome 5. A second region on chromosome 8 explained increased seed resistance via the induction of early-exiting larvae. Neither of these regions contained genes previously associated with resistance to insects that infest grain legumes. We found some evidence of gene-gene interactions affecting resistance, but epistasis did not contribute substantially to resistance variation in this mapping population. The combination of mostly high heritabilities and a relatively consistent and simple genetic architecture increases the feasibility of breeding for enhanced resistance to C. maculatus.

Inheritance of cowpea resistance to Cowpea aphid-borne mosaic vírus

The objective of this work was to evaluate the inheritance of resistance to Cowpea aphid-borne mosaic virus (CABMV) in cowpea (Vigna unguiculata). The study was performed between parental genotypes IT85F-2687 (resistant) and 'BR-14 Mulato' (susceptible), generating F1, F2, and F7 populations and backcrosses with both parental genotypes. CABMV was inoculated on plants from all generations, which were then evaluated through visual inspection and description of characteristic symptoms. A chi-square test was performed after the phenotypic classification of all plants. A segregation proportion of 1:3 (resistant:susceptible) in population F2 and of 1:1 in population F7 was accepted, showing a recessive monogenic inheritance.

Components of Cowpea Resistance to the Seed Beetle Callosobruchus maculatus (Coleoptera: Chrysomelidae: Bruchinae)

Cowpea, Vigna unguiculata (L.) Walp., serves as a major source of dietary protein in many tropical and subtropical regions around the world. To identify loci associated with agronomically desirable traits, eight elite cowpea cultivars were systematically inter-crossed for eight generations to yield 305 recombinant inbred lines. Here, we investigated whether these founder parents also possess resistance to the seed beetle Callosobruchus maculatus (F.), a highly destructive post-harvest pest. We estimated larval survival in seeds, egg-to-adult development time, adult mass at emergence, and seed acceptance for oviposition. Survival varied significantly among cowpea cultivars, but the pattern was complicated by an unexpected source of mortality; on three cultivars, mature larvae in a substantial fraction of seeds (20–36%) exited seeds prematurely, and consequently failed to molt into viable adults. Even if such seeds were eliminated from the analysis, survival in the remaining seeds varied from 49 to 92% across the eight parents. Development time and body mass also differed among hosts, with particularly slow larval development on three closely related cultivars. Egg-laying females readily accepted all cultivars except one with a moderately rugose seed coat. Overall, suitability ranks of the eight cultivars depended on beetle trait; a cultivar that received the most eggs (IT82E-18) also conferred low survival. However, one cultivar (IT93K-503-1) was a relatively poor host for all traits. Given the magnitude of variation among parental cultivars, future assays of genotyped recombinant progeny can identify genomic regions and candidate genes associated with resistance to seed beetles.