Variability assessment of fodder cowpea (vigna unguiculata (L.) walp) for mosaic resistance using SSR markers

Abstract

The research work entitled “Variability assessment of fodder cowpea (Vigna unguiculata (L.) Walp) for mosaic resistance using SSR markers” was conducted at the Department of Genetics and Plant Breeding, College of Agriculture, Vellayani during 2022- 2024 with the objective of analysing the genetic diversity of 45 fodder cowpea accessions for yield, quality, mosaic resistance and validating the reported SSR markers. A pot culture experiment, laid out in a Completely Randomized Design with three replications, was performed to assess response of the accessions to Blackeye cowpea mosaic virus (BlCMV). The 45 fodder cowpea accessions were mechanically inoculated at the cotyledonary leaf stage through sap transmission. Infected plants exhibited characteristic symptoms like vein clearing, leaf mottling, vein banding, blisters on the leaf, leaf distortion, and stunting. Disease scoring was done at 15 and 30 days after inoculation using a 0-5 scale, and disease incidence and vulnerability index values was calculated. Based on disease reaction of fodder cowpea accessions to BlCMV, 45 accessions were classified into different categories where two accessions (TNFC 0926 and C-297) were found to be immune, two (EC101973 and MFC-18-8) were resistant, five were moderately resistant, five moderately susceptible, 28 susceptible and three highly susceptible. The immune accession C-297 exhibited higher phenol content and peroxidase activity compared to the control and susceptible accession. Field screening was conducted from April to June 2024 with 30 selected accessions raised in Randomized Block Design with three replications. The accessions were evaluated for various biometric, qualitative, and biochemical traits, with significant differences observed for all traits except seed yield plant-1. High phenotypic coefficient of variation (PCV) and genotypic coefficient of variation (GCV) were observed for number of primary branches, number of leaves, dry matter yield, green fodder yield, leaf dry weight, stem dry weight and leaf area index (LAI) indicating the influence of environment. High heritability coupled with high genetic advance was observed for crude protein and crude fibre content. Green fodder yield was positively correlated with dry matter yield, leaf dry weight, LAI, stem dry weight, number of leaves, number of primary branches, and crude fibre content. Path coefficient analysis revealed that the number of primary branches had the highest direct effect on green fodder yield, followed by leaf dry weight, while the highest positive indirect effect on green fodder yield was from dry matter yield via leaf dry weight. In genetic divergence analysis using D2 statistics, 30 accessions were grouped into eight clusters. Cluster I, with 23 accessions, was the largest, while clusters II through VIII each contained a single accession. The highest intra-cluster distance was observed in cluster I, and the greatest inter-cluster distance was between clusters III and VIII. Solitary clusters V (EC546491) and VI (MFC-18-8) were identified having higher green fodder yield and quality traits respectively. In the third experiment, genomic DNA from 30 selected fodder cowpea accessions was used for PCR amplification to detect polymorphism between resistant and susceptible accessions. Out of the 10 reported SSR markers for cowpea mosaic virus tested, only two (M80 and MA80) showed polymorphism, successfully distinguishing between the resistant and susceptible accessions. Accessions identified for high green fodder yield (EC546491) and quality traits (MFC-18-8) can be further selected for release as a variety and future genetic improvement programme. Screening for BlCMV in fodder cowpea germplasm identified immune and resistant accessions, which can be utilized to transfer BlCMV resistance into high-yielding fodder cowpea accessions. Additionally, they may serve as tools for genetic diagnostics of BlCMV and for isolating resistance genes via map-based cloning which could potentially be used in genetic transformation of other legume crops to incorporate BlCMV resistance.