The obligate root parasitic weed Striga gesnerioides poses a severe constraint to cowpea productivity in the dry savannahs of West and Central Africa, where cowpea is a major crop. At least seven races of S. gesnerioides have been identified within the cowpea-growing regions of West and Central Africa, based on host differential response and genetic diversity analysis. Molecular markers linked to resistance to different races of S. gesneriodes have been identified. It was desirable to demonstrate the applicability and efficiency of such markers for use in marker-assisted selection (MAS) to fast-track the development of cowpea with resistance to S. gesnerioides. The objective of the study was to determine the suitability of two molecular markers in tracking resistance in cowpea to S. gesnerioides race SG3, the predominant race found in Nigeria. F2 mapping populations and recombinant inbred lines (RILs) derived from the cross involving IT97K-499-35 and a susceptible local landrace (Borno Brown), and another resistant parent B301 with the same susceptible land race (Borno Brown) were assayed using two linked markers. Genetic analysis showed that resistance to S. gesnerioides in cowpea is inherited as a single dominant gene. Two SCAR markers, 61RM2 and C42-2B were validated in the same F2 populations and subsequent recombinant inbred lines (RILs). The two markers were able to discriminate between resistance and susceptibility and the genotypic score was quite similar to the phenotypic score with the markers score showing greater efficiency in selection than phenotypic score. The 61RM2 had two bands in resistant cultivars and amplified a ~450 bp fragment with marker efficiency of 98% while C42-2B amplified a single ~250 bp fragment with marker efficiency of 96% in resistant cultivars and absent in susceptible cultivars. The genetic distance between 61RM2 and phenotypic score was 3.5 cM while that of C42-2B and phenotypic score was 8.5 cM. The two marker data set were significantly correlated with the phenotypic data (r=0.95). Based on the tight linkage with the resistant locus, 61RM2 was found to be a utility marker to initiate MAS in cowpea breeding for resistance to S. gesnerioides.
Key words: Cowpea, Striga, molecular marker, genetic distance, race-specific, obligate parasitic weed, Vigna unguiculata.
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