Identification of Molecular Markers Linked to Alectra Resistant Gene in Cowpea.

Identification of Molecular Markers Linked to Alectra Resistance Gene in Cowpea

L.O. Omoigui^1*, M.S.Ugbaa¹, L.L. Bello¹, B. S. Gwoda² and M.P. Timko²

¹Department of Plant Breeding and Seed Science, College of Agronomy, University of Agriculture, Makurdi

²Department of Biology, University of Virginia, Charlottesville, VA 22904, USA

Abstract

Cowpea production is constrained by a number of abiotic and biotic factors. Among the biotic constraints, the parasitic flowering plant Alectra vogelii (Benth.) is one of the most formidable limitations to cowpea production in the dry Savannas of West and Central Africa, which accounts for over 64 % of total world cowpea production. This parasite causes severe yield losses estimated between 70-100% in susceptible cultivars and translates to millions of tons annually. Among several control strategies that have been proposed, planting of cowpea genotypes with resistance to the parasite would be the most viable and economical option to the resource poor farmers in Sub-Saharan Africa. Breeding resistance cultivars will be facilitated by identification of markers linked to the resistance gene in marker-assisted selection (MAS). Our objectives were: i) to elucidate the mode of inheritance of Alectra resistance in cowpea and ii) to identify DNA markers that are closely linked to Alectra resistance in cowpea using SSR and Bulk segregant analysis (BSA). Pot screening of two F₂ populations derived from the cross B301 resistant parent (R) × Banjar susceptible parent (S), IT97K-573-1-1 resistant parent (R) × Banjar susceptible parent (S) gave a segregation of 136 resistant: 32 susceptible. χ² analysis revealed that this segregation pattern fit closely to a 13: 3 genetic ratio indicating duplicate dominant gene control the inheritance. Fifty SSR primers developed from the cowpea genome, along with forty primers from rice bean genome and fifty from asparagus bean genome, mapped previously to the cowpea genome on linkage group 6 (LG6), were used to screen DNA bulk segregant (top 5 resistant) and (top 5 susceptible) derived from B301 (R) and Banjar (R) to identify those markers showing polymorphism to Alectra resistance. Out of the 140 primers used, 20 primers were polymorphic between B301(R) and Banjar (S) and this were used in the BSA with DNA bulks of highly resistant and susceptible F₂ lines to select those that co-segregated with the resistance gene. The primers RB16 from rice bean genome and CLM0356 from asparagus bean genome was found to co-segregate with the resistance gene with similarity index of 0.70 between the two markers. The utility of the two markers were evaluated using 150 F₂ lines for marker characterization, mapping and linkage analysis.  The two markers were found to be useful in germplasm characterization, MAS for Alectra resistance and map based cloning for Alectra resistance genes.

Key word: cowpea, Alectra vogelii, SSR markers, breeding resistance

Corresponding author: lomoigui@yahoo.co.uk