Molecular Mapping of a New Wheat Gene Effective Against Race Ug99 of the Stem Rust Pathogen

Advanced wheat breeding line ND643/2*Weebil1 carries stem rust resistance gene, temporarily designated as SrND643, effective against Ug99 group of Puccinia graminis f. sp. tritici (Pgt) races at both seedling and adult growth stages. To map this gene, one hundred and forty eight recombinant inbred lines (RILs), developed by crossing  ND643/2*Weebil1 with susceptible line Cacuke, were evaluated for stem rust response in field nurseries at Njoro, Kenya, during two growing seasons in 2010, and were genotyped with DArT, SSR and SNP markers. Linkage mapping tagged SrND643 at the distal end of chromosome 4AL showing close association with SSR markers Xgwm350 (0.5 cM), Xwmc219 (4.1 cM) and Xwmc776 (2.9 cM). The race specificity of SrND643 is different from those of Sr7a and Sr7b, indicating that the resistance is conferred by a gene at a new locus or an allele of Sr7. As the flanking markers Xgwm350 and Xwmc219 were predictive of the presence of SrND643 in advanced germplasm, marker-assisted selection can be effectively carried out to develop Ug99 resistance wheat cultivars in the future.