Molecular Mapping of a Stem Rust Resistance Gene Sr40 in wheat.

Stem rust, caused by Puccinia graminis f. sp. tritici, historically was the most destructive disease of wheat worldwide. Deployment of resistant cultivars has successfully reduced losses to minimal in the past several decades. Unfortunately a new race Ug99 reemerged in Africa that defeats most resistance genes existing in commercial cultivars. Sr40, a stem rust resistance gene from Triticum timopheevii, has been transferred to common wheat and is still effective against Ug99. To characterize Sr40 in wheat, two mapping populations were developed from the crosses RL6088 / Lakin and RL6088 / 2174. RL6088 is a rust resistant parent containing Sr40, and Lakin and 2174 are highly susceptible. Simple sequence repeats (SSR) on chromosome 2B were screened for polymorphism between the parents. The F₂ population was analyzed with polymorphic markers between two parents, and F₂ and F_2:3 populations were inoculated with rust isolate RKQQ at the seedling stage. Sr40 was located on chromosome 2BS near the centromere and closely linked to Sr36, another stem rust resistance gene. Four markers, Xwmc344, Xwmc474, Xgwm374, and Xbarc230, were closely linked to Sr40 in the linkage map constructed from the population RL6088 / Lakin. Among the four markers, only Xwmc474 was mapped in the RL6088 / 2174 population about 2.5cM to Sr40. Xwmc474 and Xwmc661 are flanking markers for Sr40 in both populations. Markers linked to Sr40 will be useful for marker-assisted integration of Sr40 into elite breeding lines.