Genetic Mapping of Stem Rust Resistance Gene Sr8a in Wheat

Stem rust, caused by Puccinia graminis f. sp. tritici (Pgt), is a destructive disease of wheat (Triticum aestivum) that can be controlled by deploying effective stem rust resistance (Sr) genes in cultivars. Several races of Pgt have evolved in Africa that are virulent to many of the wheat cultivars grown in the Canadian Prairies. Recently, the genetic basis for resistance to Pgt race TRTTF found in some Canadian cultivars was described. The genes responsible for resistance to TRTTF were SrCad on chromosome arm 6DS and a second Sr gene mapped to chromosome arm 6AS. A previously identified gene, Sr8a, is located on chromosome arm 6AS but has not been accurately mapped. The purpose of this study was to genetically map Sr8a and compare the map position to the Sr gene on chromosome arm 6AS mapped in the cultivar ‘Harvest’. A cross was made between LMPG-6S and LMPG-Sr8a. Leaf tissue was collected from F₂ seedlings for DNA extraction and F₃ families were inoculated with Pgt race MCCF at the seedling stage. There were 17 homozygous resistant families, 51 segregating families, and 26 homozygous susceptible families which fitted a 1:2:1 single gene ratio (χ² = 2.40, p = 0.30). DNA markers used to map the Sr gene on chromosome arm 6AS in ‘Harvest’ were used to map Sr8a. The two genes occupy a very similar map position and it is likely that the resistance on chromosome 6AS found in Canadian wheat cultivars is either Sr8a or an allele of Sr8.