High Resolution Mapping of the Stem Rust Resistance Gene SrCad

Stem rust, caused by the fungus Puccinia graminis f. sp. tritici, is a disease of wheat that can cause devastating economic losses. The evolution and emergence of new highly virulent races, TTKSK (Ug99) and its variants, has alarmed the global wheat community. Highly effective resistance to Ug99 and its variants is present in Canadian wheat cultivars Peace (BW90*3/BW553) and AC Cadillac (BW90*3/BW553//BW90‘S’/Katepwa). The resistance gene SrCad, which confers Ug99 resistance, was previously mapped to chromosome arm 6DS in both cultivars. A lack of polymorphic markers in the 6DS region has hindered fine mapping of SrCad.  Thus, using bioinformatical tools we identified single nucleotide polymorphisms (SNP) associated with SrCad resistance using an Infinium 90K iSelect assay, publicly available SNP databases, the wheat survey sequence, and Aegilops tauschii genomic resources. SNPs were converted to Kompetitive Allele Specific PCR (KASP) assays and were mapped in two populations: 345 DH lines from RL6071/Peace and an F6-derived recombinant inbred line population of 1,872 lines from BW278/AC Foremost. The SNP markers delineated SrCad to a 0.6 cM interval using the two populations.  Five SNP markers developed from resistance gene analogs (RGAs) co-segregated with SrCad in the BW278/AC Foremost population. These RGAs are candidate genes for SrCad. The goal of this project is to develop diagnostic molecular markers useful for wheat breeding programs and to sequence the gene SrCad.