The stem rust resistance gene SrTmp carried by Triumph 64 confers resistance to Pgt race TTKSK and other members of the Ug99 race group. While some previous studies have mapped resistance postulated to be SrTmp, none used Triumph 64 as a parent. The purpose of this study was to genetically map SrTmp with DNA markers using a DH population from LMPG/Triumph 64 and compare the map position of SrTmp to previously mapped Sr genes. The DH population was tested with Pgt race TTKSK at the seedling stage. A single gene conditioned resistance to TTKSK (n = 144; χ21:1 = 0.44, p = 0.50). SrTmp was mapped to the distal region of chromosome 6DS with SSR markers. The map location of SrTmp was similar to SrCad and Sr42, which likely represent the same gene. In a concurrent study SNP markers were developed to fine-map SrCad. SNP markers were identified and/or developed using a public SNP database (http://www.cerealsdb.uk.net) and sequence information from an Aegilops tauschii genome sequencing project. Further SNP markers were developed by using resistance gene analogs from chromosome 6D to BLAST exome capture sequences from a set of Canadian wheat cultivars followed by searching for unique SNPs found in SrCad carriers. These SNP markers were added to the map of SrTmp. The map positions of SrTmp and SrCad/Sr42 are very similar. While preliminary data show functional differences between SrTmp and SrCad/Sr42, further analysis is needed to determine whether these genes are allelic or closely linked.
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Most of the current stem rust resistance genes (Sr) in Canadian wheat varieties are ineffective against the Pgt race Ug99 lineage, which pose a major threat to wheat production worldwide. Several stem rust resistance genes, including Sr33, Sr35, Sr36, SrCad/Sr42 and Sr43, are effective against race TTKSK. Although Sr36 is ineffective against Ug99 race TTTSK, it is still potentially useful for pyramiding genes to develop germplasm with durable stem rust resistance. For this purpose, we made crosses among RL5405 (Sr33), RL6099 (Sr35), Lang (Sr36), AC Cadillac (SrCad/Sr42), and RWG34 (Sr43) containing the respective Sr genes. A total of 54 doubled haploid (DH) lines were produced from the F1 from AC Cadillac/Lang//RWG34/RL5405, and 82 DH lines were obtained from RWG34/RL5405//RL6099. The DH progeny were tested at the seedling stage with race TTKSK and susceptible lines were discarded. We putatively developed 12 genotypes with multiple Sr gene combinations, including Sr33+Sr36+SrCad/Sr42+Sr43, Sr33+Sr36+SrCad/Sr42, Sr33+Sr36+Sr43, Sr33+SrCad/Sr42+Sr43, Sr36+SrCad/Sr42+Sr43, Sr35+Sr33+Sr43, Sr33+Sr36, Sr33+Sr43, Sr36+SrCad/Sr42, Sr36+Sr43, Sr35+Sr33, and Sr35+Sr43, based on positive association with linked PCR markers. Another population with 63 DH lines was derived from (Hoffman*2/RL6099)//(Hoffman*2/Lang) to combine the Fusarium head blight (FHB) resistance of Hoffman (Fhb1) with Sr35 and Sr36. We found 17 of 63 DH lines containing both Sr35 and Sr36 based also on linked PCR markers. This indicated that the combination Sr35+Sr36 was pyramided into the Canadian cultivar Hoffman; this derivative will be useful for development of cultivars resistant to Ug99 and FHB in Canada.