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Genetic relationship between wheat stem rust resistance genes Sr36 and Sr39

Stem rust resistance genes Sr39 (RL6082) and Sr36 (Cook) were transferred from Aegilops speltoides and Triticum timopheevi to chromosome 2B of wheat. Both genes are located on large translocated segments. Genotypes carrying Sr36 and Sr39 produce infection types (ITs) 0; and 2, respectively, against avirulent pathotypes. This investigation was planned to study the genetic relationship between these genes with the aim of combining them in a single genotype. Seedling tests on RL6082/Cook F3 lines showed complete repulsion linkage [25 Sr39Sr39sr36sr36 (IT2-) : 53 Sr39sr39Sr36sr36 (IT2-, IT0;) : 13 sr39sr39Sr36Sr36 (IT 0;)], and preferential transmission of the Ae. speltoides segment over the T. timopheevi segment was evident from the segregation ratio. The Sr39-carrying translocation was shortened by Niu et al. (2011; Genetics 187: 1011-1021) and the genetic stock carrying the shortest segment was named RWG1. Based on the reported location of Sr39 in the smaller alien segment in RWG1, we predicted that it should recombine with Sr36. F3 lines derived RWG1/Cook were phenotyped for stem rust response at the two-leaf stage and again complete repulsion linkage between Sr39 and Sr36 was observed [23 Sr39Sr39sr36sr36 (IT2-) : 78 Sr39sr39Sr36sr36 (IT0;, IT2-) : 68 sr39sr39Sr36Sr36 (IT 0;)]. In contrast to the cross involving the large Sr39 translocation, preferential transmission of the T. timopheevi segment was observed. These results indicated that a genetic determinant of meiotic drive had been deleted in the shortened Ae. speltoides segment. Genotyping with the co-dominant STS marker rwgs28 matched the phenotypic classification of F3 families. Marker rwgs28 was diagnostic for the Ae. speltoides segment, but the rwgs28 allele amplified in Cook was not T. timopheevi-specific.

The University of Sydney, Plant Breeding Institute, Australia
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Pyramiding genes Sr33, Sr35, Sr36, SrCad, and Sr43 for resistance to Ug99

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.

Agriculture and Agri-Food Canada, Eastern Cereal and Oilseed Research Centre, Canada
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