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The Sr2 gene has been used extensively in bread wheat improvement for durable stem rust resistance. Interestingly, the resistance of Sr2, associated with the pleotrophic gene Pbc expressed as pseudo-black chaff (PBC), is tightly linked with Yr30/Lr27/Pm genes conferring multiple disease resistance. The linkage map of chromosome 3BS revealed that Sr2 is 0.43cM away from lesion mimic (lm) locus. The RIL population (Yangmai#6 ? Sonalika) of 88 lines including parents where Sonalika carries Sr2 and lm while Yangmai#6 is deficient to both was evaluated for three years (2013-2016). The objective was to determine if this fragment is inherited as one unit and provides resistance to multiple diseases. Twenty four SSR markers distributed between 0.00 to 7.09cM on 3BS covering both Sr2 (5.57cM) and lm locus (6.0cM) were studied in the RIL population. Phenotyping was done for Sr2 associated PBC and lesion mimic along with disease severity for leaf rust, and spot blotch. Positive and significant correlations were observed between leaf stem rust resistance with Sr2 carrying PBC and lm. However, lines with lm either alone or with Sr2 (showing PBC) exhibited spot blotch susceptibility. The reverse situation does not hold not true where genotypes carrying Sr2 alone showed no correlation with spot blotch resistance. This indicates that the Sr2 complex is inherited as a single unit. Use of 24 SSR also suggest that Sr2 and lm loci are tightly linked and inherited together. The co-inheritance of Sr2 and lm ensures the stability and durability of rust resistance. However, the discouraging observation of spot blotch susceptibility due to lm gene suggests a limitation in achieving multiple disease resistance in environments where spot blotch is important. We identified two transgressive segregates in the population showing least expression of lm despite the presence of Sr2 and lm together.
Resistances conferred by Sr2 and Fhb1 are considered important in the control of stem rust and Fusarium head blight (FHB), respectively, but these genes on chromosome 3BS are known only to occur in repulsion. The objective of this study was to use a doubled haploid (DH) population of Carberry/AC Cadillac to seek a recombinant with the Sr2 and Fhb1 resistance alleles in coupling. Carberry expresses moderate resistance to FHB. AC Cadillac has marker alleles typically linked to the Sr2 resistance allele. Carberry has marker alleles associated with Fhb1. The DH population was genotyped with 578 DArT®, 55 SSR, 2 BAC-derived, 2 CAPS, and 1 STS markers. The parents and 261 DH lines were evaluated for adult plant stem rust response (Ug99) at Njoro, Kenya, and at Swift Current, Canada (Canadian Pgt races). Response to FHB was evaluated in nurseries near Portage la Prairie, Manitoba. Pseudo black chaff (PBC), known to be tightly linked to Sr2, was scored in nurseries when symptoms were expressed. Both cultivars have other Sr and Fhb resistance genes, and QTLs contributing to PBC on chromosomes other than 3BS. Using phenotypic and molecular marker data, and the very tight linkage or pleiotropic relationship of Sr2 with PBC, DHs were classified for presence of PBC and FHB response. Putative recombinant DH candidates were re-evaluated for symptoms of PBC, stem rust, and FHB in three international nurseries and genotyped with markers closest to Sr2 and Fhb1. The results will be presented.