Although many wild relatives in the Triticeae tribe have been exploited to transfer stem rust resistance genes to wheat, the derived germplasms have often not been immediately useful in wheat breeding programs. Too frequently, large chromosome segments surrounding desirable genes also harbor deleterious genes that result in unacceptable yield or quality. Recombination between chromosomes of wheat and chromosomes of distant relatives is very rare due to genetic restrictions on chromosome pairing in polyploid wheat. However, chromosome pairing can be manipulated by utilizing mutant stocks that relax this tight genetic control. The ph1b mutant produced by E.R. Sears over 30 years ago is an invaluable chromosome engineering tool, readily employed in the age of high-throughput molecular genetics. Shortened translocations have already been produced for stem rust resistance genes Sr26 and SrR using ph1b-induced homoeologous recombination. We are currently using induced-homoeologous recombination to reduce the sizes of alien chromosome segments surrounding TTKSK-effective genes Sr32, Sr37, Sr39, Sr40, Sr43, Sr47, SrTt3, Sr2S#1 and SrAeg5 to eliminate linkage drag putatively associated with these genes. Additional TTKSK-effective genes Sr44, SrHv6, SrAsp5, and SrAse3 were first targeted for development of compensating translocation stocks and then for shortening the size of each alien segment. Population development is also underway to characterize several potentially new sources of resistance.
Cytogenetic manipulation to enhance the utility of alien resistance genes
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