Development of wheat lines having a small introgressed segment carrying stem rust resistance gene Sr22

The wheat stem rust resistance gene Sr22 confers resistance to Puccinia graminis f. sp. tritici Pers. race TTKSK (also known as Ug99) that developed in Africa and is an immediate threat to world wheat production. The resistance gene is present on a chromosomal translocation derived from Triticum boeoticum Boiss., which has a genome that is partially homologous to the A genome of T. aestivum L. Sr22 has been deployed in a limited number of cultivars due to poor agronomic performance of lines carrying the resistance gene. Linkage analysis of simple sequence repeat (SSR) markers on chromosome 7A was performed to identify loci closely linked to Sr22. The most tightly linked proximal and distal SSR marker loci were Xcfa2123 and Xwmc633, respectively. A two-step process was then used to develop resistant lines having smaller chromosome segments derived from the diploid donor. First, individuals in which a single recombination event had occurred between wheat and the Sr22 introgression were identifi ed in the mapping populations. In spite of reduced recombination between T. boeoticum and T. aestivum chromosomes, suffi cient recombination events were found among 398 F_3:4 lines derived from recombinant F₂ progeny to recover multiple resistant individuals with smaller alien introgressions. Resistant lines were identifi ed having less than 6% of the chromosome arm derived from T. boeoticum. These lines may provide a more agronomically desirable source of Sr22 that can be readily deployed in cultivars resistant to Ug99.