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Although wheat can be crossed with a wide range of related species, most adapted bread wheat varieties are non-crossable because of the failure of pollen to fertilize the ovary, greatly restricting the germplasm that can be used for alien introgression. Inhibition to crossability between wheat (Triticum aestivum L.) and related species such as rye (Secale cereale L.) is genetically controlled. A number of QTL have been identified to date, including Kr1 on chromosome 5BL and SKr, a strong QTL mapped previously in our group at the distal end of chromosome 5BS. In this work, we have used 50 recombinant SSD individuals originating from a cross between the poorly crossable cultivar Courtot and the crossable line MP98 to first characterize the effect of SKr. Two classes of crossability were observed with a 1:1 segregation as expected for a single major gene in a SSD population after 6 generations of selfing, thereby confirming the major dominant effect of SKr on crossability. Additional SSR markers were mapped at the proximal end of the gene on 5BS and new markers were developed from the colinearity with barley and rice. High disruption of colinearity between wheat and rice in the telomeric region of 5BS did not allow to use rice in the first place but the use of a high-density barley EST map allowed us to indentify a new marker at the distal end of SKr and establish a syntenic relationship with rice chromosome 12L in the region. New wheat SSR markers were also derived from a BAC sequence carrying the GSP locus that was identify in the vicinity of the SKr locus. In total, 6 additional markers were mapped at the SKr locus and 400 kb of physical contig were established on both sides of the gene after screening the Chinese Spring BAC library with flanking markers. Sequence comparisons showed that the region spanning SKr in wheat corresponds to a 54 kb region carrying 6 genes on rice chromosome 12L. New high-resolution mapping populations have been developped to achieve the map-based cloning of the SKr gene. Two SSR markers completely linked to SKr were further used to evaluate a collection of crossable wheat progenies originating from a primary triticale program developed in the institute. High association was observed between the crossability and the alleles from the crossable lines at the SKr locus. This confirmed the major effect of SKr on crossability in wheat and provided very useful markers for the efficient introgression of Skr alleles in elite varieties to increase genetic diversity in wheat and triticale breeding programs.

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