The winter wheat cultivar Red Chief has been identified as the wheat cultivar most resistant to Pyrenophora tritici-repentis (Ptr). This study was undertaken to determine the inheritance, chromosomal location and molecular mapping of a tan spot resistance gene in Red Chief, χ² analysis of the F₂ segregation data of the hybrids between 21 monosomic lines of the susceptible wheat cultivar Chinese Spring and the resistant cultivar Red Chief revealed that tan spot resistance in cv. Red Chief is controlled by a single recessive gene located on chromosome 3 A. Linkage analysis using SSR markers in the Red Chief/Chinese Spring F₂ population showed that the tsr4 gene is clustered in the region around Xgwm 2a, on the short arm of chromosome 3 A. This marker has also been identified as the closest marker to the tsr3 locus on chromosome 3D in synthetic wheat lines. Validation analysis of this marker for the tsr3 and tsr4 genes using 28 resistant and 6 susceptible genotypes indicated that the 120 bp allele (the tsr3 gene) specific fragment was observed in 11 resistant genotypes, including the three synthetic lines XX41, XX45 and XX110, while the 130 bp allele was amplified only in cv. Red Chief and Dashen. Xgwm2a can be used to trace the presence of the target gene in successive backcross generations and pyramiding of the tsr3 & tsr4 genes into a commonly grown and adaptable cultivar.
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