Fine Mapping of Two Wheat Powdery Mildew Resistance Genes Located at the Pm1 Cluster

Powdery mildew caused by Blumeria graminis (DC.) f. sp. tritici (Bgt) is a globally devastating foliar disease of wheat (Triticum aestivum L.). More than a dozen genes against this disease, identified from wheat germplasms of different ploidy levels, have been mapped to the region surrounding the Pm1 locus on the long arm of chromosome 7A, which forms a resistance (R)-gene cluster. Mlm2033 and Mlm80 from einkorn wheat (T. monococcum L.) were two of the R genes belonging to this cluster. This study was initiated to fine map these two genes toward map-based cloning. Comparative genomics study showed that macrocolinearity exists between Brachypodium distachyon L. chromosome 1 (Bd1) and the Mlm2033-Mlm80-Pm1a region, which allowed us to develop markers based on the wheat sequences orthologous to genes contained in the Bd1 region. With these and other newly developed and published markers, high-resolution maps were constructed for both Mlm2033 and Mlm80 using large F-2 populations. Moreover, a physical map of Mlm2033 was constructed through chromosome walking with bacterial artificial chromosome (BAC) clones and comparative mapping. Eventually, Mlm2033 and Mlm80 were restricted to a 0.12- and 0.86-cM interval, respectively. Based on the closely linked common markers, Mlm2033, Mlm80, and MlIW172 (another powdery mildew resistance gene in the Pm1 cluster) were not allelic to one another. Severe recombination suppression and disruption of synteny were noted in the region encompassing Mlm2033. These results provided useful information for map-based cloning of the R genes in the Pm1 cluster and interpretation of their evolution.