Suppressed recombination rate in 6VS/6AL translocation region carrying the Pm21 locus introgressed from Haynaldia villosa into hexaploid wheat

Pm21 is an effective gene for powdery mildew resistance transferred from Haynaldia villosa into common wheat cultivars. No virulence against this gene has been detected so far. A set of 42 powdery mildew isolates collected in Israel and tested in the current study also revealed no virulence against this gene. Pm21 was previously reported to be located on the short arm of 6VS/6AL translocation chromosome. We constructed a high-density genetic map of chromosome 6A, consisting of 28 PCR markers and the Pm21 gene. A comparison with previously published genetic maps of wheat chromosome 6A revealed that the recombination rate in the 6VS/6AL translocation region was poor. We assume that suppressed recombination caused by the alien H. villosa genetic material is the most reasonable explanation for the tight genetic linkage and the inadequacy between the Pm21 genetic map and the Pm21 physical map of 6A. A large number of sequence-tag sites (STS) and simple sequence repeat markers, which co-segregate with or are closely linked to the Pm21 gene, and the conversion of three resistance gene analog markers into new STS markers, provide a reliable and easy-to-use molecular tool for marker-assisted selection of Pm21 in wheat breeding programs. An additional gene, Pm31, previously reported to be derived from Triticum dicoccoides, was mapped into a similar genomic location to Pm21. Screening of the parental lines and the mapping population with Pm21 diagnostic markers clearly confirmed that the donor line of Pm31 is H. villosa and not T. dicoccoides. Therefore, we conclude that Pm21 and Pm31 refer to the same gene, derived from H. villosa, and that the designation of Pm31 as a new Pm gene was erroneous.