Molecular genetic description of the cryptic wheat-Aegilops geniculata introgression carrying rust resistance genes Lr57 and Yr40 using wheat ESTs and synteny with rice

The cryptic wheat-alien translocation T5DL.5DS-5M9(g)(0.95), with leaf rust and stripe rust resistance genes Lr57 and Yr40 transferred from Aegilops geniculata ((UMg)-M-g) into common wheat, was further analyzed. Molecular genetic analysis using physically mapped ESTs showed that the alien segment in T5DL.5DS-5Mg(g)(0.95) represented only a fraction of the wheat deletion bin 5DS2-0.78-1.00 and was less than 3.3 cM in length in the diploid wheat genetic map. Comparative genomic analysis indicated a high level of colinearity between the distal region of the long arm of chromosome 12 of rice and the genomic region spanning the Lr57 and Yr40 genes in wheat. The alien segment with genes Lr57 and Yr40 corresponds to fewer than four overlapping BAC or PAC clones of the syntenic rice chromosome arm 12L. The wheat-alien translocation breakpoint in T5DL.5DS-5Mg(g)(0.95) was further localized to a single BAC clone of the syntenic rice genomic sequence. The small size of the terminal wheat-alien translocation, as established precisely with respect to Chinese Spring deletion bins and the syntenic rice genomic sequence, further confirmed the escaping nature of cryptic wheat-alien translocations in introgressive breeding. The molecular genetic resources and information developed in the present study will facilitate further fine-scale physical mapping and map-based cloning of the Lr57 and Yr40 genes.