Genomic targeting and high-resolution mapping of the Tsn1 gene in wheat

Tan spot, caused by the fungal pathogen Pyrenophora tritici-repentis (Died.) Drechs. causes severe yield losses in wheat (Triticum aestivum L., 2n = 6x = 42, AABBDD) and durum (T. turgidum L., 2n = 4x = 28, AABB). The Tsn1 gene acts dominantly to confer sensitivity to a host-selective proteinaceous toxin (Ptr ToxA) produced by the fungus. Our objectives were to: (i) target markers to the Tsn1 genomic region and (ii) develop a high-resolution map of the Tsn1 locus. The techniques of methylation-sensitive AFLP, traditional AFLP, and cDNA-AFLP were combined with bulked segregant analysis (BSA) using various wheat and durum cytogenetic stocks to target markers to the Tsn1 genomic region. Over 500 primer combinations were screened resulting in the identification of 18 low-copy markers closely linked to Tsn1. High-resolution mapping of the markers delineated the Tsn1 gene to a 0.2 cM interval in a hexaploid wheat population consisting of 1266 gametes, and to 0.8 cM in a durum wheat population consisting of 1860 gametes. Comparisons with rice BAC/PAC sequences indicated the lack of colinearity within the Tsn1 genomic region. Tsn1 was located within a gene-rich recombination hot spot region, and the physical distance separating the flanking markers may be as little as 200 kb. Therefore, these markers will serve as a basis for the map-based cloning of Tsn1. The isolation of Tsn1 will further our knowledge of wheat-tan spot interactions as well as host-pathogen interactions in general.