Distribution and haplotype diversity of WKS resistance genes in wild emmer wheat natural populations.

The wheat stripe rust resistance gene Yr36 ( WKS1 ) with a unique kinase-START domain architecture is highly conserved in wild emmer wheat natural populations. Wild emmer wheat (Triticum dicoccoides) populations have developed various resistance strategies against the stripe rust pathogen Puccinia striiformis f. sp. tritici (Pst). The wild emmer gene, Yr36 (WKS1), which confers partial resistance to a broad spectrum of Pst races, is composed of a kinase and a START lipid-binding domain, a unique gene architecture found only in the Triticeae tribe. The analysis of 435 wild emmer accessions from a broad range of natural habitats revealed that WKS1 and its paralogue WKS2 are present only in the southern distribution range of wild emmer in the Fertile Crescent, supporting the idea that wheat domestication occurred in the northern populations. An analysis of full-length WKS1 sequence from 54 accessions identified 15 different haplotypes and very low-nucleotide diversity (π = 0.00019). The high level of WKS1 sequence conservation among wild emmer populations is in contrast to the high level of diversity previously observed in NB-LRR genes (e.g., Lr10 and Pm3). This phenomenon may reflect the different resistance mechanisms and different evolutionary pathways that shaped these genes, and may shed light on the evolution of genes that confer partial resistance to stripe rust. Only five WKS1 coding sequence haplotypes were revealed among all tested accessions, encoding four different putative WKS1 proteins (designated P0, P1, P2, and P3). Infection tests showed that P0, P1, and P3 haplotypes display a resistance response, while P2 displayed a susceptible response. These results show that the WKS1 proteins (P0, P1, and P3) can be useful to improve wheat resistance to stripe rust.