Reduction in the sex ability of worldwide clonal populations of Puccinia striiformis f.sp. tritici.

Puccinia striiformis f.sp. tritici (PST), has so far been considered to reproduce asexually with until very recently no known alternate host, has a clonal population structure in the USA, Australia and Europe. However, recently, high genotypic diversity in Eastern Asia and recombinant populations in China has been reported. Variations in the ability for sexual reproduction could provide an explanation for such a geographical gradient in genotypic diversity. In order to address this hypothesis, we tested for the existence of a relationship between the ability to produce telia, sex-specific structures that are obligatory for sexual cycle, and the genetic diversity of populations measured using neutral markers, in a set of 56 isolates representative of six worldwide geographical origins. Clustering methods assigned these isolates to five genetic groups corresponding to their geographical origin, with eight inter-group hybrid individuals. Isolates representing China, Nepal and Pakistan displayed the highest telial production, while clonal populations from France and the Mediterranean region displayed very low telial production. The geographic cline in telial production corresponded to the gradient of genotypic diversity described during previous studies, showing a clear difference in telial production between clonal vs. diverse/recombinant populations. The higher mean Qst value (0.822) for telial production than the Fst value (0.317) suggested that telial production has more probably evolved through direct or indirect selection rather than genetic drift alone. The existence of high telial production in genetically diverse populations and its reduction in clonal populations is discussed with regard to evolution of sex, PST centre of origin and distribution of its alternative host.