A Comparison of Virulence Phenotypes in Wheat Stem Rust Populations Reproducing Sexually and Asexually

Abstract

The number and distribution of 16 loci expressing virulence were compared in 2 populations of wheat stem rust in the USA. In 1975, samples were collected from a population on barley and wheat in Idaho and Washington that undergoes sexual reproduction annually and from a population east of the Rocky Mountains that reproduces asexually. Virulence was determined by inoculating 16 wheat lines, each differing by a single gene for stem rust resistance. The sexual population had a larger frequency of distinct phenotypes expressed as a percentage. In the sexual population, the frequency was 23.5% with 426 isolates and in the asexual population it was 0.07% with 2377 isolates. Simpson''s measure of diversity was 0.974 and 0.501 for the 2 populations, respectively. Mean number of loci expressing virulence per isolate was about 6 and 10 for the sexual and asexual populations, respectively. The distribution of numbers of loci expressing virulence differences between pairs of isolates was nearly random in the sexual population, while it was characterized by clusters of phenotypes in the asexual population. These clusters differed from one another by 4-10 genes expressing virulence; genotypes within clusters differed by 1-2 loci. None of the loci, examined in all of their paired combinations, deviated strongly from expected mean frequencies based on products of individual locus frequencies, providing no evidence for strong positive or negative fitness effects associated with individual genes.