Virulence and isozyme diversity of sexual versus asexual collections of Uromyces appendiculatus (bean rust fungus)

Organisms that reproduce sexually have been assumed to be more genetically diverse (have more phenotypes with a more even frequency distribution) than those that are clonal. Rust fungi usually have regular sexual reproduction, but asexual species or populations are not uncommon. Two field collections of the bean rust fungus from sexually reproducing populations were compared with two collections from apparently nonsexual populations. Neither virulence on 10 differential lines of beans nor isozyme band patterns of five enzymes, as compared by the Gleason, Shannon, or Rogers indices of difference or with simple matching coefficients, supported the hypothesis of less population polymorphism in the asexual collections. The distribution of virulence differences among isolates in one of the asexual collections suggested that sexual reproduction was occurring at a low frequency which was, nevertheless, sufficient to maintain genetic variability.