DNA Markers Reveal Hybrids Between Two Diverse Background Genotypes in Australian Collections of the Bean Rust Fungus Uromyces appendiculatus

Abstract

Two classes of molecular markers, RFLPs (restriction fragment length polymorphisms) and RAPDs (random amplified polymorphic DNA) were used to assess genetic diversity among Australian bean rust isolates (Uromyces appendiculatus (Pers.) Unger var. appendiculatus) collected from diverse geographic locations spanning the period 1973-1990. Initially we screened 22 isolates using WLPs from five DNA probes. This was followed by analysis of a subset of 12 of these isolates using RFLPs from 10 cDNA probes and RAPDs from 10 arbitrary primers for a comprehensive evaluation of background genotype. Polymorphic bands revealed the existence of two divergent clusters of isolates, representing genotypes designated A and B. The RFLP markers showed 30% band dissimilarity between A and B, and RAPDs 16% dissimilarity. Isolates in a third cluster (genotype AB) exhibited most of the polymorphic bands present in A and B, but no unique polymorphic bands of their own, indicating that they had most probably arisen from recent hybridisation between isolates of genotype A and B. The subset of 12 isolates included 10 race phenotypes, but no clear correlation between background genotype (as assessed by RFLP and RAPD markers) and race phenotype was evident. We suggest that Australian races of bean rust have most probably evolved by a combination of mutation to virulence from common A and B background genotypes, and genetic recombination.