DNA MARKERS REVEAL HYBRIDS BETWEEN 2 DIVERSE BACKGROUND GENOTYPES IN AUSTRALIAN COLLECTIONS OF THE BEAN RUST FUNGUS UROMYCES-APPENDICULATUS

Two classes of molecular markers, RFLPs (restriction fragment length p olymorphisms) and RAPDs (random amplified polymorphic DNA) were used t o assess genetic diversity among Australian bean rust isolates (Uromyc es appendiculatus (Pers.) Unger var. appendiculatus) collected from di verse geographic locations spanning the period 1973-1990. Initially we screened 22 isolates using RFLPs from five DNA probes. This was follo wed by analysis of a subset of 12 of these isolates using RFLPs from 1 0 cDNA probes and RAPDs from 10 arbitrary primers for a comprehensive evaluation of background genotype. Polymorphic bands revealed the exis tence of two divergent clusters of isolates, representing genotypes de signated A and B. The RFLP markers showed 30% band dissimilarity betwe en 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 phenotyp es, but no clear correlation between background genotype (as assessed by RFLP and RAPD markers) and race phenotype was evident. We suggest t hat Australian races of bean rust have most probably evolved by a comb ination of mutation to virulence from common A and B background genoty pes, and genetic recombination.