RELATIONSHIP BETWEEN VIRULENCE VARIATION AND DNA POLYMORPHISM IN PUCCINIA-STRIIFORMIS

One-hundred, fifteen single-urediospore isolates from 23 collections (five isolates per collection) of Puccinia striiformis f. sp. tritici (stripe rust of wheat) selected to represent different regions in North America and different virulence patterns were analyzed for variation in virulence and DNA polymorphism. Virulence characterization was based on the expression of infection types on 15 wheat cultivars used to differentiate races of P. striiformis in North America. Of the 23 collections, 20 had uniformly different virulence patterns, i.e., each of the 20 collections was a different race, and all five isolates of each collection had the same virulence pattern. Two collections consisted of isolates with two virulence patterns, and one collection consisted of isolates with three virulence patterns. Of the 26 virulence patterns (races), four (designated as races CDL-47, CDL-48, CDL-49, and CDL-50) had not been detected previously. DNA poly morphism was determined by random amplified polymorphic DNA (RAPD) assay. Random primers were used to amplify DNA extracted from the 115 isolates. Of 107 RAPD bands that were amplified with 11 random primers, 47% were polymorphic. DNA polymorphism was detected among races and among single-spore isolates within races. Race CDL-21, originally collected from triticale in California, was different from other races in 13% of the RAPD bands and had only 67% similarity with other races. All other races were more closely related (about 80% similarity). Based on cluster analyses, the 115 isolates were placed in six virulence and five RAPD groups. The correlation coefficient between the virulence and RAPD similarity matrices was low (0.20). Cluster analysis based on RAPD data separated isolates virulent on stripe rust-resistance gene Yr1 from those avirulent on Yr1. Virulence groups were highly associated with epidemic regions. RAPD groups generally were not associated with geographic regions. The low association between virulence and RAPD patterns indicates that DNA polymorphisms are independent of virulence, and selection for virulence by growing hosts with race-specific resistance plays a major role in determining the race structure of the pathogen.