Segregation for avirulence/virulence in Puccinia striiformis based on an experimental genetic system using Berberis vulgaris
An experimental genetic system for Puccinia striiformis was recently developed using the alternate (sexual) host, Berberis vulgaris. Selfing of an aggressive Pst isolate resulted in an S1 generation of 16 progeny, which were confirmed by segregating SSR markers. We analyzed the inheritance of avirulence/virulence in the S1 generation using wheat genotypes representing 21 Pst resistance genes. All S1 progeny were virulent for 14 of 15 Yr genes where the parental isolate was virulent. No segregation was observed for 5 of 6 host genes for which the parental isolate was avirulent. Segregation was observed with respect to Yr8 where the parental isolate gave infection type (IT) 0, and to Yr17 where the parental isolate gave IT 5-6 (0-9 scale). Avirulence/virulence to Yr8 (Compair and AvS+Yr8) was represented by two phenotypes, and avirulence/virulence to Yr17 (VPM1, AvS+Yr17, and Baltimore) was represented by three host phenotypes. On both Yr8 host genotypes, some progenies produced IT 0 and others produced IT 1-2, suggesting that the parental isolate was heterozygous for two different Avr8 alleles resulting in different, but clearly avirulent phenotypes. On the Yr17 genotypes, two distinct phenotypes, IT 2-4 and 5-6 (occasionally 7), were observed, the latter being similar to the parental isolate. None of the progenies was considered virulent (IT 7 to 9), as observed for Yr17-virulent reference isolates. This unusual segregation pattern could be explained by the presence in the parental isolate of a heterozygous modifier gene influencing the phenotypic expression of avirulence. In order to resolve the genetics in detail, additional progeny are being produced from the parental isolate, and selfings of additional Pst isolates with divergent levels of aggressiveness are in progress.