Asexual Genetic Exchange in the Barley Pathogen Rhynchosporium secalis

The causal agent of barley scald, Rhynchosporium secalis, is a haploid anamorphic ascomycete with no known sexual stage. Nevertheless, a high degree of genetic variation has been observed in fungal populations on commercial barley cultivars and parasexuality has been suggested to contribute to this variation. In order to test whether asexual genetic exchange can occur, isolates of R. secalis were transformed to hygromycin B resistance or phleomycin resistance. Mixtures of transformants were co-inoculated either on agar or in planta and screened for the occurrence of dual-antibiotic-resistant colonies. No dual-antibiotic-resistant colonies resulted from mixing transformants of different fungal isolates. In contrast, with transformants originating from the same fungal isolate, asexual exchange of markers was demonstrated on agar plates and in planta. This is the first definitive evidence of asexual genetic exchange in R. secalis.