Genetic isolation among host races of the anther smut fungus Microbotryum violaceum on three host plant species

Genetic isolation among strains of the plant pathogenic fungus Microbotryum violaceum on three species of its host plants was examined. Fungal strains collected from a sympatric population of all three host plant species were examined for their ability to infect the other host plant species and to c ross-conjugate among each other. Genetic isolation was investigated from th e distribution of neutral microsatellite alleles. Since this is a pollinato r-transmitted disease, we examined movement patterns and flower-visitation behavior of pollinators to investigate whether they transfer fungal spores between different host species. Low infection success from the cross-inocul ation experiment limits interpretability of the results, but fungus collect ed from Silene vulgaris was capable of infecting Dianthus carthusianorum. D ifferent fungal strains were able to conjugate and form the infectious dika ryon in most combinations, so hybridization between different fungal host r aces is possible. The distribution of neutral genetic variation, however, r evealed little successful genetic exchange among the fungal host races that were clearly differentiated by host plant species. Pollinators, while show ing partial constancy, moved between plants of different host species. Poll inator behavior is therefore not adequate to explain the lack of gene now a mong the different fungal races. This indicates that the divergence among t hese fungal races that has produced incipient species sharing almost no all eles may have occurred in allopatry, unless disruptive selection can outwei gh gene flow among fungal races in sympatry.