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 cross-conjugate among each other. Genetic isolation was investigated from the distribution of neutral microsatellite alleles. Since this is a pollinator-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-inoculation experiment limits interpretability of the results, but fungus collected from Silene vulgaris was capable of infecting Dianthus carthusianorum. Different fungal strains were able to conjugate and form the infectious dikaryon in most combinations, so hybridization between different fungal host races is possible. The distribution of neutral genetic variation, however, revealed little successful genetic exchange among the fungal host races that were clearly differentiated by host plant species. Pollinators, while showing partial constancy, moved between plants of different host species. Pollinator behavior is therefore not adequate to explain the lack of gene flow among the different fungal races. This indicates that the divergence among these fungal races that has produced incipient species sharing almost no alleles may have occurred in allopatry, unless disruptive selection can outweigh gene flow among fungal races in sympatry.