XVI International Botanical Congess

We are studying the role of host-pathogen specificity in the evolution of Heterobasidion annosum, a species complex of partially intersterile species. We provide evidence that host-specific units are genetically isolated and interspecific gene flow is limited by prezygotic and postzygotic isolation mechanisms. Indirect evidence of negative selection on hybrids is provided by population genetics and phylogenetic studies. Direct evidence stems from greenhouse inoculations comparing the virulence of hybrids to that of parental types. While hybrids are less virulent than either parent on the respective adapted hosts, they can be competitive in benign environments. The evolution of this fungus thus, may be best explained by a model of environment-dependent (e.g. host-dependent), rather than unconditional postzygotic isolation of diverging units.