9707675 Milgroom The overall goal of this proposal is to examine the mating system and genetic structure of natural populations of the fungus that causes chestnut blight, Cryphonectria parasitica. This is the only fungus so far shown to both self-fertilize and outcross in nature; that is, it has a mixed mating system. While selfing occurs readily in nature, it is rare in the lab, suggesting that environment has a role in determining the frequency with which selfing occurs. The first objective for this proposal, therefore, is to determine the role played by environment in influencing how often selfing occurs, and also the extent to which the ability to self-fertilize is genetically determined. This will be accomplished by testing the potential of individuals to self and/or outcross in natural populations, under different environmental conditions, especially with regard to the ecology of chestnut trees (e.g., tree age, habit, and density, companion species), which are hosts for this fungal pathogen. The evolution of selfing depends on the intensity of inbreeding, which is mating among closely related individuals. Therefore, the second objective is to determine the degree to which natural populations of C. parasitica are inbred. The effects of selfing and inbreeding on population structure are determined in part by the potential for inbreeding depression, which is a reduction in fitness of inbred individuals relative to complete outbreeding. Therefore, the third objective is to determine the effects of inbreeding on fitness by studying laboratory crosses between individuals of varying degrees of relatedness. Mating systems, the means by which organisms reproduce and pass their genes on to the next generation, have profound effects on diversity and evolution. The fungal kingdom comprises a richly diverse group of organisms, representing a broad range of life histories and reproductive strategies. Yet our knowledge of the natural breeding systems of fungi is surprisingly sparse and incomplete, considering the rich tradition of research, dating back to Darwin, on plant breeding systems. Virtually nothing is known about the temporal or spatial dynamics of fungal mating systems in nature. The work outlined above will constitute a first step in addressing this disparity, and in so doing launch a new direction in research on fungi.
