POPULATION BIOLOGY OF THE CHESTNUT BLIGHT FUNGUS, CRYPHONECTRIA-PARASITICA

Interest in the population biology of the chestnut blight fungus Cryph onectria parasitica has been motivated largely by the potential for bi ological control of chestnut blight with fungal viruses that cause hyp ovirulence. Earlier studies gave valuable insights into the correlatio n between diversity of vegetative compatibility groups and transmissio n of hypovirulence viruses. However, inferences about evolutionary pro cesses affecting populations were not possible because vegetative comp atibility groups are not genetically defined. Using restriction fragme nt length polymorphism markers, however, progress has been made in stu dying the origin of C. parasitica in North America, gene flow among po pulations, dispersal within populations, and recombination and the mat ing system. Cryphonectria parasitica populations in North America are genetically more similar to populations in Japan than in China, which is consistent with previous speculations that this fungus was introduc ed from Japan. Populations in China and Japan are quite different, sug gesting little or no gene flow between these areas. Restricted gene fl ow and genetic drift are probably the dominant evolutionary forces sha ping North American populations, with approximately 20% of gene divers ity due to differences among populations (G(ST) = 0.20). Two populatio ns of C. parasitica in Michigan and one population in Italy are primar ily clonal in structure. In contrast, sexual reproduction appears to b e common in populations in eastern North America, although most of the se populations deviate significantly from random mating. Deviations fr om random mating are most likely due to self-fertilization (uniparenta l inbreeding), restricted dispersal of male gametes, and mating betwee n individuals that are more closely related genetically than would be expected by chance (biparental inbreeding). Aggregations of similar ge notypes in space suggest that populations of C. parasitica may be stru ctured into genetic neighborhoods by restricted dispersal. Future rese arch efforts in this system will explore isolation by distance and add ress questions of hypovirulence virus coevolution with its fungal host .