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
.