Although parasites are ubiquitous and play an important role in most natural communities, they have generally been overlooked by population biologist. The almost complete lack of knowledge of the genetic structure of natural populations of parasites is particularly noteworthy. Genetic variation among parasites in their ability to infect various species of hosts is relevant to questions concerning (1) the effects of parasites on species diversity within communities, (2) the maintenance of genetic variation and sexual reproduction within host species, (3) host- parasite coevolution, and (4) modes of speciation in parasites. Such genetic variation may also be important in the evolution and ecology of zoonoses (parasites of animals that can infect humans) and pathogenic fungi that infect agricultural plants. The research proposed here focuses on a species of nematode, Howardula aoronymphium, that in nature parasitizes mushroom-feeding species of Drosophila. Previous studies have indicated that this nematode can have major effects on individuals, populations, and communities of flies. The proposed research includes: (1) a survey of natural populations of H. aoronymphium for genetic variation in infectivity, pathogenicity, and individual growth rate as a function of host species; and (2) selection experiments to determine how patterns of host availability affect the evolution of host specialization.
