MYCOPHAGOUS DROSOPHILA AND THEIR NEMATODE PARASITES

The nematode Howardula aoronymphium (Allantonematidae) parasitizes several mushroom-feeding species of Drosophila of the quinaria and testacea species groups in eastern North America. The principal effect of nematodes on their hosts is a reduction in female fertility: females of Drosophila putrida and Drosophila testacea are sterilized by these parasites. The prevalence of parasitism varies substantially among host species and through time, showing evidence of seasonal as well as year-to-year variation. Because the principal effect of these parasites is on host reproduction and because they are not highly aggregated among hosts, this parasite-host system may be prone to unstable population dynamics. Some field evidence suggests that H. aoronymphium may influence the outcome of competition among its host species. Because these parasites and their hosts have similar population sizes, generation times. and rates of gene flow, they may interact evolutionarily in a dynamically interesting manner.