AGGREGATIONS OF NEMATODE PARASITES WITHIN DROSOPHILA - PROXIMATE CAUSES

Macroparasites almost invariably exhibit overdispersed distributions o f parasites/host, yet the specific causes of such aggregations remain poorly understood. The present study focused on the distribution of th e parasitic nematode Howardula aoronymphium among its hosts, several s pecies of mycophagous Drosophila. The distribution of parasites/host i s close to random for cohorts of flies of a given host species emergin g from single mushrooms. At the level of Howardula populations, overdi spersion of parasites among hosts results primarily from variation amo ng subgroups of hosts in their exposure to infective-stage nematodes. The sources of variation identified in this study include Drosophila h ost species, the site where flies bred, mushroom species within sites, and, most importantly, individual mushrooms within mushroom species a t a site. For the mean intensity of parasitism observed in this study, the degree of aggregation is typical of macroparasites in general. Co mbinations of random distributions with different means, resulting fro m variation among groups in exposure to parasites, may be a common cau se of the overdispersion of macroparasites among hosts.