Aggregations of nematode parasites withinDrosophila: proximate causes

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Abstract
Macroparasites almost invariably exhibit overdispersed distributions of parasites/host, yet the specific causes of such aggregations remain poorly understood. The present study focused on the distribution of the parasitic nematode Howardula aoronymphium among its hosts, several species of mycophagous Drosophila. The distribution of parasites/host is close to random for cohorts of flies of a given host species emerging from single mushrooms. At the level of Howardula populations, overdispersion of parasites among hosts results primarily from variation among subgroups of hosts in their exposure to infective-stage nematodes. The sources of variation identified in this study include Drosophila host species, the site where flies bred, mushroom species within sites, and, most importantly, individual mushrooms within mushroom species at a site. For the mean intensity of parasitism observed in this study, the degree of aggregation is typical of macroparasites in general. Combinations of random distributions with different means, resulting from variation among groups in exposure to parasites, may be a common cause of the overdispersion of macroparasites among hosts.