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PARASITES, SEX, AND EARLY REPRODUCTION IN A MIXED POPULATION OF FRESHWATER SNAILS

Two hypotheses have been presented regarding the effects of parasites on the life-history evolution of their hosts. First, a high risk of infection should favor sexual reproduction over parthenogenetic reproduction, because sexually produced progeny should be better able to evade coevolving parasites (Levin 1975; Jaenike 1978; Hamilton 1980; Bell 1982; Hamilton et al. 1990; Howard and Lively 1994); this idea has come to be known as the Red Queen hypothesis (see Bell 1982). Second, a high risk of infection is predicted to favor early reproduction, especially if infection results in host castration or death (Gadgil and Bossert 1970; Law 1979; Michod 1979; Minchella and Loverde 1981). The evidence from comparative studies have been generally favorable to both the Red Queen hypotheses (Lively 1987, 1992; Schrag et al. 1994) and to the predictions of life-history theory (Lafferty 1993). These studies have focused on comparing data from different populations of aquatic gastropods. In the present study, we examined the predictions of these theories in a single population of freshwater snails (Potamopyrgus antipodarum), which contains a mixture of obligately sexual and obligately parthenogenetic females. The results were in agreement with both theories.