Tests of Competing Hypotheses for the Maintenance of Sex and Parthenogenesis in Campeloma limum, a SE United States Snail

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Why organisms reproduce sexually instead of asexually is one of the outstanding problems in biology. Recent models suggest that, under higher dispersal rates of asexuals relative to sexuals, asexual organisms may persist for three reasons: escape from parasites (Red Queen hypothesis); relaxed selection against deleterious mutations; or local adaptation of asexuals to marginal environments. Using diploid sexual and asexual Campeloma limum, a freshwater snail from the southeastern US, this proposal outlines molecular and experimental field tests of these hypotheses. Using sequence variation in a mitochondrial gene (cytochrome b) and a nuclear intron (calmodulin), we will assess whether asexuals have higher dispersal rates than sexuals. We will also test unique predictions of the three models by field experiments. Because plant pathologists, epidemiologists, and conservation biologists realize that host genetic diversity plays an important role in the spread of diseases, the results from this study will be of considerable to these scientists. These results will also be of considerable interest to evolutionary biologists, because the study will provide critical tests of ecological and evolutionary causes of the spatial distribution of sexual and asexual organisms.