| Summary: | Evolutionary biology has yet to reconcile the ubiquity of sex with its costs relative to asexual reproduction. Here, we test the hypothesis that coevolving parasites maintain sex in their hosts. Specifically, we examined the distributions of sexual reproduction and susceptibility to local parasites within a single population of freshwater snails (Potamopyrgus antipodarum). Susceptibility to local trematode parasites (Microphallus sp.) is a relative measure of the strength of coevolutionary selection in this system. Thus, if coevolving parasites maintain sex, sexual snails should be common where susceptibility is high. We tested this prediction in a mixed population of sexual and asexual snails by measuring the susceptibility of snails from multiple sites in a lake. Consistent with the prediction, the frequency of sexual snails was tightly and positively correlated with susceptibility to local parasites. Strikingly, in just two years, asexual females increased in frequency at sites where susceptibility declined. We also found that the frequency of sexual females covaries more strongly with susceptibility than with the prevalence of Microphallus infection in the field. In linking susceptibility to the frequency of sexual hosts, our results directly implicate spatial variation in coevolutionary selection in driving the geographic mosaic of sex. coordinatesThis file contains the latitude and longitude for each of the 13 sites sampled in this study. Coordinates were obtained from Google Earth. Data columns are as follows: (1) site (n=13); (2) latitude; (3) longitude.experimental dataThis file contains the full results of the inoculation experiment from which susceptibility and the proportion of sexual females were determined for 13 sites in each of three years. Juvenile snails from each were exposed to high doses of parasites from Lake Alexandrina. Following parasite development, snails were evaluated for sex, infection and reproductive mode. Data columns are as follows: (1) year (n=3; 2013, 2014, and 2015); (2) site ...
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