Data from: Within-population covariation between sexual reproduction and susceptibility to local parasites

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.

进化生物学至今仍未能调和有性生殖（sexual reproduction）相较于无性生殖（asexual reproduction）的进化成本，与有性生殖普遍存在性之间的矛盾。本研究旨在验证协同进化寄生虫（coevolving parasites）可维持宿主有性生殖的假说。具体而言，我们针对单一淡水螺类（Potamopyrgus antipodarum）种群，分析了有性生殖的分布特征及其对本地寄生虫的易感性。在该研究体系中，对本地吸虫（trematode parasites）中的微穴吸虫属（Microphallus sp.）的易感性，可作为协同进化选择强度的相对量化指标。因此，若协同进化寄生虫确实维持宿主的有性生殖，则在寄生虫易感性较高的区域，有性螺类的占比应当更高。我们通过测定某湖泊中多个采样点螺类的易感性，在有性与无性螺类共存的混合种群中验证了这一预测。实验结果与预测一致：有性螺类的占比与本地寄生虫易感性呈显著正相关。尤为值得注意的是，仅在两年时间内，寄生虫易感性下降的采样点中，无性雌螺的占比便出现了上升。此外我们还发现，相较于野外环境中微穴吸虫属（Microphallus sp.）的感染率，有性雌螺的占比与寄生虫易感性的关联更为紧密。本研究将寄生虫易感性与宿主有性生殖占比建立关联，结果直接表明，协同进化选择的空间差异正是驱动有性生殖地理镶嵌格局（geographic mosaic of sex）形成的关键因素。