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.进化生物学领域至今尚未厘清有性生殖的普遍性与其相较于无性生殖的成本之间的核心矛盾。本研究检验了协同进化寄生虫（coevolving parasites）维持宿主有性生殖的假说。具体而言，我们在淡水螺（Potamopyrgus antipodarum）的单个自然种群中，调查了有性生殖的分布格局以及宿主对当地寄生虫的易感性。在本研究体系中，宿主对当地吸虫（Microphallus sp.）的易感性可作为协同进化选择强度的相对量化指标。据此，若协同进化寄生虫确实能够维持宿主的有性生殖，则易感性较高的区域内有性生殖螺类的占比应当更高。我们通过测定某湖泊内多个采样点的螺类易感性，在有性与无性螺类混合的种群中验证了该预测。结果与预测相符：有性生殖螺类的占比与宿主对当地寄生虫的易感性呈显著正相关。尤为值得注意的是，仅在两年时间内，易感性下降的采样点中无性生殖雌螺的占比便出现了显著上升。本研究还发现，相较于野外环境中Microphallus的感染率，有性生殖雌螺的占比与宿主易感性的关联更为紧密。本研究将宿主易感性与有性生殖宿主的占比建立起直接关联，结果表明协同进化选择的空间差异是驱动有性生殖地理镶嵌格局（geographic mosaic of sex）形成的关键因素。