Data from: Sex as a strategy against rapidly evolving parasites

Why is sex ubiquitous when asexual reproduction is much less costly? Sex disrupts coadapted gene complexes; it also causes costs associated with mate finding and the production of males who do not themselves bear offspring. Theory predicts parasites select for host sex, because genetically variable offspring can escape infection from parasites adapted to infect the previous generations. We examine this using a facultative sexual crustacean, Daphnia magna, and its sterilizing bacterial parasite, Pasteuria ramosa. We obtained sexually and asexually produced offspring from wild-caught hosts and exposed them to contemporary parasites or parasites isolated from the same population one year later. We found rapid parasite adaptation to replicate within asexual but not sexual offspring. Moreover, sexually produced offspring were twice as resistant to infection as asexuals when exposed to parasites that had coevolved alongside their parents (i.e. the year two parasite). This fulfils the requirement that the benefits of sex must be both large and rapid for sex to be favoured by selection.为何有性生殖的普遍性远高于成本更低的无性生殖？有性生殖会破坏共适应的基因复合体，同时还会带来求偶耗费、产生无法生育后代的雄性个体等相关成本。理论预测，寄生虫会驱动宿主演化出有性生殖，因为具有遗传多样性的后代能够摆脱适配前代宿主的寄生虫感染。我们以兼性有性生殖的甲壳类——大型溞（Daphnia magna）及其致不育细菌寄生虫——巴斯德芽孢杆菌（Pasteuria ramosa）为研究模型开展实验。我们从野外捕获的宿主个体中获取有性及无性生殖的后代，并将其分别暴露于同期采集的寄生虫，或是一年前从同一种群分离得到的寄生虫中。实验结果显示，寄生虫的快速适应性仅能在无性生殖后代的实验组中被重复观测到，而在有性生殖后代组中则无此现象。此外，当暴露于与其亲本共同演化的寄生虫（即隔年采集的寄生虫样本）时，有性生殖后代的感染抗性是无性生殖后代的两倍。这一结果满足了自然选择青睐有性生殖所需的核心条件：其演化优势必须足够显著且起效迅速。