Data from: Coevolutionary interactions with parasites constrain the spread of self-fertilization into outcrossing host populations

Given the cost of sex, outcrossing populations should be susceptible to invasion and replacement by self-fertilization or parthenogenesis. However, biparental sex is common in nature, suggesting that cross-fertilization has substantial short-term benefits. The Red Queen hypothesis (RQH) suggests that coevolution with parasites can generate persistent selection favoring both recombination and outcrossing in host populations. We tested the prediction that coevolving parasites can constrain the spread of self-fertilization relative to outcrossing. We introduced wild-type Caenorhabditis elegans hermaphrodites, capable of both self-fertilization, and outcrossing, into C. elegans populations that were fixed for a mutant allele conferring obligate outcrossing. Replicate C. elegans populations were exposed to the parasite Serratia marcescens for 33 generations under three treatments: a control (avirulent) parasite treatment, a fixed (nonevolving) parasite treatment, and a copassaged (potentially coevolving) parasite treatment. Self-fertilization rapidly invaded C. elegans host populations in the control and the fixed-parasite treatments, but remained rare throughout the entire experiment in the copassaged treatment. Further, the frequency of the wild-type allele (which permits selfing) was strongly positively correlated with the frequency of self-fertilization across host populations at the end of the experiment. Hence, consistent with the RQH, coevolving parasites can limit the spread of self-fertilization in outcrossing populations.鉴于有性生殖的演化代价，异交种群易被自交或孤雌生殖入侵并取代。然而，双亲生殖在自然界中仍广泛存在，这表明异体受精具备显著的短期演化优势。红皇后假说（Red Queen hypothesis, RQH）提出，宿主与寄生虫的协同进化可产生持续的选择压力，促使宿主种群同时偏好重组与异交。我们针对相较于异交，协同进化的寄生虫可限制自交的传播这一预测开展了验证实验。我们将既能自交又能异交的野生型秀丽隐杆线虫（Caenorhabditis elegans）雌雄同体，引入到纯合携带强制异交突变等位基因的秀丽隐杆线虫种群中。随后将重复构建的秀丽隐杆线虫种群置于三种处理条件下，与粘质沙雷氏菌（Serratia marcescens）共培养33代：分别为对照（无毒性）寄生虫处理组、固定（不发生进化）寄生虫处理组，以及共传代（潜在协同进化）寄生虫处理组。实验结果显示，在对照与固定寄生虫处理组中，自交快速入侵秀丽隐杆线虫宿主种群，但在整个实验期间，共传代处理组中的自交始终处于极低频率。此外，实验结束时，宿主种群中允许自交的野生型等位基因频率与自交频率呈显著正相关。综上，与红皇后假说相符，协同进化的寄生虫可限制异交种群中自交的传播。