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 (non-evolving) 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.

鉴于有性生殖的成本，异交种群极易被自交或孤雌生殖的个体入侵并取代。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优势。红皇后假说（Red Queen hypothesis, RQH）提出，与寄生虫的协同进化可产生持续的选择压力，使宿主种群同时倾向于重组与异交。我们验证了这一预测：相较于异交，协同进化的寄生虫可限制自交的扩散。我们将兼具自交与异交能力的野生型秀丽隐杆线虫（Caenorhabditis elegans）雌雄同体个体，引入到携带专性异交突变等位基因的秀丽隐杆线虫种群中。将重复的秀丽隐杆线虫种群置于三种处理条件下，与寄生虫粘质沙雷氏菌（Serratia marcescens）共同培养33代：分别为对照（无毒性）寄生虫处理、固定（非进化）寄生虫处理，以及共传代（潜在协同进化）寄生虫处理。实验结果显示，在对照与固定寄生虫处理组中，自交型个体快速入侵秀丽隐杆线虫宿主种群，但在共传代处理组中，自交型个体在整个实验周期内始终维持极低频率。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，与自交发生率呈显著正相关。综上，本研究结果与红皇后假说一致：协同进化的寄生虫可限制异交种群中自交的扩散。