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.

鉴于有性生殖所付出的演化代价，异交种群本应易于被自体受精或孤雌生殖入侵并取代。然而，双亲有性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演化可产生持续的选择压力，促使宿主种群同时倾向于重组与异交。我们针对这一预测开展了实验验证：相较于异交，协同演化的寄生虫可抑制自体受精的扩散。我们将既能进行自体受精又可实现异交的野生型秀丽隐杆线虫（Caenorhabditis elegans）雌雄同体个体，导入至全部个体均携带专性异交（obligate outcrossing）突变等位基因的秀丽隐杆线虫种群中。多组重复的秀丽隐杆线虫种群被置于三种处理条件下，与粘质沙雷氏菌（Serratia marcescens）共同培养33代：分别为无毒性寄生虫对照处理、不发生演化的固定寄生虫处理，以及潜在发生协同演化的共传代寄生虫处理。在对照与固定寄生虫处理组中，自体受精快速入侵秀丽隐杆线虫宿主种群；但在共传代处理组中，自体受精在整个实验周期内始终维持极低频率。此外，实验结束时，各宿主种群中允许自体受精的野生型等位基因频率，与种群内自体受精频率呈显著正相关。综上，本研究结果与红皇后假说一致：协同演化的寄生虫可有效限制异交种群中自体受精的扩散。