Data from: Occurrence, costs and heritability of delayed selfing in a free-living flatworm

Evolutionary theory predicts that in the absence of outcrossing opportunities, simultaneously hermaphroditic organisms should eventually switch to self-fertilization as a form of reproductive assurance. Here we report the existence of facultative self-fertilization in the free-living flatworm Macrostomum hystrix, a species in which outcrossing occurs via hypodermic insemination of sperm into the parenchyma of the mating partner. First, we show that isolated individuals significantly delay the onset of reproduction compared to individuals with outcrossing opportunities (“delayed selfing”) as predicted by theory. Second, consistent with the idea of M. hystrix being a preferential outcrosser under natural conditions, we report likely costs of selfing manifested via reduced hatchling production and offspring survival. Third, we demonstrate that selfing propensity has a genetic basis in this species, with a heritability estimated at 0.43 ± 0.11. Variation in selfing propensity could arise due to differing costs of inbreeding among families, though despite marked inter-family variation in apparent costs of inbreeding we found no evidence for such a link. Alternatively, selfing propensity might differ across families because of heritable variation in reproductive traits that determine the likelihood of selfing. We speculate that adaptations to hypodermic insemination under outcrossing, most notably a highly modified copulatory stylet (male copulatory organ) and reduced sperm complexity, could also facilitate facultative selfing in this species.进化论预测，在缺乏异交机会的情况下，雌雄同体生物最终应转向自体受精，以此作为繁殖保障策略。本研究报道了自由生活扁形动物大口涡虫（Macrostomum hystrix）中存在兼性自体受精现象——该物种的异交通过将精子皮下授精至交配伙伴的实质组织中完成。首先，我们证实，与拥有异交机会的个体相比，孤立饲养的个体显著延迟了繁殖起始时间（即“延迟自体受精”），这与理论预测相符。其次，契合大口涡虫在自然条件下为偏好异交物种的结论，我们观测到自体受精存在明显代价，表现为幼体产出率降低及后代存活率下降。第三，我们证明该物种的自体受精倾向具有遗传基础，其遗传力估计值为0.43±0.11。自体受精倾向的变异可能源于不同家系间近交代价的差异；尽管我们观测到家系间近交表观代价存在显著差异，但未发现二者存在关联的证据。另外，不同家系间自体受精倾向的差异，也可能源于决定自体受精概率的繁殖性状存在可遗传变异。我们推测，异交条件下针对皮下授精的适应性演化——最显著的是高度特化的交配针突（雄性交配器官）以及简化的精子结构——同样可能促进了该物种的兼性自体受精。