Hypodermic self-insemination as a reproductive assurance strategy

Self-fertilization occurs in a broad range of hermaphroditic plants and animals, and is often thought to evolve as a reproductive assurance strategy under ecological conditions that disfavour or prevent outcrossing. Nevertheless, selfing ability is far from ubiquitous among hermaphrodites, and may be constrained in taxa where the male and female gametes of the same individual cannot easily meet. Here, we report an extraordinary selfing mechanism in one such species, the free-living flatworm Macrostomum hystrix. To test the hypothesis that adaptations to hypodermic insemination of the mating partner under outcrossing also facilitate selfing, we experimentally manipulated the social environment of these transparent flatworms and then observed the spatial distribution of received sperm in vivo. We find that this distribution differs radically between conditions allowing or preventing outcrossing, implying that isolated individuals use their needle-like stylet (male copulatory organ) to inj...自体受精广泛存在于各类雌雄同体动植物中，通常被认为是在不利于或阻碍异交的生态条件下，作为一种繁殖保障策略演化而来的。然而，自体受精能力在雌雄同体生物中远非普遍存在，在同一个体的雌雄配子难以轻易相遇的类群中，自体受精能力可能受到约束。在此，我们报道了这类物种中一种特殊的自体受精机制：自由生活的扁形动物大口涡虫（Macrostomum hystrix）。为验证"异交环境下适配于交配对象皮下授精（hypodermic insemination）的性状，同样可促进自体受精"这一假说，我们通过实验操控这些透明扁形虫的社会环境，并活体观察了接收精子的空间分布。我们发现，在允许或阻止异交的两种实验条件下，精子的空间分布模式存在显著差异，这表明孤立个体会利用其针状交合刺（stylet，雄性交配器官）对自身……