Data from: Egg chemoattractants moderate intraspecific sperm competition

Interactions among eggs and sperm are often assumed to generate intraspecific variation in reproductive fitness, but the specific gamete-level mechanisms underlying competitive fertilization success remain elusive in most species. Sperm chemotaxis–the attraction of sperm by egg-derived chemicals—is a ubiquitous form of gamete signaling, occurring throughout the animal and plant kingdoms. The chemical cues released by eggs are known to act at the interspecific level (e.g., facilitating species recognition), but recent studies have suggested that they could have roles at the intraspecific level by moderating sperm competition. Here, we exploit the experimental tractability of a broadcast spawning marine invertebrate to test this putative mechanism of gamete-level sexual selection. We use a fluorescently labeled mitochondrial dye in mussels to track the real-time success of sperm as they compete to fertilize eggs, and provide the first direct evidence in any species that competitive fertilization success is moderated by differential sperm chemotaxis. Furthermore, our data are consistent with the idea that egg chemoattractants selectively attract ejaculates from genetically compatible males, based on relationships inferred from both nuclear and mitochondrial genetic markers. These findings for a species that exhibits the ancestral reproductive strategy of broadcast spawning have important implications for the numerous species that also rely on egg chemoattractants to attract sperm, including humans, and have potentially important implications for our understanding of the evolutionary cascade of sexual selection.

精卵间的相互作用通常被认为会导致繁殖适合度的种内变异，但在多数物种中，支撑竞争性受精成功的具体配子水平机制仍难以阐明。精子趋化性（sperm chemotaxis）——即卵子释放的化学物质对精子的吸引作用——是一种广泛存在的配子信号形式，遍布动物界与植物界。已知卵子释放的化学信号可在种间层面发挥作用（例如促进物种识别），但近期研究表明，这类信号还可通过调控精子竞争在种内层面发挥功能。本研究利用一种散播式产卵的海洋无脊椎动物的实验可操作性，对这一配子水平性选择的假定机制进行检验。我们以贻贝为研究对象，使用荧光标记的线粒体染料实时追踪精子竞争受精的动态成功率，并首次在任一物种中提供了直接证据，证明竞争性受精成功可通过差异性精子趋化性得以调控。此外，我们的研究数据支持下述假说：卵子化学引诱剂可根据核遗传标记与线粒体遗传标记推断的亲缘关系，选择性地吸引遗传相容雄性的精子群。对于采取散播式产卵这一祖先繁殖策略的物种而言，本研究的发现对众多同样依赖卵子化学引诱剂吸引精子的物种（包括人类）具有重要意义，同时也为我们理解性选择的演化级联效应提供了潜在的重要见解。