Data from: Egg chemoattractants moderate intraspecific sperm competition

Interactions among eggs and sperm are often assumed to generate variation in 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 signalling, 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 labelled 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.人们通常认为，卵子与精子间的相互作用会产生繁殖适合度（reproductive fitness）的种内变异，但多数物种中，支撑竞争性受精成功率的具体配子（gamete）层面机制仍未阐明。精子趋化性（sperm chemotaxis）——即卵子释放的化学物质对精子的定向吸引作用——是一类广泛分布的配子信号传导形式，遍及动物界与植物界。已知卵子释放的化学信号可在种间层面发挥功能，例如促进物种识别，但近期研究表明，其亦可通过调控精子竞争在种内层面发挥作用。本研究借助体外产卵（broadcast spawning）海洋无脊椎动物的实验可操作性，对这一推测的配子水平性选择机制开展验证。研究以贻贝为实验材料，使用荧光标记线粒体染料（fluorescently labelled mitochondrial dye）追踪精子竞争受精过程中的实时动态，并在任一物种中首次提供直接证据，证明竞争性受精成功率可通过差异化精子趋化性得到调控。此外，基于核与线粒体遗传标记（nuclear and mitochondrial genetic markers）推断的亲缘关系，本研究数据支持如下假说：卵子化学引诱剂可选择性吸引遗传相容雄性的精液。本研究对象为采用体外产卵这一祖先型繁殖策略的物种，其研究结果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）具有重要参考价值，同时也为我们理解性选择的进化级联（evolutionary cascade of sexual selection）提供了潜在的关键启示。