Data from: Local gamete competition explains sex allocation and fertilization strategies in the sea

Within and across taxa, there is much variation in the mode of fertilization, i.e. whether eggs and/or sperm are released or kept inside or on the surface of the parent's body. Although the evolutionary consequences of fertilization mode are far reaching, transitions in the fertilization mode itself have largely escaped theoretical attention. Here we develop the first evolutionary model of egg retention and release, which also considers transitions between hermaphroditism and dioecy as well as egg size evolution. We provide a unifying explanation for reported associations between small body size, hermaphroditism, and egg retention in marine invertebrates that have puzzled researchers for over three decades. Our model, by including sperm limitation, shows that all these patterns can arise as an evolutionary response to local competition between eggs for fertilization. This can provide a general explanation for three empirical patterns: Sperm casters tend to be smaller than related broadcast spawners, hermaphroditism is disproportionately common in sperm casters, and offspring of sperm casters are larger. Local gamete competition also explains a universal sexual asymmetry: females of some species retain their gametes while males release theirs, but the opposite ('egg casting') lacks evolutionary stability and is apparently not found in nature.

在不同类群内部及类群之间，受精模式存在诸多差异——即卵子和/或精子是被释放至体外，还是留存于亲代体内或体表。尽管受精模式的演化后果影响深远，但受精模式自身的转变在很大程度上尚未得到理论学界的充分关注。本研究首次构建了关于卵子留存与释放的演化模型，该模型同时纳入了雌雄同体（hermaphroditism）与雌雄异体（dioecy）之间的转变，以及卵子大小演化的相关过程。针对三十余年来困扰学界的海洋无脊椎动物体型偏小、雌雄同体与卵子留存三者间的关联现象，本研究给出了统一的解释框架。本模型纳入精子限制（sperm limitation）因素后表明，上述所有模式均可作为卵子间局部受精竞争的演化响应而产生。这可为三类经验模式提供一般性解释：精子施放类群（sperm casters）的体型往往小于其近缘的播撒产卵类群（broadcast spawners）；雌雄同体在精子施放类群中所占比例显著偏高；且精子施放类群的子代个体更大。局部配子竞争还可解释一种普遍的性不对称现象：部分物种的雌性留存配子，而雄性释放配子；但反之（即“卵子施放（egg casting）”）则缺乏演化稳定性，且自然界中似乎并无此类类群存在。