Data from: Do sperm really compete and do eggs ever have a choice? Adult distribution and gamete mixing influence sexual selection, sexual conflict, and the evolution of gamete recognition proteins in the sea

The evolution of gametic compatibility and the effectiveness of compatibility, within and across species depend on whether sperm from different males directly compete for an egg and whether eggs ever have a choice. Direct sperm competition and egg choice depend on whether sperm from different males arrive at an egg in the brief interval between first sperm contact and fertilization. Although this process may be relevant for all sexually reproducing organisms, it is most easily examined in aquatic external fertilizers. When sperm are released into the sea, packets of seawater at the spatial scale relevant to single eggs might contain sperm from only one male, eliminating the potential for direct sperm competition and egg choice. Field experiments and a simple heuristic model examining the degree of sperm mixing for the sea urchin Strongylocentrotus franciscanus indicate that degree of competitive fertilization depends on density and distribution of competing males and the nature of this competition influences whether males with high- or low-affinity gamete
recognition protein genotypes have higher reproductive success. These results provide a potential explanation for the generation and maintenance of variation in gamete recognition proteins and why effectiveness of conspecific sperm precedence can be density-dependent.

物种内外的配子兼容性（gametic compatibility）演化及其作用有效性，取决于不同雄性的精子是否会直接竞争同一卵子，以及卵子是否存在选择机制。直接精子竞争与卵子选择的发生，取决于不同雄性的精子能否在精子首次接触卵子至受精之间的极短窗口期内抵达卵子位置。尽管这一过程对所有有性繁殖生物均具有研究价值，但在水生体外受精生物中最易开展相关研究。当精子被释放至海水中时，与单个卵子对应的空间尺度内的海水团块中，可能仅存在单一雄性的精子，这会消除直接精子竞争与卵子选择的可能性。针对红海胆（Strongylocentrotus franciscanus）的精子混合程度开展的野外实验与简易启发式模型研究显示，竞争性受精的程度取决于竞争雄性的种群密度与分布格局，而该竞争的本质则决定了携带高/低亲和性配子识别蛋白（gamete recognition protein）基因型的雄性能否获得更高的繁殖成功率。上述研究结果为配子识别蛋白变异的产生与维持机制，以及为何同种精子优先的作用有效性会依赖种群密度，提供了潜在的解释路径。