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 that 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.

配子相容性（gametic compatibility）的演化以及种内与种间的相容有效性，取决于不同雄性的精子是否会直接竞争一枚卵子，以及卵子是否拥有配偶选择权。直接精子竞争与卵子选择的发生，取决于不同雄性的精子是否会在首次接触卵子至受精完成之间的短暂时间窗内抵达卵子所在位置。尽管这一过程对所有有性生殖生物均具备潜在研究意义，但在水生体外受精生物（external fertilizers）中最易开展相关研究。当精子被释放至海水中时，与单枚卵子对应的空间尺度下的海水团块，可能仅包含单一雄性的精子，这将消除直接精子竞争与卵子选择的可能性。针对海胆（Strongylocentrotus franciscanus）开展的野外实验与检验精子混合程度的简单启发式模型研究表明：竞争性受精的程度取决于竞争雄性的种群密度与空间分布格局，且这类竞争的性质将决定携带高亲和力或低亲和力配子识别蛋白（gamete recognition protein）基因型的雄性是否具有更高的繁殖成功率。上述研究结果为配子识别蛋白变异的产生与维持机制，以及同种精子优先（conspecific sperm precedence）的有效性为何会依赖种群密度，提供了潜在的解释。