Data from: Unravelling anisogamy: egg size and ejaculate size mediate selection on morphology in free-swimming sperm

Gamete dimorphism (anisogamy) defines the sexes in most multicellular organisms. Theoretical explanations for its maintenance usually emphasise the size-related selection pressures of sperm competition and zygote survival, assuming that fertilisation of all eggs precludes selection for phenotypes that enhance fertility. In external fertilisers, however, fertilisation is often incomplete due to sperm limitation and the risk of polyspermy weakens the advantage of high sperm numbers that is predicted to limit sperm size, allowing alternative selection pressures to target free-swimming sperm. We asked whether egg size and ejaculate size mediate selection on the free-swimming sperm of Galeolaria caespitosa¬, a marine tubeworm with external fertilization, by comparing relationships between sperm morphology and male fertility across manipulations of egg size and sperm density. Our results suggest that selection pressures exerted by these factors may aid the maintenance of anisogamy in external fertilisers by limiting the adaptive value of larger sperm in the absence of competition. In doing so, our study offers a more complete explanation for the stability of anisogamy across the range of sperm environments typical of this mating system, and identifies new potential for the sexes to coevolve via mutual selection pressures exerted by gametes at fertilisation.配子异型（anisogamy，异配生殖）定义了多数多细胞生物的性别。针对其维持机制的理论解释通常侧重精子竞争与合子存活相关的体型选择压力，且默认所有卵子完成受精后，会阻碍对提升生育力的表型的选择。然而在体外受精类群中，受精常因精子限制而无法完全完成，且多精受精的风险会削弱高精子数量所带来的优势——该优势本应限制精子体型，使得其他选择压力可作用于自由游动的精子。本研究以具有体外受精模式的海洋管栖蠕虫*Galeolaria caespitosa*为研究对象，通过操控卵子大小与精子密度，比较精子形态与雄性生育力之间的关联，以此探究卵子大小和射精量是否对其自由游动精子施加选择压力。研究结果表明，在无竞争的情境下，这些因素所施加的选择压力可通过限制大型精子的适应价值，助力体外受精类群中异配生殖的维持。据此，本研究针对该交配系统典型精子环境范围内的异配生殖稳定性提供了更为完整的解释，并揭示了两性可通过受精时配子相互施加的选择压力实现协同进化的新可能。