Data from: The maintenance of sperm variability: context-dependent selection on sperm morphology in a broadcast spawning invertebrate

Why are sperm so variable despite having a singular, critical function and an intimate relationship with fitness? A key to understanding the evolution of sperm morphology is identifying which traits enable sperm to be successful fertilizers. Several sperm traits (e.g. tail length, overall size) are implicated in sperm performance, but the benefits of these traits are likely to be highly context-dependent. Here, we examined phenotypic selection on sperm morphology of a broadcast spawning tube worm (Galeolaria gemineoa). We conducted laboratory experiments to measure the relationship between average sperm morphology and relative fertilization success across a range of sperm environments that were designed to approximate the range of sperm concentrations and ages encountered by eggs in nature. We found that the strength and form of multivariate selection varied substantially across our environmental gradients. Sperm with long tails and small heads were favored in high-concentration environments, whereas sperm with long heads were favored at low concentrations and old ages. We suggest variation in the local fertilization environment and resulting differences in selection can preserve variability in sperm morphology both within and among males.

尽管精子仅承担单一且至关重要的生理功能，并与繁殖适合度（fitness）紧密相关，但其形态为何存在如此显著的变异？解析精子形态演化的关键，在于明确哪些精子性状可赋予其成功完成受精的能力。已有多项研究表明，精子的诸多性状（如尾长、整体体型）与精子功能表现相关，但这些性状带来的收益很可能具有极强的情境依赖性。本研究以广幅产卵的管虫（*Galeolaria gemineoa*）为对象，分析了其精子形态所受的表型选择（phenotypic selection）压力。我们通过实验室实验，在一系列模拟自然环境中卵子实际遭遇的精子浓度与精子老化程度梯度的实验条件下，测定了平均精子形态与相对受精成功率之间的关联。研究发现，多变量选择（multivariate selection）的强度与模式在我们设置的环境梯度中存在显著差异。在高精子浓度环境中，尾长较长、头部较小的精子更具竞争优势；而在低浓度环境与精子老化程度较高的条件下，头部较长的精子更受选择青睐。我们认为，局部受精环境的异质性以及由此产生的选择差异，能够维持雄性个体内部以及不同雄性之间的精子形态变异。