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.

为何精子仅具备单一且至关重要的功能，且与繁殖适合度紧密相关，但其形态却存在如此显著的变异？解析精子形态演化的核心要点，在于明确哪些性状可助力精子成功完成受精过程。已有多项精子性状（如尾部长度、整体体型）被证实与精子性能相关，但这些性状带来的收益极有可能高度依赖于环境背景。本研究针对一种体外排精型管虫（Galeolaria gemineoa）的精子形态开展了表型选择（phenotypic selection）分析，通过实验室实验在一系列模拟自然环境中卵子所遭遇的精子浓度与老化程度梯度的精子环境下，测量了平均精子形态与相对受精成功率之间的关联。研究发现，多变量选择的强度与形式在不同环境梯度下存在显著差异：在高浓度精子环境中，尾部较长、头部较小的精子更具竞争优势；而在低浓度精子环境与精子老化环境中，头部较长的精子更受青睐。我们认为，局部受精环境的变异以及由此产生的选择差异，能够维持雄性个体内部及不同雄性之间的精子形态多样性。