Data from: Postcopulatory sexual selection is associated with accelerated evolution of sperm morphology

Rapid diversification of sexual traits is frequently attributed to sexual selection, though explicit tests of this hypothesis remain limited. Spermatozoa exhibit remarkable variability in size and shape, and studies report a correlation between sperm morphology (sperm length and shape) and sperm competition risk or female reproductive tract morphology. However, whether postcopulatory processes (e.g. sperm competition and cryptic female choice) influence the speed of evolutionary diversification in sperm form is unknown. Using passerine birds, we quantified evolutionary rates of sperm length divergence among lineages (i.e. species pairs) and determined whether these rates varied with the level of sperm competition (estimated as relative testes mass). We found that relative testes mass was significantly and positively associated with more rapid phenotypic divergence in sperm midpiece and flagellum lengths, as well as total sperm length. In contrast, there was no association between relative testes mass and rates of evolutionary divergence in sperm head size, and models suggested that head length is evolutionarily constrained. Our results are the first to show an association between the strength of sperm competition and the speed of sperm evolution, and suggest that postcopulatory sexual selection promotes rapid evolutionary diversification of sperm morphology.

性征的快速多样化通常被归因于性选择，不过针对这一假说的直接检验仍较为有限。精子在大小与形态上展现出显著的变异，已有研究表明精子形态（包括精子长度与形态特征）与精子竞争风险或雌性生殖道形态存在相关性。然而，交配后过程（如精子竞争与隐蔽雌性选择）是否会影响精子形态的进化多样化速率，目前仍不得而知。本研究以雀形目鸟类为研究对象，量化了不同谱系（即物种对）间精子长度分化的进化速率，并检验了这些速率是否随精子竞争水平（以相对睾丸质量估算）发生变化。研究结果显示，相对睾丸质量与精子中段、鞭毛长度以及总精子长度的表型分化速率显著正相关。与之相反，相对睾丸质量与精子头部大小的进化分化速率并无关联，模型分析表明头部长度受进化约束。本研究首次揭示了精子竞争强度与精子进化速率之间存在关联，并表明交配后性选择可促进精子形态的快速进化多样化。