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（sexual selection），不过针对该假说的明确检验仍较为有限。精子（spermatozoa）在大小与形态上存在显著变异，已有研究表明精子形态（sperm morphology，即精子长度与形态）与精子竞争风险（sperm competition risk）或雌性生殖道形态（female reproductive tract morphology）存在相关性。然而，交配后过程（postcopulatory processes，例如精子竞争（sperm competition）与隐性雌性选择（cryptic female choice））是否会影响精子形态的进化分化速率，目前仍不明确。本研究以雀形目鸟类（passerine birds）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（relative testes mass）估算）发生变化。研究结果显示，相对睾丸质量与精子中段（sperm midpiece）、鞭毛（flagellum）长度以及精子总长度的快速表型分化呈显著正相关。与之相反，相对睾丸质量与精子头部大小的进化分化速率无显著关联，模型分析表明头部长度受到进化约束。本研究结果首次揭示了精子竞争强度与精子进化速率之间的关联，并表明交配后性选择可推动精子形态的快速进化分化。