Data from: Early diversification of sperm size in the evolutionary history of the old world leaf warblers (Phylloscopidae)

Sperm morphological traits are highly variable among species and are commonly thought to evolve by post-copulatory sexual selection. However, little is known about the evolutionary dynamics of sperm morphology, and whether rates of evolutionary change are variable over time and among taxonomic groups. Here, we examine sperm morphology from 21 species of Old World leaf warblers (Phylloscopidae), a group of generally dull, sexually monochromatic birds, which are known to have high levels of extra-pair paternity. We found that sperm length differs markedly across species, spanning about 40% of the range observed across a larger selection of passerine birds. Furthermore, we found strong support for an ‘early-burst’ model of trait evolution, implying that the majority of divergence in sperm length has occurred early in the evolutionary history of this clade with subsequent evolutionary stasis. This large early divergence matches the early divergence reported in ecological traits (i.e. body size and feeding behaviour). Our findings demonstrate that rates of evolution in sperm morphology can change over time in passerine taxa, and that evolutionary stasis in sperm traits can occur even in species exhibiting characteristics consistent with moderate-to-high levels of sperm competition. It remains a major challenge to identify the selection mechanisms and possible constraints responsible for these variable rates of sperm evolution.不同物种间的精子形态性状存在高度变异，学界普遍认为其演化由交配后性选择所驱动。然而，学界对精子形态的演化动态，以及演化变化速率是否随时间和不同类群而存在差异，仍知之甚少。本研究针对21种旧大陆柳莺（柳莺科，Phylloscopidae）的精子形态展开分析，该类群鸟类通常羽色黯淡、呈性单态，且已知存在较高水平的配偶外父权现象。研究发现，不同物种的精子长度存在显著差异，其跨度约为更大范围雀形目鸟类精子长度变异范围的40%。此外，本研究为性状演化的“早期爆发”模型提供了强有力的支持，这意味着该演化支的精子长度差异大多产生于其演化历史的早期阶段，后续则进入演化停滞状态。这种显著的早期分化与生态性状（即体型大小与取食行为）中所报道的早期分化模式相一致。本研究结果表明，雀形目类群的精子形态演化速率可随时间发生变化，且即使在表现出中等至高强度精子竞争特征的物种中，精子性状也可能出现演化停滞。目前，厘清驱动精子演化速率存在差异的选择机制与潜在限制因素，仍是一项重大挑战。