Data from: The Azores Bullfinch (Pyrrhula murina) has the same unusual and size-variable sperm morphology as the Eurasian Bullfinch (Pyrrhula pyrrhula)

The Azores bullfinch is endemic to the island of São Miguel in the Azores archipelago and the sister species to the Eurasian bullfinch. Here we show that the spermatozoa of the two species have similar ultrastructure and gross morphology. Thus, the unusual and supposedly neotenous sperm morphology previously described for the Eurasian bullfinch appears to be an ancestral trait that evolved before the two taxa diverged. In addition, the coefficients of variation in total sperm length, both within and among males, were high in both species and exceed any previously published values for free-living passerines. Such high sperm-size variation is typically found in species with relaxed sperm competition. However, the high variance in mean sperm length among Azores bullfinches is surprising, because the trait has high heritability and this small, insular population shows clear signs of reduced genetic diversity at neutral loci. A possible explanation for this apparent contradiction is that the Azores bullfinch has retained more diversity at functional and fitness-related loci than at more neutral parts of the genome. Finally, we also present data on relative testis size and sperm swimming speed for the Eurasian bullfinch, and discuss the hypothesis that the small and putatively neotenous sperm in bullfinches has evolved in response to lack of sperm competition.

亚速尔红腹灰雀（Azores bullfinch）为亚速尔群岛圣米格尔岛特有物种，且与欧亚红腹灰雀（Eurasian bullfinch）互为姊妹物种。本研究表明，两个物种的精子（spermatozoa）具有相似的超微结构与整体形态。因此，此前针对欧亚红腹灰雀所报道的、罕见且被认为属于幼态持续（neotenous）的精子形态，实则为两个类群分化前便已演化形成的祖先性状。此外，两个物种的雄性个体内部及个体间的精子总长度变异系数（coefficients of variation）均处于较高水平，远超此前已发表的自由生活雀形目鸟类（passerines）的相关数值。这类高精子尺寸变异通常见于精子竞争（sperm competition）压力减弱的物种中。但亚速尔红腹灰雀种群中平均精子长度的高变异却令人费解：该性状具有较高的遗传力（heritability），且这一小型岛屿种群在中性基因座（neutral loci）上已表现出明确的遗传多样性降低迹象。针对这一表观矛盾，一种可能的解释是：相较于基因组（genome）中更多中性区域，亚速尔红腹灰雀在功能位点及适合度相关位点上保留了更多的遗传多样性。最后，本研究还提供了欧亚红腹灰雀的相对睾丸大小与精子游动速度相关数据，并针对红腹灰雀的小型且被认为属于幼态持续的精子是因缺乏精子竞争压力而演化形成的这一假说展开讨论。