Data from: Extensive variation in sperm morphology in a frog with no sperm competition

Background: Recent comparative studies of several taxa have found that within-species variation in sperm size decreases with increasing levels of sperm competition, suggesting that male-male gamete competition selects for an optimal sperm phenotype. Previous studies of intraspecific sperm length variation have all involved internal fertilizers where some other factors—e.g., sperm storage and sperm movement along the walls of the female’s reproductive tract—probably also influence and reduce sperm size variation. Thus external fertilizers, where those factors are absent, might be expected to exhibit even more variation when there is little or no sperm competition. To test that idea, we studied the sperm morphology of a North American chorus frog, the spring peeper (Pseudacris crucifer), a species in which males encounter little or no sperm competition. Results: As expected, sperm size was highly variable in the spring peeper, largely due to variation in flagellum length within and among individual males, among populations and between mitochondrial lineages in southwestern Ontario. In addition, a large proportion of spermatozoa in all males was abnormal in such a way that the ability of abnormal spermatozoa to fertilize was probably compromised. There were no differences in the frequencies of abnormalities among populations or mitochondrial lineages. Conclusions: In the absence of sperm competition, we suggest that genetic drift has probably played a role in the generation of diversity in sperm morphology in this species, potentially resulting in the observed differences among populations. Such interpopulation difference in sperm morphology might be expected to increase the degree of reproductive isolation between populations even before other isolating mechanisms evolve.

背景：近期多项针对不同类群（taxa）的比较研究显示，精子大小的种内变异程度随精子竞争（sperm competition）水平升高而降低，这表明雄配子间的竞争会筛选出最优的精子表型（sperm phenotype）。此前针对种内精子长度变异的研究均聚焦于体内受精（internal fertilizers）物种，这类研究中存在其他潜在影响因素——例如精子储存过程以及精子沿雌性生殖道壁移动的过程——这些因素同样可能影响并降低精子大小的变异程度。因此，在不存在上述因素的体外受精（external fertilizers）物种中，若精子竞争程度极低甚至完全缺失，其精子大小的变异程度可能会更高。为验证这一假说，本研究针对北美雨蛙类群中的春鸣拟蝗蛙（Pseudacris crucifer）开展了精子形态学（sperm morphology）分析，该物种的雄性个体几乎不存在精子竞争。
结果：正如预期，春鸣拟蝗蛙的精子大小存在显著变异，这种变异主要源于雄性个体内、个体间以及加拿大安大略省西南部不同种群和线粒体谱系（mitochondrial lineages）间的鞭毛（flagellum）长度差异。此外，所有雄性个体的精子中均有较大比例存在形态异常，这类异常精子的受精能力大概率已受到损害。不同种群及线粒体谱系间的异常精子频率并无显著差异。
结论：在无精子竞争的情况下，我们推测遗传漂变（genetic drift）可能在该物种精子形态多样性的产生过程中发挥了作用，这或许可以解释观测到的种群间差异。这类精子形态的种群间差异，甚至可能在其他生殖隔离（reproductive isolation）机制演化形成之前，就进一步加剧种群间的生殖隔离程度。