Natural History Museum, University of Oslo, 0318 Oslo, Norway.
Proc Biol Sci. 2013 Jan 2;280(1753):20122616. doi: 10.1098/rspb.2012.2616. Print 2013 Feb 22.
Spermatozoa exhibit considerable interspecific variability in size and shape. Our understanding of the adaptive significance of this diversity, however, remains limited. Determining how variation in sperm structure translates into variation in sperm performance will contribute to our understanding of the evolutionary diversification of sperm form. Here, using data from passerine birds, we test the hypothesis that longer sperm swim faster because they have more available energy. We found that sperm with longer midpieces have higher levels of intracellular adenosine triphosphate (ATP), but that greater energy reserves do not translate into faster-swimming sperm. Additionally, we found that interspecific variation in sperm ATP concentration is not associated with the level of sperm competition faced by males. Finally, using Bayesian methods, we compared the evolutionary trajectories of sperm morphology and ATP content, and show that both traits have undergone directional evolutionary change. However, in contrast to recent suggestions in other taxa, we show that changes in ATP are unlikely to have preceded changes in morphology in passerine sperm. These results suggest that variable selective pressures are likely to have driven the evolution of sperm traits in different taxa, and highlight fundamental biological differences between taxa with internal and external fertilization, as well as those with and without sperm storage.
精子在大小和形状上表现出相当大的种间变异性。然而，我们对这种多样性的适应意义的理解仍然有限。确定精子结构的变化如何转化为精子性能的变化，将有助于我们理解精子形态的进化多样化。在这里，我们使用雀形目鸟类的数据来检验这样一个假设，即较长的精子游得更快，因为它们有更多的可用能量。我们发现，中段较长的精子具有更高水平的细胞内三磷酸腺苷（ATP），但更多的能量储备并不意味着游泳速度更快的精子。此外，我们还发现，精子中 ATP 浓度的种间变异与雄性面临的精子竞争水平无关。最后，我们使用贝叶斯方法比较了精子形态和 ATP 含量的进化轨迹，并表明这两个特征都经历了定向进化变化。然而，与其他分类群中最近的建议相反，我们表明，在雀形目精子中，ATP 的变化不太可能先于形态的变化。这些结果表明，可变的选择压力可能导致了不同分类群精子特征的进化，并突出了内部受精和外部受精、有和没有精子储存的分类群之间的基本生物学差异。
