Briskie James V, Montgomerie Robert, Birkhead Tim R
Department of Biology, Queen's University, Kingston, Ontario, K7L 3N6, Canada.
Department of Animal and Plant Sciences, University of Sheffield, Sheffield, S10 2TN, U.K.
Evolution. 1997 Jun;51(3):937-945. doi: 10.1111/j.1558-5646.1997.tb03674.x.
Sperm size varies enormously among species, but the reasons for this variation remain obscure. Since it has been suggested that swimming velocity increases with sperm length, earlier studies proposed longer (and therefore faster) sperm are advantageous under conditions of intense sperm competition. Nonetheless, previous work has been equivocal, perhaps because the intensity of sperm competition was measured indirectly. DNA profiling now provides a more direct measure of the number of offspring sired by extrapair males, and thus a more direct method of assessing the potential for sperm competition. Using a sample of 21 species of passerine birds for which DNA profiling data were available, we found a positive relation between sperm length and the degree of extrapair paternity. A path analysis, however, revealed that this relationship arises only indirectly through the positive relationship between the rate of extrapair paternity and length of sperm storage tubules (SSTs) in the female. As sperm length is correlated positively with SST length, an increase in the intensity of sperm competition leads to an increase in sperm length only through its effect on SST length. Why females vary SST length with the intensity of sperm competition is not clear, but one possibility is that it increases female control over how sperm are used in fertilization. Males, in turn, may respond on an evolutionary time scale to changes in SST size by increasing sperm length to prevent displacement from rival sperm. Previous theoretical analyses predicting that sperm size should decrease as sperm competition becomes more intense were not supported by our findings. We suggest that future models of sperm-size evolution consider not only the role of sperm competition, but also how female control and manipulation of ejaculates after insemination selects for different sperm morphologies.
精子大小在不同物种间差异极大，但这种差异的原因仍不清楚。由于有人提出精子游动速度会随着精子长度增加，早期研究认为更长（因而更快）的精子在激烈精子竞争条件下具有优势。然而，此前的研究结果并不明确，可能是因为精子竞争强度是间接测量的。DNA 分析现在能更直接地测量配偶外交配雄性所产生后代的数量，从而提供一种更直接的方法来评估精子竞争的可能性。利用 21 种有 DNA 分析数据的雀形目鸟类样本，我们发现精子长度与配偶外交配亲子关系程度之间存在正相关。然而，路径分析表明，这种关系仅通过配偶外交配亲子关系率与雌性精子储存管（SST）长度之间的正相关间接产生。由于精子长度与 SST 长度呈正相关，精子竞争强度的增加仅通过其对 SST 长度的影响导致精子长度增加。雌性为何随精子竞争强度改变 SST 长度尚不清楚，但一种可能性是这增强了雌性对受精时精子使用方式的控制。反过来，雄性可能在进化时间尺度上通过增加精子长度来应对 SST 大小的变化，以防止被竞争对手的精子取代。我们的研究结果不支持先前预测随着精子竞争变得更激烈精子大小应减小的理论分析。我们建议未来精子大小进化模型不仅要考虑精子竞争的作用，还要考虑雌性在授精后对射精的控制和操纵如何选择不同的精子形态。
