Zoology Museum, University of Zürich, Zürich, Switzerland.
Heredity (Edinb). 2010 Jan;104(1):61-6. doi: 10.1038/hdy.2009.93.
Sperm length shows considerable phenotypic variation both inter- and intra-specifically, but a general explanation for this variation is lacking. In addition, our understanding of the genetic variation underlying sperm length variation is also limited because there have been few studies on the genetics of sperm size. One factor that could explain the variation in sperm length is that length influences sperm competitiveness, and there is some evidence for this. However, in yellow dung flies (Scathophaga stercoraria), microevolutionary responses to experimental variation at levels of sperm competition indicate that sperm length does not influence sperm competitiveness, although this lack of response may simply indicate sperm length lacks evolutionary potential (that is, it is constrained in some way), in spite of evidence that sperm length is heritable. Here we report on a laboratory study, in which we artificially selected upwards and downwards on sperm length in S. stercoraria. We found that sperm length significantly diverged after four generations of selection, but the response to selection was asymmetrical: upward selection generated a rapid response, but downward did not. We estimated the realized heritability of sperm length to be approximately 50%, which is consistent with previous sire-son estimates. We also assessed the fertility of males from upward and downward lines and found they did not differ. Results are discussed in the context of sperm competition.
精子长度在种内和种间都表现出相当大的表型变异，但缺乏对此变异的一般解释。此外，我们对精子长度变异的遗传基础的理解也很有限，因为很少有关于精子大小的遗传学研究。一种可以解释精子长度变异的因素是，长度影响精子竞争力，并且有一些证据支持这一点。然而，在黄粪蝇（Scathophaga stercoraria）中，对精子竞争水平的实验变异的微进化反应表明，精子长度并不影响精子竞争力，尽管这种缺乏反应可能仅仅表明精子长度缺乏进化潜力（即它在某种程度上受到限制），尽管有证据表明精子长度是可遗传的。在这里，我们报告了一项实验室研究，在这项研究中，我们在 S. stercoraria 中人为地向上和向下选择精子长度。我们发现，经过四代的选择，精子长度显著分化，但选择的反应是不对称的：向上选择产生了快速的反应，但向下选择没有。我们估计精子长度的实际遗传力约为 50%，这与之前的父子估计值一致。我们还评估了向上和向下选择系的雄性的生育能力，发现它们没有差异。结果在精子竞争的背景下进行了讨论。
