Bjork Adam, Pitnick Scott
Department of Biology, Syracuse University, Syracuse, New York 13244-1270, USA.
Nature. 2006 Jun 8;441(7094):742-5. doi: 10.1038/nature04683.
Research into the evolution of giant sperm has uncovered a paradox within the foundations of sexual selection theory. Postcopulatory sexual selection on males (that is, sperm competition and cryptic female choice) can lead to decreased sperm numbers by favouring the production of larger sperm. However, a decline in sperm numbers is predicted to weaken selection on males and increase selection on females. As isogamy is approached (that is, as investment per gamete by males approaches that by females), sperm become less abundant, ova become relatively less rare, and competition between males for fertilization success is predicted to weaken. Sexual selection for longer sperm, therefore, is expected to be self limiting. Here we examine this paradox in Drosophila along the anisogamy-isogamy continuum using intraspecific experimental evolution techniques and interspecific comparative techniques. Our results confirm the big-sperm paradox by showing that the sex difference in sexual selection gradients decreases as sperm size increases. However, a resolution to the paradox is provided when this finding is interpreted in concert with the 'opportunity for selection' and the 'opportunity for sexual selection'. Furthermore, we show that most of the variation in measures of selection intensity is explained by sperm length and relative investment in sperm production.
对巨型精子进化的研究揭示了性选择理论基础中的一个悖论。雄性的交配后性选择（即精子竞争和雌性隐秘选择）会通过青睐更大精子的产生而导致精子数量减少。然而，精子数量的减少预计会削弱对雄性的选择，并增加对雌性的选择。随着同配生殖的临近（即雄性每个配子的投入接近雌性），精子变得不那么丰富，卵子相对不那么稀少，预计雄性之间为受精成功而进行的竞争会减弱。因此，对更长精子的性选择预计会自我限制。在这里，我们使用种内实验进化技术和种间比较技术，在果蝇中沿着异配生殖 - 同配生殖连续统研究这个悖论。我们的结果通过表明随着精子大小增加，性选择梯度的性别差异减小，证实了大精子悖论。然而，当这一发现与“选择机会”和“性选择机会”一起解释时，就为这个悖论提供了一个解决方案。此外，我们表明选择强度测量中的大部分变异是由精子长度和精子产生的相对投入所解释的。
