Data from: Stabilizing selection on sperm number revealed by artificial selection and experimental evolution

Sperm competition is taxonomically widespread in animals and is usually associated with large sperm production, being the number of sperm in the competing pool the prime predictor of fertilization success. Despite the strong postcopulatory selection acting directionally on sperm production, its genetic variance is often very high. This can be explained by trade-offs between sperm production and traits associated with mate acquisition or survival, that may contribute to generate an overall stabilizing selection. To investigate this hypothesis, we first artificially selected male guppies (Poecilia reticulata) for high and low sperm production for three generations, while simultaneously removing sexual selection. Then, we interrupted artificial selection and restored sexual selection. Sperm production responded to divergent selection in one generation, and when we restored sexual selection, both high and low lines converged back to the mean sperm production of the original population within two generations, indicating that sperm number is subject to strong stabilizing total sexual selection (i.e. selection acting simultaneously on all traits associated with reproductive success). We discuss the possible mechanisms responsible for the maintenance of high genetic variability in sperm production despite strong selection acting on it.

精子竞争（sperm competition）在动物界各分类类群中广泛存在，且通常与高精子产量相关，竞争池中的精子数量是预测受精成功的首要因素。尽管存在针对精子产量的强定向性交配后选择（postcopulatory selection），但其遗传方差往往仍处于较高水平。这一现象可通过精子产量与求偶获取或生存相关性状间的权衡来解释，这类权衡可能共同促成了整体的稳定选择。为验证这一假说，我们首先针对高、低精子产量对雄性孔雀鱼（Poecilia reticulata）开展了为期三代的人工选择，同时移除了性选择压力。随后，我们终止了人工选择并恢复了性选择。精子产量在单一代内即对歧向选择产生响应；而当我们恢复性选择后，高低两个选育系均在两代内回归至原始种群的平均精子产量水平，表明精子数量受到强烈的总性选择稳定作用，即同时作用于所有与繁殖成功相关性状的选择。我们还探讨了在针对精子产量的强选择作用下，其仍能维持较高遗传变异水平的潜在机制。