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）在各类动物中广泛存在，通常与高精子产量相关，竞争池内的精子数量是预测受精成功率的核心指标。尽管强定向的交配后选择（postcopulatory selection）对精子产量发挥定向作用，但其遗传方差（genetic variance）往往维持在较高水平。这一现象可通过精子产量与配偶获取或生存相关性状间的权衡（trade-offs）得以解释，此类权衡或可催生整体稳定选择（stabilizing selection）。为验证该假说，我们首先对雄孔雀鱼（Poecilia reticulata）开展了三代高、低精子产量的人工选择实验，同时移除性选择（sexual selection）压力。随后中止人工选择并恢复性选择。实验结果显示，精子产量可在单一代内响应歧化选择（divergent selection）；当恢复性选择后，高低选择系均在两代内回归至原始种群的平均精子产量水平，这表明精子数量受到强烈的总性选择（total sexual selection，即同时作用于所有与繁殖成功相关性状的选择）约束。我们就尽管存在强选择作用却仍维持精子产量高遗传变异的潜在机制展开了讨论。