Data from: Experimental evolution reveals that sperm competition intensity selects for longer, more costly sperm

It is the differences between sperm and eggs that fundamentally underpin the differences between the sexes within reproduction. For males, it is theorized that widespread sperm competition leads to selection for investment in sperm numbers, achieved by minimizing sperm size within limited resources for spermatogenesis in the testis. Here, we empirically examine how sperm competition shapes sperm size, after more than 77 generations of experimental selection of replicate lines under either high or low sperm competition intensities in the promiscuous flour beetle Tribolium castaneum. After this experimental evolution, populations had diverged significantly in their sperm competitiveness, with sperm in ejaculates from males evolving under high sperm competition intensities gaining 20% greater paternity than sperm in ejaculates from males that had evolved under low sperm competition intensity. Males did not change their relative investment into sperm production following this experimental evolution, showing no difference in testis sizes between high and low intensity regimes. However, the more competitive males from high sperm competition intensity regimes had evolved significantly longer sperm and, across six independently selected lines, there was a significant association between the degree of divergence in sperm length and average sperm competitiveness. To determine whether such sperm elongation is costly, we used dietary restriction experiments, and revealed that protein-restricted males produced significantly shorter sperm. Our findings therefore demonstrate that sperm competition intensity can exert positive directional selection on sperm size, despite this being a costly reproductive trait.繁殖过程中两性差异的根本基础，在于精子与卵子之间的差异。从理论上来说，雄性面临的广泛精子竞争会催生针对精子数量投入的选择压力，而这种选择可通过在睾丸有限的精子发生资源限制下，最小化精子体型来实现。本研究以混杂交配的赤拟谷盗（*Tribolium castaneum*）为实验对象，在高、低两种精子竞争强度下对重复品系开展了超过77代的实验选择，随后实证探究了精子竞争对精子体型的塑造作用。经过该实验进化过程后，不同种群的精子竞争力已出现显著分化：在高精子竞争强度下演化的雄性，其射精中的精子所获得的父本贡献率，比低竞争强度下演化的雄性高出20%。雄性在此次实验进化后，其精子生产的相对投入未发生改变，高、低强度组的睾丸尺寸无显著差异。然而，源自高精子竞争强度组的更具竞争力的雄性，其精子长度已显著变长；在6个独立选择的品系中，精子长度的分化程度与平均精子竞争力之间存在显著关联。为明确此类精子伸长是否存在进化成本，我们开展了饮食限制实验，结果显示蛋白质摄入受限的雄性所产生的精子显著更短。综上，本研究结果表明，尽管精子体型是一项存在进化代价的生殖性状，但精子竞争强度可对其施加正向定向选择。