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.

两性在繁殖过程中的差异，本质上由精子与卵子的固有区别所奠定。针对雄性，现有理论认为，广泛存在的精子竞争（sperm competition）会催生针对精子数量投入的选择压力，该压力通过在睾丸有限的精子发生（spermatogenesis）资源中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、低精子竞争强度条件下对重复实验品系开展了超过77代的实验选择（experimental selection），借此实证探究精子竞争对精子体型的塑造作用。经过上述实验进化（experimental evolution）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，其射精液中的精子所获得的父权比例，较低竞争强度组的雄性高出20%。尽管经过该实验进化，雄性在精子生产的相对投入上未发生改变——高、低竞争强度组的睾丸尺寸并无显著差异，但来自高精子竞争强度组的更具竞争力的雄性，其精子体型已显著变长；且在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。为探明此类精子伸长是否存在演化成本，我们开展了饮食限制（dietary restriction）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演化成本的繁殖性状，但精子竞争强度可对其施加正向定向选择（directional selection）压力。