Selection for longer-lived sperm within ejaculate reduces reproductive ageing in offspring

Males produce numerous sperm in a single ejaculate that greatly outnumber their potential egg targets. Recent studies found that phenotypic and genotypic variation among sperm in a single ejaculate of a male affects the fitness and performance of the resulting offspring. Specifically, within-ejaculate sperm selection for sperm longevity increased the performance of the resulting offspring in several key life-history traits in early-life. Because increased early-life reproductive performance often correlates with rapid ageing, it is possible that within-ejaculate sperm selection increases early-life fitness at the cost of accelerated senescence. Alternatively, within-ejaculate sperm selection could improve offspring quality throughout the life cycle, including reduced age-specific deterioration. We tested the two alternative hypotheses in an experimental setup using zebrafish Danio rerio. We found that within-ejaculate sperm selection for sperm longevity reduced age-specific deterioration of fecundity and offspring survival but had no effect on fertilization success in males. Remarkably, we found an opposing effect of within-ejaculate sperm selection on female fecundity, where selection for sperm longevity resulted in increased early-life performance followed by a slow decline, while females sired by unselected sperm started low but increased their fecundity with age. Intriguingly, within-ejaculate sperm selection also reduced the age-specific decline in fertilization success in females, suggesting that selection for sperm longevity improves at least some aspects of female reproductive ageing. These results demonstrate that within-ejaculate variation in sperm phenotype contributes to individual variation in animal life histories in the two sexes and may have important implications for assisted fertilization programs in livestock and humans.

雄性单次射精所产生的精子数量远多于其潜在的卵子结合对象。近期研究表明，雄性单次射精产出的精子之间存在表型变异（phenotypic variation）与基因型变异（genotypic variation），这类变异会影响子代的适合度（fitness）与发育表现。具体而言，针对精子寿命（sperm longevity）的射精内精子选择（within-ejaculate sperm selection），可提升子代在早期阶段的多项关键生活史性状（life-history trait）表现。由于早期生殖性能提升通常与衰老加速相关，因此射精内精子选择有可能以加速衰老为代价，提升子代的早期适合度。另一种可能性是，射精内精子选择可在整个生命周期中提升子代品质，包括降低年龄特异性衰退（age-specific deterioration）。
我们以斑马鱼（Danio rerio）为实验对象，对这两种对立假说进行了验证。研究发现，针对精子寿命的射精内精子选择可降低雄性繁殖力（fecundity）与子代存活率的年龄特异性衰退，但对雄性的受精成功率（fertilization success）无显著影响。值得注意的是，射精内精子选择对雌性繁殖力的影响恰好相反：经精子寿命选择的子代雌性，其早期繁殖性能先提升后缓慢下降；而由未经过选择的精子所产生的雌性，初始繁殖力较低，但会随年龄增长而提升。有趣的是，射精内精子选择还降低了雌性受精成功率的年龄特异性衰退，这表明针对精子寿命的选择，至少可改善雌性生殖衰老的部分表现。
本研究结果表明，精子表型的射精内变异会导致雌雄个体间生活史性状的差异，该发现或许对家畜与人类的辅助受精项目具有重要的应用价值。