Developmental Environment Effects on Sexual Selection in Male and Female Drosophila melanogaster

The developmental environment can potentially alter the adult social environment and influence traits targeted by sexual selection such as body size. In this study, we manipulated larval density in male and female Drosophila melanogaster, which results in distinct adult size phenotypes–high (low) densities for small (large) adults–and measured sexual selection in experimental groups consisting of adult males and females from high, low, or a mixture of low and high larval densities. Overall, large adult females (those reared at low larval density) had more matings, more mates and produced more offspring than small females (those reared at high larval density). The number of offspring produced by females was positively associated with their number of mates (i.e. there was a positive female Bateman gradient) in social groups where female size was experimentally varied, likely due to the covariance between female productivity and mating rate. For males, we found evidence that the larval environment affected the relative importance of sexual selection via mate number (Bateman gradients), mate productivity, paternity share, and their covariances. Mate number and mate productivity were significantly reduced for small males in social environments where males were of mixed sizes, versus social environments where all males were small, suggesting that social heterogeneity altered selection on this subset of males. Males are commonly assumed to benefit from mating with large females, but in contrast to expectations we found that in groups where both the male and female size varied, males did not gain more offspring per mating with large females. Collectively, our results indicate sex-specific effects of the developmental environment on the operation of sexual selection, via both the phenotype of individuals, and the phenotype of their competitors and mates.发育环境可潜在改变成年个体的社会环境，并影响性选择所靶向的诸多性状，例如体型。本研究中，我们对黑腹果蝇（Drosophila melanogaster）雌雄个体的幼虫密度进行了操控，由此得到了差异化的成虫体型表型——幼虫密度高（低）对应小型（大型）成虫——并在由高、低幼虫密度，或高低混合幼虫密度处理组的雌雄成虫组成的实验组中，对性选择进行了量化。总体而言，与小型雌性（由高幼虫密度培育而来）相比，大型雌性（由低幼虫密度培育而来）的交配次数更多、交配对象更多，且产下的后代数量也更多。在雌性体型经实验操控的社会群组中，雌性产下的后代数量与其交配对象数量呈正相关（即存在正向雌性贝特曼梯度（Bateman gradient）），这可能源于雌性繁殖力与交配率之间的协方差。对于雄性而言，我们发现发育环境会通过交配对象数量（贝特曼梯度（Bateman gradient））、交配繁殖成效、父权份额及其协方差，影响性选择的相对重要性。相较于所有雄性体型均一致的社会环境，在雄性体型混合的社会环境中，小型雄性的交配对象数量与交配繁殖成效均显著降低，这表明社会异质性会改变针对该类雄性亚群的选择压力。学界通常认为雄性与大型雌性交配可获得演化收益，但与预期相悖的是，我们在雌雄体型均存在差异的群组中发现，雄性与大型雌性交配后并未获得更多的每交配后代数。综上，本研究结果表明，发育环境会通过个体自身的表型以及其竞争者与交配对象的表型，对性选择的运作产生性别特异性的影响。