Data from: Differential investment in pre- versus post-copulatory sexual selection reinforces a cross-continental reversal of sexual size dimorphism in Sepsis punctum (Diptera: Sepsidae)

Theory predicts that males have a limited amount of resources to invest in reproduction, suggesting a trade-off between traits that enhance mate acquisition and those enhancing fertilization success. Here we investigate the relationship between pre- and post-copulatory investment by comparing the mating behavior and reproductive morphology of four European and five North American populations of the dung fly Sepsis punctum (Diptera) that display a reversal of sexual size dimorphism (SSD). We show that the geographic reversal in SSD between the continents (male-biased in Europe, female-biased in North America) is accompanied by differential investment in pre- versus post-copulatory traits. We find higher re-mating rates in European populations, where larger males acquire more matings and consequently have evolved relatively larger testes and steeper hyper-allometry with body size. American populations, in sharp contrast, display much reduced, if any, effect of body size on those traits. Instead, North American males demonstrate an increased investment in mate acquisition prior to copulation, with more mounting attempts and a distinctive abdominal courtship display that is completely absent in Europe. When controlling for body size, relative female spermathecal size is similar on both continents, so we find no direct evidence for the co-evolution of male and female internal reproductive morphology. By comparing allopatric populations of the same species that apparently have evolved different mating systems and consequently SSD, we thus indirectly demonstrate differential investment in pre- vs. post-copulatory mechanisms increasing reproductive success.

理论预测雄性可投入繁殖的资源总量有限，这提示在提升配偶获取能力的性状与提升受精成功率的性状之间存在进化权衡。本研究以展现性大小二型性（sexual size dimorphism, SSD）反转现象的斑粪蝇（Sepsis punctum，双翅目）为研究对象，通过对比其4个欧洲种群与5个北美种群的交配行为及生殖形态，探究了交配前与交配后繁殖投入之间的关联。研究发现，两大洲间SSD的地理反转（欧洲种群为雄性偏态，北美种群为雌性偏态），伴随着交配前与交配后性状的差异化投入。欧洲种群的再次交配率更高，在此类种群中，体型更大的雄性可获得更多交配机会，因此演化出相对更大的睾丸，且睾丸大小随体型增长呈现更显著的超异速生长模式。与之形成鲜明对比的是，北美种群中体型对上述性状的影响大幅减弱，甚至近乎无影响。取而代之的是，北美雄性在交配前的配偶获取投入有所增加，表现出更多的攀附尝试，以及一种欧洲种群完全缺失的独特腹部求偶展示。在控制体型变量后，两大洲雌性的相对受精囊大小并无显著差异，因此未发现雄性与雌性内部生殖形态协同演化的直接证据。本研究通过对比同一物种的异域种群——这些种群显然已演化出不同的交配系统，并因此形成了差异化的SSD——间接证实了，提升繁殖成功率的交配前与交配后繁殖机制存在差异化投入。