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

Abstract

Theory predicts that males have a limited amount of resources to invest inreproduction, suggesting a trade-off between traits that enhance mate acquisitionand those that enhance fertilization success. Here, we investigate therelationship between pre- and post-copulatory investment by comparing themating behaviour and reproductive morphology of four European and fiveNorth American populations of the dung fly Sepsis punctum (Diptera) thatdisplay a reversal of sexual size dimorphism (SSD). We show that the geographicreversal in SSD between the continents (male biased in Europe,female biased in North America) is accompanied by differential investmentin pre- vs. post-copulatory traits. We find higher remating rates in Europeanpopulations, where larger males acquire more matings and consequentlyhave evolved relatively larger testes and steeper hyper-allometry with bodysize. American populations, in sharp contrast, display much reduced, if any,effect of body size on those traits. Instead, North American males demonstratean increased investment in mate acquisition prior to copulation, withmore mounting attempts and a distinctive abdominal courtship display thatis completely absent in Europe. When controlling for body size, relativefemale spermathecal size is similar on both continents, so we find no directevidence for the co-evolution of male and female internal reproductivemorphology. By comparing allopatric populations of the same species thatapparently have evolved different mating systems and consequently SSD,we thus indirectly demonstrate differential investment in pre- vs. post-copulatorymechanisms increasing reproductive success.