Quantifying episodes of sexual selection: insights from a transparent worm with fluorescent sperm

Sexual selection operates through consecutive episodes of selection that ultimately contribute to the observed variance in reproductive success between individuals. Understanding the relative importance of these episodes is challenging, particularly because the relevant post-copulatory fitness components are often difficult to assess. Here we investigate different episodes of sexual selection on the male sex function, by assessing how (pre-copulatory) mating success, and (post-copulatory) sperm-transfer efficiency and sperm-fertilising efficiency contribute to male reproductive success. Specifically, we used a transgenic line of the transparent flatworm, Macrostomum lignano, which expresses green fluorescent protein (GFP) in all cell types, including sperm cells, enabling in vivo sperm tracking and paternity analysis. We found that a large proportion of variance in male reproductive success arose from the post-copulatory episodes. Moreover, we also quantified selection differentials on ...