Résumé
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 ten morphological traits. Testis size and seminal vesicle
size showed significant positive selection differentials, which were
mainly due to selection on sperm-transfer efficiency. Overall, our results
demonstrate that male reproductive success in M. lignano is not primarily
limited by the number of matings achieved, but rather by the ability to
convert matings into successful fertilisations, which is facilitated by
producing many sperm.