Sex-limited experimental evolution, mating, and sexual conflict in a hermaphrodite

Sexual selection in simultaneous hermaphrodites can act through male–male competition and female choice, often favouring male-biased traits, yet remains understudied relative to gonochorists. We tested how sex-limited experimental evolution alters mating behaviour and the male copulatory organ (the stylet) in Macrostomum lignano. After more than 40 generations of selection for male- or female-specific fitness, we recorded mating behaviour and quantified stylet morphology. Mating frequency and duration did not differ significantly among lines, though female-selected lines tended to mate slightly less. In contrast, we detected pronounced divergence in post-copulatory ‘sucking’ by wild-type partners, in which a worm covers its female antrum with its mouth, attempting to remove the ejaculate. Partners sucked most after mating with male-selected worms, and least after mating with female-selected worms. The probability of sucking decreased with increasing mating duration, but increased with the number of matings. These patterns match predictions if sucking reflects sexual conflict over fertilization, expected to be especially salient in hermaphrodites. Finally, the stylet shape diverged among lines, indicating rapid evolution of the male organ under sexual selection. Our results demonstrate that sex-limited selection can reshape genital morphology and post-copulatory behaviour in a simultaneous hermaphrodite in this free-living flatworm model.