Résumé
Sexual selection is considered a potent evolutionary force in all sexually
reproducing organisms, but direct tests in terms of experimental evolution
of sexual traits are still lacking for simultaneously hermaphroditic
animals. Here, we tested how evolution under enforced monogamy affected a
suite of reproductive traits (including testis area, sex allocation,
genital morphology, sperm morphology and mating behaviour) in the
outcrossing hermaphroditic flatworm Macrostomum lignano, using an assay
that also allowed the assessment of phenotypically plastic responses to
group size. The experiment comprised 32 independent selection lines that
evolved under either monogamy or polygamy for 20 generations. While we did
not observe an evolutionary shift in sex allocation, we detected effects
of the selection regime for two male morphological traits. Specifically,
worms evolving under enforced monogamy had a distinct shape of the male
copulatory organ and produced sperm with shorter appendages. Many traits
that did not evolve under enforced monogamy showed phenotypic plasticity
in response to group size. Notably, individuals that grew up in larger
groups had a more male-biased sex allocation and produced slightly longer
sperm than individuals raised in pairs. We conclude that, in this
flatworm, enforced monogamy induced moderate evolutionary but substantial
phenotypically plastic responses.