Résumé
Sex allocation theory is considered as a touchstone of evolutionary
biology, providing some of the best supported examples for Darwinian
adaptation. In particular, Hamilton’s local mate competition theory has
been shown to generate precise predictions for extraordinary sex ratios
observed in many separate-sexed organisms. In analogy to local mate
competition, Charnov’s mating group size model predicts how sex allocation
in simultaneous hermaphrodites is affected by the mating group size (i.e.,
the number of mating partners plus one). Until now studies have not
directly explored the relationship between mating group size and sex
allocation, which we here achieve in the simultaneously hermaphroditic
flatworm Macrostomum lignano. Using transgenic focal worms with ubiquitous
expression of green-fluorescent protein (GFP), we assessed the number of
wild-type mating partners carrying GFP-positive sperm from these focal
worms when raised in different social group sizes. This allowed us to test
directly how mating group size was related to the sex allocation of focal
worms. We find that the proportion of male investment initially increases
with increasing mating group size, but then saturates as predicted by
theory. To our knowledge, this is the first direct test of the mating
group size model in a simultaneously hermaphroditic animal.