Asymmetric evolutionary responses to sex-specific selection in a hermaphrodite

NICOLÁS BONEL; ELSA NOËL; TIM JANICKE; KEVIN SARTORI; ELODIE CHAPUIS; ADELINE SÉGARD; STEFANIA MECONCELLI; BENJAMIN PELISSIÉ; VIOLETTE SARDA; PATRICE DAVID

Paris

Workshop; Simultaneously Hermaphroditic Organisms Workshop; 2018

Universidad de Paris 13

Sex allocation theory predicts that simultaneous hermaphrodites evolve to an evolutionary stable resource allocation, whereby any increase in investment to male reproduction incurs a disproportionate cost on female reproduction and vice-versa. However, empirical evidence for the evolution of sex allocation and sexual tradeoffs in hermaphroditic animals is still surprisingly limited. Here, we tested how male and female reproductive traits evolved under conditions of reduced opportunity for selection on either the male or the female sex function for 40 generations in the hermaphrodite freshwater snail Physa acuta (Basommatophora). The tested selection regimes favor a reinvestment of resources from the sex function under relaxed selection towards the other function. We found no such evolutionary response. Instead, we observed a strong decrease in juvenile survival and male reproductive success in lines where selection on the male function (including sexual selection) was relaxed but no significant response to relaxation of selection on the female function. This is consistent with standing variation being dominated by a load of mutations that affect survival traits (hence both male and female fitness), but have at the same time a strong sex-specific effect on male reproduction. Our results suggest that most polymorphisms on which selection has acted in these lines are not of the antagonistic type??that is, increasing one sexual function at the expense of the other. Rather, they were deleterious mutations with larger effect on male than on female fitness, which accumulated in response to relaxation of purifying selection on male reproduction. This supports the idea that sexual selection in hermaphrodites is a powerful force that contributes to purge the mutation load from the genome as in separate-sex organisms.