A trade-off between traits that contribute to male and female function in hermaphrodites

Sex allocation theory assumes that male and female reproductive functions share a common limited resource pool and are negatively correlated in hermaphrodites. Here we report on the first artificial selection experiment designed to test the existence of genetically-based correlations between sex functions in hermaphroditic animals. The polychaete worm Ophryotrocha diadema has a long juvenile male phase, and then shifts to the simultaneously hermaphroditic phase. We selected two sets of lines of worms for a short male phase and, after four generations, worms had a significantly shorter male phase than their generation-0 ancestors. As negatively correlated responses, generation-4 worms spent more time maturing eggs and produced a higher number of eggs at 1st laying than worms of generation 0. Both traits contributed to the female function and were not the target of the selection experiment. In contrast, selection was ineffective in the lines descending from phenotypically-hermaphroditic worms that reproduced only via their male function. Our results provide the first empirical support of a genetic basis for a trade-off between traits related to the male and female function in hermaphroditic animals and highlight that these trade-offs are complex. Our results also suggest that the trade-off between male and female functions breaks up as hermaphrodites evolve some sexual specialization where resources are channeled towards a single sexual function.