Labile Sex Expression and the Evolution of Dioecy in Ophryotrocha Polychaete Worms

Labile sex expression is considered to play a key role in the evolution of breeding systems and in the transition from hermaphroditism to dioecy, according to the evolutionary models proposed for plants. While in hermaphrodites sex allocation within the individual can be plastically adjusted in response to social environment, in dioecious species it is predicted to be fixed. However, labile sex expression in the form of gender plasticity can still be present in dioecious species of animals with envi- ronmental sex determination. It is still unclear how gender plasticity is involved in the evolution of breeding systems and what its role is in the transition from hermaphroditism to dioecy. We assessed the degree of plasticity in gender expression in three dioecious species of polychaete worms of the genus Ophryotrocha. We found sexual polymor- phism and plasticity in sex expression during the juvenile phase to be a response to social environment. The majority of juveniles reared with an adult female or male expressed the gender opposite of that of the partner, so as to form heterosexual pairs. On the basis of these findings we out- line a possible evolutionary pathway of the transition from hermaphroditism to dioecy in the genus Ophryotrocha.