MEPS 269:163-171 (2004)  -  doi:10.3354/meps269163

ABSTRACT: For some organisms, sex is not fixed by genotype at conception but determined later by environmental conditions encountered during development (environmental sex determination, ESD). According to theoretical models, selection should favour ESD when the environment experienced by offspring is patchy and unpredictable and affects the relative fitness of males and females differentially. In a field study, we analysed the environmental heterogeneity displayed among individuals of the sea squirt Ciona intestinalis, a host for the copepod Pachypygus gibber, which is characterised by ESD. Consistent with theoretical predictions, environmental parameters that affect sex-specifically the fitness of the parasite (food and sexual partners) vary significantly in space and time. The spatial genetic structure of populations of P. gibber in the Thau lagoon, France, was investigated from allozyme data. No differentiation was found between distant populations, indicating that the larvae disperse intensively in the lagoon. Since their environment has been shown to be heterogeneous in space, and since the mothers cannot leave the host in which they developed, we conclude that both parent and offspring have little control over the quality of the environment the offspring enters. All the conditions are thus validated to support the adaptive nature of ESD in P. gibber.

KEY WORDS: Sex-determining mechanism · Selection · Heterogeneity · Unpredictability · Parasite · Copepod · Pachypygus gibber