Divergence in thyroid hormone concentrations between juveniles of marine and stream ecotypes of the threespine stickleback (Gasterosteus aculeatus)

Background: Hormones regulate the expression of multiple phenotypic traits. Therefore, divergence in hormone concentrations may lead to evolutionary changes in the coordinated physiological and behavioural traits that comprise an organism's integrated phenotype. Adults of marine ecotypes of threespine stickleback (Gasterosteus aculeatus) have higher concentrations of the thyroid hormone thyroxine (T-4) than adults of stream-resident ecotypes (Kitano et al., 2010). Thyroid hormones are well-established mediators of osmoregulation and migratory behaviours in fish, and the difference in T-4 concentrations indicates that changes in thyroid hormone signalling may underlie the evolutionary and ecological divergence of migratory and non-migratory ecotypes. Questions: Is the variation in T-4 concentrations present in earlier life stages where it could contribute developmentally to differences in phenotype? Do T-4 concentrations change in marine ecotypes before seaward migration? Organisms: A parapatric pair of marine and stream ecotypes of threespine stickleback collected from British Columbia, Canada, and a marine ecotype collected from Washington State, USA. Methods: We collected juvenile fish of both marine and stream ecotypes on the same day in a single river to compare the whole body concentrations of T-4 using radioimmunoassay. We also sampled juvenile fish of the marine ecotype in another river at three different times to determine whether these fish exhibit temporal changes in T-4 concentrations before seaward migration. Results: Juvenile stickleback of the marine ecotype had higher T-4 concentrations than the parapatric stream-resident juveniles. The T-4 concentrations in another marine population varied slightly across sampling times before seaward migration. Conclusions: T-4 concentrations differ consistently between marine and stream ecotypes in both juvenile and adult life stages consonant with the hypothesis of evolutionary changes in thyroid signalling.