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 (T4) 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 T4 concentrations indicates that changes in thyroid
hormone signalling may underlie the evolutionary and ecological divergence of migratory and
Questions: Is the variation in T4 concentrations present in earlier life stages where it
could contribute developmentally to differences in phenotype? Do T4 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
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 T4 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 T4 concentrations before seaward migration.
Results: Juvenile stickleback of the marine ecotype had higher T4 concentrations than
the parapatric stream-resident juveniles. The T4 concentrations in another marine population
varied slightly across sampling times before seaward migration.
Conclusions: T4 concentrations differ consistently between marine and stream ecotypes
in both juvenile and adult life stages consonant with the hypothesis of evolutionary changes in
Available at: http://works.bepress.com/slema/33/