Phenotypic plasticity and divergence in physiological traits during freshwater colonization in threespine stickleback (Gasterosteus aculeatus)

Colonization of new environments exposes organisms to novel combinations of abiotic factors that have the potential to negatively affect fitness. Organisms may be able to cope with these changes in abiotic factors using existing phenotypic plasticity, or the novel environment may drive adaptive divergence, but the role of phenotypic plasticity in assisting or hindering the process of local adaptation remains unclear. This dissertation contributes to addressing this topic by examining the interactive effects of multiple abiotic factors on phenotypic plasticity and the evolution of physiological traits, which is an area that has received relatively little study. Specifically, I explored the roles of salinity and temperature in driving divergence during freshwater colonization using marine, anadromous, and derived freshwater populations of the threespine stickleback, Gasterosteus aculeatus. In north-temperate freshwater habitats, stickleback experience a combination of low salinity and low winter temperatures that is not experienced by the ancestral marine and anadromous forms which overwinter at sea. Overall, the results of this work are consistent with adaptive evolution in response to the interactive effects of low salinity and low temperature during freshwater colonization. My results showed that both salinity and temperature, and the interaction between them, had stronger negative effects on the growth of marine and anadromous populations compared to the freshwater population. Using a whole-transcriptome approach, I also detected differentiation in gene expression patterns between populations, particularly in processes important for changes in gill structure and permeability. Based on these data I hypothesize that freshwater stickleback have less permeable gills in fresh water, which may result in less energy use for osmoregulation, providing a physiological mode by which freshwater stickleback save energy, resulting in superior growth in cold fresh water. Both marine and freshwater stickleback showed interactive effects of low temperature and salinity on gill morphology, and marine stickleback exhibited substantial increases in the expression of Na⁺,K⁺-ATPase in cold fresh water, whereas more modest responses were observed in the freshwater ecotype, which may indicate increased energetic costs of osmoregulation in the marine population and potentially contribute to the growth deficits exhibited by these fish in cold fresh water.Science, Faculty ofZoology, Department ofGraduat