Comparison of freshwater tolerance during spawning migration between two sympatric Japanese marine threespine stickleback species

Background: The colonization of empty niches can trigger phenotypic diversification. For example, the colonization of newly formed freshwater environments by marine ancestors led to phenotypic diversification in threespine sticklebacks. However, not all lineages of threespine stickleback have taken advantage of these ecological opportunities; all Japanese freshwater populations are derived from the Pacific Ocean lineage rather than the Japan Sea lineage of Gasterosteus. Hypothesis: Marine ancestors of these two lineages differed in their ability to survive in freshwater environments. Japan Sea and Pacific Ocean marine sticklebacks may differ in freshwater tolerance and transcript levels of the prolactin (PRL) gene, which encodes a hormone important for freshwater osmoregulation. Methods: We collected Japan Sea and Pacific Ocean marine sticklebacks migrating upstream near the mouth of a brackish water lake in early May and challenged them with fresh water. We then compared the survival rates, plasma sodium concentrations, and PRL expression levels of the two species. Results: When challenged with fresh water, Japan Sea fish showed significantly higher death rates and a trend towards a greater reduction in plasma sodium concentration than the sympatric Pacific Ocean fish. Levels of PRL were consistently higher in the Pacific Ocean fish both before and after the freshwater challenge.