Concluding remarks
The saltwater fish in this study were collected from the Oslofjord, where they experience seasonal variation in salinity, due to periods of high freshwater influx from rivers after heavy rain and snow-melting. It is therefore even more surprising that the saltwater fish exhibited such relatively low significant expressional plasticity when exposed to freshwater, although theoretical studies have shown that fluctuating environments can reduce plasticity (Leung et al. 2020). Further, the genetic background for the saltwater fish is likely more diverse. Having a more diverse genetic makeup can likely also lead to a higher variation expression (higher standard deviations), which again will impact the false discovery rate and estimations of significance between experimental groups when filtering on expressional differences as in this study. On the other hand, recent studies suggest that a reduced level of genetic diversity can even increase the expessional diversity, and possibly buffer some of the loss of adaptive potential given with a higher genetic variation (Liu et al. 2019; Mazzarella et al. 2015; Morris et al. 2014). More comprehensive studies on physiological changes and osmoregulatory transcriptional responses are needed to understand how the stickleback, especially the saltwater stickleback, are able to tolerate short-term salinity changes. The low genetic expressional differences for the saltwater fish in this study indicates that they invoke some alternative strategy than gene regulation to handle changing salinities, like reversing the orientation of the proteins in the cell membrane (Hartmann et al. 1989), changing the activity state and/or function of cells and cell types, or proteins after interacting with other proteins (Pertl et al. 2010; Szczesnaskorupa et al. 1988), and/or mitochondrial activity/morphology or numbers (Austin and Nowikovsky 2021); as short term cellular adjustments are needed in order for the cell volumes to remain stable when moved abruptly from 30‰ to 0‰.