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‰.