Discussion
Here, we have investigated the transcriptomic response of an acute
change in salinity for an allopatric freshwater and a saltwater ecotype
of the threespine stickleback in Norway that have previously shown
little energetic costs of salinity transfer (Grøtan et al. 2012). Within
the six-hour exposure in this study, very few genes were significantly
differentially expressed in saltwater stickleback transferred to fresh
water (comparison 1), whereas about 1500 transcripts were differentially
regulated in freshwater stickleback transferred to saltwater (comparison
2). Further, over 1300 transcripts differed between the controls,
including ~500 transcripts that did not express
significant changes in regulation within salinity, but between ecotypes.
These results indicate that the ability to adjust following a change in
salinity is maintained by both ecotypes, but the gene expression cost of
the transition seems much larger for the freshwater ecotype. As there
were so few genes with significant plastic expression in contrast 1, we
could not compare expression patterns toward higher or lower salinity
for these ecotypes (salinity and ecotype interactions), and we could
hence not find dominating gene-groups equally important for abrupt
salinity transfers in either direction. Many of the genes found to be
differentially regulated within freshwater and between ecotypes in this
study are known to be critical for ion regulation, as they facilitate
transport through energy conversion, or are directly involved in
building ion channels, ion pumps or supress passive ion diffusion. Gills
consists of different cell types, and some differentially expressed
genes are involved in restructuring the gill tissue, through e.g.
tightening the junctions between chloride cells in freshwater and likely
increasing the density of the chloride cell type itself in saltwater
(Perry 1997). By also moving the control fish over to a new aquarium,
the observed stress-responses in this study should hence only relate to
the salinity changes and the handling itself. Many genes in the
HSP-family, in addition to other stress-related genes, were found to
increase when freshwater fish were exposed to saltwater, but not for the
saltwater fish when they were exposed to freshwater. Taken together,
contrasting results between the two ecotypes strengthens the theory of
many different evolutionary pathways to physiological freshwater
adaptations in stickleback (DeFaveri et al. 2011; Gibbons et al. 2017)
and other fish species (Velotta et al. 2017). Many genes linked to
complex physiological regulatory mechanisms showed evidence of adapted
expression profiles between the two ecotypes, supporting evolutionary
adaptation via genetic assimilation and overall genomic reduction in
phenotypic plasticity within the gill transcriptome, similarly to
findings in other fish species (Velotta et al. 2017).