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).