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Salmonid fishes, chum salmon (Oncorhynchus keta) have the developed adaptive strategy to withstand wide salinity changes from the early life stage. This study investigated gene expression patterns of cell membrane proteins in the gill of chum salmon fry on the transcriptome level by tracking the salinity acclimation of the fish in changing environments ranging from freshwater (0 ppt) to brackish water (17.5 ppt) to seawater (35 ppt). Using GO analysis of DEGs, the known osmoregulatory genes and their functional groups such as ion transport, transmembrane transporter activity and metal ion binding were identified. The expression patterns of membrane protein genes, including pump-mediated protein (NKA, CFTR), carrier-mediated protein (NKCC, NHE3) and channel-mediated protein (AQP) were similar to those of other salmonid fishes in the smolt or adult stages. Based on the protein-protein interaction analysis between transmembrane proteins and other related genes, we identified osmotic-related genes expressed with salinity changes and analyzed their expression patterns. The findings of this study may facilitate the disentangling of the genetic basis of chum salmon and better able an understanding of the osmophysiology of the species. Salinity is one of the critical factors limiting the distribution patterns of all aquatic organisms 1-4. Salmonid fishes display diverse life-history traits; anadromous individuals that mature in the river from hatching through to juveniles acquire the capacity to tolerate salinity associated with parr-smolt transformation and undergo ocean migrations before returning to rivers for spawning, whereas landlocked types spend their entire life within freshwater 5,6. Although migration between habitats is common among salmonid fishes, the seawater acclimation period varies even within anadromous species. Therefore, the timing of river to ocean migration varies from species to species 5,7,8. Chum salmon (Oncorhynchus keta) possess an excellent osmotic plasticity in coping with hyperosmotic or hypoosmotic environments 9-11. During the late embryonic stage, chum salmon have already acquired the hypo-osmoregulatory mechanism by the mitochondria-rich cells (MRCs) in the yolk-sac membrane 12. In addition, chum salmon fry whose habitat is freshwater begin to show remarkable seawater adaptability prior to seawater entry, which is not observed in the fry of other salmonids 13. Chum salmon begin to activate MRCs in the gill at an earlier stage (alevin-fry) and show higher salinity resistance at the fry stage than at the late alevin stages 11,14,15. Most of the salmonid fishes currently in the market are dominated by cultured Atlantic salmon (Salmo salar), whereas the production of chum salmon mostly depends on fishing (FAO, 2019). It is also noteworthy that chum salmon have been studied less when compared to other salmonid fishes. However, chum salmon are a major species of salmonid fishes that return to Korea, and if the feed and the aquaculture system are improved with the help of research on seawater ...

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