Integration of comparative transcriptome and physiological adaptations reveals the osmoregulation mechanism of grass carp (Ctenopharyngodon idella) skin response to hyper-salinity stress

Making full use of the growing saline-alkali water for aquaculture is an important measure to solve the limited space of freshwater aquaculture. Grass carp (Ctenopharyngodon idella), one of China's most important freshwater fishes, has potential for aquaculture in saline-alkali water. However, there is poor knowledge about the adaptation mechanisms to salinity changes. In the present study, we characterized the transcriptome, physiological and morphological changes of skin in grass carp treated with different salinity (0, 4 and 8 ppt) stress for one month. Based on transcriptome data, we identified 1489, 1944 and 3055 differentially expressed genes (DEGs) in three groups of comparisons (S4 vs S0, S8 vs S0 and S8 vs S4, respectively). These DEGs were mainly enriched in transmembrane transport, signal transduction and energy metabolism. Furthermore, the WGCNA results showed that the turquoise module positively correlating with high-salinity stress formed an interaction network with psma/b/c/d as the core genes. In addition, the CAT and GPx activities in the high-salinity group were significantly decreased compared with the control and low-salinity group, indicating that chronic salinity stress caused oxidative damage. It is noteworthy that the cortisol content increased with the increasing salinity, suggesting that it could be used as an indicator of salinity stress. The changes in number and type of mucous cells were observed in the high-salinity group, which may be a strategy to maintain osmotic homeostasis. In conclusion, our findings provide new insights into the physiological responses and potential adaptive strategies to salinity stress, and contribute to the selective breeding of salinity tolerance of grass carp and the utilization of saline-alkaline waters.