Kidney transcriptome analysis reveals the molecular responses to salinity adaptation in largemouth bass (Micropterus salmoides)

Recently, against the background of increasing land salinization and global warming, many studies have examined the mechanisms of freshwater fish adaptation to elevated salinity. However, the mechanisms underlying salinity tolerance in the kidney of Micropterus salmoides, a popular saline aquaculture species, remain poorly understood. We used RNA-seq to explore the differentially expressed genes (DEGs) in the kidney of M. salmoides at 0 ‰, 5 ‰, and 10 ‰ salinity for 24 d and 48 d. These DEGs mainly affected metabolism-related pathways, such as secondary metabolite biosynthesis, arachidonic acid metabolism, etc., and immunity-related pathways, such as IL-17 signaling and ECM-receptor interaction. Trend analysis on days 24 and 48 showed that, as salinity increased, the up-regulated genes were notably enriched in the cytochrome P450 xenobiotic metabolic pathway, and down-regulated genes substantially linked to cell cycle, phagosome, etc. More importantly, we identified a total of 22 genes enriched in the cytochrome P450 xenobiotic metabolic pathway, including seven UDP-glucuronosyltransferase genes (UGTs) and five glutathione S-transferase genes (GSTs). We speculated that M. salmoides kidneys removed toxic substances produced due to salinity stress and mitigated oxidative damage by up-regulating UGTs and GSTs, hence maintaining normal physiological function. In addition, genes such as Cystatin A1, significantly up-regulated with increasing salinity stress and duration, favoured the recovery of kidney injury. This research delved into the molecular processes involved in the adaptability of M. salmoides to high salinity stress and provided valuable information for the future breeding of salinity-tolerant strains.