Genome-Wide Analysis of Urea Transporter (ut) Gene Family in Spotted Sea Bass (Lateolabrax maculatus): Evolution and Differential Expression After Salinity Adaptation

Urea is a major end product of nitrogen catabolism, serving as an osmolyte to regulate osmotic stress in fish exposed to varying water environments. It has been well known that urea transporters (UTs) facilitate the rapid movement of urea across cell membranes. However, researches on ut genes were predominantly focused on elasmobranchs and early developmental stages of fish. In this investigation, a total of three ut genes were identified in spotted sea bass. Phylogenetic, homology, and syntenic analyses were conducted to validate the annotation and assess the evolutionary relationships among ut genes. Both ut-a and ut-b genes have retained their evolutionary stability, demonstrating a significant level of homology between them. To gain deeper insights into the evolution of ut genes in spotted sea bass, we performed selective pressure analysis using site, branch, and branch-site models. The results suggested that positive selection likely played a significant role in shaping the evolution of the ut gene family. Furthermore, tissue-specific expression analyses revealed high expression levels of ut genes in osmoregulatory tissues such as the gill and kidney. Additionally, all three ut genes exhibited salinity-related expression patterns in gill and kidney tissues during both seawater-to-freshwater (SF) and freshwater-to-seawater (FS) adaptation. In situ hybridization results demonstrated the localization of both ut-a and ut-c mRNAs on the gill lamellae and adjacent gill filament epithelium. In summary, our study establishes a solid foundation for future research elucidating the evolutionary relationships and functional significance of ut genes during salinity acclimation in spotted sea bass and other teleost species.