Comparative Evolution of SLC12 Transporters in Tilapias and Salinity-Responsive Expression in Nile Tilapia

Tilapia species differ markedly in salinity tolerance, providing a useful system for investigating the molecular basis of osmotic adaptation. The solute carrier family 12 (SLC12) encodes cation-chloride cotransporters that play central roles in ion homeostasis and osmoregulation. Here, we compared the SLC12 gene family among Nile tilapia (Oreochromis niloticus), Mozambique tilapia (O. mossambicus), and blue tilapia (O. aureus) using comparative genomic, phylogenetic, protein-structural, and chromosomal analyses. Separately, we re-analyzed a published long-term salinity RNA-seq dataset from GIFT Nile tilapia maintained for 3.5 months at 0, 17, or 27 ppt, focusing on SLC12 expression in the gill, intestine, and skin. All three tilapia species contained 17 SLC12 loci with identical subfamily copy numbers and broadly conserved chromosomal distributions. In the Nile tilapia transcriptomic dataset, paralogous SLC12A2, SLC12A7, and SLC12A10 copies displayed distinct tissue- and salinity-dependent expression patterns. SLC12A8 also showed salinity-responsive expression in Nile tilapia, prompting further comparative sequence and structural analysis. The three SLC12A8 orthologs were broadly similar in sequence and predicted structure, although several species-specific residue differences were observed. These findings reveal overall conservation of the SLC12 family among tilapias, identify salinity-responsive expression differences among paralogous members in Nile tilapia and interspecific sequence variation in SLC12A8, and provide candidate features for further functional investigation.