Evolutionary characterization and pathogen-responsive expression of the Annexins in bivalves.

Annexins (ANXs) are a family of Ca2+-dependent phospholipid-binding proteins that participate in membrane dynamics and stress responses; however, their evolutionary patterns and immune functions in bivalves remain poorly understood. In this study, a total of 206 ANX genes were identified from 29 bivalve species. Their physicochemical properties, conserved motifs, gene structures, and phylogenetic relationships were systematically analyzed. The results revealed that the ANX family is highly conserved in bivalves and can be classified into seven major clades (ANX1-ANX7). In the noble scallop Chlamys nobilis, tissue expression analysis showed that CnANX3 and CnANX4 were highly expressed in immune-related tissues, including the gill and blood. Following Vibrio parahaemolyticus challenge, CnANX4 was rapidly induced during the early stage of infection, suggesting a potential role in membrane repair and cellular homeostasis maintenance. In contrast, CnANX3 exhibited sustained upregulation during the later stages of infection, implying its involvement in inflammation regulation and tissue homeostasis restoration. Furthermore, comparative transcriptomic analyses in oysters demonstrated that CgANX4 was consistently induced in response to both bacterial and viral infections, indicating a conserved pathogen-responsive function across species. Collectively, these findings suggest that ANX4 may represent a key conserved immune-responsive factor that plays an important role in innate immunity and cellular membrane homeostasis in bivalves.