Lysozyme genes in bivalves: Evolutionary insights and functional roles in host defense against Vibrio infections

Bivalves are economically and ecologically significant species that are constantly exposed to environmental stressors and pathogens, particularly Vibrio species, which pose persistent threats to bivalve aquaculture. Lysozymes, key immune effectors, play crucial roles in host defense against microbial invasion. However, systematic studies on bivalve lysozymes across multiple species remain limited. In this study, we identified 192 lysozyme genes across 35 bivalve species. Notably, oysters, mussels, and clams exhibited a larger repertoire of lysozyme genes, possibly reflecting gene expansion in species inhabiting the highly variable intertidal zone. Phylogenetic analysis revealed bivalve lysozymes clustered into three major types-C-type, G-type, and I-type-consistent with the traditional classification of animal lysozymes. Mussels and clams retained C-type lysozymes, whereas G-type lysozymes were absent in sand-buried bivalves, suggesting evolutionary selection in lysozyme type retention. Conserved motif analysis showed distinct structural features among the three types, each possessing unique conserved motifs. Synteny analysis revealed no gene duplication within species, and collinearity was observed only among closely related species, indicating substantial variation among distantly related taxa. Expression analysis showed that under Vibrio challenge, the expression levels of CnGLys1, CgILys2, McoCLys1, RpCLys3/4, and RpILys2/5 were significantly upregulated, suggesting they may play key roles in the response to Vibrio stress. This implies that the functional lysozyme types involved differ among species. Additionally, different Vibrio strains induced similar expression levels, while tissue-specific responses were observed and LPS effectively stimulated lysozyme expression. These findings suggest that lysozyme expression in bivalves may be not Vibrio strain specific, exhibits tissue specificity, and is strongly induced by LPS. In conclusion, this study provides a comprehensive analysis of bivalve lysozyme genes, offering new insights into the host-pathogen interactions and immune regulation in marine invertebrates under Vibrio stress.