Comparative sequence, structural, and phylogenetic analyses provide insights into the salinity-associated evolution...

Antimicrobial peptides (AMPs) are essential components of innate immunity in teleost fishes and play a critical role in host defense under diverse environmental conditions. However, how habitat salinity influences the molecular evolution and structural conservation of AMP families remains poorly understood. In this study, a comprehensive comparative analysis was performed on four representative AMP genes, including Defbl1, Defbl2, Hamp, and Nkl, across freshwater, marine, and euryhaline teleost species. mRNA sequence alignment revealed strong conservation within coding regions, particularly in domains encoding functional antimicrobial motifs, whereas untranslated regions exhibited pronounced habitat-associated divergence, raising the hypothesis that these variations may potentially contribute to regulatory adaptation. Amino acid sequence analysis showed strict conservation of cysteine residues and disulfide bond patterns in Defbl1, Defbl2, and Hamp, while Nkl displayed relatively weaker sequence constraint. Three-dimensional homology modeling combined with RMSD-based structural comparison demonstrated high structural conservation of Defbl1, Defbl2, and Hamp across species, especially in marine and euryhaline fishes, whereas Nkl exhibited greater structural variability. Phylogenetic analyses further indicated lineage-specific diversification and suggested that salinity adaptation of certain AMP genes in euryhaline fishes may have evolved independently under convergent selective pressures. Collectively, these findings provide new insights into the evolutionary and structural basis of innate immune adaptation in teleost fishes across distinct salinity environments.