Molecular characterization and expression analysis of three complement genes: C4B, C6 and C7 in Mastacembelus armatus under Aeromonas veronii infection.

To investigate the immune functions and evolutionary features of complement genes in Mastacembelus armatus, three complement genes (MaC4B, MaC6 and MaC7) were cloned and characterized for the first time, and their expression profiles upon Aeromonas veronii infection as well as their positively selected sites were analyzed. The full-length coding sequences (CDSs) of MaC4B, MaC6 and MaC7 were successfully obtained. The encoded proteins consist of 1708, 936 and 840 amino acids, respectively, and possess conserved protein domains identical to those of other teleost fish. Tissue expression analysis revealed that the three genes were constitutively expressed in nine tested tissues, with relatively high transcript levels in the liver, muscle, fins and gills. Upon challenge with A. veronii, the three genes displayed distinct tissue-specific expression patterns. Their transcript levels in the spleen and kidney decreased initially and then increased, whereas expression in the liver was persistently downregulated. Phylogenetic analysis showed that M. armatus clustered closely with other Perciformes species. Positive selection analysis demonstrated that the three genes were mainly subjected to purifying selection at the whole-gene level, while several sites were identified to be under significant positive selection. These findings indicated that MaC4B, MaC6 and MaC7 participate in the antibacterial immune response of M. armatus and have undergone adaptive evolution under pathogen stress. This study provides a theoretical basis for exploring the immune defense mechanisms of the complement system in teleosts.