Genomic characterization and expression pattern of Bf/C2 and C4 in miiuy croaker and molecular evolution analysis on mammals and fishes

• We characterized the Bf/C2 and C4 genes from miiuy croaker.
• The expression of Bf/C2 and C4 were up-regulated in liver after bacterial infections.
• Comparative genomic structure reflects the evolutionary conservation between vertebrates.
• The ancestral of Bf/C2 and C4 genes in mammals and fishes underwent positive selection.
• The Bf/C2 and C4 genes experience different evolutionary patterns in mammals and fishes.

The complement system plays an important role in both innate and adaptive host defense against the invading microorganisms in vertebrates. It can be activated by three pathways: the classical, alternative and lectin pathways. Bf/C2 and C4, as members of complement, play a pivotal role in the activation of the complement system. In our study, we identified Bf/C2 and C4 genes and genomic structure in miiuy croaker, and expression patterns of Bf/C2 and C4 genes was analyzed. In healthy miiuy croaker tissues, Bf/C2 and C4 genes were found to be ubiquitously expressed in all ten tested tissues. Analysis of expression of Bf/C2 and C4 genes after bacterial infection showed a significant up-regulated in liver. The evolutionary analysis showed that the ancestral lineages of Bf/C2 and C4 genes in mammals and fishes experienced positive selection indicated that the ancestors of mammals and fishes had further evolved to adapt to their environment, respectively. A series of maximum likelihood (ML) methods were used to study the evolution on vertebrates' Bf/C2 and C4 genes. One and five positive selection sites were found in mammals of Bf/C2 and C4 genes, but no positive selection site was found in fishes of Bf/C2 and C4 genes, indicating that Bf/C2 and C4 genes in mammals and fishes underwent different evolutionary patterns.