Characterization and evolutionary analysis of duplicated C7 in miiuy croaker

The complement system, as one of the most sophisticated innate immune system, plays an important role in defense against invading microorganisms. The complement component C7 participates in the cytolytic phase of complement activation through a series of polymerization reactions with other terminal complement components. In this study, we derived two C7 genes from the whole genome of miiuy croaker which were the consequence of the fish-specific genome duplication. Our data showed that miiuy croaker C7-1 and C7-2 genes shared same structure domains. The analysis of gene synteny showed that high degree conserved of synteny was retained between miiuy croaker and other teleosts, and miiuy croaker had a relatively closer relationship with fugu. The expression of C7-1 and C7-2 in miiuy croaker healthy tissues revealed that they were ubiquitously expressed in all ten tested tissues. Besides, the immune response of C7-1 and C7-2 were different in spleen with Vibrio anguillarum, Staphylococcus aureus, poly I:C and LPS at 24 h post-injection, respectively. Furthermore, the expression patterns of C7-1 and C7-2 were different in liver, spleen and kidney after infected with V. anguillarum at different time-point. Evolutionary analysis showed that all the ancestral lineages underwent positive selection except for the ancestral lineages of fish C7-2, indicated that the ancestral lineages of fish C7-1 genes undertook more pressures than C7-2 in defense against the invading microorganisms. Meanwhile, a series of maximum likelihood methods were used to explore the evolutionary patterns on extant vertebrates' C7 genes. Three and one positive selection sites were found in extant mammalian C7 genes and fish C7-2 genes, but no positive selection site was found in extant fishes C7-1 genes. The result showed that extant fish C7-2 genes undertook more pressures compared with C7-1. In conclusion, fish C7-1 and C7-2 gene underwent different evolutionary patterns.