FOXO genes in channel catfish and their response after bacterial infection.

FOXO proteins are a subgroup of the forkhead family of transcription factors that play crucial roles in lifespan regulation. In addition, FOXO proteins are also involved in immune responses. After a systematic study of FOXO genes in channel catfish, Ictalurus punctatus, seven FOXO genes were identified and characterized, including FOXO1a, FOXO1b, FOXO3a, FOXO3b, FOXO4, FOXO6a and FOXO6b. Through phylogenetic and syntenic analyses, it was found that FOXO1, FOXO3 and FOXO6 were duplicated in the catfish genome, as in the zebrafish genome. Analysis of the relative rates of nonsynonymous (dN) and synonymous (dS) substitutions revealed that the FOXO genes were globally strongly constrained by negative selection. Differential expression patterns were observed in the majority of FOXO genes after Edwardsiella ictaluri and Flavobacterium columnare infections. After E. ictaluri infection, four FOXO genes with orthologs in mammal species were significantly upregulated, where FOXO6b was the most dramatically upregulated. However, after F. columnare infection, the expression levels of almost all FOXO genes were not significantly affected. These results suggested that either a pathogenesis-specific pattern or tissue-specific pattern existed in catfish after these two bacterial infections. Taken together, these findings indicated that FOXO genes may play important roles in immune responses to bacterial infections in catfish.