Genome-wide characterization of Toll-like receptor gene family in common carp (Cyprinus carpio) and their involvement in host immune response to Aeromonas hydrophila infection.

The Toll-like receptor (TLR) gene family is a class of conserved pattern recognition receptors, which play an essential role in innate immunity providing efficient defense against invading microbial pathogens. Although TLRs have been extensively characterized in both invertebrates and vertebrates, a comprehensive analysis of TLRs in common carp is lacking. In the present study, we have conducted the first genome-wide systematic analysis of common carp (Cyprinus carpio) TLR genes. A set of 27 common carp TLR genes were identified and characterized. Sequence similarity analysis, functional domain prediction and phylogenetic analysis supported their annotation and orthologies. By examining the gene copy number of TLR genes across several vertebrates, gene duplications and losses were observed. The expression patterns of TLR genes were examined during early developmental stages and in various healthy tissues, and the results showed that TLR genes were ubiquitously expressed, indicating a likely role in maintaining homeostasis. Moreover, the differential expression of TLRs was examined after Aeromons hydrophila infection, and showed that most TLR genes were induced, with diverse patterns. TLR1, TLR4-2, TLR4-3, TLR22-2, TLR22-3 were significantly up-regulated at minimum one timepoint, whereas TLR2-1, TLR4-1, TLR7-1 and TLR7-2 were significantly down-regulated. Our results suggested that TLR genes play critical roles in the common carp immune response. Collectively, our findings provide fundamental genomic resources for future studies on fish disease management and disease-resistance selective breeding strategy development.