Identification of genes expressed in the pectoral fins and skin of Takifugu rubripes to reveal candidate genes involved in host recognition by the crustacean ectoparasite Caligus fugu

The initial detection of host-derived chemical cues is a key physiological event that strongly influences infection success in parasitic copepods such as Caligus fugu. Although ionotropic receptors were previously identified in C. fugu as potential host-sensing molecules, their ligands and specific roles in host recognition remain unknown. Since the infective copepodid I stage preferentially attaches to the fins of the puffer host, genes expressed at higher levels in fins may contribute to host recognition by C. fugu. In this study, differentially expressed genes between the pectoral fins and skin of Takifugu rubripes were comprehensively identified through RNA-seq to expand the list of candidate host-recognition genes beyond those detected earlier. To refine this list, genes expressed at higher levels in pectoral fins that were annotated with the gene ontology term "extracellular region" (Gene Ontology:0005576) were examined for predicted protein features, with emphasis on secreted, soluble molecules that the parasite could encounter in the surrounding water. From an initial set of 126 genes, intracellular and membrane-associated proteins were removed, and those possessing signal peptides or globular domains were retained. The remaining 92 proteins were considered likely secreted, water-soluble host-derived ligands. Relative qPCR analysis highlighted col10a1a, stc2a, mmp9, fibinb, bmp5, c4b, bmp6, and lipg as potential secreted cues for C. fugu. Antimicrobial peptides such as hamp and db1 were also identified as candidates. Overall, this study expands the pool of molecules that may function as chemoattractants for C. fugu and provides targets for future experimental validation.