De novo transcriptomic analysis to identify candidate genes potentially related to host recognition during infective stage of Caligus fugu (Crustacea: Copepoda)

Attachment site recognition for infecting a definite host is crucial for obligate ectoparasites. Caligus fugu, a marine copepod parasite primarily found on pufferfishes, has a unique host-/organ-specificity. The infective stage of the species exclusively infects host fins as the attachment site. However, so far, little is known about the mechanism underlying the specificity of C. fugu. In this study, transcriptomic profiles of the infective first copepodid stage (CI) along with the pre- and a post-infective stage, i.e., second nauplii (NII) and second copepodids (CII), respectively, were investigated. The de novo assembled transcriptome of C. fugu showed that a high number of transcripts showed high homology to those of relative species, Caligus rogercresseyi (94.7 %) and Lepeophtheirus salmonis (91.0 %), suggesting that only a small portion of species-specific genes contribute to interspecific differences, such as host-seeking. Importantly, no gene was noted from the odorant receptors and gustatory receptors families in C. fugu transcripts, similar to L. salmonis genome. Genes related to chemosensing such as the ionotropic glutamate receptors (iGluR) or ionotropic receptors (IRs), viz., GRIA2, GRIA3, GRID2, GRIK2, GRIK3, IR21a, IR25a, IR40a, and IR93a, likely involved in host-seeking, were highly expressed during CI among the three stages. In addition, inositol 1,4,5-triphosphate receptor-associated 2, another potential candidate gene involved in host-seeking, was significantly upregulated in CI compared with that in NII and maintained at the same level in CII. Our present transcriptomic data should offer a foundation for further investigations on various biological aspects, such as the host-/organ-specificity of sea lice.