Screening the V2R-type putative odorant receptor gene repertoire in bitterling Tanakia lanceolata

Fish odorant receptors comprise several distinct families of G protein-coupled receptors. The repertoires of fish vomeronasal receptor family 2 (V2R) genes differ substantially among species, which seems to be related to the different odor sensitivity among fish species. In this study, the number and diversity of V2R genes in a cyprinid fish, bitterling Tanakia lanceolata, were examined by gene cloning and genomic Southern blot analysis. Fifty-six distinct V2R partial sequences were identified by extensive cloning of degenerate PCR fragments. Phylogenetic analysis of the V2R genes in bitterling and other model fishes showed that the bitterling V2Rs were subdivided into eight subfamilies that diverged before the separation of cyprinids and other fishes. Most bitterling V2Rs belonged to two major subfamilies, while only one or a few genes were identified in the remaining six subfamilies. The repertoire of bitterling V2Rs was quite similar to that of zebrafish, although some bitterling-specific gene expansions have also occurred. Based on the genomic Southern blot analysis, the copy number of the bitterling V2R gene was estimated at least 60, similar to the number of V2R genes found in the zebrafish genome. This also suggests that bitterling has relatively larger number of V2R genes than other model fishes.