A novel olfactory receptor gene family in teleost fish:phylogenomics, cellular localization and comparison with other olfactory receptor gene families

While for two of four mammalian olfactory receptor families, all of them G protein-coupled receptors, ortholog teleost families have been identified and well-characterized (OR and V2R), two other families (V1R and TAAR) lack to date a systematic study in non-mammalian vertebrates. By data mining I identified a total of six V1R-like genes in five teleost species plus four orthologs in one jawless and one cartilaginous fish species each. In the phylogenetic analysis these ora genes (olfactory receptor, class A-related) form a single clade with three subclades, one of them including the entire mammalian V1R superfamily. The Ora family originates early in the vertebrate lineage, before the separation of the jawless from jawed fish. A similar search was performed also for taar genes in genomes of five teleosts, two basal fish and seven higher vertebrates. Taar genes segregate into three classes and their family size ranges from 18 to 112 genes in teleosts (pufferfish and zebrafish, respectively), while mammalian families contain at most 19 genes (opossum). The TAAR family originated in the common ancestor of bony and cartilaginous fishes, after its divergence from jawless fish. In these and other properties the ora and taar gene families turn out to be at opposite poles of the spectrum of olfactory receptor families. All the six teleost Ora family members are evolutionarily much older than the speciation events in the teleost lineage, while most extant teleost taar genes have emerged late in evolution, well after the split between basal teleosts (zebrafish) and neoteleostei (stickleback, medaka, pufferfish). Taar genes are largely arranged according to phylogenetic proximity in two big clusters (both syntenic to the single sarcopterygian gene cluster), whereas the ora genes are organized as singletons or symmetrical gene pairs. TAAR genes are mostly monoexonic, whereas two ora genes exhibit a highly conserved multi-exonic structure. Furthermore, the ora genes are under strong negative selection (minute dN/dS values), whereas the teleost taar genes display a relaxed pattern of global negative selection and an unprecedented degree of local positive selection. Taken together, the ora gene repertoire is highly conserved across teleosts, in striking contrast to the frequent species-specific expansions observed in mammalian V1Rs. The inverse is observed for the taar gene repertoire, which is rather conserved across mammalian species, but exhibits frequent and large species-specific expansions in teleosts. Thus, the transition from teleosts to tetrapods may parallel a transition in function as well as regulation of both the ora/V1R and TAAR gene families. Consistent with a function as olfactory receptors all zebrafish ora and all analyzed taar genes (except taar1) were expressed in sparse subsets of olfactory receptor neurons. The olfactory epithelium contains three subtypes of olfactory receptor neurons, ciliated, microvillous and crypt cells, the latter so far without known receptors, but with both cilia and microvilli. I found the ora genes to be expressed in the crypt cells, thereby deorphanizing this third type of OSN. Furthermore, the ora genes follow the monogenic rule of expression previously reported for members of other olfactory receptor gene families. Ora genes co-express both Gαi and Gαo, supporting the hypothesis that crypt cells might possess two distinct olfactory signaling pathways, one via their cilia and the other via microvilli