Species-Specific Duplications of Trace Amine-Associated Receptors in Vertebrate Genomes

Chemosensory receptors (CRs) are used to detect a wide range of chemicals and are a crucial gateway between perception and environment. Expansion or loss of CRs would reflect the adaptation to the organism's life at the molecular level. Life-history traits such as foraging behavior and type of foods are expected to play a central role in driving variation in the number of CRs. Evolution of CRs may be also correlated with that of other detection functions (e.g., vision and audition). CRs, especially the olfactory receptors (ORs), represent the largest gene family in animal genomes (Nei et al., 2008). Vertebrate ORs are predominantly expressed in the main olfactory epithelium (MOE) in the nasal cavity. Interestingly, Liberles and Buck (2006) demonstrated that mouse Trace Amine-Associated Receptors (TAARs) are also expressed in the MOE and can function as ORs for volatile amines found in urine. TAAR-expressing olfactory sensory neurons could be involved in the detection of social pheromone cues such as β-phenylethylamine, isoamylamine, and trimethylamine that elicit innate behaviors. In order to examine how the number of TAARs varies among vertebrates and elucidate the evolutionary history of these proteins, we identified all TAARs from twenty two vertebrate genomes. We found that the number of TAAR genes is much smaller than the number of ORs. On the other hand, extremely large variation in the number of TAARs among vertebrate species was observed. For example, while the megabat (Pteropu