The molecular evolution of teleost olfactory receptor gene families

Four olfactory receptor gene families, all of them G protein-coupled receptors, have been identified and characterized in mammals--the odorant (OR), vomeronasal (V1R and V2R) and trace amine-associated (TAARs) receptors. Much less attention has been directed towards non-mammalian members of these families. Since a hallmark of mammalian olfactory receptors is their remarkable species specificity, an evaluation of the non-mammalian olfactory receptors is instructive both for comparative purposes and in its own right. In this review I have compiled the results currently available for all four olfactory gene families and discuss their phylogenomic properties in relation to their mammalian counterparts. Representatives of all four families are found in cartilaginous fish and/or jawless fish, allowing a minimal estimate for the evolutionary origin as preceding the segregation between cartilaginous and bony fish or cartilaginous and jawless fish, respectively. Gene repertoires of teleost olfactory receptors are smaller in size (OR, ORA), comparable (olfC), or even larger (TAAR) than the corresponding mammalian gene repertoires. Despite their smaller repertoire size, the teleost OR and ORA families show much larger divergence than their mammalian counterparts. Evolutionary rates vary greatly between families, with evidence for positive selection in teleost OR genes, whereas the ora genes are subject to strong negative selection, and in fact are being conserved among all teleost species investigated. With one exception, ligands are unknown for any of the four teleost olfactory receptor gene families, and so the considerable knowledge about the odor responses of the olfactory epithelium and the olfactory bulb can only be linked indirectly to the receptor repertoires.