Olfactory receptors in aquatic and terrestrial vertebrates

In species representing different levels of vertebrate evolution, olfactory receptor genes have been identified by molecular cloning techniques. Compa ring the deduced amino-acid sequences revealed that the olfactory receptor gene family of Rana esculenta resembles that of Xenopus laevis, indicating that amphibians in general may comprise two classes of olfactory receptors. Whereas teleost fish, including the goldfish Carassius auratus, possess on ly class I receptors, the 'living fossil' Latimeria chalumnae is endowed wi th both receptor classes; interestingly, most of the class II genes turned out to be pseudogenes. Exploring receptor genes in aquatic mammals led to t he discovery of a large array of only class II receptor genes in the dolphi n Stenella Coeruleoalba; however, all of these genes were found to be non-f unctional pseudogenes. These results support the notion that class I recept ors may be specialized for detecting water-soluble odorants and class II re ceptors for recognizing volatile odorants. Comparing the structural feature s of both receptor classes from various species revealed that they differ m ainly in their extracellular loop 3, which may contribute to ligand specifi city. Comparing the number and diversity of olfactory receptor genes in dif ferent species provides insight into the origin and the evolution of this u nique gene family.