Olfactory receptors in aquatic and terrestrial vertebrates

Abstract

In species representing different levels of vertebrate evolution, olfactory receptor genes have been identified by molecular cloning techniques. Comparing 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 only class I receptors, the 'living fossil' Latimeria chalumnae is endowed with both receptor classes; interestingly, most of the class II genes turned out to be pseudogenes. Exploring receptor genes in aquatic mammals led to the discovery of a large array of only class II receptor genes in the dolphin Stenella Coeruleoalba; however, all of these genes were found to be non-functional pseudogenes. These results support the notion that class I receptors may be specialized for detecting water-soluble odorants and class II receptors for recognizing volatile odorants. Comparing the structural features of both receptor classes from various species revealed that they differ mainly in their extracellular loop 3, which may contribute to ligand specificity. Comparing the number and diversity of olfactory receptor genes in different species provides insight into the origin and the evolution of this unique gene family.

Somatosensation, Echolocation, and Underwater Sniffing: Adaptations Allow Mammals Without Traditional Olfactory Capabilities to Forage for Food Underwater

“…Semiaquatic mammals, on the other hand, have variably maintained their olfactory receptor genes and olfactory structures (Kishida et al, 2007). For instance, while Trichechus manatus (West Indian manatee) displays evidence of a rudimentary olfactory system (Mackay-Sim et al, 1985), seals have a main olfactory bulb and an accessory olfactory bulb associated with a vomeronasal organ (Freitag et al, 1998). In the case of cetaceans, Kishida and Thewissen (2012) presented evidence suggesting that cetacean olfactory capabilities decreased gradually after adaptation to water, and some of these animals, like the baleen whales, still use olfaction to detect above-water odor plumes (e.g., Hagelin et al, 2012).…”

The olfactory receptor gene repertoire in primates and mouse: Evidence for reduction of the functional fraction in primates

“…Actually, it is likely that the different hypotheses evoked to explain the loss of function would be not exclusive but that a parallel might exist between the reduction of the anatomical structures devoted to olfaction, the decrease in the number of functional OR genes, and the reduction of the sense of smell. This hypothesis is strongly supported by the fact that aquatic mammals such as dolphin, which has a reduced olfactory apparatus, have only OR pseudogenes (15). These animals live in water and do not need to smell volatile odorants.…”

Evolutionary dynamics of olfactory and other chemosensory receptor genes in vertebrates

“…For this reason, they called class I and class II genes ''fish-like'' and ''mammalian-like'' OR genes respectively. In Freitag et al (1998), they extended this view and proposed that fishes have only class I genes, mammals have only class II genes, and amphibians have both of them. For this reason, class I genes were assumed to be specialized for water-soluble odorants and class II genes were for airborne odorants.…”