Understanding metamorphosis of the amphibian olfactory system at the morphological, molecular, and functional level

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A major challenge for the vertebrate sense of smell has been the transition from aquatic to terrestrial olfaction, since odor classes are distinctly different. In this respect amphibians represent an evolutionary transition point between teleost fish and e.g. mammals. A second, ontogenetic transition is specific to amphibians: metamorphosis. This process generates a terrestrial adult frog from an aquatic tadpole, and involves a drastic reconstruction of the olfactory system. However, very little is known about morphological, molecular, and functional changes during metamorphosis of the olfactory system. We have previously described several novel features of the olfactory system of Xenopus laevis tadpoles, which all reflect the evolutionary transition point of this species, chief among them the exclusive expression of a large group of V2R receptors in the larval MOE, co-localized with functional amino acid responses. Thus the amphibian V2R family is divided between vomeronasal and main olfactory epithlium, a very unusual feature. Here we suggest to study the development of this feature during metamorphosis, together with an analysis of amino acid odor responses in both main noses generated during metamorphosis, the principal and the middle cavity (air nose and water nose, respectively). To complement these studies we suggest two independent approaches to deorphanize Xenopus V2R receptors, measuring odor responses of V2Rs heterologously expressed in Xenopus oocytes, and in vivo imaging after knockout of an essential co-receptor for V2Rs.