Abstract
The olfactory receptor (OR) multigene family is responsible for the sense of smell in vertebrate species. OR genes are scattered widely in our chromosomes and constitute one of the largest gene families in eutherian genomes. Some previous studies revealed that eutherian OR genes diverged mainly during early mammalian evolution. However, the exact period when, and the ecological reason why eutherian ORs strongly diverged has remained unclear. In this study, I performed a strict data mining effort for marsupial opossum OR sequences and bootstrap analyses to estimate the periods of chromosomal migrations and gene duplications of OR genes during tetrapod evolution. The results indicate that chromosomal migrations occurred mainly during early vertebrate evolution before the monotreme-placental split, and that gene duplications occurred mainly during early mammalian evolution between the bird-mammal split and marsupial-placental split, coinciding with the reduction of opsin genes in primitive mammals. It could be thought that the previous chromosomal dispersal allowed the OR genes to subsequently expand easily, and the nocturnal adaptation of early mammals might have triggered the OR gene expansion.
Highlights
Tetrapods can recognize various environmental odors using olfactory receptors (ORs)
Large scale degeneration of OR genes is found in cetaceans, which have secondarily adapted to a marine habitat and have lost or greatly reduced their sense of smell acquired in terrestrial environments [9]
These findings suggest that the number of intact OR genes reflects the ability of odor recognition in tetrapod species
Summary
Tetrapods can recognize various environmental odors using olfactory receptors (ORs). ORs belong to the superfamily of seven transmembrane G-protein coupled receptors (GPCRs) and consist of one of the largest multigene families in vertebrate genomes [1,2,3]. An SWS2 class opsin gene, which encodes one of the four spectrally distinct classes of vertebrate cone pigment and has never been found in marsupial or placental mammals, was found in a monotreme platypus, suggesting that placental mammals lost their sense of color vision gradually during early mammalian evolution between the mammal-bird split and the placentalmarsupial split [12].
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