Evolution of Olfactory Receptors
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Olfactory receptors are a specialized set of receptor cells responsible for the detection of odors. These cells are G protein-coupled
receptors and expressed in the cell membranes of olfactory sensory neurons. Once a cell is activated by a ligand, it initiates
a signal transduction cascade that produces a nerve impulse to the brain where odor perception is processed. Vertebrate olfactory
evolution is characterized by birth-and-death events, a special case of the stochastic continuous time Markov process. Vertebrate
fish have three general types of receptor cells (two dedicated to pheromones). Terrestrial animals have different epithelial
biology due to the specialized adaptation to detecting airborne odors. Two general classes of olfactory receptor gene reflect
the vertebrate marine heritage (Class I) and the derived amphibian, reptile, and mammal terrestrial heritage (Class II). While
we know much about olfactory receptor cells, there are still areas where our knowledge is insufficient, such as intra-individual
diversity throughout the life time, epigenetic processes acting on olfactory receptors, and association of ligands to specific
cells.