Evolution

Coming Up for Air

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Science  13 May 2005:
Vol. 308, Issue 5724, pp. 927
DOI: 10.1126/science.308.5724.927e

The emergence of organisms from the ocean onto land was made possible by a suite of adaptations, not least of which were the integrated biomech-anical changes that were required for bodily support and locomotion in a 1g world, as compared to a buoyant and incompressible aquatic environment. Sensory systems also had to adapt, and Niimura and Nei have examined the evolutionary dynamics of the genes encoding olfactory receptors in a phylo-genetic analysis based on draft genome sequences from fish, frog, and chicken, along with already available genome data from mouse and human. They find that the most recent common ancestor (MRCA) of fishes and tetrapods carried at least nine distinct groups of olfactory receptors. Present-day fish have inherited eight of these, but have lost group α, which is one of two groups extant in mammals and birds, the other being group γ. The number of olfactory receptors in groups α and γ has expanded enormously (numbering about 1000 in mouse), and they are proposed to have become specialized for the detection of airborne molecules, whereas olfactory receptor genes (presumed to have retained their competence for sensing water-soluble odorants) in the other groups were discarded. — GJC

Proc. Natl. Acad. Sci. U.S.A. 102, 6039 (2005).

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