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M.L. Porter, T.W. Cronin, D.A. McClellan, K.A. Crandall; The Molecular Evolution of Invertebrate Opsins . Invest. Ophthalmol. Vis. Sci. 2006;47(13):3748.
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© ARVO (1962-2015); The Authors (2016-present)
Investigations of invertebrate opsin evolution have long been focused on insect visual pigments, while other groups have received little attention. Furthermore, few studies have explicitly investigated the selective influences across all known invertebrate opsins. In this study, we contribute to the knowledge of crustacean opsins by sequencing the opsin gene from six previously uncharacterized crustacean species (Euphausia superba, Homarus gammarus, Archaeomysis grebnitzkii, Holmesimysis costata, Mysis diluviana, and Neomysis americana).
Visual pigment spectral sensitivities were characterized using microspectrophotometry for A. grebnitzkii (496 nm), H. costata (512 nm), M. diluviana (501 nm), and Neomysis americana (520 nm). These novel crustacean opsin sequences were included in a phylogenetic analysis with previously characterized (both genetically and spectrally) invertebrate opsin sequences to determine the evolutionary placement of our opsin sequences relative to the well–established insect spectral clades (long–/middle–/short wavelength sensitive). Phylogenetic analyses indicate these novel crustacean opsins form a well–supported clade with previously characterized crayfish opsin sequences, and form a sister–group to insect middle–/long–wavelength sensitive opsins. The reconstructed invertebrate opsin phylogeny was used to investigate selective influences within invertebrate opsin evolution using standard dN/dS ratio methods and more sensitive techniques investigating the amino acid property changes resulting from nonsynonymous replacements in a historical (i.e. phylogenetic) context.
While the conservative dN/dS methods did not detect any selection, four amino acid properties (coil tendencies, compressibility, power to be at the middle of an alpha helix, and refractive index) were found to be under destabilizing positive selection. Based on homology modeling using bovine rhodopsin, ten amino acid sites relating to these properties were found to be facing the binding pocket within 4Å of the chromophore, with potential to affect spectral tuning.
The identification of these sites confirms that amino acids affecting spectral tuning are being selected for impacts on structural aspects of the helices. These amino acid properties in particular may affect the internal packing of the chromophore binding site and thereby affect spectral tuning of the chromophore and signal propagation of the opsin protein.
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