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Molecular evolution of color vision in vertebrates.
Yokoyama S
.
Abstract
Visual systems of vertebrates exhibit a striking level of diversity, reflecting their adaptive responses to various color environments. The photosensitive molecules, visual pigments, can be synthesized in vitro and their absorption spectra can be determined. Comparing the amino acid sequences and absorption spectra of various visual pigments, we can identify amino acid changes that have modified the absorption spectra of visual pigments. These hypotheses can then be tested using the in vitro assay. This approach has been a powerful tool in elucidating not only the molecular bases of color vision, but the processes of adaptive evolution at the molecular level.
Fig. 1. In vitro assays of the absorption spectra of the wild-type and mutant bovine RH1 pigments. (A) The opsin cDNAs in an expression vector, pMT5, are expressed in COS1 cells by transient transfection. (B) The visual pigments were then regenerated by incubating the opsins with 11-cis-retinal in the dark. (C) The resulting visual pigments are then purified by immunoaffinity chromatography by using monoclonal antibody 1D4 Sepharose 4B. The absorption spectra of the visual pigment are recorded using a spectrophotometer. (D) The amino acid change A292S has been obtained by site-directed mutagenesis. The absorption spectrum in inset shows the λmax determined by dark-light spectrum.
Fig. 2. A composite tree topology of selected RH1 and RH2 pigments and amino acid replacements inferred by a likelihood Bayesian method (Yang et al., 1997). The numbers after P refer to λmaxs of visual pigments. Amino acid replacements are shown next to different. The pigments with blue-shifted λmaxs are indicated by rectangles. Cavefish, Astyanax fasciatus; goldfish, Carassius auratus; coelacanth, Latimeria chalumnae; chameleon, Anolis carolinensis; chicken, Gallus gallus; bovine, Bos taurus; dolphin, Tursiops truncatus; gecko, Gekko gekko.
Fig. 3. Secondary structure of bovine RH1 opsin, showing naturally occurring amino acid mutations that cause significant λmax-shifts. The model is based on Palczewski et al. (2000). Blue, red and black circles indicate the amino acid sites that are involved in the spectral tuning of SWS1, LWS/MWS, and RH1/RH2 pigments, respectively. The 11-cis-retinal is shown in orange.
Fig. 4. A composite tree topology of selected SWS1 pigments and critical amino acid replacements at sites, 46, 49, 52, 86, 90, 3, 114, and 118, along the goldfish and avian branches. The UV pigments are boxed. UV and V indicate UV and violet sensitivities, respectively. Amino acid replacements next to the goldfish and avian branches show those at the eight critical sites. The eight letters next to the vertebrate ancestor and 13 contemporary pigments show the eight critical amino acids. Clawed frog, Xenopus laevis; pigeon, Columba livia; budgerigar, Melopsittacus undulatus; zebra finch, Taeniopygia guttata; human, Homo sapiens; mouse, Mus musculus; rat, Rattus norvegicus; and salamander, Ambystoma tigrinum. For other species, see Fig. 2.
