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Shozo YokoyamaAsa Griggs Candler Professor B. S. (Plant Breeding), Miyazaki University, Miyazaki, Japan, 1968 E-mail: syokoya@emory.edu |
The ability of animals to see the entire visible spectrum ranging from UV to far red is controlled by a small number of sets of photosensitive molecules, known as visual pigments. Each of these visual pigments generally consists of a chromophore, 11-cis-retinal (vitamin A1 aldehyde), and a transmembrane protein, opsin. Vision researchers have described a large number of fascinating examples of vertebrate visual systems, where ecology and evolution have been recognized as important forces in generating such variability. For example, 1) many species in the bony fishes, birds, reptiles, and primates have full-fledged trichromatic color vision, while many extant vertebrates are color-blind, 2) many amphibian and fish species detect longer wavelengths by using 3-dehydroretinal (vitamin A2 aldehyde), instead of 11-cis-retinal, as a chromophore, in some cases only in certain ecological conditions and/or developmental stages (example: marine lamprey), 3) vision of many amphibians, reptiles, and birds is affected by the colored oil droplets that are lodged in their photoreceptor cells (example: pigeon), and 4) some organisms have developed retinas with only one photoreceptor cell type, as exemplified by the all-rod nocturnal gecko and the all-cone American chameleon.
We have conducted the genetic analyses of the visual pigments of a diverse range of organisms, inlcuding marine lamprey (Petromyzon marinus), Mexican cavefish (Astyanax fasciatus), coelacanth (Latimeria chalamnae), American chameleon (Anolis carolinensis), gecko (Gekko gekko), pigeon (Columba livia), zebra finch (Taeniopygia guttata), different mammalian species, and several deep-sea fishes. Our recent work on "adaptive evolution of color vision of the coelacanth" may be suffice to describe our typical approach to the evolutionary genetic analyses of color vision. The coelacanth, a 'living fossil', lives near the coast of the Comoros archipelago in the Indian Ocean. Living at the depth of about 200m, the Comoran coelacanth receives only a narrow range of light at about 480 nm. We have cloned all opsin genes from this species, two of which are found to be functional. The light sensitivities of the RH1 and RH2 visual pigments are shown to have the maximaum wavelengths of absorption 478 nm and 485 nm, respectively. These values are about 20 nm blue-shifted compared to the orthologous pigments in other species. Thus, in order to detect the entire range of 'color' available to them, the coelacanths use only two types of visual pigments. The fascinating part of these two values is that they were predicted almost exactly from the animal's photic environment. Furthermore, mutagenesis experiment shows that the adaptation of these RH1 and RH2 pigments are achieved by only two amino acid replacements Q122E/S292A and Q122E/L207M, respectively.
In the immediate future, we plan to continue to work on three major areas:
1) the molecular genetics and evolution of UV vision; 2) color vision of deep-sea fishes; and 3) functional genomics using eyed and blind cavefishes, Astyanax fasciatus, from different Mexican caves. This project involves the characterization of expressed sequence tags (ESTs) and microarray analyses.
Obviously, vision has profound effects on the evolution of organisms by affecting survivorship through such basic behaviors as mate choice and foraging strategies. The long-term goal of our research is to elucidate evolutionary changes of the structure-function relationships of molecules involved in vision. Thus, once molecular genetic bases of color detection by visual pigments and photoreceptor cells are elucidated, we will be ready to elucidate the relationships between colorations and patterns of animals and color vision-oriented animal behavior. Then, we will be closer to fully appreciating the complexity of colors and patterns exhibited by animals and plants in nature.
Yokoyama, S, Radlwimmer, F. B. and Blow, N. S. (2000)
Ultraviolet pigments in birds evolved from violet pigments by a single amino acid change. Proc. Natl. Acad. Sci. U S A. 97: 7366-7371.
[PubMed: Abstract] [PDF: Fulltext]Yokoyama, S. and Radlwimmer F.B. (2001)
The molecular genetics and evolution of red and green color vision in vertebrates. Genetics. 158: 1697-710.
[PubMed: Abstract] [PDF: Fulltext]Shi Y., Radlwimmer F.B. and Yokoyama, S. (2001)
Molecular genetics and the evolution of ultraviolet vision in vertebrates. Proc Natl Acad Sci U S A. 98: 11731-6.
[PubMed: Abstract] [PDF: Fulltext]Yokoyama, S. (2002)
Evaluating adaptive evolution (News & Views). Nature Genet. 30: 350-1.
[PubMed: Abstract] [PDF: Fulltext]Shi Y. and Yokoyama, S. (2003)
Molecular evolution of ultraviolet vision in vertebrates. Proc. Natl. Acad. Sci. U S A. 100: 8308-8313.
[PubMed: Abstract] [PDF: Fulltext]
Full publication list is Here
Principle Investigator
Shozo Yokoyama, Ph.D.Research Associates
Ruth W. Yokoyama, Ph.D.
Takashi Tada, Ph.D.
Yusuke Takahashi, Ph.D.
Laboratory Technician
Naomi Takenaka, MS.