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The newspaper of The Johns Hopkins University May 8, 2006 | Vol. 35 No. 33
'Third eye' of Lizard sheds light on evolution of color vision

By Audrey Huang
Johns Hopkins Medicine

Lizards have given Johns Hopkins researchers a tantalizing clue to the evolutionary origins of light-sensing cells in people and other species.

Published in the March 17 issue of Science, their study describes how the "side-blotched" lizard's so-called third, or parietal, eye distinguishes two different colors, blue and green, possibly to tell the time of day.

Specialized nerve cells in that eye, which looks more like a spot on the lizard's forehead, use two types of molecular signals to sense light: those found only in simpler animals, like scallops, and those found only in more complex animals, like humans.

Although the blue-green color comparison method used by the parietal eye is not one shared by humans, it does reveal one potential step in the evolution of color vision, the Johns Hopkins researchers say.

Human light-reception cells responsible for color vision are called cone cells or photoreceptors, and they contain only one kind of pigment — red, green or blue — per cell. A color image results when light-triggered signals in the three different types of cone cells are compared by other nerve cells in the retina as well as the brain.

The lizard's parietal eye photoreceptors contain two pigments per cell, blue and green. Having two different pigments allows the cell to respond to two different colors of light and process that information within the same cell.

According to the researchers, when the lizard's third eye sees blue light, the blue pigment triggers a molecule called gustducin, which is very similar to a molecule found in human photoreceptors as well as in the lizard's lateral eyes, those on the sides of its head. But when the lizard's third eye sees green light, the green pigment triggers a different molecule called Go, known as "G-other," which also signals light responses in the light-sensing cells of the scallop and other creatures without a backbone. That Go is found in spineless creatures suggests it is the evolutionarily more ancient light-triggering signal.

Although gustducin and Go are dif-ferent molecules, they are similar and considered "related" proteins. However, gustducin and Go activate different molecular pathways that work against each other physiologically. Blue light and gustducin generate an "off" response in the nerve cell, while green light and Go generate an "on" response.

"It may seem strange to have two opposing signals in the same cell," says the study's senior author, King-Wai Yau, a professor in the Solomon H. Snyder Department of Neuroscience, "but the unique mechanism renders these parietal photoreceptors most active at dawn and dusk."

"So incorporating two different pigments and two separate signaling molecules in one cell may have been an economical way, in a primitive eye with relatively few cell types, to tell the transitions of the day based on changes in the spectrum of sunlight," continues Chih-Ying Su, the first author of the study and a former neuroscience graduate student at Johns Hopkins.

The researchers propose that the lizard's parietal eye photoreceptor cells — by sharing features found in human photoreceptors as well as those found in simpler organisms like the scallop — represent a "missing link" between the light-sensing apparatus in lower animals and ours.

It turns out that some frogs and fish also have a spot on their foreheads that might play the role of a light-sensing third eye. Yau says he hopes to pursue these structures to obtain more clues about how our photoreceptor cells, the rods and cones, came about. He's most curious, he says, about how the same function can be achieved in different ways in different animals.

The researchers were funded by the National Eye Institute and the Allene Reuss Memorial Trust.

Authors on the paper are Su, Dong-Gen Luo, His-Wen Liao and Yau, all of Johns Hopkins; Akihisa Terakita and Yoshinori Shichida, of Kyoto University; and Manija Kazmi and Thomas Sakmar, of the Rockefeller University.


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