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 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 per cell - red, green, or blue. 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 the lateral eyes of the lizard - 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 different molecules, they are similar and considered "related" proteins. However, gustducin and Go each 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, Ph.D, a professor in the Solomon H. Snyder Department of Neuroscience at Hopkins, "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," says Chih-Ying Su, Ph.D., the first author of the study and a former neuroscience graduate student at Hopkins.
"It's just like in a small company," says Yau. "You have to delegate each person to do more things."
By sharing features found in human photoreceptors as well as those found in simpler organisms like the scallop, the researchers propose that the lizard's parietal eye photoreceptor cells 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 hopes to pursue these structures to obtain more clues about how our photoreceptor cells, the rods and cones, came about. As he says, he's most curious about how the same function can be achieved in different ways in different animals.