The parietal eye of a lizard sheds light on the evolution of the human eye.

Johns Hopkins University researchers have come to a better understanding of how our eyes may have evolved by studying the side-blotched lizard’s parietal eye.

The researchers—Hopkins neuroscientist King-Wai Yau and his former Hopkins grad student/current Yale University post doc Chih-Ying Su— studied the process of color vision in the parietal eye on a cellular level to better understand its function. They found that the reptile’s third eye, which they believe is used to sense the passing of time over a day, combines attributes of invertebrate eyes and vertebrate eyes.

“This in fact is a missing link between the lower animals and the vertebrates,” said Yau, who was the senior author of a study published in a recent issue of Science; Su served as a primary author.

Unlike the human eye, which makes use of five different kinds of neurons called photoreceptors to analyze light, the parietal eye, which is also found in frogs and fish, only has two kinds of neurons.

Human photoreceptors absorb wavelengths of light in a single pigment—either red, green or blue—and pass the information to the retina, which analyzes it, compares the signals from different photoreceptors and then synthesizes an image. Photoreceptors in the lizard’s parietal eye, however, pull double duty: Each cell absorbs two pigments—blue and green—and the comparison between the different pigments takes place at the photoreceptor level, instead of in the retina.

“The comparison of the color signals now begins at the photoreceptor, rather than in the retinal neurons as in a regular eye,” Yau said. “And in so doing, these photoreceptors in the parietal eye are able to give information about the passage of time, because the color spectrum changes over time during the day. And so the signal that comes out of the photoreceptor is already a read-out of the time of day.”

Yau and Su’s principal discovery relates to the exact kind of protein used to pass information from the pigment on the photoreceptors to other neurons. This protein is part of the family known as G proteins, but its precise identity varies between different types of organisms. The lateral eyes of vertebrates use a G protein known as transducin, while invertebrates, like scallops, use a protein called Go, for G-Other.

For the two pigments used in the parietal eye of the side-blotched lizard, the researchers found two different structures of protein communication: One pigment communicated with transducin-like protein called gustducin, as vertebrates would, while the other pigment used Go protein, in an invertebrate fashion.

“Chih-Ying has found both [protein structures] in this parietal eye photoreceptor,” said Yau. “So what Chih-Ying has found is this eye that seems to have both characteristics.”

Their finding suggests that a structure like the parietal eye may represent an evolutionary conjunction between the invertebrate and the vertebrate ways of seeing color.

“The idea is that, early on in evolution, the Go pathway is probably the norm,” Yau said. “And as evolution progresses, then the transducin pathway is added on. And then as evolution progresses further—when you move up to the lateral eyes which are actually very highly specialized structures because they are for acute vision—then the Go pathway is dropped, and we retain only the transducin pathway.”

Originally published April 19, 2006

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