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
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
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
of Neuroscience, "but the unique mechanism renders
these parietal photoreceptors most active at dawn and
"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
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.