Exploring the evolution of sight
Lizards and several other lower
vertebrates have a third, or parietal, eye. Whereas their other two eyes provide
high-level visual functions such as image processing, this evolutionary vestige
essentially tells time. At dawn, the composition of wavelengths in the sunlight
differs considerably from that in the middle of the day. The same is true at dusk.
Thus, by comparing the relative numbers of these wavelengths, the parietal eye can
mark the passing of each day.
The precise mechanisms by which this occurs long
remained unknown, however. In a 1993 Nature paper, researchers at Syracuse
University in New York described the parietal eye. They found that, although the
photoreceptors involved in image processing in conventional vision hyperpolarize
when exposed to light — that is, they cause the electrical potential inside
the cell to turn more negative — the photoreceptor of the parietal eye can
either hyperpolarize or depolarize, depending on the wavelength. Furthermore, they
noted different light-signaling pathways for these mechanisms. To date, no other
photoreceptor has shown this sort of “chromatic antagonism.”
Scientists have described the “third eye” that can be found in lizards and in some
other lower vertebrates. Unusual photoreceptors, which mark the passing of the day
rather than provide high-level visual function, may be the missing link (with respect
to seeing) between vertebrates and our invertebrate evolutionary forebears.
King-Wai Yau, a researcher at Johns
Hopkins University in Baltimore, took up the question in 1998: Specifically, he
wanted to determine which molecular components are involved in the pathways. He
and colleagues at Kyoto University in Japan and at Rockefeller University in New
York reported their findings.
Their study showed that each of the
pathways consists of a pigment, a G-protein, an enzyme and an ion channel. The pigment
is different in each. The blue-sensitive pigment is called pinopsin and also can
be found in chickens. The green-sensitive pigment turned out to be completely new.
Named parietopsin by the researchers, it does not bear a strong resemblance to any
other pigments. In fact, Yau said, “it seems to be a rather ancient pigment.”
The G-protein also is different in
each of the pathways. In the blue-sensitive pathway, pinopsin interacts with a G-protein
called gustducin, closing the ion channel and causing hyperpolarization. In the
green-sensitive pathway, it interacts with a Go (the o stands for other), opening
the ion channel and causing depolarization.
“What is really interesting,”
Yau noted, “is that the parietopsin-Go signaling pair has a parallel in the
scallop,” an invertebrate. As vertebrates emerged from invertebrates, the
question has always been: How did they evolve the novel components of the image
processing eye? “There must be a transition somewhere,” he continued.
“In fact, this parieto-receptor represents the missing link,” as it
embodies the lower animal pathway as well as a precursor to the pathway currently
found in higher animals. Early vertebrates likely had both pathways. As the latter
continued to evolve, the Go pathway was, in most cases, dropped.
Although Yau’s work is primarily
in the area of biomedicine — he describes the current study as a “naturalist
adventure” — he plans to continue his investigations of the parietal
eye. He and his colleagues have identified the components of the light-signaling
pathway, but many of the dynamics of the signaling process remain unknown.
Understanding these dynamics would
satisfy a general “biological curiosity” and could shed light on signaling
in the retina, one of the major emphases of his overall research.
Science, March 2006, pp. 1617-1621.
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