Salmon migrating from the open ocean to inland waters do
more than swim upstream. To navigate the murkier freshwater streams and reach a
spot to spawn, the fish have evolved a means to enhance their ability to see
infrared light. Humans lack this evolutionary adaptation.
For nearly a century, scientists have puzzled over how
salmon as well as other freshwater fish and amphibians, including frogs, easily
shift their vision from marine or terrestrial environments - where the light
environment is blue-green - to the waters of inland steams. In such streams,
mud, algae and other particles filter out light from the blue end of the visual
spectrum, creating a light environment that shifts to the red and infrared end
of the spectrum.
Now, scientists at Washington University School of Medicine
in St. Louis report in the journal Current Biology http://www.cell.com/current-biology/home
that they have solved the mystery.
"We've discovered an enzyme that switches the visual
systems of some fish and amphibians and supercharges their ability to see
infrared light," said senior author Joseph Corbo, MD, PhD, associate
professor of pathology and immunology. "For example, when salmon migrate
from the ocean to inland streams, they turn on this enzyme, activating a
chemical reaction that shifts the visual system, helping the fish peer more
deeply into murky water."
As it turns out, the enzyme - called Cyp27c1 - is closely
linked to vitamin A, long known to promote good vision, especially in low
light. The enzyme converts vitamin A1 to vitamin A2; the latter has remarkable
properties to enhance the ability to see longer wavelength light such as red
and infrared light.
The findings could lead to advances in biomedical research,
particularly in optogenetics, a hot, new field in which light is used to
control the firing of neurons in the brain. Optogenetic applications currently
are limited to visible light, which penetrates only the top layer of neural
But if scientists are able to incorporate the newly
discovered enzyme, they may be able to activate photosensitive neurons with
infrared light, which penetrates much deeper. "Just as the enzyme helps
fish peer into murky water, it could help us peer deeper into the brain,"
Corbo and his team made the enzyme discovery in zebrafish -
tiny, transparent freshwater fish that remain a staple of laboratory research.
They confirmed their findings in bullfrogs, whose eyes are uniquely designed
for the light environments of both air and freshwater.
Bullfrogs sit with their eyes at the water's surface so that
they can look up into the air and down into the water at the same time. The
researchers found vitamin A2 and the enzyme Cyp27c1 right where they expected:
in the upper half of the bullfrog's eyes that peer down into the water, but not
in the lower half which looks upward into the air.
Furthermore, the scientists showed that zebrafish with
normal copies of the cyp27c1 gene move toward infrared light shined into a dark
aquarium. But fish with disabled cyp27c1 genes continue to behave like they are
in the dark, whether or not the infrared light is on.
Humans have a form of the same gene, but it is not turned on
in the eye. Thus, people are not able to enhance their infrared vision in the
same way fish can. To do so, they must wear night-vision goggles. "We
don't know yet how this enzyme is utilized in the human body," Corbo said.
"But just because our eyes don't make vitamin A2
doesn't mean we can't use it," he said. Research on medical students in
the 1940s showed that people who consume vitamin A2 have an enhanced ability to
detect red and infrared light. In 2013, a group of "biohackers"
successfully crowdfunded an experiment to try to extend their vision into the
near-infrared spectrum by eating a diet supplemented with vitamin A2.
"I wouldn't necessarily recommend following their
dietary advice, but the concept is sound," Corbo said.