Researchers at the
Johns Hopkins School of Medicine and their colleagues
have found that mice simply expressing a human light
receptor in addition to their own can acquire new color
vision, a sign that the brain can adapt far more rapidly to
new sensory information than anticipated.
This work, appearing March 23 in Science, also
suggests that when the first ancestral primate inherited a
new type of photoreceptor more than 40 million years ago,
it probably experienced immediate color enhancement, which
may have allowed this trait to spread quickly.
"If you gave mice a new sensory input at the front
end, could their brains learn to make use of the extra data
at the back end?" asks Jeremy Nathans, professor of molecular biology and
genetics,
neuroscience, and
ophthalmology at Johns Hopkins. "The answer is,
remarkably, yes. They did not require additional
generations to evolve new sight."
Retinas of primates such as humans and monkeys are
unique among mammals in that they have three visual
receptors that absorb short (blue), medium (green) and long
(red) wavelengths of light. Mice, like other mammals, have
only two: one for short and one for medium wavelengths.
In the study, the researchers designed a "knock-in"
mouse that has one copy of its medium wavelength receptor
replaced with the human long wavelength receptor, so both
were expressed in the retina. The human receptors were
biologically functional in the mice, but the real question
was whether the mice could use the new visual
information.
To address this question, the researchers used a
classic preference test. Mice set before three light panels
were trained to touch the one panel that appeared to differ
from the other two; a correct answer was rewarded with a
drop of soy milk.
To circumvent thorny issues related to the subjective
nature of color perception — everyone who has had a
discussion as to whether the "green" he sees is the same as
the "green" his friend sees can attest to this —
the researchers tested only whether the mice could
discriminate among the lights.
"Each photoreceptor absorbs a range of wavelengths,
but the efficiency changes with wavelength," Nathans said.
"For example, one photoreceptor might absorb green light
only half as efficiently as red light. If an animal had
only this type of photoreceptor, then a green light that
was twice as bright as a red light would look identical to
the red one. But if the animal adds a second photoreceptor
with different absorption properties, then by comparing
both receptors, the red and green lights could always be
distinguished."
Normal mice failed to discriminate yellow vs. red
lights when the light intensities were set to give equal
activation of their middle wavelength receptor. However,
mice with both the human long wavelength and the mouse
middle wavelength receptors learned to tell the difference,
although it took more than 10,000 trials to learn to make
the distinction.
Nathans suggests that these knock-in mice mimic how
our earliest primate ancestors acquired trichromatic
vision, color vision based on three receptors. At some
point in the past, random mutations created a variant of
one receptor gene, located on the X chromosome, producing
two different receptor types. Present-day New World (South
American) monkeys still use this system, which means that
in these monkeys only certain females can acquire
trichromatic color vision.
In contrast, among Old World (African) primates such
as humans, the two different X chromosome genes duplicated
so that each X chromosome now carries the genes for both
receptor types, giving both males and females trichromatic
color vision.
"You could say that the original primate color vision
system, and the one that New World monkeys still use today,
is the poor man's — or, to be accurate, poor
woman's — version of color vision," Nathans
said.
The research was funded by the National Eye Institute
and the Howard Hughes Medical Institute.
Authors on the paper are Nathans and Hugh Cahill, both
of Johns Hopkins; and Gerald Jacobs and Gary Williams, of
the University of California, Santa Barbara.
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