Chickens 'one-up' humans in ability to see color
Thursday, 03.11.2010, 09:54pm (GMT+3)
Researchers at Washington University School of
Medicine in St. Louis have peered deep into the eye of the chicken and
found
a masterpiece of biological design. Scientists mapped
five types of light receptors in the chicken's eye. They discovered
the receptors were laid out in interwoven mosaics that
maximized the chicken's ability to see many colors in any given part
of the retina, the light-sensing structure at the back
of the eye. "Based on this analysis, birds have clearly one-upped us
in several ways in terms of color vision," says Joseph
C. Corbo, M.D., Ph.D., senior author and assistant professor of
pathology
and immunology and of genetics. "Color receptor
organization in the chicken retina greatly exceeds that seen in most
other
retinas and certainly that in most mammalian retinas."
Corbo plans follow-up studies of how this organization is established.
He says such insights could eventually help scientists
seeking to use stem cells and other new techniques to treat the nearly
200 genetic disorders that can cause various forms of
blindness. Scientists published their results in the journal PLoS One.
Birds likely owe their superior color vision to not
having spent a period of evolutionary history in the dark, according to
Corbo. Birds, reptiles and mammals are all descended
from a common ancestor, but during the age of the dinosaurs, most
mammals
became nocturnal for millions of years. Vision comes
from light-sensitive photoreceptor cells in the retina. Night-vision
relies on receptors called rods, which flourished in
the mammalian eye during the time of the dinosaurs. Daytime vision
relies
on different receptors, known as cones, that are less
advantageous when an organism is most active at night. Birds, now widely
believed to be descendants of dinosaurs, never spent a
similar period living mostly in darkness. As a result, birds have more
types of cones than mammals. "The human retina has
cones sensitive to red, blue and green wavelengths," Corbo explains.
"Avian
retinas also have a cone that can detect violet
wavelengths, including some ultraviolet, and a specialized receptor
called
a double cone that we believe helps them detect
motion."
In addition, most avian cones have a specialized
structure that Corbo compares to "cellular sunglasses": a lens-like drop
of oil within the cone that is pigmented to filter out
all but a particular range of light. Researchers used these drops to
map the location of the different types of cones on
the chicken retina. They found that the different types of cones were
evenly distributed throughout the retina, but two
cones of the same type were never located next to each other. "This is
the
ideal way to uniformly sample the color space of your
field of vision," Corbo says. "It appears to be a global pattern created
from a simple localized rule: you can be next to other
cones, but not next to the same kind of cone." Corbo speculates that
extra sensitivity to color may help birds in finding
mates, which often involves colorful plumage, or when feeding on berries
or other colorful fruit. "Many of the inherited
conditions that cause blindness in humans affect cones and rods, and it
will
be interesting to see if what we learn of the
organization of the chicken's retina will help us better understand and
repair
such problems in the human eye," Corbo says.
Science Daily
|