News: Skin Color Evolution In Fish And Humans
- From: Ye Old One <usenet@xxxxxxxxx>
- Date: Fri, 14 Dec 2007 14:18:10 GMT
Skin Color Evolution In Fish And Humans Determined By Same Genetic
Machinery
http://www.sciencedaily.com/releases/2007/12/071213121006.htm
ScienceDaily (Dec. 13, 2007) ? When humans began to migrate out of
Africa about 100,000 years ago, their skin color gradually changed to
adapt to their new environments. And when the last Ice Age ended about
10,000 years ago, marine ancestors of ocean-dwelling stickleback fish
experienced dramatic changes in skin coloring as they colonized newly
formed lakes and streams. New research shows that despite the vast
evolutionary gulf between humans and the three-spined stickleback
fish, the two species have adopted a common genetic strategy to
acquire the skin pigmentation that would help each species thrive in
their new environments.
The researchers, led by Howard Hughes Medical Institute investigator
David Kingsley, published their findings in the December 14, 2007,
issue of the journal Cell. Kingsley and first author Craig Miller are
at the Stanford University School of Medicine, and other co-authors
are from the University of Porto in Portugal, the University of
British Columbia, the University of Chicago, and the Pennsylvania
State University Further studies of stickleback, they say, may reveal
other malleable pieces of genetic machinery both fish and humans have
used for adaptation.
The stickleback has become a premier model organism for studying
evolution because of its extraordinary evolutionary history, said
Kingsley. "Sticklebacks have undergone one of the most recent and
dramatic evolutionary radiations on earth," he said. When the last Ice
Age ended, giant glaciers melted and created thousands of lakes and
streams in North America, Europe, and Asia. These waters were
colonized by the stickleback's marine ancestors, which subsequently
adapted to life in freshwater. "This created a multitude of little
evolutionary experiments, in which these isolated populations of fish
adapted to the new food sources, predators, water color, and water
temperature that they found in these new environments," Kingsley
explained.
Among those adaptations were new colorations that helped the fish
camouflage themselves, distinguish species, and attract mates in their
new environments. Until now, however, scientists had not understood
what genetic factors drove the changes in skin pigmentation.
Human populations have also undergone pigmentation changes as they
have adapted to life in new environments. The ecological reasons for
those changes may be quite different from the forces driving the
evolution of pigmentation in sticklebacks, said Kingsley. As human
populations migrated out of Africa into northern climates, the need
for darker pigmentation necessary to protect against the intense
tropical sun diminished. With skin that was more transparent to
sunlight, humans were better able to produce sufficient vitamin D in
their new climate.
To begin to understand the genetic basis of skin pigmentation changes
in fish, Kingsley and his colleagues crossed stickleback species that
had different pigmentation patterns and used genetic markers and the
recently completed sequence map of the fish's genome to search for the
mechanism regulating stickleback pigmentation. They searched for
chromosome segments in the offspring that were always associated with
inheritance of dark or light gills and skin. Through detailed mapping
of one such segment, Kingsley and his colleagues found that a gene
called Kitlg (short for "Kit ligand") was associated with pigmentation
inheritance. Kitlg was an excellent candidate for regulating
pigmentation because mutant forms of the corresponding gene in mice
produce changes in fur color, said Kingsley.
The Kitlg gene is involved in a variety of biological processes,
including germ cell development, pigment cell development, and
hematopoiesis. Light-colored fish have regulatory mutations that
reduce expression of the Kitlg gene in gills and skin, but that do not
reduce the gene's function in other tissues. "By altering expression
of this gene in one particular place in the body, the fish can fine
tune the level of expression of that factor in some tissues but not
others," said Kingsley. "That lets evolution produce a big local
effect on a trait like color while preserving the other functions of
the gene."
Humans also have a Kitlg gene, and Kingsley and his colleagues
wondered if it played a role in regulating the pigmentation of human
skin. One clue they had came from previous research by other groups
that had revealed that the human Kitlg gene has undergone different
changes among different human populations, suggesting that it is
evolutionarily significant.
Kingsley and his colleagues tested whether the different human
versions of the Kitlg gene are associated with changes in skin color.
Humans with two copies of the African form of the Kitlg gene had
darker skin color than people with one or two copies of the new Kitlg
variant that is common in Europe and Asia.
Knowing that people had also adapted lighter skin when they migrated
north, Kingsley wondered whether mutations in the same gene accounted
for light pigmentation in people living in northern climes. In the
north, where less sunlight reaches the ground, lighter coloring helps
people absorb enough sunlight to produce vitamin D.
Kingsley and his colleagues collected DNA from people with a variety
of skin colors to look for alterations in the Kit ligand gene. Sure
enough, people with lighter skin had an altered form of the gene. He
said this gene isn?t alone in controlling a person?s skin color, but
it does seem to account for about 20 percent of the differences in
pigmentation between people of African and northern European descent.
?It is the same genetic mechanism between organisms that are very
different from each other,? Kingsley said. This gene is known to make
a protein that plays a role in maintaining the melanocyte skin cells
that control pigmentation.
In terms of how evolution progresses, this gene would be a large ladle
of dye that helps set the paint color apart from the original.
Additional genetic changes account for the exact color of each
person?s skin.
"Although multiple chromosomal regions contribute to the complex trait
of pigmentation in both fish and humans, we have identified one gene
that plays a central role in color changes in both species," said
Kingsley.
"Since fish and humans look so different, people are often surprised
that common mechanisms may extend across both organisms," he said.
"But there are real parallels between the evolutionary history of
sticklebacks and humans. Sticklebacks migrated out of the ocean into
new environments about ten thousand years ago. And they breed about
once every one or two years, giving them five thousand to ten thousand
generations to adapt to new environments."
Although modern humans arose in Africa, they are thought to have
migrated out of Africa in the last 100,000 years. "Humans breed about
once every 20 years, giving them about 5,000 generations or so to
emerge from an ancestral environment and colonize and adapt to new
environments around the world," Kingsley added. "So despite the
difference in total years, the underlying process is actually quite
similar. Whether it be fish or humans, there were small migrating
populations encountering new environments and evolving significant
changes in some traits in a relatively short time. And the genetic
mechanisms that can produce these changes may be so constrained that
evolution will tend to use the same sorts of genes in different
organisms."
Kingsley and his colleagues are now exploring the genetic basis of
other evolved traits in the stickleback that could find a parallel in
humans. "And given the degree to which evolutionary mechanisms appear
to be shared between populations and organisms, we're optimistic about
finding the particular genes that underlie other recent adaptations to
changing environments in both fish and humans," he said.
Adapted from materials provided by Howard Hughes Medical Institute.
--
Bob.
.
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