Adaptation coincides with the '60s cleanup of toxic pollution in Seattle's
Lake Washington
SEATTLE, May 15 /PRNewswire/ -- Evolution is supposed to inch forward
over eons, but sometimes, at least in the case of a little fish called the
threespine stickleback, the process can go in relative warp-speed reverse,
according to a study led by researchers at Fred Hutchinson Cancer Research
Center and published online today ahead of print in the May 20 issue of
Current Biology (Cell Press).
"There are not many documented examples of reverse evolution in
nature," said senior author Catherine "Katie" Peichel, Ph.D., "but perhaps
that's just because people haven't really looked."
Peichel and colleagues turned their gaze to the sticklebacks that live
in Lake Washington, the largest of three major lakes in the Seattle area.
Five decades ago, the lake was, quite literally, a cesspool, murky with an
overgrowth of blue-green algae that thrived on the 20 million gallons of
phosphorus-rich sewage pumped into its waters each day. Thanks to a $140
million cleanup effort in the mid-'60s -- at the time considered the most
costly pollution-control effort in the nation -- today the lake and its
waterfront are a pristine playground for boaters and billionaires.
It's precisely that cleanup effort that sparked the reverse evolution,
Peichel and colleagues surmise. Back when the lake was polluted, the
transparency of its water was low, affording a range of vision only about
30 inches deep. The tainted, mucky water provided the sticklebacks with an
opaque blanket of security against predators such as cutthroat trout, and
so the fish needed little bony armor to keep them from being eaten by the
trout.
In 1968, after the cleanup was complete, the lake's transparency
reached a depth of 10 feet. Today, the water's clarity approaches 25 feet.
Lacking the cover of darkness they once enjoyed, over the past 40 years
about half of Lake Washington sticklebacks have evolved to become fully
armored, with bony plates protecting their bodies from head to tail. For
example, in the late '60s, only 6 percent of sticklebacks in Lake
Washington were completely plated. Today, 49 percent are fully plated and
35 percent are partially plated, with about half of their bodies shielded
in bony armor. This rapid, dramatic adaptation is actually an example of
evolution in reverse, because the normal evolutionary tendency for
freshwater sticklebacks runs toward less armor plating, not more.
"We propose that the most likely cause of this reverse evolution in the
sticklebacks is from the higher levels of trout predation after the sudden
increase in water transparency," said Peichel, whose Hutchinson Center lab
has established the stickleback as a new model for studying complex genetic
traits. By examining multifaceted traits in the fish, such as body type and
behavior, Peichel and colleagues shed light on the genetic networks at play
in other complex traits, such as cancer and other common human diseases.
The ability of the fish to quickly adapt to environmental changes such
as increased predation by the cutthroat trout is due, Peichel believes, to
their rich genetic variation. The sticklebacks in Lake Washington contain
DNA from both marine (saltwater) fish, which tend to be fully plated, and
freshwater sticklebacks, which tend to be low-plated. When environmental
pressures called for increased plating, some of the fish had copies of
genes that controlled for both low and full plating, and so natural
selection favored the latter.
"Having a lot of genetic variation in the population means that if the
environment changes, there may be some gene variant that does better in
that new environment than in the previous one, and so nature selects for
it. Genetic variation increases the chance of overall survival of the
species," she said.
The researchers' findings challenge a widely held theory behind rapid
evolutionary change, the idea of "phenotypic plasticity" -- when an
organism can take on different characteristics independent of genetic
influences. Body type is one such example. "There is some genetic component
to body size, but if you eat more nutritious food as a child you're
probably going to grow taller than someone who has the same genes but may
not have had as good of a diet growing up," Peichel said. "Our findings
challenge the primary role of phenotypic plasticity in rapid evolutionary
change."
The gene that controls for plating is called Eda, which comes in two
forms: one causes low plating and the other complete plating. Peichel was
the first person to home in on the neighborhood where the Eda gene lives
while a postdoctoral researcher in the laboratory of David Kingsley, Ph.D.,
at Stanford University.
In humans, mutations in this gene cause a syndrome called ectodermal
dysplasia, a group of more than 100 inherited disorders that impact the
ectoderm, the outer layer of tissue involved in the formation of many parts
of the body, including the skin, nails, hair, teeth and sweat glands.
"There's probably a developmental correlation between these external
structures in humans and the bony plates on the fish," Peichel said. "It
also looks like the Eda gene was probably important for human evolution
although we don't really know in what context," she said.
Collaborators on the study included researchers from the University of
Washington in Seattle; the University of Texas in Austin; Gifu Keizai
University in Ogaki, Gifu, Japan; and the Research Institute for Humanity
and Nature in Kyoto, Japan. The work was supported by a Uehara Memorial
Fellowship, the Packard Foundation, Seattle Public Utilities, Water and
People Project, and a Burroughs Wellcome Fund Career Award in the
Biomedical Sciences.
At Fred Hutchinson Cancer Research Center, our interdisciplinary teams
of world-renowned scientists and humanitarians work together to prevent,
diagnose and treat cancer, HIV/AIDS and other diseases. Our researchers,
including three Nobel laureates, bring a relentless pursuit and passion for
health, knowledge and hope to their work and to the world. For more
information, please visit fhcrc.org.
CONTACT
Kristen Woodward
206-667-5095
kwoodwar@fhcrc.org
SOURCE Fred Hutchinson Cancer Research Center
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Related links:
http://www.fhcrc.org
CONTACT:
Kristen Woodward of Fred Hutchinson Cancer
Research Center, +1-206-667-5095, kwoodwar@fhcrc.org
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