Download Spineless Fish and Dark Flies Prove Gene Regulation Crucial

NEWS OF THE WEEK
EVOLUTION
1612
18 DECEMBER 2009
VOL 326
SCIENCE
Published by AAAS
www.sciencemag.org
CREDIT: MARK REBEIZ ET AL., SCIENCE
Almost 3 years ago, biologists got
University of Wisconsin, Madiinto a tussle over what drives morson, and their colleagues followed
phological evolution: changes in
up on work into why the normally
the protein-coding portions of
yellow abdomens of fruit flies
genes or changes in the DNA
(Drosophila melanogaster) have
regions that regulate gene activity.
become dark in some African
At the time, some researchers felt
populations living at high elevathere was little hard evidence to
tions. Earlier work done by collabsupport the idea that regulatory
orators John Pool of the Univerchanges were indeed important
sity of California, Berkeley, and
(Science, 8 August 2008, p. 760).
Charles Aquadro of Cornell UniNow, on page 1663 and in last
versity suggested that this color
week’s Science Express (www.
change was due to a change in the
sciencemag.org/cgi/content/
activity of a gene called ebony.
abstract/science.1182213), two
The new work narrows down
teams not only independently
the cause to an enhancer upstream
report that changes in regulatory Color coordinated. In Africa, lowland fruit flies are light-colored, whereas those of the gene. By dissecting the
DNA were responsible for an adap- at high altitudes are dark, all because of a change in gene regulation.
function of this region in different
tation in natural populations of fish
Drosophila populations, ones
and insects, but each group has also pieced in both fish types. But it’s active in the pelvic with either dark or light abdomens, the
together details of the underlying genetic alter- region of the saltwater animals and not in the researchers identified five mutations that
ations in those animals. “They provide beauti- freshwater ones, Kingsley’s team found. Yet reduced ebony expression to varying degrees.
ful and convincing examples of how [certain] the gene’s coding region is virtually unchanged
Three of those mutations are present in
regulatory elements can be lost or modified to between fresh- and saltwater fish, suggesting Drosophila with light abdomens, but the
reduce [gene] expression, ultimately causing regulatory DNA is the difference.
dark flies from high altitudes also have two
morphological change,” says Hopi Hoekstra,
The new work confirms that suspicion. newer mutations. These two have the biggest
an evolutionary biologist at Harvard University When Kingsley, his graduate student Yingguang effects on squelching ebony expression, but
and one of the chief skeptics.
Frank Chan, now at the Max Planck Institute all five mutations combine to create the full
In one case, the same piece of regulatory for Evolutionary Biology in Plön, Germany, color change.
DNA was lost in different freshwater fish and their colleagues studied fish with and
The work “leads to two very important
populations, each time causing the loss of without a pelvis, they determined that DNA conclusions about classic evolutionary
pelvic spines. In the other case, the darkening upstream of Pitx1 was responsible for genetic questions,” says William Cresko, an
of a fruit fly took place through an accumula- silencing the gene. They broke that DNA evolutionary biologist at the University of
tion of small mutations in regulatory DNA. into fragments and determined exactly Oregon, Eugene. One is that a “big” evoluTaken together with other discoveries of non- which piece, an enhancer called Pel, turned tionary step—a color change—actually can
coding regions involved in evolution, “there Pitx1 on in the pelvis. When the researchers come about because of several little steps:
is broad support now” that changes in regula- compared the sequence of Pel in saltwater multiple mutations in the enhancer. Secondly,
tory DNA can generate morphological varia- sticklebacks and freshwater ones from nine it shows that adaptive mutations sometimes
tion, says Günter Wagner, an evolutionary different lakes, they found the latter popula- exist unnoticed in a population.
developmental biologist at Yale University.
tions each had various amounts of missing
Others are also in hot pursuit of evolutionThe fish study took place at Stanford Uni- DNA bases in the enhancer region, includ- ary changes based on regulatory elements.
versity, where David Kingsley has spent the ing an apparently key 488-base region that is Wittkopp is homing in on mutations in a regpast decade tracking down the genetic basis for absent in most of the lake fish.
ulatory region that change body color
why three-spined sticklebacks (Gasterosteus
“The fact that multiple alleles, with unique between two closely related species of fruit
aculeatus) that live in salt water have spines mutations, were observed is surprising and flies (Science, 23 October, p. 540). And
jutting from their belly whereas some of their emphasizes the tinkering nature of the evo- Hoekstra’s group has a regulatory region in its
freshwater counterparts do not. In salt water, lutionary process,” says Patricia Wittkopp, an sights that seems to underlie mouse color
the pelvic spines help make the stickleback evolutionary biologist at the University of changes (Science, 28 August, p. 1095). But
difficult for a predator to swallow, but the pro- Michigan, Ann Arbor.
developing a broad understanding of the relatuberances become a hindrance in fresh water
To conf irm Pel’s importance, the re- tive roles of shifts in gene regulation versus
as they could serve as handholds onto which searchers added a saltwater stickleback’s ver- changes in the proteins encoded by genes
dragonflies and other carnivorous insects grab. sion of the enhancer and Pitx1 to the fertilized “will require many more case studies from
In 2004, Kingsley and his colleagues eggs of freshwater sticklebacks that don’t nor- across a wide array of taxa,” cautions Cresko.
tracked the disappearance of the pelvis in mul- mally sport pelvic spines. The resulting trans- Given how much work the two new studies
tiple populations of freshwater sticklebacks genic freshwater fry developed the structures.
took, he adds, it may require a “quantum
primarily to a gene called Pitx1. The gene is
For the insect study, evolutionary biolo- leap” in technology for that to be possible.
expressed during development in many places gists Sean Carroll and Mark Rebeiz of the
–ELIZABETH PENNISI
Downloaded from www.sciencemag.org on January 10, 2010
Spineless Fish and Dark Flies Prove Gene Regulation Crucial