50 THE SCIENTIST | the-scientist.com
© JULIA MOORE, WWW.MOOREILLUSTRATIONS.COMT
he tale of the peppered moth (Biston betularia)
is, quite literally, a textbook story of adaptive evo-
lution. Back in the early 19th century, only one
form of these now-iconic moths was known: a
light variety speckled with dark spots (hence the
name). In 1864, though, a naturalist in England
first documented an all-dark moth—what seemed
at the time to be little more than a curious example of melanin
overproduction. Then came the Industrial Revolution, and dark
moths took over, their inky wings reducing the odds of the noc-
turnal insects being eaten while they rested on soot-stained trees
during the day. More than a century later, scientists discovered
that the genetic tweak underscoring the moths’ dark pigmenta-
tion was a transposable element (TE).^1
About 200 years ago, researchers estimate, nearly 22,000
nucleotides leapt into the first intron of a moth gene called cortex,
and in doing so, dramatically increased the production of the pro-
tein it codes for—a key player in wing development and coloration.
TEs, also called transposons or jumping genes, are often cast
in a negative evolutionary light. And there is a reason for that:
when these sequences insert themselves into new places in the
genome, they can mess up genes or alter their expression. They’re
sometimes called junk DNA, or worse, genomic parasites, the idea
being that they would mutate their host genomes into oblivion if
they weren’t almost always silenced by epigenetic modifications
such as methylation. But recent research is illuminating the intri-
cacies of TE function and adding texture to this simplistic model.
“I don’t see them as parasites,” says Marie Mirouze, a plant gen-
omicist at the French National Research Institute for Sustainable
Development (IRD) in Marseille. “I rather see them as living in
symbiosis with the host.” Indeed, some TEs become so embedded
in their hosts’ biology that they are considered “domesticated,” los-
ing the ability to jump around. For some time now, researchers
have uncovered various ways in which that symbiosis has benefited
organisms, from the development of placentas in mammals to the
existence of adaptive immunity in most vertebrates.
But domestication is just one way TEs can drive evolution.
Increasingly, scientists are uncovering examples of “wild” TEs—
ones that remain autonomous and move about if not actively
repressed by the cell—that influence the biology of their hosts
in intriguing ways. “So many studies these days [are] pointing
to transposons as the answer,” says Edward Chuong, a genomi-cist with the BioFrontiers Institute at the University of Colorado
Boulder. Almost no matter what genetic question is asked, he
adds, TEs “tend to be there in the end.”
Chuong points out that TE-derived mutations likely play a sig-
nificant role in evolution because of their selfish nature: to repli-
cate, transposons need to “convince” the cell’s machinery to help,
so their sequences contain lots of elements that can recruit tran-
scription factors and otherwise regulate gene expression. That
means that every time a TE moves, it carries regulatory motifs
with it that can immediately begin influencing the expression of
nearby genes, with potential functional outcomes that influence
the overall fitness of the organism.
“If a gene were to evolve a new regulatory element simply by
base pair changes, that would presumably take a lot of steps,”
Chuong says. But with TEs, “in a single event... a gene could
acquire a whole new regulatory element.” And that’s just if the ele-
ment lands near a gene. If it lands within one, it can directly add
to the code of exons, or alter intron motifs associated with splic-
ing, or otherwise impact the gene itself. (See illustration on page
17.) Plus, TEs don’t always jump alone: sometimes they sweep up
sections of nearby code as they leap, which can create duplicates
of whole genes or other functional sequences.According to Chuong and others, TEs’ outsized potential
both to affect expression and to alter the genetic code make them
important players in evolution, alongside other forms of mutation
that are major sources of genetic diversity—the raw material of
natural selection. While TEs can sometimes be detrimental, “in
the long term they can also be beneficial, and the host can get
some advantage [from] the presence of transposable elements,”
Mirouze says. With more and more examples now coming out, she
adds, it’s becoming clear that TEs are “an engine for evolution.”From useful to junk and back again
The notion that TEs are vital to genomes, and not parasites or
trash, harks back to the 1950s and Barbara McClintock, who won
a Nobel Prize in 1983 for the discovery of transposons in maize:
she proposed that TEs play an important role in gene expres-
sion in the very first paper on them.^2 But the idea that genetic
elements could be mobile clashed with the prevailing view of an
organism’s genome as fixed. It would be decades before transpo-
sons were described from other organisms and their near-universal
presence in genomes became clear. By then, researchers had fig-I don’t see them as parasites. I rather see
them as living in symbiosis with the host.
—Marie Mirouze, French National
Research Institute for Sustainable Development