Swallowtails Show How To Co-Opt Nature's Palette
Article originally published October 1, 1998
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MADISON - How some animals change their appearance to better blend in
with their environments or mimic other animals to avoid being eaten, has
always been a puzzle.
But now UW-Madison scientists have found an important piece of that puzzle,
a biochemical switch that helps regulate color pattern formation in the
wings of a swallowtail butterfly that sometimes takes on the appearance
of an unappetizing cousin.
The finding, published earlier this summer in the journal Development,
helps illustrate the underlying, genetically controlled biochemical pathway
that leads to melanism, the abnormal development of dark pigmentation, a
phenomenon seen not only in insects but also in birds and mammals, including
humans.
Importantly, the work helps fill in the big picture of how adaptive traits
such as mimicry and insecticide resistance arise and spread in insect populations.
It also helps illustrate how something as complex as changing the pattern
of a butterfly wing arises from a single gene, said Richard ffrench-Constant,
a UW-Madison professor of entomology and the leader of the team reporting
the discovery. [Dr. ffrench-Constant is now at the University of Bath, UK]
Lending itself to the study was the eastern tiger swallowtail, a butterfly
common in eastern North America and that appears in two forms. It typically
develops an elegant yellow and black "tiger-stripe" wing pattern.
But some female eastern tiger swallowtails appear as all black, possibly
mimicking the look of the pipevine swallowtail, an animal that feeds on
toxic plants and that is distasteful to birds. The black form, said ffrench-Constant,
is a striking example of Batesian mimicry, a survival strategy where some
animals attempt to recreate the look of another animal that is avoided by
predators.
In all butterflies, wing patterns are like mosaics where tiny scales
are individually colored and used to create an astounding array of decoration.
Wings develop in the larvae from embryonic cells and form wing buds. During
the pupal stage, when wings develop into the colorful sails characteristic
of butterflies, wing scale cells develop and are colored just before the
butterfly emerges from its chrysalis.
To see how the swallowtail lays down its colors, ffrench-Constant's team
followed a pathway, a cascade of genetically-triggered biochemical events
that plays out early in development. It is through this pathway, a biochemical
chain reaction initiated by a gene, that the swallowtail determines the
timing and placement of pigments in the scales on the surface of its developing
wings.
"Initially, the attempt was to see if colors are always laid down
in the same order in a range of different butterfly species, and indeed
they are - colored pigments first and then black melanin," said ffrench-Constant.
"In eastern tiger swallowtails this means that the yellow-colored pigments
are deposited before the black."
The ordered appearance of pigments thus appears to be a universal feature
of butterfly wing pattern formation, according to ffrench-Constant. In addition
to the tiger swallowtail, ffrench-Constant's group found the same sequence
of events in three other species of butterfly >from divergent families.
"We've verified the predicted biochemical pathway, and cloned and
examined a key player, dopa decarboxylase (DDC), an enzyme that supplies
dopamine to both yellow and black pigments," ffrench-Constant said.
"It is expressed early in yellow tissues and then later in black. This
difference in the timing of expression in different tissues allows the same
enzyme to be involved in delivering pigments to different color pathways.
"The different pigments are synthesized in common order, always
finishing with black melanin," he said. "This ordered appearance
of pigments appears to be a universal feature of butterfly wing pattern
formation, suggesting that temporal as well as spatial regulation of pigment
synthesis plays a key role in patterning."
But are the same switches then involved in allowing a butterfly to change
its look?
The answer, said ffrench-Constant, seems to be yes.
In the swallowtails that mimic the foul-tasting pipevine swallowtail,
the Wisconsin group observed the early suppression of DDC enzyme activity
and, correspondingly, the shutting down of yellow pigment production in
the developing butterfly's wings. That blank slate is later filled in with
the black pigment melanin when the DDC enzyme switch is again flipped on.
"The presence or absence of DDC activity seems to be correlated
with the decision of tissues to be either yellow or black," said ffrench-Constant.
Other authors of the paper include P. Bernhardt Koch of the University
of Ulm, Germany; Thomas Rocheleau, Katherine Aronstein and Michael Blackburn
of the UW-Madison Department of Entomology; and David N. Keys and Sean B.
Carroll of the Howard Hughes Medical Institute at UW-Madison.
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Writer: Terry Devitt, (608) 262-8282
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