Butterfly wing-spots--Keep your eyes open

Rcjohnsen rcjohnsen at aol.com
Sun Feb 27 19:57:42 EST 2000

Genetic Diversion, Evolutionary Diversity
E. Pennisi

   When Sean Carroll set out more than 5 years ago to find out how the buckeye
butterfly got its spots, he had only a slight inkling of what this tale could
contribute to an emerging discipline called evo-devo. But he soon discovered
that the eyespot that adorns many butterflies' wings--where it helps confuse
predators looking for a tasty snack--evolved through the reuse of genes already
known to be important for the development of the wing. Because this result
demonstrated that nature could co-opt genes for completely different purposes,
the work guaranteed the story a place in the evo-devo history books.
   Since then, the plot has become even more intriguing. About a year ago,
Carroll, an evolutionary developmental biologist at the University of
Wisconsin, Madison, and his team showed that not just single genes, but an
entire developmental control pathway involving a suite of genes--the one
through which the key gene hedgehog exerts its effects--had been recruited to
specify where eyespots would appear. This suggested that evolution of new
features doesn't require the evolution of new genes or pathways, just a change
in how those pathways are used.
   In new results presented at the meeting, Carroll and his colleagues have now
taken the work a step further, identifying some of the genes that take over
after the eyespot location has been established to determine the sizes, and,
very likely, the colors, of the central spot and any surrounding rings. The
work fills in a "missing link" in understanding how the butterfly sets up the
details of spot formation, says Scott Gilbert, a developmental biologist at
Swarthmore College in Pennsylvania.
   The results show that some of the same genes involved in determining the
eyespot locations are called upon again, this time to set up the exact eyespot
pattern. There's also a great deal of flexibility in how the genes are used.
Whereas all four butterfly species studied appear to use the same patterns of
gene expression to set up a spot, they each use the genes differently to
determine the spot's details. "Everything looks very fixed and conserved to a
certain step, and then there's a little riot going on," Carroll said.
   For the current work, Carroll and Wisconsin's Craig Brunetti decided to
track the activity of three genes, called engrailed, spalt, and Distalless,
during the stage of development when the outlines of the spot and its rings are
actually defined. They just happened to pick those genes from among the many
that help the wing form. "We got really lucky," Carroll reported, as all three
proved active at this time.
   The genes' expression patterns indicated that they help define the spots and
rings. For example, in the East African butterfly, Bicyclus anynana, all three
genes are active in what becomes the white center of the spot, while just spalt
and Distalless are turned on in the black ring flanking it. And in the outer
ring, only engrailed was active.
In contrast, in the buckeye butterfly, Distalless, spalt, and engrailed are
turned on in both the central white spot and its adjacent ring, leaving it up
to another, still unidentified, gene to set up the ring. The combinations of
active genes were different yet again in two other species examined. "What
we've lifted the lid on is a very flexible system," Carroll concluded.
   He suggested that such flexibility in gene usage is tolerable because the
butterfly has already set up the wing and other critical aspects of its body,
so it can tolerate deviations in more superficial characteristics, such as the
appearance of the wing decorations. What results is a rapid and continual
experimentation with new eyespots and eyespot patterns, some of which persist
because, in the context of entire populations, one distracts predators better
than another.
   Despite the progress, many questions remain about eyespot evolution. For
one, Carroll has yet to identify the mutations that enabled butterflies to
co-opt the same genes for so many different functions. Still, he and others are
pleased with what's been learned so far. "[The work] is a very good example of
comparative developmental studies and how it is now possible to do detailed
studies down to the molecular level in nonmodel organisms like butterflies,"
notes Lennart Olsson, an evolutionary developmental biologist at Uppsala
University in Sweden. "Studying later parts of development will become more
common, I hope, [because] from an evolutionary perspective, later parts of
development are more interesting because this is where the viable variation

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Volume 287, Number 5453 Issue of 28 Jan 2000, pp. 570 - 572 
©2000 by The American Association for the Advancement of Science. 
Copyright © 2000 by the American Association for the Advancement of Science.   

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