Science News - July 17, 2010

wings will grow, and helping set up part of the digestive system. At some point in evolution, Carroll says, an ancestor of D. guttifera and some related fly species co-opted the Wingless system to create color patterns.

The hijacking was accomplished by inserting a new switch into the DNA control panel that governs activity of a pigment-producing gene. Wherever Wingless helps build sensory organs on the wing, the switch flips on pigment production, and a spot or shadow appears.

Carroll doesn’t claim to have solved
the riddle of all animal patterns with
this new work. “I’m happy we planted a
flag on polka-dotted wings,” he says, “but
there’s a whole world of color patterning
left to understand.”
Still, the mechanism might also
occur in other insects. Nijhout says
that butterflies, for instance, might
use Wingless to create stripes on their
wings, since the protein is made in the
same places where bands of color later
appear. A similar mechanism may paint
the eyelike spots on some butterfly
wings, using proteins called Distal-less
and Notch instead of Wingless.

Just because Turing’s models fail to predict how insects decorate their wings doesn’t mean he was completely wrong about all aspects of animal patterning, scientists say. In vertebrates, including fish and mammals, pigment cells may self-organize into patterns the way Turing’s interacting chemicals do.

Animals such as fish, tigers and zebras don’t seem to position their spots and stripes over any particular body structures. And the pattern can be slightly different from one side of the animal to the other. Such clues suggest that pigment cells, which are born in one part of the body and migrate to their eventual location on the skin, assemble themselves into patterns according to a Turing-like mechanism.

“Mathematically, the cellular behaviors [in these animals] meet the behavior of the Turing predictions,” says David Parichy, a developmental and evolutionary biologist at the University of Washington in Seattle.

Still, he says, “it’s quite clear that you
need some type of a prepattern there to
orient the cells.”
Parichy’s work in zebrafish supports
the idea that multiple mechanisms are
in play. He studies the way zebrafish
form multicolored stripes along their
bodies and on their fins. Along with col-
league Jessica Turner, Parichy found
that delaying the development of yel-
low pigment cells as fish transitioned
from larvae to adults could cause their
tail stripes to switch from horizontal to
vertical. Some unknown factor, which
the researchers are investigating now,
must orient pigment cells in the right
direction. And once pigment cells begin
migrating, something has to tell them
where to settle down.

One protein Parichy’s group knows to be involved in making fish patterns is called basonuclin- 2, which helps keep pigment cells healthy and allows the stripes to form. Fish that lack basonuclin- 2 in their skin also lack stripes, the researchers reported last year in

PLoS Genetics. “If the pigment cells are paints, the basonuclin- 2 is essentially priming the canvas to receive these paints,” Parichy says.

Until his team discovered basonuclin- 2’s role in the skin, all of the other proteins known to affect stripe development were found in the pigment cells themselves. So fish may deploy a combination of prepatterning along with a Turing-like mechanism to create their stripes, Parichy says.

With mammals, it remains to be seen if the Turing mechanism alone is at work. Insects and fish are easier to work with in the lab than large cats like tigers or leopards, so scientists know much more about smaller creatures. For now, no genetic evidence indicates mammals might make patterns differently, or that leopard spots are fundamentally different from butterfly dots.

One day, research into color patterns could help illuminate wider questions about the animal world, says Parichy. For instance, pigment patterns are tied to animal behavior, such as mating signals, and can reflect the state of an animal’s health. Studying the relatively simple regulation of color patterns could give biologists clues to how organisms change and adapt other body parts.

And the research raises questions such as how the Wingless prepattern is laid out, what draws the blueprints for that pattern’s prepattern, and so on. It could be an infinite loop that will take many years of colorful research to understand. 

“There’s a
whole world
of color
patterning
left to
understand.”
sEan B. carroll
Glowing gene expression The wing of a young genetically modified fruit fly (Drosophila
guttifera) displays underlying genetic activity where spots (green) and shadows (red) will form.