All Patterns Great and Small

How the leopard got its spots and the zebra its stripes might not be just-so stories much longer. Biologists are beginning to pinpoint the molecular mechanisms animals use to deck themselves out with colorful swirls, stripes, spots and dots.

Insects, fish and mammals may have different tricks. Butterflies and fruit flies, for instance, paint their wing decorations on top of underlying patterns, such as wing veins. Fish may similarly arrange their colored scales according to a “prepattern” but probably decorate their fins according to mathematical principles laid out on a blank slate. And mammals’ beauty marks may arise like those of fish — or via an entirely different mechanism.

British mathematician Alan Turing was one of the first scientists to explain how color patterns might form. In a 1952 paper, he envisioned patterns as self-assembling products of molecular reactions created as two chemicals spread across a uniform surface. Turing’s mathematical models could replicate any pattern found in nature, and scientists soon began hunting for the chemicals responsible for painting butter­fly wings and tiger stripes.

But “Turing has led biologists astray,” charges Fred Nijhout, a developmental physiologist at Duke University in Durham, N.C. “Mathematically it’s completely correct, and there are some physical systems in which it can occur,” he says. Turing formulated his theory before the modern era of molecular genetics, though, and biological systems don’t always work the way his models predict.

For one thing, skin, scales and fur aren’t the blank canvases upon which Turing envisioned color patterns painting themselves. “Sometimes biology is a bit uncooperative because it uses more components than models tell us are necessary,” says Sean B. Carroll, a developmental and evolutionary biologist at the University of Wisconsin–Madison.

A team led by Carroll recently found molecular evidence that preexisting patterns are important in directing color patterns to form. The researchers studied a species of fruit fly called Drosophila guttifera, which sports 16 black spots and four gray shadows on each wing. The black spots develop where wing veins cross, while the shadows form in the spaces between veins.

The mechanism that creates these spots and shadows piggybacks on a system that lays out the wing veins and other body parts, Carroll’s team reported in the April 22 Nature.

Every time scientists found a fly with a new black spot on its wing, the spot always appeared in the place where a new sensory organ had formed, or where wing veins made a new junction. Flies that lacked certain wing veins or sensory organs were also missing spots.

Molecular detective work revealed that a protein called Wingless helps draw the spots. Wingless has many different jobs during fruit fly development, including properly orienting the fly’s body segments, directing where legs and 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 colleague Jessica Turner, Parichy found that delaying the development of yellow 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.