How and why nature created an enormous number of butterfly species has both enthralled scientists and stumped them. Now, University of Wisconsin researchers report that they have found the key: a set of genes that also may solve a baffling puzzle in human evolution.
"It's a mystery that has captured the imagination of scientists over the decades," said Sean Carroll, who headed the research team.
The exquisite splendor of diversity -- the brilliant yellows, oranges and blues painted on the winged canvases of over 17,000 species of butterflies -- may be controlled by the same genetically coded sequence that helps design methods of propulsion for all of creation.
For the wings of fruit flies or the arms and legs of humans, the genes that make such structures are yielding their secrets to scientists. And the 200-million-year-old genetic blueprint is proving to be nearly identical in all organisms.
Perhaps even more remarkably, the genes can have more than one role, the Wisconsin researchers reported in the journal Science.
For the first time, the scientists showed that ancient genes created to perform one function (making limbs) were remodeled to do something completely different (designing wing patterns) by fragile insects whose existence depends on their markings.
"There are not many examples of how animals develop novelty, or invent new things," said Mr. Carroll, a developmental biologist with the Howard Hughes Institute who formerly studied wing genes in fruit flies. "Our study showed the way an organism -- in this case, the butterfly -- used existing tools in new combinations."
The greatest mystery in all of biology is called cell differentiation -- how a fertilized egg divides and develops all the different kinds of cells that constitute an organism. The genes responsible for the programming of cell types are gradually being discovered by scientists.
The new discovery traces a major aspect of differentiation down to a handful of master genes. It may be regarded as a landmark in developmental biology.
Genes are specially coded sequences of DNA that can turn on and off like light switches at specific times during development.
Because all cells are descendants of a single fertilized egg, an organism's genetic code is identical in every cell. However, the genes that are active differ among cells -- even those situated side by side. This phenomenon explains how muscle cells differ from nerve cells even though they both contain exactly the same DNA.
Among the key genes Mr. Carroll studied is one named "distal-less" that switches on in butterfly wing precursor cells when the insect is only a cluster of cells growing inside a tiny egg.
"We were studying the period when the larva figures out that it's time to make adult wings," said Mr. Carroll, noting that the genetic programming of a wing begins long before the structure forms.
In the beginning, the distal-less gene sits in the center of the pre-wing cellular mass called the wing disk. The gene provides instructions so that the surrounding clumps of cells will know how to line up side by side to form a double-layered butterfly wing.
The two layers make it possible for the same butterfly to have drastically different markings on the top and underside of its wings -- a trait that allows the creature to hide from enemies in one posture and attract a mate in the other.
In other organisms, such as humans, a related gene governs the development of limbs by targeting different cells, the scientists said. And the gene switches off once the programming is complete.
But eons ago, butterflies discovered a way to reactivate the gene and make good use of it, Mr. Carroll said.
"In making a butterfly wing, there are actually two steps -- sculpting, and painting that sculpture. The second phase, the embroidering or fine patterning, is where the butterfly has developed something unique."
The phase occurs simultaneously with the most drastic change in the insect's life: metamorphosis. Entombed within its silk cocoon, a caterpillar prepares to face the world as an entirely different creature.
It is then that the dormant distal-less gene switches back on and busily begins to direct the intricate patterning of wing markings.
Some of the same wing cells that were earlier positioned by the gene now secrete pigments such as melanin in a multitude of variations on one basic theme: four dark bands, an "eyespot," a chevron shape and a final edge band. Those are the components of each segment of a butterfly wing and are repeated many times.
Distal-less and other genes discovered by the researchers control the exact pattern of each segment, cell by cell, down to the finest detail. The result is much like a painting of tiny multicolored dots that, when viewed from a distance, become a complete portrait.
Wing patterns, in addition to providing aesthetic pleasure to nature lovers, are crucial to butterfly survival. Markings provide defense against predators, as well as signals and displays for courtship and communication. In short, the patterns encompass the whole history of butterfly evolution.
"Butterflies may display their patterns to either hide from or intimidate birds," explained Dr. H. Frederik Nijhout, a zoologist at Duke University who studied butterfly behavior for 15 years.
According to Dr. Nijhout, eyespot wing patterns appear like large staring eyes to a bird -- hence their name. By fluttering their wings, he said, the insects present a blinking-eye effect, scaring away predators.
The genetic wing-marking program may become altered if conditions such as temperature and humidity change. The result: a radical butterfly makeover to equip the delicate insect better for survival.
For example, Mr. Carroll described a species of West African butterfly whose offspring rework their wing patterns -- either adding or deleting eyespots -- depending on the season in which they are born.
"The gene that controls butterfly markings represents the molecular biology of diversity," Mr. Carroll summed up. "It shows how animals can evolve in order to survive."