Colorful Nature

Some people and animals are color blind. For those who can see the world of colors there is endless wonder. So how do these colors happen in nature? How do you choose to be pink, black or violet? How different creatures in the animal kingdom — from colorful birds and reef fish to butterflies and snakes — make and deploy their artful designs is one of nature's many deep secrets. Now, however, a team of researchers from the Howard Hughes Medical Institute at the University of Wisconsin-Madison has exposed the fine details of how animals make new body ornamentation from scratch. The work, the result of years long and laborious experimentation, was published April 7 in the journal Nature.

How do you tell one cell or part of a body to be white and the next adjacent area or cell black as in a zebra? In essence a pigment selection is made and the amount that is needed is added to the cells in question.

Colors are useful often for mating or camouflage. The colors selected are a result of sexual/environmental adaptation.

"How do you generate complex patterns? This is a question that has interested biologists for a really long time," says Sean Carroll, a UW-Madison molecular biologist and the senior author of the Nature report. "In this case, we at first had no clue. But now we think we've figured out all the key ingredients and we believe they are generally applicable (to many animals)."

The new study is important because it is the first to provide concrete evidence for a long hypothesized system for generating animal color patterns, be they stripes, spots or any of the myriad designs animals use to camouflage themselves or find a mate. In particular, the Wisconsin group is the first to identify a color-inducing morphogen, a diffusible protein that tells certain cells to make pigment.

To ferret out the secret of animal ornamentation, Carroll and his UW-Madison colleagues, Thomas Werner and Shigeyuki Koshikawa, and Thomas Williams, now at the University of Dayton, pried loose the molecular details and evolutionary history of how a species of North American fruit fly, Drosophila guttifera, generates a complex pattern of 16 wing spots.

The fruit fly is a little insect about 3 millimeters long, of the kind that accumulates around spoiled fruit. It is also one of the most valuable of organisms in biological research, particularly in genetics and developmental biology. Drosophila has been used as a model organism for research for almost a century, and today, several thousand scientists are working on many different aspects of the fruit fly. It is short lived and easy to study in its multiple generations.

The group discovered a morphogen, a protein present in embryonic tissue and encoded by a gene known as Wingless, which seems to be a linchpin of wing decoration. Late in wing development, the Wingless morphogen is produced and diffuses through tissue where it prompts cells in certain areas of the wing to make pigment. "It acts by triggering responding cells to do things, in this case make color," Carroll explains.

In Drosophila guttifera, the morphogen acts in proximity to existing physical landmarks such as the intersections of veins and cross veins on the wing. The positioning of the spots, in short, is dictated by these pre-existing patterns, notes Carroll: "The Wingless molecule is deployed in this species at specific points in time and in specific places — the places where the spots are going to be."

The role of the Wingless morphogen was detailed by the painstaking genetic manipulation of flies that took three years and the injection of nearly 20,000 fly embryos to accomplish. Complicating the project is the fact that Drosophila guttifera is little used in research and its genome has not been sequenced.

However, by inserting the Wingless gene into different parts of the fly's genome, the team was able to successfully manipulate the decoration of the fly's wing, creating stripes instead of spots, and patterns not seen in nature. "We can make custom flies," notes Carroll. By manipulating the gene, "we can make striped flies out of spotted flies."

In addition to working out the molecular details of how the fly colors its wings, Carroll's group was also able to deduce the evolutionary history of wing coloring in Drosophila guttifera.

In short, says Carroll, the patterns found on the wings of Drosophila guttifera came about through the use of the Wingless gene acting as a color trigger.

Although the study was conducted in a lowly fruit fly, the principles uncovered may well be applicable to other species. Unique or "unnatural" color combinations may be possible once the science is unraveled.

For further information: http://www.eurekalert.org/pub_releases/2010-04/uow-cfa040110.php