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30 November 2000
Dress For Reproductive Success
by George Atkinson

In many animals, the difference between male and female is strikingly apparent. This is especially true in birds, fish and some insects where colors and other adornments can spell the difference between mating success and failure.

Now, thanks to the lowly fruit fly and a team of scientists at the University of Wisconsin-Madison and at Washington University in St. Louis, one genetic circuit that governs sexual dimorphism - the diagnostic differences between the sexes - has been found and characterized.

The discovery, described in the Thursday, Nov. 30, edition of the British scientific journal Nature, is important because it not only shows how and why animals dress for reproductive success, but provides a glimpse of the genetic changes that, over time, lead to the evolution of new animal species.

"Fundamentally, the difference between species is in their DNA," says Sean B. Carroll, a professor of genetics at the Howard Hughes Medical Institute at UW-Madison and an author of the Nature paper. "And this genetic circuit holds the gene that makes a key difference between fly species."

In the fruit fly, specifically the laboratory workhorse Drosophila melanogaster, one of the obvious visual signals of its sex is body pigmentation: the rear end of the melanogaster male is heavily pigmented and the female's is not. This new "fruit fly fashion" has evolved only recently in a relatively small subset of Drosophila species, according to Carroll, and co-authors Artyom Kopp, also of HHMI at UW-Madison, and Ian Duncan of Washington University.

The researchers found that a gene called "bric-a-brac" establishes the difference between melanogaster females and males by suppressing pigmentation in females. However, the same gene functions in both sexes in other fly species where male-specific pigmentation is absent and males and females look pretty much the same.

Beginning with Darwin, scientists have believed that animals assume gaudy colors to promote themselves as potential mates, and that this dressing up is a major force in animal evolution. The present role of bric-a-brac, says Carroll, has probably been shaped by the process of "sexual selection" because the pigmentation patterns specified by this gene affect mating preferences.

But in investigations of the genetic controls for gender-based pigmentation in fruit flies, the HHMI team at Wisconsin found that, for the female, the sex appeal of a pattern or color wears off over time.

In experiments with male flies engineered to have the same abdominal stripes as the female melanogaster, the courted females were smitten no less than when confronted with a male flying all the colors of machismo. According to Kopp, what this suggests is that the male is constantly under pressure to evolve something new in order to stay competitive in the mating game. It is very much a sexual arms race, he says.

"What we found was that the female didn't care, and that makes sense under the arms race scenario," says Kopp. "The pigmentation has lost its significance to the female - it is last year's fashion - and males are probably forced to evolve new ones all the time."

Accumulated over time, it is these kinds of wardrobe changes that lead to morphological evolution and the establishment of new species, Carroll argues.

The discovery moves science closer to understanding the genetic architecture of change, and "it gets us significantly closer to understanding when and how changes in DNA lead to phenotypic changes," Carroll says. "It is a reconstruction of the evolutionary process at the root of the tree, not the tip, the root."

Despite losing their power to sway the romantic interests of female fruit flies, the gender-based pigmentation patterns may still serve a useful purpose by allowing males, who have precious little time to find a mate and pass along their genes, to avoid wooing other males.

"Courtship, for a fruit fly, is a very expensive activity, and males don't want to waste their time approaching other males," says Carroll. "They want to find Jill, not Jack."

But the fundamental importance of the new discovery, Carroll maintains, is that it provides a window to ongoing evolutionary processes.

"Looking at these flies, we could see that this is something that evolved very quickly, and that it continues to evolve," says Kopp. "Such rapid change give us a chance to observe evolution in progress, rather than just look at the end result."

"It's like going from a vacation post card to a home video," says Carroll. "We've captured a genetic event responsible for morphological change and we're beginning to understand how changes at the molecular level affect development and translate into the diversity of forms we see around us."

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