by Tony Fitzpatrick
Telling the boys from the girls in the animal kingdom is usually pretty apparent. Striking colors and other showy displays in birds, fish and insects are meant to advertise sexual availability and robustness so that males fighting Father Time are able to quickly and successfully attract female mates. Such differences are called sexual dimorphisms.
Now, a researcher at Washington University and colleagues at the University of Wisconsin-Madison, all with a connection to the University, have found and characterized a gene that governs a striking sexual dimorphism in the fruit fly.
The discovery, described in the Nov. 30 edition of the British scientific journal Nature, shows how and why animals go to such narcissistic lengths, but also provides insight into the genetic changes that eventually 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 the University of Wisconsin-Madison and an author of the Nature paper. He said the discovered gene makes a key difference between fly species.
Carroll is an alumnus of the University, as is co-author Artyom Kopp, Ph.D., a post-doctoral researcher at the University of Wisconsin-Madison, and former student of co-author Ian Duncan, Ph.D., professor of biology in Arts & Sciences.
Kopp made important discoveries on Drosophila pigmentation working with Duncan at the University in the late 1990s. Carroll did undergraduate research in the biology department's evolution program and worked closely with Owen Sexton, Ph.D., professor emeritus of biology in Arts & Sciences, on snakes.
In the fruit fly, specifically 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 difference has evolved only recently in a relatively small subset of Drosophila species.
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 similar.
Scientists have long 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 researchers say the present role of bric-a-brac was probably shaped by the sexual selection process because the pigmentation patterns specified by this gene affect mating preferences. But looking at the genetic controls for gender-based pigmentation in fruit flies the team 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 attraction. This suggests that the male is constantly under pressure to evolve something to be competitive in the "mating game."
Kopp likens this scenario to a "sexual arms race."
"What we found was that the female didn't care, and that makes sense under the arms race scenario," said 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 display changes that lead to morphological, or body evolution and the establishment of new species, Carroll said. The gender-based pigmentation patterns may still serve a useful purpose by allowing males, who have a very short time to mate and pass their genes, to avoid mistakenly mating with males.
"Courtship, for a fruit fly, is a very expensive activity, and males don't want to waste their time approaching other males," said Carroll. "They want to find Jill, not Jack."
But the fundamental importance of the new discovery, the biologists maintain, is that it provides a window to ongoing evolutionary processes.
"What is exciting about this is that male-specific pigmentation evolved quite recently," said Duncan. "Artyom did a beautiful job in Sean's lab of comparing bric-a-brac expression in closely related Drosophila species. Within the melanogaster species subgroup and the closely related oriental species subgroups, he finds a perfect correlation between male-specific bric-a-brac repression and male-specific pigmentation. This suggests that by comparing bric-a-brac regulatory regions in closely related species that do and do not have dimorphic pigmentation, it should be possible to identify the DNA sequence changes that were responsible for this evolutionary innovation."
"Looking at these flies, we could see that this is something that evolved very quickly, and that it continues to evolve," said Kopp. "Such rapid change give us a chance to observe evolution in progress, rather than just look at the end result."