Fighting styles: gene gives flies his, her conflict moves.

A female fruit fly fights like a guy and a male fly fights like a girl after researchers switch forms of one gene, according to fly-fight specialists.

That gene, fruitless, is the same one that scientists had found to control roles in fruit fly courtship, says Edward A. Kravitz of Harvard Medical School in Boston. Switching between two forms of the gene can create same-sex flirtations in the insect. In the new work, that same genetic alteration swaps sex-determined styles of aggression, Kravitz and his colleagues report in an upcoming Nature Neuroscience.

Only in the past decade have fruit flies emerged as a model animal for studying aggression. In the well-fed, plush life in laboratories, fruit flies rarely fight. So, Kravitz and his team had to invent ways to provoke conflicts.

They've found that a tiny dab of mouth-watering yeast paste incites male flies to strike threatening wing poses at each other and then come to blows. At times, males actually box, rising upright and pummeling each other with their forelegs. They also lunge at each other, striking the top of an opponent's body with their legs. Slow-motion video shows that such a blow temporarily flattens the bottom fly, which splays out his legs, Kravitz reports.

Females confronted with the same small treat typically don't box or lunge. Instead, they butt heads and shove each other with their forelegs.

Coauthor Barry Dickson of the Research Institute of Molecular Pathology in Vienna and his colleagues genetically engineered female flies to express the fruitless gene in a small percentage of their brain cells, as males normally do. Normal females don't produce any of the proteins encoded by fruitless. The researchers also created male flies that don't make the proteins.

The altered female flies lunged like males, regardless of the sex of the fly they fought. The altered males displayed feminine head butts and shoves, the researchers report.

The altered males also failed to form the normal dominance hierarchy. That is, after a normal male wins a bout, its odds of winning increase in its next match, even against another opponent. In contrast, losers tend to keep losing. This effect doesn't appear in normal females and didn't show up in altered males.

There's no human version of the fruitless gene, says Kravitz. However, understanding fighting behavior among invertebrates will inspire new questions about aggression in other species, he predicts.

The new work is "really wonderful," says Hadley W. Horch of Bowdoin College in Brunswick, Maine, who is studying cricket fights.

Hans Hofmann of the University of Texas at Austin, who has studied cricket fights, says that in many animals, aggressive behaviors differ between the sexes. Kravitz' study is among the first to apply modern genetic tools to these distinctions, Hofmann notes.