What makes flies attack?

Pity the poor female fruit fly. Being a looker is simply not enough, it seems. If you're to get a date, much less a proposal, you must also smell and act like a girl. Otherwise, you might just have a fight on your hands. Read more in next week's issue of the online, open access journal PLoS Biology.

As with most animals, Drosophila face the problem of distinguishing between a potential mate and a potential competitor. If, when meeting a second fruit fly, a male fly thinks 'female,' he'll begin courting her. But if he senses another male, he'll fight. What triggers these sex-specific responses? According to new research by scientists at Harvard Medical School, it's pheromone profiles and behavioural patterns. The researchers discovered the separate and combined effects of smell and action by manipulating the expression of a gene that produces a protein that both governs the sex specificity of fruit fly pheromones and the sex-linked behaviour cues in a fly brain. The results show that not only pheromonal but also behavioural cues can serve as triggers of aggression, underscoring the importance of behavioural feedback in the manifestation of social behaviour.

Would a male fruit fly ever attack a female of the species? A research team led by Maria de la Paz Fernandez and Yick-Bun Chan in the laboratory of Edward Kravitz, the George Packer Berry Professor of Neurobiology at Harvard Medical School, used Drosophila as a model system in which to address this question. In fruit flies, sex is cell specific, yet the sex of cells and tissues can be changed by altering the expression of genes that determine their sexual identity. One of these genes, called transformer, makes a protein that is active in females but not in males.

For their investigation, the researchers first chose to masculinise females by inhibiting the expression of transformer. When the pheromone profile for females was masculinised, the researchers found that males would attack the newly altered females despite having copulated with them, indicating that pheromonal cues alone could label another fly as a competitor. Likewise, males attacked females that had been masculinised for both smell and behaviour. But the researchers were surprised to discover that males also attacked 'aggressive females' - flies that still looked and smelled female but had been genetically altered to display male-like behaviour. When the researchers turned the tables by triggering transformer in males so as to feminize both pheromone and behaviour profiles, control males showed no aggression toward the transformed males and instead began to court them. These results demonstrate that by presenting different types of sensory cues, the behavioural response of a male can be switched.

'Future studies will aim at unravelling the neuronal circuitry that governs this type of decision-making behaviour, as such decisions are essential for survival. With the powerful genetic methods available in fly neurobiology, it should be possible to dissect the decision-making circuitry at far greater levels of detail than have heretofore be possible on other species,' said Kravitz.