The sexual preferences of fruit flies have been switched from gay to straight and back again with drugs and genetic engineering.

Scientists in the US found that a mutation in a gene known as “gender-blind”, or GB, can make flies bisexual, and that manipulating its activity can switch this sexual trait on and off.

The findings, from a team based at the University of Illinois, suggest that homosexuality has a clear biological basis, but that this is not necessarily hard-wired by the genes. A combination of genetic and environmental factors seems to be required.

While the research does not necessarily have direct implications for humans, whose brain and nervous system are far more complex, it does show that changes in biology can directly affect sexual behaviour. Consensus scientific opinion on homosexuality holds that biological factors such as genes or prenatal exposure to hormones may create a predisposition that can be activated later by environmental influences. The study, led by David Featherstone, is published in the journal Nature Neuroscience.

The GB gene is involved in the function of glial cells, a type of brain cell that supports the active nerve cells. When the gene was inactivated, the strength of the synapses between nerve cells was affected, and the scientists noted that male flies began courting other males. “The GB mutant males treated other males the same way normal males would treat a female,” Dr Featherstone said. “They even attempted copulation.”

Drugs and genetic engineering were then used to manipulate synapse strength independently of the GB gene, with similar results. “I never thought we’d be able to do that sort of thing, because sexual orientation is supposed to be hard-wired,” he said. “This fundamentally changes how we think about this behaviour.”

Times Recommends

• The bright side of this winter rain
• Story behind the picture: Snow Leopard, 2008
• Times Charity Appeal: The Place2Be