A Swiss scientist has discovered that sexual orientation in fruit flies is controlled by a previously unknown regulator of chemicals in the brain.
While the evolutionary basis for homosexuality remains a mystery, researchers have succeeded in using genetic manipulation or drugs to rapidly turn the flies' homosexual behaviour on and off.
"We discovered that by manipulating a gene we called genderblind, even in the adult flies we were able to modify their courtship towards other males," said Yaël Grosjean from the Centre for Integrative Genomics at Lausanne University.
"So one big point of this is that sexual behaviour – being able to recognise a sexual partner – is not fixed during development: it can be turned on and off, even in the adult," said Grosjean, who started the work at the University of Illinois, Chicago.
The research could open up new insights into how sexual attraction and responses in other animals, including humans, are shaped and specified.
Using geneticists' favourite guinea pigs, drosophila melanogaster fruit flies, the scientists focused on a mutant gene called genderblind, which causes flies to be indiscriminate about gender when courting.
Grosjean – whose research is scheduled for the January issue of Nature Neuroscience - found the gene interesting initially because it has the unusual ability to transport the neurotransmitter glutamate, out of glial cells – cells that support and nourish nerve cells but do not fire like neurons do.
But the genderblind gene became even more interesting when Grosjean noticed that all the genderblind mutant male flies were courting other males.
It's still unknown why a male brain chooses to do male things and a female brain does female things. The discovery of genderblind provided an opportunity to understand why males choose to mate with females.
"Based on our previous work, we reasoned that genderblind mutants might show homosexual behaviour because their glutamatergic synapses were altered in some way," said fellow researcher David Featherstone at the University of Illinois.
Synapses are specialised junctions through which the cells of the nervous system signal to each other and to non-neuronal cells such as those in muscles or glands.
"Homosexual courtship might be sort of an 'overreaction' to sexual stimuli," he said.
To test this, the scientists genetically altered synapse strength independent of genderblind and also fed the flies drugs that can alter synapse strength. As predicted, they were able to turn fly homosexuality on and off – and within hours.
They reasoned that adult fly brains have dual-track sensory circuits: one that triggers heterosexual behaviour, the other homosexual. When the genderblind gene suppresses glutamatergic synapses, the homosexual circuit is blocked.
Further work showed precisely how this happens – without genderblind to suppress synapse strength, the flies no longer interpreted smells, chemicals known as pheromones, the same way: in effect, the genderblind mutant males were no longer turned off by male pheromones.
Human behaviour is a lot more complex, but Grosjean thinks parts of the research can be extrapolated beyond flies.
"We need more experiments and knowledge to be sure that this is exactly the same in humans, but we have some data arguing that pheromones also play a role for courtship and partner recognition in humans."
swissinfo, Thomas Stephens
The Centre for Integrative Genomics has brought together interactive research groups performing research in three major areas:
The structure and function of genomes and their evolution.
The regulation of gene expression by the transcription machinery, chromatin structure, transcription factors and signal transduction cascades.
The genomics of complex functions such as embryonic development, physiological functions in the context of the organism and behaviour.
In October 2006 a display, "Against Nature?", opened at the University of Oslo's Natural History Museum, presenting 51 species of animals exhibiting homosexuality.
"Homosexuality has been observed in more than 1,500 species, and the phenomenon has been well described for 500 of them," said Petter Bockman, project coordinator.
In 1999 Silo and Roy, two male penguins, made headlines around the world when they came out with their same-sex relationship. Since then, the pair successfully hatched and raised an adopted chick – after trying to incubate a rock. In 2005 however Silo ran off with a female penguin called Scrappy.
But scientists warn against drawing conclusions for humans based on animal behaviour. Some argue that if something is natural, it is ethically acceptable or desirable.
Others point out that infanticide for example is also widespread in the animal kingdom.