Why flies have sex

July 06, 2005

Or, one gene is enough to make things happen.

Copulating flies

IMBA's Barry J. Dickson and his team proved that a single gene in the fruit fly specifies essentially all aspects of the fly's sexual orientation and behaviour.

Normally, male flies chase after female flies. By various manipulations of a gene called “fruitless”, Dickson and his team generated males that court other males, females that court other females, and females that court males. These research results are probably groundbreaking as they produce hard evidence that genetically scripted information not only defines how living beings are built, but also, to a certain extent, how they behave.

Dickson and his team focused on the “fruitless” gene of the fruit fly Drosophila melanogaster which shows distinct differences in males and females. They succeeded in altering this specific gene at will, thus producing males and females bearing the sexual gene of the opposite sex. To everybody’s surprise, these flies instantly started to produce the respective sexual behaviour while all the rest of their body, including the sexual organs, had remained unchanged.

Thus females, who ordinarily wait passively to be courted by males, began to actively chase other females, and males, who would normally court females, now ran after other males.

The significance of such amazing behavioural patterns reaches far beyond the odd sexual behaviours of these flies. So far, it was clear that such a complex process as sexual behaviour must require the action of many different genes, and mutations in any one of these genes could disrupt this behaviour. The surprising finding, however, is that just one of these genes is enough to produce the behaviour in the other sex.

Barry Dickson’s 'sufficiency experiment’ clearly shows that in the case of thefruit fly, a single ‘switch gene’ that sits on top of the hierarchy is responsible to trigger complex behaviour. This is a major step in learning to understand how innate behaviours are specified, a key area for biology that so far is hardly understood.

Males and females of the fruit fly generally have dramatically distinct and innate sexual behaviours. These behaviours are essential for their reproductive success, and so strong selective pressure is likely to have favoured the evolution of genes that ‘hardwire’ them into the brain.

Male courtship in Drosophila is an elaborate ritual that involves multiple sensory inputs and complex motor outputs. It is largely afixed-action pattern, in which the male orients toward and followsthe female, taps her with his forelegs, sings a species-specific courtship song by extending and vibrating one wing, licks her genitalia, and finally curls his abdomen for copulation. If the female is sufficiently aroused and has not recently mated, she accepts his advances by slowing down and opening her vaginal plates to accept copulation. An obvious but nonetheless remarkable aspect of this behaviour is that mature males court only females, never other males, whereas females do not court at all.

The concept that a switch gene can specify an entire innate behaviour in no way denies the critical role of complex gene networks, just as, for the physical building plan of an animal, the concept of a morphogenetic switch does not deny the existence of complex regulatory networks among the genes it regulates. These networks add both detail and robustness to the behavioural or morphological pattern initially laid down by the switch gene at the top of the hierarchy. Given the appropriate genetic tools, behavioural instincts should ultimately succumb to the same kind of molecular genetic analysis that has so successfully revealed the principles of morphological development.

Barry J. Dickson’s findings were published as a cover story in "Cell" on June3rd, 2005 (www.cell.com ).

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