Researchers from Oxford University and the University of Glasgow have shown that a gene known as 'doublesex' (dsx) plays a role in determining different brain circuitry in male and female fruit flies, resulting in gender-specific behaviours.
The courtship behaviour of the fruit fly has long been used to study the relationship between genes and behaviour. The male chases an initially unreceptive female, and 'woos' her through tapping and licking and using wing vibration to generate a 'courtship' song. If successful, the female will slow and present a receptive posture, which allows copulation to occur.
These courtship behaviours are innate in the fruit fly, not learned. And for some time now, the gene 'fruitless' (fru), which is specific to the adult male fruit fly, was thought to be the key to male behaviour and the development of male-specific neural circuitry in the brain.
However, the researchers have shown that fru does not explain the complete repertoire of male behaviours in the fly. Female flies in which the fru gene has been activated demonstrate some but not all of the characteristic courtship behaviour of males.
The researchers demonstrate that dsx, which is known to determine the shape and structure of the male and female body in the fruit fly, also plays an important role in shaping the neural circuitry involved in this behaviour.
‘The dogma was that dsx made fruit flies look the way they did and fru made them behave the way they did,’ explains Dr Stephen Goodwin from the Department of Physiology, Anatomy and Genetics at the University of Oxford. ‘We now know that this is not true. dsx and fru act together to form the neuronal networks – the wiring – for sexual behaviour.’
While fru has only been found in insects, dsx is found throughout the animal kingdom, where it plays a fundamental role in sex determination. This makes the gene of particular interest to researchers.
Dr Goodwin and colleagues were able to map where the dsx gene is active throughout the fly's development using a fluorescent protein marker. This highlighted profound differences in neural architecture between male and female fruit flies.
In males, the researchers were able to show that dsx complements fru activity to create a 'shared' male-specific neural circuit. In females (where fru is inactive), dsx forms a female-specific circuit. Importantly the researchers were able to show that these circuits are responsible for behaviours of the different sexes.
‘It has been suggested that there are only minor trivial differences between the neural circuits that underlie behaviour in males and females,’ explains Dr Goodwin. ‘We have shown that in fact there is quite a bit of difference in the number of neurons and how these neurons connect, or “talk”, to each other. These differences can have big consequences on the structure and function of the nervous system.’
While dsx was previously known to establish the gender of the adult fly, where the dsx gene is active in the fly was unknown. Dr Goodwin and colleagues have shown that the gene is not active everywhere, but rather is restricted in a specific and telling manner.
Some tissues, such as blood cells, may not require a defined gender in order to function. Others do, such as the 'fat body', which in the adult fly functions in part to produce hormones, and the oenocytes, which produce sex-specific pheromones. It was unsurprising to Dr Goodwin and colleagues to find dsx expressed in these tissues in both males and females, as it would be key to establishing a normal sexual physiological state.
‘Determining gender in a fruit fly seems to be about adding different splashes of ‘‘colour” here or there,’ he says. ‘It's not like the canvas, meaning the nervous system, needs to be all blue or pink, just a little bit of blue over here or a little bit of pink over there. Not all cells need to know what sex they are, but those that do need to know will be ones that are important for sex-specific behaviours.’