Sexuality, that powerful and most mysterious of forces, has always been a good candidate for lively discussion - nowhere more so than in the arena of genetics and behaviour. Recent studies suggesting there is a genetic component to homosexuality, for example, have provoked huge controversy in the media, and the debate continues unabated.

"On the one hand, brain formation and function must be controlled in part by genes - so it's hard to see how they can fail to have an impact on all aspects of our personalities, including our sexual behaviour," says Dr Stephen Goodwin at the University of Glasgow. "On the other hand, understanding how genes influence something as complex and individual as sexuality - proving that a single gene or genetic mutation controls a precise, discernible aspect of behaviour - is notoriously difficult."

Dr Goodwin and his team are investigating the relationship between genes and sexuality in the fruit fly, Drosophila. "Flies are less complex than humans, and their sexual behaviour is easier to watch and measure because it is very stereotypical: every step is precisely known. The male shows off. He vibrates his wings to create a courtship 'song' and chases the female, who is coy and runs away. The male touches and licks her and eventually, if he does all the right things, she allows him to copulate with her."

Previous studies have shown that much of fly sexual behaviour is genetically determined. "If you rear isolated males and females in separate vials then put them together they immediately begin courting, so we know this behaviour is innate, it's not learnt. However, before we get carried away with genetic determinism, we should appreciate that Drosophila sexual performance, just like humans, is affected by all sorts of environmental factors and is modifiable by good and bad experiences."

The role genes play in controlling the fruit fly's sexual behaviour was highlighted by a serendipitous discovery made in 1963 by Dr Kulbir Gill. While researching the genetic causes of female sterility at Yale University, Dr Gill suddenly noticed that a group of mutant male flies were courting other males. Moreover, unlike non-mutant male fruit flies, who typically reject courtship from other males - they flick their wings, kick and box energetically - these mutant males form elaborate male-male courtship 'chains' in which each male is simultaneously courting and being courted.

Every Drosophila gene mutation is given a name, and Dr Gill christened his flies 'fruity'. Inspired by a Mel Brooks and Carl Reiner comedy sketch, Jeff Hall at Brandeis University renamed the mutation the more politically correct 'fruitless'.

The new name reflects another idiosyncratic aspect of the mutant flies: although they court both other males and females indiscriminately, they never actually copulate with members of either sex. "There seems to be a way but no will," says Dr Goodwin. "All the physiology appeared in place - they go through the motions - but for some unknown reason they stop short and don't go through with it."

Dr Goodwin and his team, in collaboration with Jeff Hall's lab, are investigating how the fruitless gene influences the development of the nervous system and ultimately how it works to control the emergence of particular male behaviours.

"fru is a complex gene with several functional pathways in male and female flies. If you remove the gene completely, neither sex survives. Other parts of the gene are expressed only in the male's nervous system. Mutations in fruitless that affect the amount and distribution of these male-specific proteins, disrupt male sexual behaviour. By studying a gene like fruitless, we have the potential to understand how a single gene expressed in the nervous system acts to give rise to a complex behavioural circuit."

This is no easy task. "It's hard to figure out, even in a relatively simple fly, because the fru gene plays so many complicated, intricate roles in development, survival and reproduction," says Dr Goodwin.

And it will be no easier disentangling the relationship between human sexuality and genes. Certainly, however complex a single gene might be, it is unlikely to be the sole determinant of an experience that encompasses, among other things, factors as immeasurable and compelling as love, pain, loneliness and the search for identity.

See also

• Why the fly?
• Further details on model organisms including the fruit fly

External links

• Dr Stephen Goodwin at the University of Glasgow: Research interests

Further reading

Billeter J C, Goodwin S F, and O'Dell K M C. (2002) Genes mediating sex-specific behaviors in Drosophila. Advances in Genetics, 47: 87-116.

Song H-J, Goodwin S F, Reynaud E, Carlo T, Billiter J C, Spana E P, Perrimon N, Baker B S, and Taylor B J. (2002) The fruitless gene is required for the proper formation of axonal tracts in the embryonic CNS of Drosophila. Genetics, 162(4): 1703-24.

Anand A, Villella A, Ryner L C, Carlo T, Goodwin S F, Song H-J, Gailey D A, Morales A, Baker B S, Hall J C and Taylor B J. (2001) Molecular genetic dissection of the sex-specific and vital functions of the Drosophila melanogaster sex determination gene fruitless. Genetics, 158(4): 1569-95.

Lee G, Foss M, Goodwin S F, Carlo T, Taylor B J, and Hall J C. (2000) Spatial, temporal and sexually dimorphic expression patterns of the fruitless gene in the Drosophila central nervous system. Journal of Neurobiology, 43(4): 404-26.

Goodwin S F, Taylor B J, Villella A, Foss M, Ryner L C, Baker B S, and Hall J. (2000) Aberrant splicing and altered spacial expression patterns in fruitless mutants of Drosophila melanogaster. Genetics, 154(2): 725-45

Goodwin S F. (1999) Molecular neurogenetics of sexual differentiation and behaviour. Current Opinion in Neurobiology, 9(6): 759-65