The Biology of Sexual Preference and Behaviour: Perspective from Drosophila

Mating Fruit Flies, originally uploaded by arbil.

Drosophila mating. (Above.)


Smell plays an important role in who a fruit fly chooses for a mate. That's because fruit flies rely on smell to tell them the sex (male or female) of the other Drosophila they encounter.

Typically, when a male fly smells a female he will engage in sexual behaviour and approach her. She will reciprocate by engaging in correspondent female sexual behaviours. Why do they do this? Let's explore the genetic basis of sexual behaviour in Drosophila.

Fruit flies have a gene called the fruitless gene. The fruitless gene produces a protein called Fru. The Fru protein is expressed (that means that it's working in and on) areas of the antennal lobe, in particular - Fru works on brain cells (mAL neurons) located there. The fruitless gene is only expressed in male fruit flies. A fruit fly smells through it's antennal lobe.

Catch all that? The fruitless gene in fruit flies affects cells that process smell in Drosophila, but only in males. Smell is very important in mate selection and sexual behaviour.

So what exactly does fruitless do?

Early in development both males and females have the same number of mAL neurons in their antennal lobe. As both sexes develop, mAL neurons die. Mature females typically have 5 mAL neurons remaining and these have a female-specific shape. Mature males on the other hand, have 30 mAL neurons remaining and these have a male-specific-shape. Only male mAL neurons grow extensions that connect to the subesophageal ganglion, an area of Drosophila's brain that processes smell and initiates male sexual behaviour. These male differences are all due to the work of Fru.

In other words, the fruitless gene causes brain cells that process smell to connect to more brain cells that initiate male sexual behaviour in a fly.

So, what do you think will happen if the fruitless gene is turned on in a female fly? What do you think will happen if the fruitless gene mutates so that fru is not expressed in male flies?

• In the first case, the expression of fru will cause the female to develop mAL neurons like a typical male, and she will exhibit male sexual behaviour when she smells another female.

• In the second case, the lack of fru expression will cause the male to develop mAL neurons like a typical female, and he will exhibit female sexual behaviour when he meets up with another male.

I hope you guessed the correct answer! Fru may be expressed atypically in nature, as well as in the lab.

At this point you may be wondering what happens to the male sexual area of the subesophageal ganglion in female flies. It becomes a vestigial area. You may be surprised to learn that many female animals have vestigial brain areas that control male sexual behaviour, including most mammals. Hmmn!

So, we've learned that a single gene can control what sort of sexual behaviour a fruit fly exhibits - male typical or female typical. Isn't that neat?

Of course, human sexuality is far more complicated than Drosophila's! (And even Drosophila's is more complicated than described here. Other genes and mechanisms have important roles to play, as do environmental factors.) Experts agree (based on quite a lot of good evidence) that when it comes to humans, both biological and environmental factors play an important role in sexual preference. And the role these factors play in sexual orientation varies considerably between individuals.

All the same, it's not unlikely that similar mechanisms are at work in creating human sexual behaviour and researchers are on the hunt for our own copy of the Fruitless gene.

Glossary

Fruitless gene - A gene studied in fruit flies that produces the protein Fru.
Fru - A protein that alters the development of mAL neurons in the antennal lobes of Drosophila.
Antennal lobe - For Drosophila, this is the first stop in processing smells. It is the rough equivalent of the olfactory bulb in humans.
Neuron - Cells of the nervous system are called neurons. When people talk about brain cells they usually mean neurons.
Subesophageal ganglion - An area of the fruit fly brain, part of which controls male sexual behaviour.

Recommended reading:
- Seattle Times: Born gay? How biology may drive sexual orientation.
- Wikipedia
- Nelson's "An Introduction to Behavioral Endocrinology" 3rd ed. p.209-213

Also see:
Fruitless specifies sexually dimorphic neural circuitry in the Drosophila brain


Written for Homosecular Gaytheist in response to some comments on Reed's post on homosexuality in penguins. Unfortunately, some folks hope homosexuality is simply a product of lonliness, while others think that it's about aggression (and irresponsibly enough, feel quite confident asserting that scientists do too). Questions are welcome.