Giovanna Di Sauro

In today’s study, scientists tried to activate certain neurons using what could basically be thought of as a “light switch”, and observed female flies performing a characteristic male courtship song. This study is the logical consequence of others which were conducted on fruitless mutants not so long ago. Scientists studying fruit flies mutant in a gene called fruitless realized that “fruitless” females display male courtship behavior when expressing certain versions of the protein product of fruitless, which seems to be needed in a specific set of neurons for this behavioral effect of the mutation to show up.

These studies on “fruitless” flies are some of the coolest studies on gender-specific behavior around. Although the BBC suggestively calls this a “mind-control sex swap“, this is definitely not a sex swap. One characteristic of fruitless flies is that their sexual characteristics are perfectly normal - the males are fertile, and so are the females, and there is no deformation of their bodies. So we are really not looking at the sex of the flies - but rather, at their gender. Which is what makes these studies so intriguing.

First of all, let me briefly explain how courtship works in Drosophila melanogaster. The male is the only one that can be said to display courtship behavior: when a male intends to court a female, it uses wing vibrations to produce a “courtship song”. This song can be identified because of the specific frequencies of the sounds emitted by the male. There are two “modes” of emission, known as the sine song, at ∼140–170 Hz, and the pulse song, constituted of brief and repetitive amplitude modulations in a range of ∼150–300 Hz. Females respond to this by allowing the male to copulate (or not; whether the male is successful depends on a variety of other factors, including winning “fights” with other males).

What about these fruitless (fru) flies? It was already known that fruitless encodes a gene product necessary for the determination of sex-specific courtship behavior in flies, and that the products of this gene exist in two main versions produced by alternative splicing: a male version and a female version. A previous study done in Barry Dickson’s lab showed that the male version is necessary for the determination of male courtship behavior and sexual orientation. The gene is in fact also able to induce male courtship behavior in females when spliced in a male-specific manner (causing the male version of the protein to be produced in females). On the other hand, loss of male-specific splicing in males causes the disappearance of all typical male courtship behavior.

This whole situation is complicated by the fact that there are several different alleles (variants) of the fru gene, able to alter not only courtship, but sexual orientation as well. For example, males with certain mutant forms of fru display an enhanced form of homosexual behavior. What do I mean by that? You might want to see for yourself in this video from one of the Dickson papers. Click on the image to go to the video, which gives a new dimension to the phrase “love train”.

Now we can get back to today’s study. Researchers working at Yale and Oxford have decided to look at the neuronal circuitry expressing the products of the fru gene in male and female flies, and to see what would happen when one activates this neuronal circuit in fru mutants of both sexes. The experiments showed that activating this circuit results in different responses in males and females, and that the responses are dictated by the version of the fru protein expressed by the animal. Thus, wild-type males and male-fru-expressing females can produce the wing movements and the courtship song; wild-type females will move their wings and produce a sound, but not a courtship song.

The neuronal circuitry was artificially activated by shining light on the neurons, which were made to express a light-sensitive ion channel.

What does this mean? When ions are allowed into neurons by ion channels, the neurons depolarize - and the depolarization wave moves all along the membrane of the neuron, till it reaches the synapse, where neurotransmitter release occurs. All you need to to do activate a neuron in a controlled way is to make it express an ion channel you can control. Ion channels need energy in the form of ATP to work, and therefore providing ATP can induce depolarization. You can then use a form of ATP linked to another compound: this compound makes the ATP “inactive”, but when you shine light on it, the “normal” ATP is released, and the ions channels open.

How do you make sure that the channels are present in the neurons you are studying? You use a promoter specific to those neurons - so that the gene coding for the channels makes protein only there. And what better promoter to study fru-expressing neurons…than the fru promoter? In fact, that is the one used in this study.

This short video summarizes the results of the study. I love when the narrator says that “to make a female sing like a male, all you need to do is to turn on one gene, and chop her head off”. It does give the wrong impression, doesn’t it? The head is chopped so that the female does not stop “singing”, as that behavior usually requires interaction with another fly, and it is intermittent. Which is not helpful if you want to analyze the song of that one fly in the experiment.

Now, apart from the results and the hype, you might still be wondering what are the implications of the study, from a more neurobiological point of view. What the results suggest is that behavioral differences between the sexes might not be necessarily due to differences in neural circuitry, but in the presence or absence of sex-specific regulators of such circuitry. Although it is usually true that there are differences in the overall neuronal structure of males and females (with males usually having extra neurons dedicated to male-specific behavior, which is something that is surely true of the nematode C. elegans), the fact that some shared circuitry might also be part to fundamental sex-specific behaviors is also something that needs to be considered.

As a last note, in case you are wondering: while some sources (which do not attach any citation to this statement) say that the fruitless gene is not present in mammals, I have run my own search, and it turns out that humans might have a homologue of the fly gene. The name of the homologue is ZBTB22 (which tells me this gene has not been well characterized), a transcription factor, just like fruitless is. Does this mean that it has the same function in humans? Well, given that we do not make courtship songs using wings, it probably doesn’t.

Curious facts: we have known about gay fly males for quite a while. In fact, the fru mutants were initially created because of X-ray radiation in 1963, and there is a 1989 paper describing the effects of the fruitless defects in males, and tracking them back to a chromosomal inversion, which must have probably been caused by DNA breaks and repairs following radiation exposure in the original strain. Nowadays people are looking at different mutated alleles of fru, not at the original version created through radiation exposure.

Citation

Clyne, D., Miesenböck, G. (2008). Sex-Specific Control and Tuning of the Pattern Generator for Courtship Song in Drosophila. Cell, 133(?), 354-363.

Male fly image courtesy of Wikipedia Commons

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