Pharyngula

Blogging on Peer-Reviewed Research

On the one hand, this is a strange tale of mutant, bisexual, necrophiliac flies, and you’ve got to love it for the titillating nature of the experiments. But on the other, much more interesting hand, it’s a story about drilling down deeply into the causes of a complex behavior, and tracing it to a single gene product — and it also reveals much about the way the chemicals sloshing about in the brain can modulate responses to stimuli. Work by Grosjean and others on a simple Drosophila mutant, genderblind, which causes flies to be indiscriminate about gender in their courtship, opens up a window into how sexual responses are shaped and specified.

Think about human sexual responses. Some of us, when we see an attractive woman, are at least mildly aroused; others are have their sexual interest picqued when they see an attractive man; still others might feel sexual urges when they see a shoe, or a plush animal, or a pot of baked beans. No matter what the stimulus, these are all biological responses, with something in the environment matching some trigger in our brains and initiating a cascade of neural, neurochemical, and hormonal activity that leads to sexual behaviors. The question we want to address is what every step in the biology is doing; unfortunately, human behaviors are both too complex and not amenable to ethical experimentation, so we turn instead to simpler organisms that allow us to find simpler causes and carry out thorough experiments to probe the behavior.

Like flies. A fly’s desires are relatively straightforward: they want to mate with a member of the same species and of the opposite sex. Males tend to be the initiators, and they approach other flies, and test sex and species by tasting — they secrete specific pheromones from their cuticles — and by singing species-specific courtship songs. Females strike males or run away if they don’t want to mate, so interaction between the potential partners is also important. Male flies secrete several substances that proclaim their maleness, 7-tricosene and cis-vaccenyl acetate, which other males find distasteful and discourage them from attempting to mate.

Well, wildtype males find it discouraging; there are a lot of fly mutants that modify this behavior. There are mutations that cause a female pattern of brain organization to develop in a male body, and vice versa, and which cause males to wait for other males to start courting them, or that create females that try to initiate sex with other females. The mutant of interest here, genderblind, creates an unusual pattern of behavior illustrated in the test below.

i-4a4c61c85e0b94990f0df51b0399310f-fly_test.jpg

Here’s the test. Put an experimental male fly in a petri dish (the fly in the center above) with a choice of two potential mates, a male on the left and a female on the right. Courtship is, of course, a matter of two interacting flies, so to simplify things, the potential mates have been decapitated to make them unresponsive (they’re still alive, but they aren’t going to be doing much, like rejecting potential suitors). Then watch and see what the fly does.

A wildtype (WT) male fly pretty much completely ignores the decapitated male fly, but spends 60% of his time (the black bar) trying to court the decapitated female fly; perhaps he thinks she’s playing hard to get. A genderblind male fly (gb[KG07905]), on the other hand, splits his time between the decapitated male and female flies (gray bars), spending even more time in frustrated attempts to coax a response out of either one.

Genderblind flies are perfectly normal in other ways. If their partners are not headless, they can successfully court and mate female flies. They also aren’t indiscriminately trying to mate with anything that moves; they retain species specificity and are indistinguishable from wildtype flies in the frequency of attempted matings with other species of flies, showing diminished attempts with increasing species disparity.

i-ad24fa1bdc5e836dcff7f04b56356538-species_specific.jpg

What all this means is that these are truly bisexual flies — they have normal sexual responses except that these male flies don’t find other male flies unattractive. Some change has simply stripped away some attribute of their brain that inhibits wildtype flies from same-sex courtship, while leaving other proclivities and preferences intact. Now, because it is in that easily manipulable experimental animal, the fly, investigators can also trace the change back to a single gene…and this is where it gets really interesting. How can a gene’s activity be translated into a behavior?

Genderblind is a gene for a glial amino acid transporter. That is, it makes a protein that is inserted into the cell membrane of glia, not neurons, and is responsible for secreting amino acids (in particular, glutamate) from the glia into the extracellular spaces of the brain. The micrographs of the fly brain below were made with a fluorescent marker for genderblind that is pseudocolored magenta. In addition, all neurons are green with green fluorescent protein (GFP) in (a), all glia are GFP-green in (b), and a subset of neurons that use glutamate are GFP-green in (c). If a cell has both the GFP marker and the genderblind marker, it will appear white in the photograph. Notice where genderblind colocalizes: not with the neurons, but with the glia in the brain.

i-1b6b4ba163a085288749e097af49435d-gb_map.jpg
Genderblind (genderblind) protein is
expressed in central glia surrounding
glutamatergic neurons. (a-c) Single fluorescence
confocal microscopy sections from male adult
brains, stained with antibodies to genderblind
(magenta) and CD8 (green). Colocalization is
represented by white color. For each image, the
transgenic transmembrane protein CD8::GFP was
expressed in a specific tissue-type using the Gal4/
UAS system. CD8 expression was driven with the
neuronal driver Elav-Gal4 (a), Repo-Gal4 (which is
expressed in a subset of glia, b) or the weak
glutamatergic neuron driver OK371-Gal4 (c).
Selected brain structures are indicated in each
panel. AL, antennal lobe; CA, calyx; CC, central
complex; MB, mushroom body; SOG,
subesophageal ganglion (see also Supplementary
Fig. 3). Scale bars represent 25 µm.

Maybe you’re still baffled. How does glutamate from glia translate into bisexuality in flies? Another part of the story is that the normal fly brain is awash in an environment containing glutamate, which is also a neurotransmitter. All the glutamate desensitizes the glutamate receptors on neurons so that they are less attentive to incoming signals — it’s basically jamming synapses with a lot of background noise. In the genderblind mutant, ambient glutamate concentrations are cut in half, so it’s as if the nervous system is a little quieter, receptors are not desensitized as much, so neurons are suddenly more sensitive to stimuli. These glia are affecting neuronal activity indirectly.

Fitting this model are some other observations: the genderblind flies overreact to other chemosensory stimuli. Olfactory traps that use scent to lure flies are much more effective on genderblind vs. wildtype flies, suggesting that sensory processing in these flies is a bit more responsive in general.

In other clever experiments, they used a temperature-inducible RNAi to selectively block synthesis of the protein by simply dialing up the temperature on the incubator, the bisexual behavior could be switched on just by warming them up. Bisexual flies could also be switched to wildtype heterosexuality by giving them apple juice doped with γ-DGG, a glutamate-receptor antagonist, that also desensitized receptors. Within hours, they can turn bisexuality off and on in Drosophila — and the cool part is that this doesn’t involve manipulation of specific, individual neurons, but is done by modifying a diffuse chemical balance in the brain, a balance maintained by populations of glia.

Here’s the proposed model. The authors suggest that there is a general response to chemosensory stimuli that is selectively switched off in wildtype flies by desensitization of glutamate receptors to specifically male signals — so flies have a general courtship urge that is modified by a sex-specific downregulation.

i-bcecf98fbc8a82a8866c1b3179e8cec2-flymodel.jpg
In addition to demonstrating a behavioral role for genderblind, our
results also suggest a physiological model for Drosophila sexual preference that parallels a model recently proposed for mice. In this model, wild-type flies are ‘pre-wired’ for both heterosexual
and homosexual behavior, but genderblind-based transporters suppress the glutamatergic circuits that promote homosexual behavior. In
gb mutants, the repression of homosexual behavior does not occur and
flies become bisexual. Heterosexual courtship is not altered in gb
mutants, indicating that circuits driving heterosexual courtship are
not regulated by genderblind. This could be because circuits promoting
heterosexual courtship are not glutamatergic, or because they are
perfused by a different ambient extracellular glutamate pool than the
one that is regulated by genderblind-based transporters.

Humans are definitely messier and less well-defined, so don’t even think that you might be able to get that hot same-sex person to be receptive to your signals with a few chemicals in their apple juice — it’s just not going to work in exactly the same way. The general concept, however, is intriguing: the work is saying that the extracellular chemical environment of your nervous system has an extremely important role in modulating significant behavioral properties.


Grosjean Y, Grillet M, Augustin H, Ferveur J-F, Featherstone DE (2007) A glial amino-acid transporter controls synapse strength and homosexual courtship in Drosophila. Nat Neurosci.Dec 9; [Epub ahead of print].

Comments

  1. #1 David Marjanovi?, OM
    December 14, 2007

    Wait, so did the bisexual fruit flies go for the pot of sexy baked beans, too?

    No: “they retain species specificity”.

  2. #2 Asbi
    January 30, 2009

    I wouldn’t be too worried about a cure. Most likely you’d have straight people making their kids straight, gay people making their kids gay, and bisexual people making their kids bisexual. In fact as a bisexual myself I’d want to have bisexuals if possible. Even though there’s discrimination I consider it a gift. I can decide whether to have a relationship with a person based on their personality without letting gender get in the way. But to each their own and I think it will be very little time maybe not any time at all between when parents can choose genes for their children and adults can change their genes. Gene therapy (or protein therapy) could even become inexpensive and changing your sexual orientation could become like dying your hair. Rarely is anyone ever prejudiced against somebody for their hair color, so it might actually be good for tolerance. I think with a more tolerant society more people will choose bisexuality. There’s nothing wrong with heterosexuality or homosexuality it’s just why limit your options if you don’t have to?

    As for not being able to tell men and women apart of course we can at a conscious level just like we can tell races apart, but that doesn’t mean racial recognition is inborn. There’s the concept of gender that children learn. But attraction takes place at the subconscious level. People don’t think “I’d like to be attracted to that person” and then they’re attracted. Attraction is uncontrollable.

  3. #3 Asbi
    February 11, 2009

    Something else I thought of. Would the genderblind gene blind any flie to gender or just flies that would have otherwise been heterosexual? I think they should do another experiment. This time modifying the white gene as one poster mentioned, making the flies gay, then feeding them spiked juice to see if it makes them try to mate with both males and females.

    Also even if this makes bisexuals smelling work better as far as noticing that something smells doesn’t not being able to tell the difference between male and female pheromones mean that there is a decrease in ability to distinguish between different kinds of smells or is it just for the pheromones? Regardless I’d be interested to see scientific studies into the olfactory function of bisexual males.