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.
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.
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.
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.
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].