So a fruit fly goes to a bar ...
The news last week that scientists induced homosexual courtship in male fruit flies by changing levels of a neuro chemical was greeted with predictable headlines: “Scientists make fruit flies gay, then straight again.” On science blogs, discussion raged about whether this meant that a drug altering sexual orientation would, or should, be developed by the demon Big Pharma. Others trotted out arguments about whether homosexuality was learned or genetic, and about its existence elsewhere in the animal kingdom, and then meandered into why places with large contingents of gays -- such as San Francisco and Boulder, Colo. -- are usually nice places to live.
As someone who studies animal behavior for a living, I’ve been at least as interested as anyone else in the emerging discoveries that many animals exhibit homosexual behavior. And, as it happens, I’ve become something of a go-to person on gay penguins (a subject for another day). But people seem to be missing the real reason the discovery is important, which has little or nothing to do with sexual orientation.
What David Featherstone and his colleagues at the University of Illinois at Chicago found is that male flies with a mutation -- a form of a gene called genderblind -- will court other males rather than females. What is exciting is that the researchers pinned down how and why.
They discovered that the gene controls transport of the neurotransmitter glutamate out of so-called glial cells, which are nervous system cells that do not conduct electrical signals but support other transmitter cells. Glutamate in turn can control the synapses, or junctions between other nerve cells, and synapse strength is important in determining many aspects of behavior. By experimenting with altering synapse strength chemically, independent of the mutation, the researchers homed in on exactly what was at work. Manipulating glutamate transmission, they discovered, allowed them to alter -- sometimes within hours -- whether the flies courted males or females. The altered males interpreted the odors of other flies (the primary come-hither signal) differently from their wild counterparts.
If what’s sauce for the fly is sauce for the human, this could mean that chemicals in our own nervous systems are involved with sexual orientation too. And I’ll admit that it’s entertaining to imagine popping a pill to swing one way for a party, the other for a get-together at grandma’s. But that dystopian possibility probably isn’t in the cards. The truth is that chemicals no more control who we are sexually attracted to than they do anything else. Which is to say, they control everything and nothing.
What does that mean? Chemicals are how our genes exert their influence. We can link genes to traits like eye color, digestive speed or whether we like mangoes, and we’ve been able to do that for many years. We’ve also long known that somewhere, somehow, genes were signaling chemicals to get the job done. But until recently in genetic research, we’ve been stalled: What chemicals work via what genes, how and why?
What Featherstone’s lab did was figure out one such transaction in its details. It joins a handful of other such work in behavioral genetics that have connected a gene to the proteins it codes for and the action of the substances those proteins in turn control. For example, we’ve also pinned down the chemical mechanisms behind foraging in honeybees and paternal behavior in rodents. Knowing the details is the first prerequisite to unraveling the workings of genetics.
But none of that detracts from something else we have known for many years: All behaviors are a combination of genes and the environment, even in simple organisms like flies. Finding the substance that enacts a genetic code doesn’t alter the fact that that code is expressed in a particular environment, and how it is expressed will be influenced by that environment. Interestingly, the Chicago researchers assessed sexual orientation by recording how much time a treated fly spent courting either a male or female test subject -- but the test subjects were always decapitated. This presumably helped control for the inevitable variation that would have been introduced by allowing the other flies to respond. In other words, that aspect of the environment might have affected the behavior that the genes, and the chemicals they deploy, produced.
All traits are like that. They are neither 100% genetically (or chemically) determined, or 100% environmentally determined (or learned). Think of a trait as if it were violin playing. It would be hopelessly pedantic to argue about whether the sound is produced by the violinist or the instrument. It’s just as impossible to tease out the exact source of a trait like homosexuality: genetic or environmental. So the straight-to-gay-and-back-again chemical dose for humans (or fruit flies) outside the lab? Highly improbable.
Here’s what Featherstone’s research could produce. He speculates that it might be used not to produce a sexual-orientation drug but much more narrowly to change the way insects respond to odors: For instance, making insects that usually don’t respond to flower odors become pollinators.
I don’t know about you, but I find the thought of a cockroach flitting from flower to flower a lot more unsettling than straight/gay flies.
Marlene Zuk is professor of biology at UC Riverside and author of “Riddled with Life: Friendly Worms, Ladybug Sex, and the Parasites That Make Us Who We Are.”
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