Random Walk
Of Flies and Men
The poet and mystic William Blake saw a world in a grain of sand. A fly’s brain is scarcely larger, yet Caltech scientists see in it a window for exploring the biological roots of our own behavior and emotions. The brain of Drosophila melanogaster, the common fruit fly, contains barely 20,000 neurons—yet two recent papers from the lab of David Anderson, Benzer Professor of Biology and an investigator with the Howard Hughes Medical Institute, offer glimpses into its genetic hardwiring that may throw light on what makes us tick.
For example, both inconsiderate boors and unthinking flies will elbow their way to the front of the press at a crowded lunch counter, causing the less assertive to go elsewhere. Now grad student Liming Wang and Anderson, writing in the January 10 edition of Nature, have identified an aggression-promoting pheromone that appears to help drive competitors away from a crowded piece of, say, overripe banana, and pinpointed the neurons in the fly’s antennae that detect it.
Pheromones are chemicals used by particular species to communicate with their own kind, but proving that a pheromone released by the insects themselves—rather than being provided in a synthetic form by inquisitive scientists—normally controls aggressive behavior “required the ability to experimentally interfere with the insects’ capacity to sense the pheromone,” Anderson notes. “And that, in turn, meant identifying the receptor molecules that detect aggression pheromones, and finding the olfactory sensory neurons that express these receptors.” According to Wang, the paper’s first author, the only insect meeting these requirements was Drosophila melanogaster. “The genetic and molecular architecture of Drosophila’s olfactory system is well understood,” he explains. “One can easily test whether a specific receptor or neuron is involved in a given behavior.”
Wang discovered that 11-cis-Vaccenyl Acetate (cVA), a pheromone present in the male fly’s cuticle, or exoskeleton, promotes aggression in pairs of male flies. An aggressive fly will “lunge,” rearing up on its hind legs and snapping its forelegs down on its opponent. When Wang and Anderson added synthetic cVA to the “arena” in which combatant flies were tested, the frequency of lunges dramatically increased. Building on earlier work elsewhere that had identified cVA’s receptors, Wang next showed that silencing the cVA-sensitive neurons in the antennae mellowed the flies out.
To find out whether natural cVA from other flies had the same effect, Wang and Anderson then trapped between 20 and 100 “donor” male flies—so called because they donate their pheromones into the surrounding environment—in a tiny cage surrounded by a fine mesh screen. The screen allowed the pheromones to escape, but not the flies. A pair of “tester” males would be placed on top of the cage, where they could sense the pheromone but not interact with the donors. “Remarkably,” says Anderson, “the presence of the caged donor flies strongly increased aggression between the tester flies, and this aggression-promoting effect increased with the number of donors.” And again, the testers’ testiness was assuaged by inactivating their cVA-sensing neurons.
Which brings us back to the lunch counter—or more aptly, the free food at happy hour. Male flies are attracted to food not only to eat, but also to mate with feeding females. And, of course, the more guys there are, the harder it gets to score. Since feisty flies tend to chase away their competitors, an aggression-promoting pheromone might keep the number of males down to an equitable level.
Wang tested this hypothesis by allowing a small number of flies to compete for a limited food supply, after genetically manipulating their cVA-receptor neurons to make them more excitable. The flies quickly dispersed. “They fought one another until a dominant fly became ‘king of the hill’ and drove the others away,” Anderson explains.
According to Wang and Anderson, this suggests that when the population of male flies reaches a certain density, the concentration of cVA rises to a level that promotes aggression, forcing some of the flies off the food. Their departure decreases the ambient concentration of the pheromone, decreasing aggression. “The population becomes stabilized at an optimal density until more flies become attracted to the food, and the cycle repeats itself,” says Wang.
Because pheromones evolved as “private” communications channels within a given species, it’s unlikely the fly pheromone would work on us. However, that doesn’t necessarily mean that humans lack aggression pheromones, Anderson notes. They’ve been discovered in mice, which are evolutionarily closer to us than flies, so it’s possible we might have our own as well. But whether such pheromones can keep lines short at the buffet, Anderson remarks, “only time will tell.”
Anderson’s lab has also seen signs of a primitive emotion-like behavior, specifically a state of agitation, that might illuminate the relationship between the neurotransmitter dopamine and attention deficit hyperactivity disorder (ADHD). Most of Drosophila’s genes are also found in humans—including those for the neurons that produce dopamine and serotonin, both of which have been implicated in psychiatric disorders.
A team led by then-postdoc Tim Lebestky found that a rapid succession of brief, brisk puffs of air caused flies to run around their test chamber in what Anderson calls a “frantic manner” for several minutes after the last puff. “Even after the flies had calmed down,” he adds, “they remained hypersensitive to a single air puff.” These “hyperactive” flies were picked out from the crowd via an automated machine-vision-based system developed in the lab of Anderson’s colleague Pietro Perona, the Puckett Professor of Electrical Engineering. These flies proved to have a mutation called DopR that inactivated a dopamine receptor known as D1—a result that was published in the November 25, 2009, issue of Neuron.
This discovery dovetails with what is known about ADHD, which is characterized by impulsivity, hyperactivity, and a short attention span, and is often treated with drugs such as Ritalin that increase dopamine levels in the brain. The way the mutant flies responded to the air puffs is, moreover, “reminiscent of how individuals with ADHD display hypersensitivity to environmental stimuli and are more easily aroused by such influences,” says Anderson. Furthermore, ADHD often goes hand in hand with learning disabilities, and Anderson’s collaborators at Penn State have shown that flies with the DopR mutation can’t learn to associate a particular odor with an electric shock. They don’t avoid the odor afterward, while flies without the mutation quickly catch on.
It’s often assumed that ADHD kids have difficulty learning precisely because they are hyperactive and easily distracted. But this work shows that hyperactivity and learning disabilities are unconnected—in flies, at least. “We could separately ‘rescue’ the hyperactivity and learning deficits in a completely independent manner,” says Anderson, “by genetically restoring the dopamine receptor to different regions of the fly’s brain.” If it turns out that ADHD works in a similar way, Anderson believes that it may be better to develop drugs to treat the two issues separately. The broad-spectrum pharmaceuticals now used to attack both at once tend to have undesirable side effects.
Besides Lebestky, Anderson, and Perona, the other people involved in the work are Caltech biology research technician Jung-Sook Chang, then-postdocs Heiko Dankert and Lihi Zelnik; Young-Cho Kim and Kyung-An Han from Penn State; and Fred Wolf from UC San Francisco.
That flies exhibit emotion-like behaviors controlled by some of the same brain chemicals as in humans “opens up the possibility of applying the powerful genetics of this model organism to understanding how these chemicals influence behavior through their actions on specific brain circuits,” says Anderson. “While the specific details of where and how this occurs are likely to be different in flies and in humans, the basic principles are likely to be evolutionarily conserved, and may aid in our understanding of what goes wrong in disorders such as ADHD.”
The research described in both papers was supported by grants from the National Science Foundation and the Howard Hughes Medical Institute. —LO
