How the touch of a female fly can pacify males of the fruit fly fight club
An explosion rocks the background as a burning building crumbles to the ground. Screams of pain and a hail of gunfire follow shortly thereafter. A manly heavily muscled figure emerges through the smoke, carrying a scantily clad lady in his arms. He is the Hero of our action film and he has just saved the day in a bloody fashion and will soon reap the rewards of saving the girl as well. If this scene triggers cliché warnings in your head, it could be because sex and violence are pervasive and topics of profound interest. But a new finding published in the journal Nature Neuroscience on the interaction between female touch and male aggression might complicate this scene. Dr. Yuh Nung Jan’s research group from the University of California at San Francisco discovered that, in fruit flies, male aggression was reduced by prolonged physical contact with females. They further discovered a set of neurons found only in the brains of male flies that are involved in reducing aggression.
In their natural habitats, male flies are aggressive toward one another. When two male flies are placed into a chamber, the flies engage one another in combat. Through a high-speed camera, the fighting style and prowess of the fly would not look out of place in a televised martial arts tournament. When a fly attacks, it raises its two forelimbs above its head while simultaneously splaying back its wings and firmly planting its back legs to form a tripod for balance. Then, it snaps downwards, forelimbs blurring. The fly on the receiving end is momentarily stunned from the force of the blow (3). This attack is called the lunge, and it is the most commonly used move in the male fruit fly fighting arsenal. Viewed in real-time, a lunge, with all its intricacies of movement, would be over in an eye blink. Looking down on the fight chamber, these physical exchanges would occur in rapid-fire succession and appear reminiscent of miniature pistons launched back and forth at one another as flies exchange lunges. The fighting can become even more intense as more attacks are drawn from the instinctual repertoire of Fruit Fly Style Kung Fu, such as boxing and tussling. The researchers videotaped the flies fighting, assessing the duration of these fighting behaviors as a measure of fly aggression levels. From extensive analysis of these fight videos, the Dr. Jan and his colleagues were able to observe how the flies’ previous social interactions influenced their aggression.
Normally, when females are present, males of all species are spurred towards higher levels of aggression. Though male flies also naturally fight for other limited resources, such as food, these fights are markedly less intense than when a potential mate is involved. But surprisingly, a long-term exposure to female flies had the exact opposite effect on aggression as a short-term one. Males with female roommates who were potential mating partners in the week leading up to the fight exhibited decreased aggression. Males that were sexually naïve (and weren’t housed with female flies) showed enhanced aggression, but only when females were present during fights compared to sexually experienced flies. One potential problem with how the experiments were conducted is that experienced males were pitted against only other males exposed to females, and naïve males were paired with other naïve males. Fighting naïve males against males previously exposed to females could have allowed a more direct comparison in aggression. However, it is apparent that the decrease in aggression of males exposed to female flies is not simply due to more social interaction prior to the fight; naïve flies that were previously housed with other male flies still fought more when females were present than sexually experienced flies that also had male roommates on a separate occasion. Importantly, there was no difference in aggression levels between naïve and experienced flies when there were no females to be fought over. This suggests that prolonged exposure to females somehow inhibits male aggression in sex-related contexts.
So how do female flies work their magic in making males more pacifistic? Is it due to some behavioral interaction between the males and females or the result of exposure to a signal produced by female flies? Male flies naturally court and copulate with female flies, so it is possible that these behaviors are in turn modifying fighting behaviors. Fly courtship is elaborate and dancelike. The male fly approaches and prods the female with his foreleg to get her attention. Then he follows her and begins to sing by extending one wing and vibrating it in her direction at a specific frequency. He then switches to his other wing and sings the next verse. This process repeats as he encircles the female with his serenade. Following the song, he extends out his mouth part, the proboscis, and kisses and licks the female to make sure she is indeed female and of the right species. Should the female fly be impressed by the would-be fly Don Juan’s moves, she will be receptive to the male fly’s inevitable attempts to copulate with her. As the prominent researcher of fly behavior and genetics Seymour Benzer noted, this courtship ritual is “only too embarrassingly anthropomorphic” (4).
When researchers housed male flies with female flies, courtship and copulation were par for the course (what else could one do when one lives in a tube?). But copulation was not causing the female-mediated decrease in aggression. When males were housed with female flies that already mated, the males were summarily rejected in their amorous advances by the females. But despite their unsuccessful copulation attempts, these male flies still became less aggressive. Conversely, successful mating alone did not diminish the flies’ fighting: when the female flies were removed shortly after they copulated with the males, the mated males remained aggressive like their counterparts who weren’t exposed to females at all. The behavioral interactions between male and female flies were not causing the decreased aggression.
Instead, the researchers discovered that the male flies were responding to a chemical signal on the female flies. Male flies of the species Drosophila melanogaster would court other species of flies. However, the aggression reduction only occurred when they were housed with the species with an outer chemical profile similar to that of females of its own species. These hydrocarbons – organic compounds made entirely of hydrogens and carbons – act as pheromones, or chemical communication signals, which are crucial for flies to identity one another’s sex and species (5). The researchers found that mutant flies with defects in a type of pheromone receptor or sensor, ppk29, were less able to detect female pheromones through direct physical contact and also did not decrease their aggression following female exposure. These pheromone-sensors are found on the sensory hairs on the flies’ legs and were male-specific. Thus, it appears that male sex-related aggression is reduced by prior exposure to females due to pheromone detection from direct contact.
But a pheromone alone cannot alter behavior if information of its sensation does not enter the brain somehow. The researchers thus searched for neurons in the fly brain that could receive the pheromone information and then affect the male flies’ aggression. Using genetic tricks, the scientists inactivated distinct groups of neurons in the fly nervous system and found a subset of neurons that was essential for aggression inhibition after female contact. That is, the male flies no longer decreased their aggression after being housed with female flies when these neurons were shut off. The neurons that controlled this effect were covered with receptors for a certain neurotransmitter – a chemical used to signal from neuron to neuron – called serotonin. This chemical is also found in humans and is known to regulate many functions, such as sleep, mood, and appetite, as well as aggression in fruit flies and mammals. A small fraction of these serotonin receptor neurons are found only in the male brain between the regions involved in taste and smell. When these male-specific neurons were shut off, the male flies did not have much decreased aggression even with prolonged contact with females. And conversely, when the neurons were activated, the male flies had reduced aggression.
Serotonin was not the entire story however. When the researchers elevated the levels of serotonin in flies, the flies’ baseline aggression increased as expected. However, these hyper-aggressive male flies were still pacified by contact with females despite the increased serotonin and baseline aggression. Instead, another neurotransmitter was the key player: gamma-aminobutyric acid (GABA). This chemical is known to inhibit neuronal activity and GABA-producing neurons may be involved in modulating aggression in flies. Blocking male-only GABA neurons led to a smaller inhibition of aggression in males following female contact, much as was the case for shutting off male-specific serotonin receptor neurons. Similarly, activating male-specific GABA neurons diminished baseline aggression, just like activating serotonin receptor neurons did. There is some overlap between the male-specific GABA neurons and the male-specific serotonin receptor neurons. Indeed, when GABA production is stopped in these male-specific serotonin receptor neurons, blocking or activating the serotonin receptor neurons failed to produce any aggression effects. Thus, the researchers argue, these male-specific GABA-producing neurons with serotonin receptors are probably responsible for the inhibition of male aggression caused by female contact.
Aggression is a complicated behavior that can be influenced by prior social experience. Previous studies have shown that male flies that experienced long-term male-male interactions had a reduced aggression baseline. This inhibition of aggression was mediated by a pheromone produced only in males and only if exposure to the pheromone was chronic; a short-term experience would actually increase fighting behaviors (6). Now it is known that exposure to females also changes male flies’ aggression, though only in contexts pertaining to sex-related competition. The change is also effected through a sex-specific pheromone and the outcome is dependent on the length of exposure. The number of neurons in the brain that drive this complex behavior is surprisingly small – only about 20 cells in a brain that contains approximately 100,000. Built into the male fly brain, it seems, is a system that reduces the male fly’s urge to fight given prolonged exposure to flies in general. This could potentially be an evolutionary mechanism to promote the survival of groups of flies by diminishing conflict between flies that live closely together. Excessive fighting over resources and potential mates can be damaging to the individuals involved and reduce their survival chances, which in turn weakens the group. In addition, if male flies become less aggressive competing for females after being exposed to them previously – and thus having the chance to copulate – this allows flies that are sexually naïve to breed as well. Aggression reduction would increase the genetic diversity of the group and thus improve chances for the group’s survival. While this evolutionary speculation is intriguing, knowing that more social interaction with both sexes can reduce innate male aggression is already quite touching.
References
1) Yuan, Q., Song, Y., Yang, C.-H., Jan, L.Y. & Jan, Y.N. Female contact modulates male aggression via a sexually dimorphic GABAergic circuit in Drosophila. Nat. Neurosci. 17, 81–88 (2014).
2) Chen et al. Fighting fruit flies: A model system for the study of aggression. PNAS 2002 99 (8) 5664-5668
3) Hoeyer et al. Octopamine in Male Aggression in Drosophila. Current Biology 18 (3), 159-167 (2008). (http://www.youtube.com/watch?v=4pDU-cqvKJc for video)
4) Weiner, Jonathan. Time, Love, Memory: A Great Biologist and His Quest for the Origins of Behavior. New York: Knopf, 1999. Print.
5) Manoli et al. Neural control of sexually dimorphic behaviors. Current Opinion in Neurobiology 2013, 23(3) 330-338.
6) Wang, L. & Anderson, D.J. Identification of an aggression-promoting pheromone and its receptor neurons in Drosophila. Nature 463, 227–231 (2010).