Introductory note by JAC: Matthew has produced a terrific post here, and I hope people will read it. The results of this scientific analysis are amazing and the genetic tools required to produce them are breathtaking—tools I couldn’t have imagined were possible when I was a graduate student. If I had one hope for 2014, it would be that more people would read the science posts. (Maybe you do, but just don’t comment on them!) kthxbai

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by Matthew Cobb

In 2007, the genomes of 12 closely-related Drosophila species were published in Nature. This was an important moment because it enabled researchers to look beyond their traditional friend, D. melanogaster, and to study the similarities and differences between the behaviour and genes of ‘the’ fly and those of its relatives, for some species had separated only a million or so years ago, while others were much more distant: up to 40 million years of divergence.

Accompanying the genomes was a thought-provoking review by Drosophila neurobiologist Leslie Vosshall from Rockefeller University. In her article, entitled ‘Into the mind of the fly’, Leslie summarised the varied behavioural studies that had been made on Drosophila, most of them deriving from the work of Seymour Benzer, who was behind the discovery of the first biological clock gene as well as mutations affecting learning. Not only did such complex behaviours and genes exist,  it turned out that they are highly conserved: the same genes control fly clocks that control part of the human biological clock, for example. In other words, you share some really interesting bits of your physiology with a fly.

Leslie concluded her article with this bold statement:

 It now seems possible to approach in the fly more complex behaviours and even emotions, the neurobiological basis of which are not well understood at the genetic or functional level in any animal: sociality, common sense, altruism, empathy, frustration, motivation, hatred, jealousy, peer pressure, and so on. The only a priori limitation to studying any of these traits is the belief that flies can show such emotions and the design of a plausible behavioural paradigm to measure them.

At the time I thought this was fly-neuroscience hubris. But I now think she was probably right – all of our specific genetically-influenced human characters must have some vague predecessor in animals; many of those may have deep evolutionary roots, and may therefore be shared even by quite distant relatives such as flies.

One indication that Leslie was right is a paper that’s just appeared in Cell from one of her ex-students, Kenta Asahina, working in the lab of David Anderson.  Asahina, Anderson and their colleagues have shown that the biochemical basis of aggression in flies appears to involve a neuropeptide that is also involved in aggression in mammals, including humans.

Drosophila flies are normally a pretty peaceful bunch, spending their time hanging around on rotting fruit, mating and so on. But if you give a couple of males a limited amount of food, then they can get very aggressive and apparently territorial, as shown in this neat video from Amber McCartney (it goes on for nearly 4 minutes – you’ll get the idea after 15 seconds or so; at around 2:15 a female turns up, but the guys are initially more interested in beating the crap out of each other than in flirting with the gal, who isn’t up for it anyway).

The aggression is shown primarily by male flies and involves chemical signals – if you create a female fly covered in male pheromones (this is the kind of thing fly scientists love doing) the male treats her aggressively rather than trying to mate with her. The cells that are involved in controlling this behaviour express a gene called fruitless, which is very complex – it has different forms in males and females – and does lots of things to do with sex-specific fly behaviour.

Kenta and his colleagues (most of them from Cal Tech) did a neat experiment; they reasoned that neuropeptides might be involved in controlling aggression in flies, because these substances are also found in the neural circuits that control other complex behaviours. They then took 40 fly lines, representing a total of around 20 neuropeptide genes, and engineered them so that the genes would only work at 29º C (this is also the kind of thing that fly people like doing). They put males together in pairs at eclosion (hatching from the pupal case), and then automatically observed their movement.

On Day 6 they shifted the temperature to 29ºC and observed what happened. As the figure shows, two of these lines expressing a neuropeptide called Tachykinin showed high levels of lunging behaviour (this is a key part of male aggression), suggesting that this neuropeptide is involved in controlling aggression.

They then looked at the way the Tachykinin (‘Tk’) gene was expressed in the brains of the flies and found that only a small number of cells in male flies expressed this gene. It turned out that these cells were also specific for the male form of the fruitless gene. This figure shows you the exquisite science neuroanatomists can do these days – the green labels Tk-expressing cells in the fly brain, while the magenta shows the pattern of male fruitless expression (A2, B2).

To prove that these cells are actually involved in controlling aggression, they then manipulated the flies so they could turn the Tk on and off only in those specific cells. Sure enough, they found that when the gene was turned on, the flies showed aggressive behaviour.

There’s a whole lot more in the paper, including showing that only aggression was affected, that a mutation in the Tk gene abolished aggression, activating the Tk cells made the flies aggressive and so on, but for the general reader we can proceed to the pièce de résistance. (My apologies to the scientists who did so much work, don’t mean to belittle your effort!)

In the Discussion, the authors highlight that what fly folk call Tachykinin is homologous to what in mammals is called ‘Substance P’, which is involved in controlling aggression in mammals. In other words, the neuropeptide that enables these flies to be aggressive is also found in you and me. The authors go on to make some bold claims for what is an incredibly detailed and well-argued study. Claims that a few years ago I might have dismissed, but now take extremely seriously. I’ve reproduced the final paragraphs here, without the references. Some of it might be hard going, but this is terrific stuff, with potentially massive implications, if they are right:

Among three species of vertebrate Tachykinin neuropeptides, Substance P has been implicated, directly or indirectly, in various forms of aggression, including defensive rage and predatory attack in cats, and intermale aggression in rats. Although not all functions of Substance P are necessarily conserved (such as nociception in mammals and olfactory modulation in the fly), these data suggest that this neuropeptide is broadly involved in the control of agonistic behavior in both vertebrates and invertebrates. They therefore add to the growing list of neuropeptide systems that show a remarkable evolutionary conservation of functions in the regulation of innate “survival behaviors” such as feeding and mating. Biogenic amines also control aggression across phylogeny. However, in the case of serotonin, the directionality of its influence is opposite in flies and humans.

Our findings indicate that studies of agonistic behavior in Drosophila can identify aggression-regulating genes with direct relevance to vertebrates. Interestingly, in humans, the concentration of Substance P-like immunoreactivity in cerebrospinal fluid has been positively correlated with aggressive tendencies in patients with personality disorders. Substance P antagonists have been tested in humans as anxiolytic and antidepressant agents, although they failed to show efficacy. The present findings, taken together with mammalian animal studies, suggest that it may be worthwhile to investigate the potential of these antagonists for reducing violent aggression in humans.

References

Leslie B. Vosshall (2007) Into the mind of a fly Nature 450:193-197 (FREE!)

Kenta Asahina et al. (2014) Tachykinin-Expressing Neurons Control Male-Specific Aggressive Arousal in Drosophila. Cell 156:221-235. (abstract free, article = $$$)