From protein to behaviour – lessons from the fruit fly mating ritual
All animals are born with instinctive or innate behaviours, such as nest building in birds or web weaving in spiders. But what is the biological basis of such behaviours? Barry Dickson, who spent around 12 years of his career at the IMP, set out to unravel neural circuits in the brain that control complex innate behaviours. As a model system, he chose to study courtship in the fruit fly, becoming a pioneer in elucidating the organization and operating principles of complex neural circuits.
Barry Dickson joined the IMP as a group leader in 1998, where he continued work on axon guidance first begun during his postdoc (1994-1995) with Corey Goodman (then at Berkeley), and subsequently pursued in his first independent research group at the University of Zurich. In 2003 he was appointed a Senior Scientist at IMBA, an Austrian Academy of Sciences institute founded based on an initiative of Kim Nasmyth and with the help of the IMP. Since IMBA did not yet have a building and all IMBA scientists were hosted by the IMP, Barry was able to stay in his lab at the IMP. Around that time, he started to change his research focus – he admits that he was getting a bit bored of axon guidance. He became interested in the neural basis of complex behaviours.
“I was very much inspired by reading a book by Jonathan Weiner: ‘Time, love, memory’, about Seymour Benzer”, Barry says. In the 1960s and 1970s Benzer and colleagues had used forward genetic screens in the fruit fly Drosophila to isolate mutants for particular behaviours, focusing on circadian rhythms, courtship and learning/memory (hence the title of the book). These screens had also picked up mutants defective in other behaviours like vision and locomotion, and several labs had focused on these in an attempt to unravel complex behaviours at a molecular and cellular level. But progress had been slow, and Barry suspected that these might not be the most suitable systems for the job. He decided to focus on the elaborate courtship ritual performed by males for females. Courtship seemed like a perfect model to study a complex behaviour – robust and amenable to genetic manipulation. In particular, Barry realised that its sexual dimorphism was a big advantage: “Then you could just ask, what’s the difference between males and females?”
The first courtship mutant to be discovered (in 1963) was fruitless – males with this mutation courted other males. Barry wanted to test the hypothesis that the fruitless gene was sufficient to program courtship behaviour in the brain. The gene was known to be spliced differently in males and females, so he reasoned that converting fruitless to the male spliced form in females might wire brains in such a way that courtship behaviour would be initiated in the female mutants. However, this would require genetic manipulation using homologous recombination, a technique that had only been successfully used in flies a handful of times. Ebru Demir, who joined the lab as a PhD student at around this time, took on the rather risky project. She set up a genetic screen, looking through thousands of flies for a red-eye phenotype, which would indicate that the homologous recombination had worked. To her relief and excitement, she found positive progeny, although the recombination event was a rare one – only around 1 in 6,000 females had the male splicing form.
Ebru couldn’t wait to see how these females would behave – working late in the evening, she tested their interaction with other females. “It was amazing – they were performing the courtship ritual like a male would do”, she recalls. “It was so exciting to see that.” The experiment showed that splicing of a single gene specified an entire complex behaviour. It was around 11 pm, so Ebru wasn’t planning to tell Barry until the morning, but her friends in the lab persuaded her to call him. About 5 minutes later Barry was in the lab too (at the time he lived just across the road) – he wanted to see the behaviour for himself! “It remains the most exciting day of my career”, Barry recalls. “It was wonderful.”
Petra Stockinger, a master’s student who joined the lab around the same time as Ebru, also recalls Barry’s almost childlike excitement when it came to new results. Her project was to make a reporter for the fruitless gene to be able to visualise and manipulate the neural circuit of courtship behaviour. Like Ebru, she used homologous recombination, this time inserting the reporter gene GAL4 into the endogenous fruitless locus. Sure enough, although again with low success rates (around 1 in 3,000 flies), she found expression that overlapped with signals obtained with the fruitless antibody. “It was really very exciting”, she recalls. “The homologous recombination had worked, and we could actually start looking at these circuits.” Together with her colleague Duda Kvitsiani, Petra showed that the neurons expressing GAL4 were directly and specifically involved in male courtship behaviour.
The researchers published their findings in two papers that appeared in the same issue of Cell (Stockinger at al., Cell 2005; Demir & Dickson, Cell 2005). The work created headlines beyond the scientific community, partly because they provoked discussion on potential genetic determination of sexual orientation in humans. But Barry has largely stayed out of such discussion – “I don’t think for a moment that this has any relevance for sexual differentiation in anything outside insects”, he says. The goal was to provide insights into animal behaviour in general, and fly courtship was simply an ideal model to do so. But as with all good science, the findings threw up more questions than they answered. “I remember thinking at the time, I’m probably going to spend the rest of my career trying to figure out why Ebru’s experiment worked”, Barry recalls. Petra and Duda’s paper was more descriptive, but it opened up the way to progress with the problem – it was now possible to study all the neurons involved.
Not long after these results were published, in 2006, Barry was appointed Scientific Director of the IMP, succeeding Kim Nasmyth. Given that his lab had not physically moved from the IMP during his time as a Senior Scientist at IMBA, he was able to continue his research without interruptions. Under Barry’s guidance, the IMP set up the world’s largest fly library – today, the “Vienna Drosophila Resource Center” consists of more than 38,000 independent transgenic fly lines available to researchers anywhere in the world. It was also his idea to start a ‘summer school’ at the Vienna BioCenter, which today attracts more than 2,000 applications from all over the world every year.
But the responsibilities that came with being Scientific Director did not detract from Barry’s research. His group continued to work on fruitless (amongst other things), showing that its sex-specific splicing is critical not only for courtship behaviour, but also for aggression and dominance (Vrontou et al., Nat Neurosci 2006). The researchers also showed that activation of a single class of fruitless-expressing olfactory receptor neurons is necessary and sufficient to mediate behavioural responses to a Drosophila sex pheromone (Kurtovic et al., Nature 2007) and – together with Richard Axel’s group at Columbia – that a fruitless-dependent difference between the male and female neurons is responsible for the different responses to the same pheromone (Datta et al., Nature 2008). More in-depth analyses of the neural circuits began to reveal more and more examples of fruitless-dependent sexual dimorphism in key neurons (e.g., Yu et al., Curr Biol 2010), resolving the puzzling observation made in Petra and Duda’s paper that the circuit looked remarkably similar in both male and female brains.
Barry’s research enabled construction of one of the first cellular-resolution wiring diagrams of an entire neural circuit. What was still a research field in its infancy when Barry began at the IMP, the neural basis of behaviour is now being studied by hundreds of groups around the world, with the ultimate goal of understanding how information processing in the brain guides behaviour. In 2013, Barry moved to the Janelia Farm Research Campus of the Howard Hughes Medical Institute, where he continued to use Drosophila as a model system for the study of neural circuits. And more than half a century after its discovery, fruitless is still proving to defy its name when it comes to providing insights into the neural basis of behaviour.
References
fruitless splicing specifies male courtship behavior in Drosophila.
Demir E, Dickson BJ; Cell 2005
Neural circuitry that governs Drosophila male courtship behavior.
Stockinger P, Kvitsiani D, Rotkopf S, Tirián L, Dickson BJ; Cell 2005
fruitless regulates aggression and dominance in Drosophila
Vrontou E, Nilsen SP, Demir E, Kravitz EA, Dickson BJ; Nat Neurosci 2006
A single class of olfactory neurons mediates behavioural responses to a Drosophila sex pheromone.
Kurtovic A, Widmer A, Dickson BJ; Nature 2007
The Drosophila pheromone cVA activates a sexually dimorphic neural circuit.
Datta SR, Vasconcelos ML, Ruta V, Luo S, Wong A, Demir E, Flores J, Balonze K, Dickson BJ, Axel R; Nature 2008
Cellular organization of the neural circuit that drives Drosophila courtship behaviour.
Yu JY, Kanai MI, Demir E, Jefferis GS, Dickson BJ; Curr Biol 2010