Lukas Breitzler (Group Larsch) writes about research from colleagues in the Benton lab.

This is a new blog section. If you are interested to contribute, just contact Nicole Vouilloz

On the third floor of the Genopode, many doors open to fruit flies. A first-time visitor might walk past rows of incubators and mistake them for the usual industrial-sized refrigerators and freezers that line the hallways of the CIG. But behind their doors are hundreds of plastic tubes teeming with numberless small fruit flies. Another door opens to a room with two large utility shelves stacked with more boxes of the same tubes. Foam plugs prevent their inhabitants from escaping, while food at the bottom allows them to feed ad libitum. At first glance, the flies look identical. But handwritten notes on labelling tape indicate that the tubes house several different species and genetic strains. Unassuming as they initially appear, these flies are central to the Benton lab’s efforts to answer a fundamental question in biology: how does behavior evolve?

“As a geneticist, I am interested in relating gene changes to neural circuits and behavior,” says Richard Benton, principal investigator of the Benton lab. Richard occupies one of the corner offices on the floor with a view of Lac Léman. Next to his desk, a table holds more boxes of tubes filled with flies. Richard explains that in the past 10 to 20 years, many labs have made impressive advances explaining how genes and neural circuits influence behavior in Drosophila melanogaster—the most commonly studied species of fruit fly. Researchers typically manipulate genes to alter neuronal processes and observe their impact on behavior. “We’re taking a slightly different approach, which is looking at natural variation in behavior to learn how circuits function and how they’re built,” says Richard. Animal genomes contain tens of thousands of genes, some of which influence behaviors. Pinpointing a single gene’s role often involves a fair amount of educated guesswork; not only do you have to pick a gene, but you also have to know what behavior to look for. Richard’s group prefers to approach this problem by working backward: identify differences in behavior first, then investigate the “nuts and bolts of what’s changing genetically.” That’s precisely the approach the lab took in their latest study, published in Nature, which revealed how changes in gene regulation enabled Drosophila to adapt their behavior to varying day lengths. A key aspect to this approach was examining the behavior of not only D. melanogaster but also that of closely related Drosophila species.

One of these species will be familiar to those who have joined the Benton lab’s progress reports: Drosophila sechellia; a close cousin to D. melanogaster. The two species are so closely related, in fact, that they are capable of producing hybrid offspring. But despite their genetic similarities, their habitats are strikingly different. D. sechellia feeds and lays eggs exclusively on Noni fruit, a plant toxic to other Drosophila species. (The fruit also emits a horrendous odor once it reaches a certain stage of ripeness, earning it the nickname ‘vomit fruit’ in regions where it grows. The Benton lab keeps a small, airtight incubator filled with them.) Additionally, while D. melanogaster can be found virtually all over the world—including in northern latitudes, where they experience highly variable day lengths throughout the year—D. sechellia live exclusively on the Seychelles islands near the equator, where the sun rises and sets at nearly the same time year-round. Previously, the group detailed some of the genetic differences that enabled D. sechellia’s specialization to noni fruit. However, whether the pronounced differences in seasonal daylight drove evolutionary adaptation had never been investigated—until Michael Shahandeh, the recent study’s first author, decided to take a look.

Indeed Michael and collaborators conducted experiments demonstrating that D. sechellia barely adjusted their behavior to longer photoperiods. The flies were most active around the same time every day, regardless of whether days were 12 or 20 hours long. D. Melanogaster on the other hand adjusted their activity without problem. Importantly, other species closely related to D. sechellia also adjusted their behavior without problem, suggesting that D. sechellia lost its ability to regulate daily rhythms through evolution as it adapted to a stable environment.

With a behavioral phenotype in hand, the difficulty now lay in identifying the genetic mechanism. Fortunately, the team had decades of research on the circadian biology of D. melanogaster to draw on. This allowed them to identify a list of candidate genes to test.

To find the gene relevant for regulating daily activity, “Michael devised a clever approach,” says Richard. By taking D. melanogaster strains with mutations for the different candidate genes and interbreeding them with D. sechellia, the lab created hybrid offspring that carried one gene copy from each species. In most cases the D. sechellia allele compensated for the missing D. melanogaster gene and the hybrids behaved normally. But when one specific gene—called Pdf—was disrupted, the sechellia allele failed to make up for the loss; the hybrids lost their ability to shift activity under longer days. This indicated that Pdf, which encodes a neuropeptide that transmits signals generated by core molecular timekeeping mechanisms in Drosophila, regulated behavioral plasticity.

Follow-up experiments found that Pdf expression was weaker and less variable in neurons of D. sechellia compared to D. melanogaster and pinpointed those differences to changes in the gene’s regulatory region. However, the lab needed to establish that those differences in gene regulation actually caused the behavior they observed. Using genetic engineering, they therefore cloned the Pdf regulatory sequences of both species and inserted them into mutant flies lacking their own functional Pdf regulatory sequence. And indeed, flies with the D. sechellia sequence showed reduced plasticity compared to flies with the melanogaster sequence. This final experiment completed the picture, showing how a subtle gene regulatory change could alter neuronal function and, ultimately, the way the flies behave.

It is exceedingly difficult to understand how evolution links genetic changes to their effects on neurons and behavior. The Benton lab achieves this by asking the right questions in the right system. “It’s really Drosophila as a whole and Drosophila sechellia as a specialized species. And it pays off to have an ecological framework,” says Richard. Genetic mechanisms can shape behavior through subtle and unexpected pathways. The Benton lab’s latest study is one of several examples from their research demonstrating how this framework not only deepens our understanding of evolution, but also uncovers mechanisms that might otherwise remain hidden.