How flies and humans see

9 February 2020

Professor Marion Silies joined the Faculty of Biology at Johannes Gutenberg University Mainz (JGU) in early 2019. Here she has been investigating the organization and function of circuits in the visual system of the fruit fly. Her work has already earned her numerous awards.

At the beginning of 2019, Professor Marion Silies moved into her new rooms in the newly-constructed BioZentrum I building on the Gutenberg campus. "I really like it here," she says. "It’s so open and friendly." A picture by the caricaturist and illustrator Frank Hoppmann, in which a red frog is stretching out to catch a black fly, is an eye catcher in her office. "I like his work. Plus, the artist grew up in the same village as I did, in Emsbüren. So I thought that would be fitting. He mostly draws flies, pigs, and politicians." And flies are the subject of this conversation. Silies and her team study visual processing using the fruit fly Drosophila melanogaster as a model organism.

On the table in front of her are several sheets of paper containing microscopy data. "They show how individual cells within the visual system of Drosophila respond to moving stimuli," explains Silies. Next to a photographic microscope image, which looks rather confusing to the untrained eye, there are four separate curves, one for each direction of perception: up, down, left, and right. For each cell in the image, only one of these curves shows a significant signal recorded in that cell. This means that each cell registers movement in a particular direction.

Neural networks for motion perception

Visual processing is Silies' main research topic. She briefly explains how it works – in both flies and humans: “First of all, the photoreceptors in the eyes measure luminance. This then helps to calculate contrast and to distinguish bright from dark contrasts. We begin to detect edges, followed by movement and orientation. Motion is basically nothing more than a change in contrast over time and space.”

Silies is interested in what lies behind this process at the neuronal level. She studies the neural networks in the fly visual system that filter information from the environment in order to control behavior. "My group investigates the way in which the nerve cells are organized into circuits in order to perform their tasks. What we have been most interested in recently is discovering what molecular mechanisms give certain neurons their unique properties." By now, scientists have a pretty clear understanding of how neurons are organized in networks to detect motion. "It's the molecular processes that we don't understand quite so well."

"As it turns out, these neural mechanisms for detecting motion are very similar in the eye of both fruit flies and vertebrates." However, since the eyes of insects and vertebrates have developed separately over the course of evolution, Silies concludes: "Apparently, there are preferred solutions for the same problem in very different visual systems."

From Münster to Mainz via Stanford and Göttingen

It made perfect sense for Silies to concentrate on Drosophila for her research. Very few organisms have been so thoroughly studied and are so easy to work with – although the fruit fly is anything but simple: "The fly visual system houses around 100,000 neurons," explains Silies. The biologist had already used Drosophila for her doctorate in Münster as a model for investigating the development of the nervous system. As a postdoc at Stanford University in California, she focused on the fly's motion vision and subsequently decided she wanted to continue to pursue the subject. In 2014, she established an Emmy Noether Junior Research Group at the European Neuroscience Institute Göttingen dedicated to studying the cellular and molecular basis of motion detection. She was also awarded an ERC Starting Grant, the most prestigious award given to junior researchers in Europe, in order to study the organization of microcircuits in the fly visual system.

"In Drosophila, we have a vast repertory of different genetic tools to selectively target each nerve cell. We can switch them on and off or mark them with proteins. This is all done non-invasively simply by crossing flies to generate a new stock. The offspring are then completely normal – except for the specific neuron type we are interested in."

With the help of these tools, her team has recently found that visual processing can still function accurately even in constantly changing light conditions. "So far we have concentrated on Drosophila melanogaster," Silies points out. "Now I would really like to investigate how the visual processing strategies differ in fly species that have different environmental constraints than Drosophila melanogaster."

New institute provides the ideal environment

At JGU, the biologist has found the ideal environment to advance her research. "A few years ago, Mainz University wasn’t on my radar as a hub for neuroscience, but thanks to the strategic realignment within its Faculty of Biology a lot has changed. Now there is the Institute of Organismic and Molecular Evolution, the Institute of Developmental Biology and Neurobiology, and the Institute of Molecular Physiology. When I read about this, I thought: These are all relevant to my interests, I’ll fit in really well."

When Silies accepted a professorship in Neurobiology at JGU in 2018, she was also appointed a Fellow of the university's Gutenberg Research College (GRC), which promotes academic excellence at Mainz University. "This would have given me the opportunity to take a five-year leave from teaching to concentrate on my research," she adds. "However, for me it is important to engage with students and invest time in their education as well as to further advance the neurosciences in Mainz. I would also like to see us set up a dedicated Master’s degree program in neuroscience."

So she instead used the GRC award to recruit one of her colleagues to JGU. “I got to know Dr. Carlotta Martelli in Göttingen. She studies olfactory processing in the fruit fly. We complement each other very well, and we share the teaching." Silies feels perfectly at home among her JGU colleagues. "Everything fits together really well. And because the new institute has just formed, I've got the chance to be involved in building something special here."