What Makes Them Tick: The Fruit Fly’s Internal Gyrosope

What Makes Them Tick: The Fruit Fly’s Internal Gyroscope

Illustration by Hannah Kennedy

By Harris Davis

There’s something special about flying. Birds, reptiles, mammals, and insects have been flying for millions of years, and yet until very recently in our evolution, the physics of flying have eluded human beings. Animal flight mechanisms have fascinated biologists since Darwin, and have been studied in a diverse range of species. For the first time, neuroscientific research at UNC is revealing the role of otherwise poorly understood mechanisms of flight control in fruit flies. The fruit fly (D. melanogaster) has long been of particular interest to researchers for a unique and deceptively simple aspect of its flight. A few centuries ago, a pair of tiny structures that twitch and vibrate during flight were discovered on the fly’s thorax. It was later found that the removal of these structures completely compromises the fly’s

Figure 1: (A) Map of haltere muscles, where hDVM is the power muscle. Steering muscles include basalares (hB1and hB2) and axillaries (hI1–hIII3). (B) Experimental setup of LED arena, which tracks wing motion and images muscle activity. (C) amplitude and muscle activity in axillaries and basalares when visual field simulates rotation to the left (red) and right (blue). ability to get off the ground.1 And unlike many insects—which have two sets of wings—flies have only one, along with a pair of these appendages that came to be known as halteres. Despite these observations, the function of halteres in flies has largely remained a mystery. Approximately 80 years ago, however, it was proposed that halteres may function as a kind of biological gyroscope, maintaining the fly’s balance midair.3 This is one of the hypotheses that Dr. Brad Dickerson and his team have spent the last year exploring at UNC. When they began their work, the lab didn’t just want to verify the halteres’ function as a gyroscope; they wanted to know exactly how sensory input causes changes in muscle motion during flight. Furthermore, they wanted to investigate the halteres’ potential role in the timing of wing motion.2 Because of the high speed at which the fruit fly beats its wings—200 to 250 times per second, the team knew that motor circuits commonly involved in timing the gaits of other animals were unlikely to control fruit fly wing strokes. Thus, the fly must have a faster, more reflexive way to control motor activity. First, Dr. Dickerson’s team explored the relationship between visual inputs that trigger neural signals to the haltere muscles. They then looked at how those signals in turn provide immediate feedback to the wing muscles to determine if neural information from the halteres to the wing muscles plays a significant role in controlling wing motion. Figuring out which inputs trigger feedback is key to interpreting the function of any biological structure. To understand how halteres regulate flight, it is first necessary to consider the nerves that make them tick. Each of the halteres has one “power muscle” for generating force, and a group of “control muscles” for fine tuning the direction of that force.1,2

Brad Dickerson Ph.D., Assistant Professor in UNC Biology Department and Kenan Honors Fellow

Each muscle is controlled by a single motor neuron. A “firing” of that neuron triggers a contraction of the corresponding muscle. These firings are triggered by physical input from sensors, which line the fly’s wings and halteres, and from visual input transmitted from the eyes to the brain.1 While this may seem relatively straightforward, the key to completely understanding how motor changes occur in response to stimuli has remained hidden in the difficult-to-analyze neural feedback mechanisms between halteres and wings. Past analyses of the motion of haltere muscles in response to stimuli have largely failed because the physical movement of the structures is so finely

Figure 3I and 3J: Wingbeat amplitude (bottom) and muscle activity (top) in haltere (right) and wing (left). Note similarities in muscle activity during cardinal rotations. tuned that changes are too small to be observable.1 Dr. Dickerson’s team, however, took a different approach, which did not attempt to see the motion of the halteres, per se. Instead, they analyzed fluctuations in the chemistry of motor neurons and the muscles they control, and their latest experiments have employed cutting-edge neural imaging technology to observe the activity of fly’s muscles during flight—down to the millisecond—in response to physical and visual stimuli. One experiment the team conducted involved placing a fly in an LED arena and broadcasting images to that fly to create the illusion of motion, thereby providing a visual stimulus. The team tracked changes in the fly’s direction in response to visual stimuli, simultaneously observing activity in the motor neurons of the wings and halteres. When an action potential fires across a motor neuron, there is an influx of calcium ions by nearby muscle tissue. Using a fluorescent microscope, the lab was able to record these calcium spikes and dips, which correspond to motor neuron activity, and thus show changes to the motion of the haltere muscle. Imaging the axon terminals of the motor neuron instead of the haltere itself allowed Dickerson and his team to not only record these changes during flight, but to correlate them with specific shifts in the fly’s visual stimulus. A caveat to this type of observation is that calcium fluctuations only show that something about the motion of the haltere is changing—they don’t tell you what. Hence the other advantage of calcium imaging. The wing nerve of the fly is homologous to that of the haltere; it has the same kinds of sensory structures, and it sends similar sensory inputs to the brain. It can also be imaged in the same manner as the haltere, using calcium. While changes in haltere motion are difficult to observe physically, changes in the wing motion are not. Thus, if physical changes in wing motion can be correlated with specific fluctuations in calcium activity in the muscles, the same fluctuations in the haltere muscles can be interpreted as similar motion changes. The data were pretty hard to argue with. First, the Dickerson Lab found a clear correlation between the simulation of motion in the fly’s visual field and changes in wingbeat amplitude. Changes in the fly’s perceived cardinal directions resulted in haltere responses that would cause the fly to change its flight angle to correct the change. Second, the link between haltere feedback and wing motion was verified when activation of haltere muscles resulted in immediate, pronounced changes in wing steering. This made sense given the similarities muscle activity between the wings and halteres.2 Not only does this relationship between feedback from the haltere muscles and wing movement support the hypothesis that halteres are involved in timing; the interlocked nature of the two systems also provides further evidence that halteres in flies evolved from a pair of hindwings present in their ancestors.1 For Dr. Dickerson, this validated countless hours of painstaking literature

life science

review, confirming in a matterof weeks a hypothesis that had taken years to generate. More broadly, these data are a major step in resolving a question about flight mechanisms that has persisted for centuries. Evidence seldom comes together quite this neatly. Biology is messy. Stable physiology in living systems requires order, forcing organisms to grapple with the second law of thermodynamics. Constantly shifting ecological pressures demand that populations adjust and readjust to their environments, often with but a handful of molecular accidents in their toolbox. This is the kind of chaos that biologists work to understand and interpret, and research is often fraught with strife, stubborn data, and dead ends. Every once in a while, however, a researcher will experience what Dr. Dickerson calls a “brief moment of extreme simplicity,” when the data just make sense. The data the Dickerson Lab presented last year are not by any means an end to the study of the fruit fly’s flight mechanisms. However, they are representative of a large-scale paradigm shift in the way we think about neural timing systems. This shift opens the door to answering entirely new questions. If halteres are involved in wingbeat timing in flies, how do insects without halteres solve timing problems? How can we begin to explore the relationships between timing systems and motor systems in other animals? These data imply a certain elegance to the seemingly complex systems we have yet to understand. Still, the road to elegant solutions is paved with struggle and frustration, and every new question presents a new set of challenges to which scientists must adapt.

References

1. Interview with Brad Dickerson, Ph.D. 2. Bradley H. Dickerson, Alysha M. de Souza, Ainul Hudal, Michael H. Dickinson. Current Biology. 2019, Volume 29, Issue 20, pp. 3517-3524. “Flies Regulate Wing Motion via Active Control of a Dual-Function Gyroscope.” 3. J.W.S. Pringle. Philosophical Transactions of the Royal Society of London, Series B, Biological Sciences. 1948, Volume 233, Issue 602, pp. 347384. “The gyroscopic mechanism of the halteres of Diptera.” 4. R. Thompson, M. Wehling, J. Evers, W. Dixon. Journal of Comparative Physiology. 2008, Volume 195, pp. 99-112. “Body Rate Decoupling Using Haltere Mid-Stroke Measurements for Inertial Flight Stabilization in Diptera.”