Researchers find how fruit flies sense leg movements

To control our arms and legs smoothly, our bodies rely on special sensors called proprioceptors, which help us know where our limbs are and how they are moving without relying on visual cues. These sensors come in various types, and they work similarly in both vertebrates and invertebrates. All walking animals use proprioception to control limb movements.

In Drosophila or fruit flies, the femoral chordotonal organ (FeCO) has different types of sensory neurons. Some of these sensors help the fly know where its leg is, others tell it which way the leg is moving, and others sense things like vibrations. These sensors help animals move their legs effectively. The question is, how does each sensor detect different aspects of movement?

In order to find answers, scientists from the University of Washington in Seattle (USA) started by using X-ray holographic nano-tomography at the ESRF’s ID16A beamline to reconstruct the structure of FeCO in the Drosophila leg. “The data from our experiments at the ESRF was critical because it allowed us to visualize and understand how all the different parts of the proprioceptive organ fit together”, explains John Tuthill, corresponding author of the publication and researcher at the University of Washington.

Tuthill collaborates with ESRF scientist Alexandra Pacureanu, who is developing a technique to study neuronal systems using the Extremely Brilliant Source in the framework of her ERC grant (BRILLIANCE –Bright, coherent and focused light to resolve neuronal circuits). “At the ESRF, on ID16A, we have now succeeded to push the limits of resolving power and scalability to support connectomics research and we know how to overcome the challenges that this type of experiments present, namely acquiring high resolution data of large and complex 3D samples and generating a seamless image volume”, explains Pacureanu.

The team then combined the data with RNA sequencing to study the sensory neurons in fruit fly legs. “We were surprised to discover that the sensors are very similar genetically, but we noticed that they are connected to the leg structure in a very different mechanical way”, says Tuthill.

Tuthill and collaborators turned then to modelling to explain how the mechanical differences help the sensors detect the angles of the leg joints. The model, combined with the technique of calcium imaging, confirmed that a “map” in the fruit fly's leg keeps track of these joint angles. “We discovered that the way these sensors are built and how they interact with the leg's structure is more important for their job than differences in gene expression”, says Tuthill.

The findings could apply to other sensory systems organized in a similar way in different animals.

Reference:

Mamiya, A., et al, Neuron, In press Corrected Proof. https://doi.org/10.1016/j.neuron.2023.07.009