2026 Theses Doctoral

The Neural Circuit Basis for Directional Escape Behavior in Drosophila Larvae

Wood, Abby J.

Escape behavior is crucial for the avoidance of painful stimuli and survival. The sensory stimuli that initiate and modulate escape behavior are well studied, as well as some of the neuromuscular mechanisms underlying escape locomotion. How specific nociceptive circuit nodes mediate specific modules of escape remains understudied. Drosophila larvae present an ideal model to study this question due to the available tools for circuit manipulation, the synapse-level connectome, and stereotyped escape motor sequence.

In response to parasitic wasps or other noxious stimuli, larvae perform a stereotyped escape sequence consisting of C-shaped body bending followed by circumferential rolling around the body axis and fast forward crawling, which results in four different patterns and direction of escape rolling. Several circuits governing the execution of escape have been described and involve second-order interneurons that integrate input from mechanosensory and nociceptive sensory neurons. Additionally, the muscle activation patterns underlying escape rolling have been described, as well as some of the potential premotor circuits that support circumferential rolling. However, the circuit basis underlying the sensorimotor transformation from multisensory input to motor output to employ directionally precise escape in larvae is not yet known.

This work identifies nociceptive circuit nodes that encode bend and/or roll direction during escape. I first describe how four escape patterns are elicited in response to naturalistic parasitic wasp attack through high-speed imaging of interactions between larvae and parasitoids, and I found that larvae preferentially perform two escape patterns. I characterize the attack behavior of wasps and determine which body regions are targeted during attack and sustain puncture wounds, revealing a potential modality-specific role for dorsolateral nociceptors in mechanical nociception.

Using a sparse optogenetic activation technique, I activated different nodes of the nociceptive circuit to ask if their lateralized activation biases bend or roll direction. I find that the activation of primary nociceptors and Basin-2 interneurons result in ipsilateral bending and rolling behavior, while lateralized Basin-4 activation results in contralateral bending and ipsilateral rolling. Ipsilateral bending and rolling following nociceptor and Basin-2 activation resembles wasp attack data. Diverging connectivity between cIVs and Basins suggest modality-specific functional roles for distinct Basin subtypes. Further, I also show that unilateral activation of the command-like interneuron Goro biases contralateral rolling, establishing a sensorimotor circuit for directional escape in the larval escape circuit. Finally, statistical modeling of sparse neuron activation data provides a foundational, methodological framework for assessing the contributions that specific cells and small subsets of cells have on driving bending initiation and direction. A series of regression analyses reveals that individual abdominal Basin-2 neurons, which each receive input from ipsilateral mechanosensory and nociceptive neurons in the same body segment, are associated with ipsilateral bending.

Together, these data reveal how natural mechanonociceptive stimuli evokes distinct escape motor sequences and identifies the nociceptive circuit nodes that are involved in engaging specific directional behavioral modules during escape. Future work will be important for determining the selective advantages of each of the distinct motor sequences and identifying other circuit nodes that employ asymmetrical motor outputs in response to noxious stimuli. Experiments testing how lateralized muscle activation is achieved by distinct premotor-motor circuits will demonstrate how asymmetrical sensorimotor transformations can drive flexible, yet stereotyped escape behavior.