Connectome simulations identify a central pattern generator circuit for fly walking

Animal locomotion relies on rhythmic body movements driven by central pattern generators (CPGs): neural circuits that produce oscillating output without oscillating input. However, the circuit structure of a CPG for walking is not known in any animal. To identify the cells and synapses that underlie rhythmic leg movement in walking flies, we developed dynamic simulations of the Drosophila ventral nerve cord (VNC) connectomes. We used a computational activation screen to identify descending neurons from the brain that drive rhythmic activity in leg motor neurons, including a command neuron for walking (DNg100). By synthetic pruning of the VNC network, we isolated a minimal three-neuron rhythm-generating circuit consisting of one inhibitory and two excitatory interneurons. A model of this core CPG circuit is sufficient to generate motor rhythms, and the two excitatory neurons are necessary in the VNC network model. Connectome simulations also predicted that parallel descending neurons (DNb08) produce rhythmic leg movements, which we experimentally confirmed using optogenetics in behaving flies. Our results reveal the cellular identity and synaptic structure of a putative CPG circuit for fly walking.