A Drosophila Tonic Motor Neuron Reinnervates Ectopic Muscles Fully Deprived of Native Tonic and Phasic Inputs

Motor neurons (MNs) form precise neuromuscular junctions (NMJs) during development, but the extent to which individual MNs can reinnervate fully denervated muscles in vivo remains poorly understood. In the Drosophila larva, each muscle is co- innervated by a tonic and a phasic glutamatergic MN. Here, we show that the tonic MN1 undergoes robust heterosynaptic sprouting and forms ectopic NMJs when neighboring muscles are deprived of both their native tonic and phasic inputs. This structural plasticity is not induced by silencing, but instead requires the physical ablation of adjacent MNs. Live imaging of the same MN1 axons in individual animals reveals that sprouting initiates early and expands progressively across larval stages. In contrast, phasic MNs show minimal remodeling, indicating that tonic MNs possess a greater intrinsic capacity for neuroplasticity. Notably, as MN1 establishes new synapses on targets it does not normally innervate, it redistributes pre-synaptic territory across both native and ectopic muscles. These findings identify a subtype-specific, injury-induced rewiring program in an intact motor circuit.