Triggering and modulation of a complex behavior by a single peptidergic command neuron in Drosophila

At the end of their growth phase, Drosophila larvae remodel their bodies, firmly glue themselves to a substrate, and harden their cuticle in preparation for metamorphosis. This process is termed pupariation and it is triggered by a surge in the steroid hormone ecdysone. Substrate attachment is achieved by a recently-described pupariation subprogram called glue expulsion and spreading behavior (GSB). An epidermis-to-CNS Dilp8-Lgr3 relaxin signaling event that occurs downstream of ecdysone after pupariation initiation is critical for unlocking progression of the pupariation motor program towards GSB, but the factors and circuits acting downstream of Lgr3 signaling remain unknown. Here, we screened for such factors using cell type-specific RNA interference (RNAi) and behavioral monitoring. We identify Myoinhibiting peptide (Mip) and its highly conserved neuronal receptor, Sex peptide receptor (SPR), as a critical neuropeptidergic signaling pathway required to trigger and modulate multiple action components of GSB. In addition, we find that Mip is specifically required in a pair of descending neurons, whose optogenetic activation at a specific competence window triggers GSB-like behavior and whose neurogenetic silencing completely abrogates GSB without overtly affecting other pupariation components. This strongly suggests that these descending Mip neurons are developmentally-regulated GSB command neurons. Dissection of the GSB action components via muscle calcium-level monitoring coupled with cell-type specific RNAi indicates that Mip acts on multiple SPR-positive neuronal populations, which collectively define and pattern the sequence and timing of GSB actions. Hence, we have identified a pair of descending command neurons that utilize both synaptic transmission and neuropeptidergic signaling to trigger and modulate a complex innate behavior in Drosophila. Our results advance our molecular and cellular understanding of pupariation control, reveal the complexity of glue expulsion and spreading behavior control, provide insight into conserved aspects of Mip-SPR signaling in animals, and contribute to the understanding of how multi-step innate behaviors are coordinated in time and with other developmental processes through command neurons and neuropeptidergic signaling.