Neuronal activity modulates synapse formation in the developing PDE axon. a, Schematic representation and line scan of the PDE axon and its synapses labeled using a combinatorial approach. Endogenous FLP-on (FRT) GFP::ELKS-1 labels active zones and endogenous FLP-on (FRT3) mScarlet::TBA-1 labels microtubules (for axon morphology) when combined with a transgene…