Postsynaptic Notum secretion limits presynaptic structural development. (A) Representative confocal images of muscle 4 NMJs co-labeled for presynaptic HRP (green) and postsynaptic DLG (red) with cell-targeted Notum RNAi knockdown (top, ubiquitous UH1-Gal4/+ versus UH1>Notum:RNAi; middle, postsynaptic muscle 24B-Gal4/+ versus 24B>Notum:RNAi; bottom, presynaptic neuron elav-Gal4/+ versus elav>Notum:RNAi). (B) Quantified NMJ area, branch number and synaptic bouton number for the six genotypes. *P≤0.05, **P≤0.01, ***P≤0.001; n.s., not significant.