A navigating axon faces complex choices when selecting postsynaptic partners in a three-dimensional (3D) space. In this work, we discovered a principle that can establish the 3D glomerular map of the fly antennal lobe by reducing the higher dimensionality serially to 1D projections. During development, olfactory receptor neuron (ORN) axons first contact their partner projection neuron dendrites on the spherical surface of the antennal lobe, regardless of whether the adult glomeruli lie near the surface or inside. Along this 2D surface, axons of each ORN type take a specific, arc-shaped trajectory that precisely intersects with their partner dendrites. Altering axon trajectories compromises synaptic partner matching. A 3D search is thus reduced to one dimension, simplifying partner matching.
