Wiring economy constrains olfactory glomerulus placement in fly larva

Olfactory sensory neurons expressing the same receptor converge their axons to a common locus called a glomerulus. The glomeruli placement are largely stereotyped. A plausible theory to explain the placement is the wiring economy principle, where neuronal placement is an optimal solution to minimizing the wiring cost given a synaptic connectivity pattern. Recently, the complete wiring diagram of a glomerular olfactory system, the insect antennal lobe, has been reconstructed using serial section electron microscopy and the fly larva. The reconstruction provides a detailed map of glomerulus placement as well as the underlying synaptic connectivity. Interneuron synaptic connectivity patterns within the antennal lobe are strikingly and consistently heterogeneous in their glomerular innervations. We developed a coarse-grained model to describe the glomeruli placement and calculate the wiring cost to generate this synaptic connectivity. The wiring cost of observed glomerular position is significantly smaller than random glomerular arrangements. We also searched for the theoretical minimal wiring cost, which compares well with the observed glomerular arrangement. Wiring economy provides an explanation in terms of physical optimization for the organization of an olfactory system.