Semaphorin-1a controls receptor neuron-specific axonal convergence in the primary olfactory center of Drosophila

In the olfactory system of Drosophila, 50 functional classes of sensory receptor neurons (ORNs) project in a highly organized fashion into the CNS, where they sort out from one another and converge into distinct synaptic glomeruli. We identified the transmembrane molecule Semaphorin-la (Sema-la) as an essential component to ensure glomerulus-specific axon segregation. Removal of sema-la in ORNs does not affect the pathfinding toward their target area but disrupts local axonal convergence into a single glomerulus, resulting in two distinct targeting phenotypes: axons either intermingle with adjacent ORN classes or segregate according to their odorant receptor identity into ectopic sites. Differential Sema-1 a expression can be detected among neighboring glomeruli, and mosaic analyses show that sema-la functions nonautonomously in ORN axon sorting. These findings provide insights into the mechanism by which afferent interactions lead to synaptic specificity in the olfactory system.