Inter-axonal recognition organizes Drosophila olfactory map formation.

Olfactory systems across the animal kingdom show astonishing similarities in their morphological and functional organization. In mouse and Drosophila, olfactory sensory neurons are characterized by the selective expression of a single odorant receptor (OR) type and by the OR class-specific connection in the olfactory brain center. Monospecific OR expression in mouse provides each sensory neuron with a unique recognition identity underlying class-specific axon sorting into synaptic glomeruli. Here we show that in Drosophila, although OR genes are not involved in sensory neuron connectivity, afferent sorting via OR class-specific recognition defines a central mechanism of odortopic map formation. Sensory neurons mutant for the Ig-domain receptor Dscam converge into ectopic glomeruli with single OR class identity independent of their target cells. Mosaic analysis showed that Dscam prevents premature recognition among sensory axons of the same OR class. Single Dscam isoform expression in projecting axons revealed the importance of Dscam diversity for spatially restricted glomerular convergence. These data support a model in which the precise temporal-spatial regulation of Dscam activity controls class-specific axon sorting thereby indicating convergent evolution of olfactory map formation via self-patterning of sensory neurons.