Olfactory responses ofDrosophilaare encoded in the organization of projection neurons

Abstract

The projection neurons (PNs), reconstructed from electron microscope (EM) images of the Drosophila olfactory system, offer a detailed view of neuronal anatomy, providing glimpses into information flow in the brain. About 150 PNs constituting 51 glomeruli in the antennal lobe (AL) are bundled together in the axonal extension, routing the olfactory signal received at the AL to the mushroom body (MB) calyx and the lateral horn (LH). Here we quantify the neuronal organization by inter-PN distances and examine its relationship with the odor types sensed by Drosophila. The homotypic PNs that constitute glomeruli in AL are tightly bundled and stereotyped in position; however, such a glomerular-like PN organization in AL is no longer sustained in the higher brain center. Instead, odor-type dependent clusters consisting of multiple different homotypes become more prevalent in MB calyx and LH. We find that there are statistically significant associations between the spatial organization among a group of homotypic PNs and certain stereotyped behavioral responses. In particular, pheromone-encoding homotypes are segregated from other neurons not only in LH but also in MB calyx. Our findings suggest that before neural computation in the inner brain, some of the olfactory responses are already encoded either by the homotype or in the spatial organization of PNs, illuminating that the labeled-line strategy is at work in the Drosophila olfactory system.