The olfactory pathway of adult and larval Drosophila: conservation or adaptation to stage-specific needs?

Tracing of olfactory projections based on odorant receptor expression has led to an almost complete receptor-to-glomerulus map in adult Drosophila. While most of the glomeruli may be involved in processing of food odors, others appear to be more specialized, for example, responding to CO(2) or to pheromonal cues. Recent studies have shed light on signal processing in the antennal lobe and in higher centers. Newly detected cholinergic excitatory local interneurons in the antennal lobe appear to provide substrates for the broad odor tuning properties of projection neurons. In the mushroom bodies, projection neurons establish an intricate divergence-convergence network with their target cells, allowing complex modes of signal transfer. In the lateral horn, projection neurons innervating candidate pheromone glomeruli appear to segregate from those innervating "normal" glomeruli. Hence, pheromone and food information may be handled by separate channels, consistent with discrete behavioral meanings of the two kinds of signals. The olfactory pathway of the larva shares the general layout of its adult counterpart, with a number of simplifications. The presence of only 21 glomeruli suggests a reduction of primary olfactory "dimensions" compared to adults. The existence of a pheromone-sensing subsystem is unlikely. Larval glomeruli are targets of single, unique sensory neurons rather than being sites of convergence as in the adult. Projection neuron outputs are restricted to single glomeruli in the mushroom body. Their target cells either innervate one or several of them creating substrates for elementary odor coding and coincidence detection. In conclusion, olfactory discrimination capacities of the larva are very likely reduced, consistent with the requirements of a substrate feeder.