Development of Wiring Specificity of the Drosophila Olfactory System

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Abstract
The central problem of neural circuit assembly is how wiring specificity is achieved. The Drosophila olfactory neural circuit presents a fascinating system to attack this problem. As in mammals, the Drosophila olfactory receptor neurons (ORNs) that express a given receptor converge their axons onto a common glomerulus in the antennal lobe, creating an odor map in this first olfactory structure of the central nervous system (Ressler et al., 1994; Vassar et al., 1994; Mombaerts et al., 1996; Gao et al., 2000; Vosshall et al., 2000). Antennal lobe projection neurons (PNs) send their dendrites into glomeruli and axons to higher brain centers including the mushroom body and the lateral horn (Stocker, 1994). Using MARCM-based systematic clonal analysis, we found that PNs are prespecified by lineage and birth order to send dendrites to specific glomeruli and thereby carry specific olfactory information (Jefferis et al., 2001). Further, we demonstrated that according to glomerular class, PNs have stereotyped axon branching patterns and terminal fields in the lateral horn (Marin et al., 2002; Wong et al., 2002). Thus during the construction of the fly olfactory system, a given ORN must target its axons to one of ∼50 glomeruli, while a given PN must also target its dendrites to one of ∼ 50 glomeruli and furthermore the PN must coordinate its dendritic target choice with its axon terminal arborization pattern in higher olfactory centers.