1. Neurosciences Program,
2. Department of Biological Sciences, Stanford University, Stanford, California 94305, USA
3. Department of Biology and Program in Neuroscience, University of Fribourg, CH-1700 Fribourg, Switzerland
4. These two authors contributed equally to this work

Correspondence to: Liqun Luo^1,2 Correspondence and requests for materials should be addressed to L.L. (e-mail: Email: lluo@stanford.edu).

Abstract

In Drosophila and mice, olfactory receptor neurons (ORNs) expressing the same receptors have convergent axonal projections to specific glomerular targets in the antennal lobe/olfactory bulb, creating an odour map in this first olfactory structure of the central nervous system^{1, 2, 3}. Projection neurons of the Drosophila antennal lobe send dendrites into glomeruli and axons to higher brain centres⁴, thereby transferring this odour map further into the brain. Here we use the MARCM method⁵ to perform a systematic clonal analysis of projection neurons, allowing us to correlate lineage and birth time of projection neurons with their glomerular choice. We demonstrate that projection neurons are prespecified by lineage and birth order to form synapses with specific incoming ORN axons, and therefore to carry specific olfactory information. This prespecification could be used to hardwire the fly's olfactory system, enabling stereotyped behavioural responses to odorants. Developmental studies lead us to hypothesize that recognition molecules ensure reciprocally specific connections of ORNs and projection neurons. These studies also imply a previously unanticipated role for precise dendritic targeting by postsynaptic neurons in determining connection specificity.