Fig. 1. AL organization and development. (A) Organization of the mature Drosophila antennal lobe (AL) with vertebrate counterparts in parentheses. Each colour represents olfactory receptor neurons (ORNs) expressing a particular seven-transmembrane-span receptor or their post-synaptic projection neuron (PN) partners. (B) Three hypotheses for the cellular basis of highly specific wiring between ORN axons and PN dendrites. (Model 1) ORNs pattern the lobe, then PN dendrites grow in. (Model 2) PN dendrites pattern the lobe, serving as a template for ORN axons. (Model 3) The lobe is patterned by the coincident interaction of both cell types. (C) Model equivalent to Model 1 above, illustrating that the order of growth and patterning might be distinct: PNs could ramify extensively through the lobe but remain unpatterned until the arrival of ORNs – models 2 and 3 could be similarly extended. (D) Time course of adult lobe development and larval lobe degeneration. Red structures outline adult lobes; the larval lobes are shown in purple.