Fig. 1. Development of the olfactory lobe in Drosophila. (A) Olfactory lobes (encircled with broken red lines) are visualized in the brain (marked with broken white lines) by expression of GFP driven by SG18.1-Gal4. Neurons enter the lobe through the antennal nerve (AN) and cross over to the contralateral side through the antennal commissure (AC). Scale bar: 100 µm. (B) Olfactory lobes of Or47b-Gal4/+;UAS-NsybGFP (UNG12)/+ stained with mAbnc82 (red). Projections to the glomerulus VA1 can be seen (arrow indicates the AC). Scale bar: 50 µm. (C,D). Schematic diagrams summarizing cellular events occurring during lobe development in the early (C) and late pupa (D). In the early pupa (18-30 hours APF), sensory neurons enter the brain via the antennal nerve (arrowhead) and traverse in the outer nerve layer in close association with glial cells (red). Ato-derived `pioneer' neurons arrive at the lobe first (purple) and guide the rest of the neurons (green) to the antennal commissure (arrow). At this time, the projection neurons (blue) are already present within the lobe. Sensory neurons invade the lobe from about 25 hours APF. (D) Termination of sensory neurons (green) and their contact with appropriate projection neurons (blue) is followed by ensheathing of the neural elements to form glomeruli.