Fig. 3. Robo receptors regulate targeting of olfactory neurons to specific glomeruli. Lobes are oriented as indicated in C,F: M, medial; D, dorsal; V, ventral. Imaging was carried out from anterior to posterior and a few 1 µm confocal sections are stacked in each picture. (A) Olfactory lobes of or22a-Gal4;UAS-N-SybGFP stained with mAbnc82 (red). Or22a-expressing neurons project to a single glomerulus DM2 and also cross over in the inter-antennal commissure (arrow). (B,D,E) The MARCM method was used to positively mark neurons mutant for robo2¹ (B) or robo3¹ (D,E). Axons lacking Robo2 function terminate within the midline creating a `roundabout' appearance (arrowheads in B; diagram in C). Ectopic axonal arbors are immunoreactive for mAbnc82 (arrow in B). Axons lacking Robo3 function terminate aberrantly (broken lines in D,E; diagram in F). (G-L) Sensory neuron terminals in the olfactory glomeruli are visualized using a SG18.1-UAS-GFP recombinant strain in different conditions. (G,J) Appropriate confocal sections have been selected to visualize normal glomeruli (G, ^*) and antennal commissure (J, arrow). (H) SG18.1 UAS-GFP/UAS-comm. (I) SG18.1 UAS-GFP/UAS-abl. In both H and I, glomerular patterning is significantly disrupted while the antennal commissure is present (arrow), although axons appear somewhat loosely fasciculated. (K,L) SG18.1 UAS-GFP/UAS-Dcdc42^v12 (constitutively active). Glomeruli cannot be discerned within the lobes (K). The commissural region (boxed in K) is examined at better resolution in L. Stacking of fewer sections shows a clear glomerular-like organization (^*) within the commissure.