Fig. 6. Model for the role of Slit-dependent Robo signaling in glomerular patterning. (A) Sensory neurons enter the lobe via the antennal nerve (AN) and radiate over the olfactory lobe. Neurons expressing Robo (green) and Robo3 (purple) segregate from those expressing Robo alone and decussate onto the medial side. Robo2 (pink) expression is elevated after neurons have crossed in the commissure. Regions of Slit expression at the midline and in the brain are indicated by yellow bars. The yellow shading indicates the proposed Slit gradient. (B) Sensory neurons invade the lobe starting at 25 APF. The positioning of terminals within the lobe is based on the Robo code (green, Robo; purple, Robo3) that they express and the differential response to Slit levels (yellow). The targeting of sensory neurons to specific positions allows short-range interactions with the projection neurons which are already present in the lobe. This leads to consolidation of the glomeruli, which become ensheathed by glial cell processes. The ventral, medial and lateral axis are indicated.