Figure 8. The co-receptor co-expression map of olfaction in Drosophila melanogaster.

(A) Summary of antennal lobe (AL) expression for all co-receptor knock-in lines (from all brains examined in Figures 3—5; Orco N = 8, Ir8a N = 15, Ir76b N = 11, Ir25a N = 15). The previously reported innervation pattern for each co-receptor is shown in green; new innervation reported here is color-coded according to strength of glomerular labeling, from strong (orange), to intermediate (yellow), to weak (pink). Glomeruli labeled in <50% of brains examined for a given knock-in line are designated variable (gray); glomeruli not labeled are white. The novel VM6 glomerular subdivisions reported here are indicated by gray stripes. (B) Overlap of chemosensory modalities in the AL. In the Venn diagram (left), IR co-receptors are color-coded in shades of purple, while Orco is in teal, as in Figure 1. Numbers indicate how many glomeruli are found in the given intersection of co-receptors out of 58 total glomeruli. Variably labeled glomeruli were excluded from these analyses. The table lists the names of the glomeruli in each section of the Venn diagram. The new glomerular subdivisions are indicated with an asterisk. (C) New view of olfaction in Drosophila. Left: in the periphery, all four co-receptors are expressed in the antenna (top), while palpal neurons express Orco and Ir25a (bottom). Middle: many different classes of olfactory sensory neurons (OSNs) express various combinations of chemosensory receptors and co-receptors. While some neurons express only IrCos (purple, #1) or Orco (teal, #2), many neurons co-express these chemoreceptors (indicated with striped fill, #3 and 4). Within the latter group, there may be OSN populations in which IRs are the dominant receptors, and OR expression is sparse (#3), and other populations where ORs are the primary receptors and IR expression is infrequent (#4). GR+ neurons (dark blue) also express Ir25a (#5, dark blue and purple striped fill), and some of these neurons additionally express Orco (#5, dark blue, purple, and teal striped fill). Question marks indicate potential instances of co-convergence of different subtypes of OSNs onto the same glomeruli. Right: a comprehensive map of the antennal lobe shows that most glomeruli are innervated by OSNs that co-express multiple chemoreceptors. Compass in (A) and (C): D = dorsal, L = lateral, P = posterior. See also Table 3 and Figure 3—source data 1 and Figure 3—source data 2.