Figure 2.

Retinotopic map in the M6 stratum. A: Frontal view of the GMR-GAL4 expression pattern in the optic lobe. B: Rear view of terminals of GMR-GAL4-labeled neurons in the medulla. Locations of closeup images in G and H are indicated. Arrow indicates the axonal terminal in the accessory medulla. C: The sample M6 surface model is warped into the medulla of the standard M6 model. D: GMR-GAL4-labeled neurons are warped into the standard M6 volume model, using the transformation matrix generated in C. E: Schematic illustration of an R7 neuron terminating at the M6 stratum. F: The M6 retinotopic map generated by the intersection (overlap) between terminals of GMR-GAL4 neurons and the M6 volume model. G: Transverse section of the ventral medulla, showing an array of the typical R7 terminals that occur in most parts of M6 stratum. H: Transverse section of the dorsal medulla, showing dual terminals (arrows) in the dorsal-posterior part of the M6 stratum. Other R8s in inner columns terminate at M3 (arrowhead). I: Preferential expression of PanR7-GAL4, showing that the dual terminals at the dorsal-posterior M6 stratum (arrows) are derived from DRA R7 and R8 neurons. PanR8-GAL4 (J) and Rh6-GAL4 (K), both expressing in R8, innervating M6 (arrow) in the dorsal-posterior region and M3 (arrowhead) in all other regions. L: Dual expression of Rh3-LexA (yellow) and Rh6-GAL4 (cyan) labels the dual terminals (arrows) at dorsal-posterior M6 with different colors. Rh6-GAL4 also labels R8s terminating at M3 (arrowhead) in other medulla regions. M: Ectopic expression of hth in all photoreceptor neurons driven by GMR-GAL4 induces dual terminals (arrows) at medial M6 stratum. N: Overexpression of hth^HM in all photoreceptor neurons driven by GMR-GAL4 induces single terminals (arrow) at dorsal-posterior M6. Scale bars = 10 μm.