Photoreceptor axon projection defects associated with increased Tor signaling.

<p>(A–D) Dorsal-posterior views of third instar optic lobes stained with MAb24B10 to visualize photoreceptor projections. (A) Mitotic clones in an <i>FRT82B</i> control background show proper termination of photoreceptor axons R1-6 at the lamina plexus (LP), and termination of photoreceptors R7 and R8 in the medulla (Med). (B) <i>Tsc1²⁹</i> mutant axons terminate at incorrect positions above and below the lamina (arrowheads) and produce a broadened lamina plexus. (C) Neuronal expression of Rheb creates axon termination defects similar to those seen in <i>Tsc1</i> mosaics (D) <i>Pten^dj189</i> mutant photoreceptors leave gaps and holes (arrowhead) in the lamina plexus, which is broader and noticeably “peaked.” The medulla contains axon projections which are thicker and much longer than in controls (arrow). (E–H′) Dorsal view of optic lobes from 40h pupae stained with MAb24B10. E′–H′ are lower optical planes of the optic lobes shown in E–H, respectively. (E, E′) Control photoreceptors R7 and R8 show two distinct layers of termination in the medulla (labels), and are arranged in a highly regular pattern (arrowhead). (F) Animals with <i>Tsc1²⁹</i> mutant photoreceptors show severe disruption of the R7 and R8 termination layers. Instead of terminating at the correct positions, the axons fail to de-fasciculate, forming dense bundles (arrowheads) that project beyond the medulla. (G, G′) Neuron-directed expression of Rheb causes axon bundles to project beyond the medulla in a fashion similar to <i>Tsc1</i> mosaics (arrowheads), but the phenotype is much less severe. (G, inset) Individual Rheb-overexpressing axons show an intermediate termination defect, stopping several microns beyond their normal targets (arrowheads in inset). (H) <i>Pten^dj189</i> mutant axons exhibit gaps and collapses in the R7/R8 termination zone (arrowhead). Thick axon bundles can be seen that bypass their usual stopping points and then loop back to terminate at other locations in the R7/R8 layers (arrows). (H′, F′) Axon bundles in <i>Pten^dj189</i> mosaics are not as densely packed as those of <i>Tsc1²⁹</i> mosaics (arrowheads), but are still disorganized. All scale bars are 50 microns.