Two live imaging experiments are shown. (1) All photoreceptors are labeled with myr-mRFP and R7 photoreceptors are sparsely labeled with CD4-tdGFP using GMR-FLP. Imaging started at P + 30% and continued for 18 hr, with 30 min intervals. Two R7 growth cone tips (red arrow) were followed. At the 2.5 hr mark a varicosity starts to develop from the axon shaft and expands over the next 15 hr, contributing to the elongation of the R7 axon. Note that being able to follow the same growth cone tip based on its unique filopodial structure allows us to verify lack of active extension without a stationary landmark. (2) R7 and R8 photoreceptors were sparsely labeled with CD4-tdGP using hs-FLP. Imaging started at P42% and continued for 21 hr. We used an alternating slow (30 min intervals) imaging of the general structure and fast (1 min interval) imaging of two neighboring R7 and R8 growth cones at higher resolution at different time points. R8 axon relocates to its final layer by sending a single filopodia proximally, which is initially very dynamic but later stabilizes and expands in the new layer, forming the new R8 terminal. In contrast, R7 terminal elongates along the axon shaft, but no directed extension activity is observed on the growth cone.
DOI:
http://dx.doi.org/10.7554/eLife.10721.020