Figure 7. Live imaging of L4 neurons.

Frames (MIPs) from live imaging of MARCM-generated L4 neurons (cyan) at the indicated time points from 16–26 h APF (see Movies S1 and S2). In Movies S1 and S2 (left panels) myr-tdTom driven by a GMR promoter highlights the R cell lattice in red. Here, however, this red signal was inverted such that R cell lattice appears black and the holes in the lattice, i.e. positions of the lamina fascicles, are in red (see also Movies S1 and S2, right panels). This was done to allow for easier visualization of the thin and numerous filopodia in L4s. L4 axons (Ax) are also indicated. Scale bar, 5 μm.

(A) Two neighboring wild-type L4 neurons undergoing three phases in their dendritic targeting:1. Exploration: In this early phase (~16 hr to 17.3 h APF) dynamic filopodia branch out from the axon in all directions occasionally contacting their target fascicles. Target fascicles for left and right L4 neurons are indicated by orange and white dotted circles, respectively; 2. Anchoring: While some filopodia continue to make transient contacts with the target fascicles between 17.5 and 18.5 h APF, (yellow regions in schematics below), others become progressively anchored to the target fascicles (red regions in schematic; arrowheads in MIPs); 3. Consolidation: Filopodia continue to make stable contacts with the target fascicles.

(B) Dscam2 mutant L4 neurons have an ‘exploratory phase’ indistinguishable from wild type but fail to anchor their dendrites to their target fascicles between 17.5 and 18.5 h APF. Instead, their dendrites extend along the D-V axis in tight apposition to the R cell lattice (black).