F-actin–enriched foci specifically localize in FCMs and undergo shape changes during their lifespan. (A–F) F-actin foci specifically reside in FCMs. Stage 14 wild-type embryos expressing GFP-actin with muscle-specific drivers double labeled with α-GFP (green) and phalloidin (red). Boxed areas in A, C, and E are enlarged in B, D, and F. In this and all subsequent figures, single-slice confocal images are shown and selected muscle cells are outlined (dashed lines) by tracing the cell cortical phalloidin labeling. Note the colocalization of GFP-actin foci with phalloidin-labeled F-actin foci when GFP-actin was expressed in all muscle cells (with twi-GAL4; A and B, arrowheads) or in FCMs (with sns-GAL4; C and D, arrowheads), and the absence of GFP-actin enrichment at sites of fusion (arrows) when it was expressed in founder cells (with rP298-GAL4; E and F). (G) Time-lapse imaging of an F-actin focus in a stage 14 wild-type embryo expressing GFP-actin in all muscle cells (with twi-GAL4). Horizontal panels are stills from a time-lapse sequence, and vertical panels are two adjacent optical slices of the same F-actin focus along the Z-axis. (H–K) Four examples of F-actin foci with irregular shapes in fixed wild-type embryos labeled with α-Duf (founder cell; enriched at sites of fusion; red), phalloidin (green), and α-Lmd (FCM; in nuclei; blue; Duan et al., 2001). Note the different foci shapes (indicated by arrow [H], asterisk [I], double-headed arrow [J], and arrowhead [K]) in fixed embryos corresponding to those at different time points (300, 400, 500, and 600 s, respectively) of the live F-actin focus shown in G. Bars: (A, C, and E) 20 µm; (B, D, and F) 10 µm; (G–K) 5 µm.