Dispersed morphology of the F-actin foci and defective PLS invasion in dpak mutant embryos. (A) Actin foci morphology visualized by confocal microscopy. Stage 14 embryos triple labeled with phalloidin (green), α-Duf (red), and α-Lmd (blue). Compared with the dense morphology in wild-type embryos (a, wt), the F-actin foci appeared fuzzy and dispersed in dpak3^zyg (b), dpak3^mat/zyg (c), and dpak1^mat/zyg,dpak3^mat/zyg (d) mutant embryos. Note that the dense wt actin focus caused a V-shaped inward curvature in the founder cell membrane (a, arrow). In contrast, the F-actin foci in dpak mutant embryos appeared dispersed and did not change the membrane curvature of the apposing founder cells (b–d, arrowheads). (B) F-actin foci visualized by EM. (a) Stage 14 wt embryo. An FCM (pseudo-colored pink) projecting multiple F-actin–enriched invasive fingers (the longest one indicated by arrowhead) into the adjacent trinucleated myotube. The protruding tip of this FCM is enlarged in (a′). Note that the F-actin–enriched area (delineated with white dashed line in a′) is almost devoid of ribosomes (small black dots) and intracellular organelles, indicating the presence of a densely packed F-actin network (also see Sens et al., 2010). (b) Stage 14 dpak3^zyg mutant embryo. An FCM was in the process of invading a binucleated myotube and generated a wide, shallow dent on the myotube membrane, without projecting long, thin protrusive fingers. The tip area of the FCM is enlarged in (b′). Note that the F-actin–enriched area (delineated with white dashed line in b′) contained more ribosomes than that of wt (a′), indicating the presence of loosely organized actin filaments. Also note that the cell membranes remained intact at the muscle cell contact site. n: muscle cell nuclei. Bars: (A) 5 µm; (B) 500 nm.