Disorganized F-actin foci and failure of fusion pore formation in dpak3 mutant embryos. (A) Actin foci morphology visualized by live imaging. Stills from time-lapse imaging of a wild-type (a, wt) and a dpak3^zyg mutant (b) embryo expressing GFP-actin with twi-GAL4. Single focal planes at representative time points are shown. Note that the wt focus maintained its dense core throughout its lifespan (a). However, the dpak3^zyg mutant focus appeared to be loosely organized with clearly discernable projections and comet tail-like structures (b, arrowheads). (B) Fusion pores fail to form between founder cells/myotubes and the adherent FCMs in dpak3^zyg mutant. Cytoplasmic GFP was expressed in founder cells with rP298-GAL4 in dpak3^zyg mutant embryos. A stage 15 embryo double labeled with α-GFP (green) and α-MHC (red). Note that the GFP signal was retained in the elongated founder cells/myotubes, without diffusing into the adherent FCMs (some indicated by arrowheads). Occasionally, rP298-GAL4 drives leaky expression in ∼7–8% FCMs (Sens et al., 2010; Haralalka et al., 2011). Correspondingly, we observed ∼6% (22/355) GFP-positive FCMs in rP298-GAL4::UAS-GFP-actin;dpak3^zyg mutant embryos (not depicted), presumably due to the leaky expression of the rP298-GAL4 driver. Bars: (A) 2.5 µm; (B) 20 µm.