Myoblast fusion is mediated by a single-channel fusion pore and the WASP–Sltr complex is required for fusion pore formation. (A and B) A single-channel macro fusion pore revealed by HFP/FS electron microscopy. Boxed area in A is enlarged in B. Arrows indicate boundaries of the fusion pore. Note the even distribution of ribosomes in the lumen of the fusion pore and the absence of F-actin and membrane sacs/vesicles. A small piece of cellular debris between the two cells is outlined in red. n: myotube nuclei. (C–F) Cytoplasmic GFP expressed in founder cells does not diffuse into FCMs in sltr mutant embryos. A stage 15 sltr mutant embryo expressing GFP in founder cells driven by rP298-GAL4 triple labeled with α-GFP (green), α-MHC (red), and α-Ants (blue). Mononucleated FCMs do not contain GFP, even though many of them (a few are indicated by arrowheads) have attached to elongated founder cells/myotubes. Bars: (A) 500 nm; (B) 200 nm; (C–F) 20 µm.