DPak3 and DPak1 have partially redundant functions in myoblast fusion and are enriched at sites of fusion. (A) Myoblast fusion is defective in dpak3 and dpak1 mutant embryos. Stage 15 wild-type (a, wt), Df(3R)Exel7330 (b), dpak3^zyg (c), dpak3^mat/zyg (d), dpak1^mat/zyg,dpak3^mat/zyg (e), dpak1^mat,dpak3^mat/zyg (f), and dpak1^mat/zyg,dpak3^mat (g) embryos labeled with a myosin heavy chain antibody (α-MHC; green). Arrowheads indicate randomly selected unfused FCMs. (B) Quantification of the Eve-positive nuclei in the DA1 muscles of the different genotypes shown in A. Statistical significance was determined by unpaired student’s t test (*, P < 0.05; **, P < 0.01; ***, P < 0.001). Error bars: standard deviations. (C) DPak3 is specifically required in FCMs. Stage 15 dpak3^zyg mutant embryos expressing indicated transgenes double labeled with α-MHC (green) and α-V5 (red). Note that V5-DPak3 expression in all mesodermal cells driven by twi-GAL4 (a) or in FCMs driven by sns-GAL4 (c) rescued the fusion defect. However, V5-DPak3 expression in founder cells driven by rP298-GAL4 did not rescue the fusion defect (b). Also note that V5-DPak1 expression in the mesoderm with twi-GAL4 resulted in a slight, but significant, rescue (d). Results of these transgenic rescue experiments are quantified in D. (E) Enrichment of DPak3 and DPak1 at sites of fusion. Stage 14 embryos triple labeled with phalloidin (green; F-actin foci), α-DPak3 or α-DPak1 (red), and α-Duf (blue; enriched at muscle cell contact sites). Note that DPak3 colocalized with the F-actin foci (arrowheads) associated with Duf accumulation in wt (a), but not dpak3^zyg mutant (b) embryos. Also note that DPak1 was not enriched at sites of fusion (arrowheads) in wt embryo (c), but was recruited to muscle cell contact sites in dpak3^zyg mutant embryo (d). Bars: (A and C) 25 µm; (E) 5 µm.