The WASP and Scar complexes are required for F-actin foci formation at sites of fusion. Stage 14 wild-type (wt) (A) and stage 15 mutant (B–F) embryos triple labeled with phalloidin (green), α-Duf (red), and α-Lmd (blue). Several muscle cell adhesion sites (marked by Duf enrichment) in each panel are indicated by arrows. In wt embryos, most fusion events occur at stage 14 and there is a decrease in the F-actin foci number in stage 15 (Beckett and Baylies, 2007). Note the persistence of F-actin foci in stage 15 scar (B, zygotic null; partial loss of fusion [Fig. S4 A, c]), scar^mat/zyg (C, eliminating most, but not all, maternal and zygotic Scar function; near complete loss of fusion [Fig. S4 A, d]), and sltr single mutant embryos (D), and the dramatic reduction of F-actin foci in scar,sltr (E) and scar^mat/zyg,sltr (F) double-mutant embryos. In scar,sltr double mutant embryos, a large percentage (76%; 35/46) of muscle cell adhesion sites are not associated with any F-actin enrichment. The average size for the remainder (11/46) of F-actin foci is 1.2 ± 0.6 µm². Arrowheads in E and F indicate actin polymerization in nonmuscle cells (Duf- and Lmd-negative). Bar, 15 µm.