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Fig. 6. SCAR and Arp2/3 are required for myoblast fusion and regulate actin foci dissolution during fusion. (A-D) Lateral views of stage 16 embryos stained with antibody against myosin heavy chain to visualize body wall muscles. Scale bar: 20 µm. (A) Wild-type embryo. (B) SCAR³⁷ embryo. Approximately 10-20 free myoblasts (arrowhead) are seen in each hemisegment indicating a myoblast fusion defect. (C) Embryo from SCAR^k13211 germline clones with reduced levels of maternal and zygotic SCAR protein. Increased numbers of free myoblasts (arrowhead) are seen, along with thinner muscles, indicating a more severe myoblast fusion defect. (D) Arp3^EP3640 embryo. These embryos display a myoblast fusion defect, with approximately 10-20 free myoblasts (arrowhead) seen in each hemisegment. (E-J) Lateral views of stage 14 embryos stained with phalloidin to label F-actin (red). F-actin labels the foci as well as cortical actin. Scale bar: 5 µm. (E) SCAR³⁷ embryo. Actin foci (arrowhead) appear larger than in wild type. The focus shown is an example of the larger foci seen in these mutants, although the average focus size is similar to wild type (Table 1). (F) SCAR^k13211 germline clone embryo with reduced levels of maternal and zygotic SCAR protein. Large accumulations of F-actin have formed at the site of adhesion between FC/myotubes and FCMs (arrowhead). (G) Arp3^EP3640 embryo. Actin foci appear larger than in wild type (arrowhead). The focus shown is an example of the larger foci seen in these mutants, although the average focus size is similar to wild type (Table 1). (H-I'') Embryos stained with antibodies against β-galactosidase to label FCs/myotubes (blue) and SCAR (green). (H) rP298-lacZ embryo. SCAR protein partially colocalizes with F-actin foci (arrowhead) in both FCMs and FCs/myotubes. (I) rP298-lacZ; kette^J4-48 embryo. SCAR protein is virtually undetectable in this mutant background. Residual protein is mislocalized (compare with H). (J-J'') SCAR^k13211 germline clone embryo stained against Rols (green). Dotted line indicates FC/myotube membrane and was drawn based on Rols localization. Rols partially overlaps with actin focus (arrowhead), indicating that enlarged actin foci localize across both FCs/myotubes and FCMs.

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