Model for progress of IFM myoblast fusion. (I) Wing disc–derived myoblasts (green) migrate toward the nascent DLM myotubes (red) and congregate in their vicinity at a distance of 50 nm or more. (II) An initial phase of myoblast–myotube apposition is mediated by cell adhesion elements such as the myoblast cell surface protein Sns. Cell surfaces are positioned 20–50 nm from each other. (III) A branched actin–dependent cell shape change, also involving ELMO, flattens the myoblast surface. (IV) A second branched actin–based process, dependent on the activity of the NPFs WASp and SCAR/WAVE, as well as on the membrane protein Sing, brings the cells in close apposition (<10 nm). This tight association allows for the formation of multiple cell–cell contacts, which serve as sites for initiation of nascent fusion pores. (V and VI) The surface membranes merge and vesiculate as the pores expand, so that eventually full cytoplasmic continuity is obtained, and the myoblast is incorporated into the DLM myotube.