Drosophila dumbfounded: A myoblast attractant essential for fusion

Aggregation and fusion of myoblasts to form myotubes is essential for myoge nesis in many organisms. In Drosophila the formation of syncytial myotubes is seeded by founder myoblasts. Founders fuse with clusters of fusion-compe tent myoblasts. Here we identify the gene dumbfounded (duf) and show that i t is required for myoblast aggregation and fusion. duf encodes a member of the immunoglobulin superfamily of proteins that is an attractant for fusion -competent myoblasts. It is expressed by founder cells and serves to attrac t clusters of myoblasts from which myotubes form by fusion.