Fig. 2. lmd mutant phenotype involves loss of Mef2 and sns expression specifically in fusion-competent myoblasts. Late stage 13 wild-type and lmd¹ mutant embryos, both of which also carry a twist-dependent Mef2-lacZ insertion (A,B) (Nguyen and Xu, 1998) or the enhancer trap insertion rP298-lacZ (C,D) (Nose et al., 1998) were double-stained with antibodies against MEF2 and lacZ, and analyzed by confocal microscopy. Control embryo (A) exhibits coincident nuclear MEF2 (red) and cytoplasmic ß-gal (green) expression in somatic myoblasts and cardioblasts at the dorsal margin while mutant embryo (B) shows a significant number of lacZ-positive somatic myoblasts that do not exhibit nuclear MEF2 expression. Control embryo (C) shows founders that are rP298-lacZ positive/MEF2 positive (yellow signals) and fusion-competent myoblasts that are only MEF2-positive (red), while mutant embryo (D) exhibits MEF2 expression only in lacZ-positive founders and cardioblasts. (E,F) Embryos were stained with an anti-Kr antibody. Kr-positive multinucleate muscle precursors in control embryo (E) are equivalent in position and number to Kr-positive mononucleate muscle precursors in lmd¹ mutant embryo (F). (G,H) Embryos were hybridized with a digoxigenin-labeled sns RNA probe. In lmd mutant embryo, sns expression in fusion-competent myoblasts is completely abolished. Residual sns expression is in presumed garland cells (arrowheads).