TWIST IS REQUIRED FOR MUSCLE TEMPLATE SPLITTING DURING ADULT DROSOPHILA MYOGENESIS

The basic helix-loop-helix transcription factor Twist is required for normal development of larval and adult somatic muscles in Drosophila. Adult flies normally have six pairs of dorsal longitudinal indirect ni ght muscles (DLMs), whereas when Twist function is reduced, only three pairs of DLMs are formed. Although twist is expressed in precursors o f adult muscles throughout the larval and early pupal stages, we demon strate that Twist function is required only during the late larval sta ge for DLM patterning. In wild-type flies, this is just prior to the t ime when three pairs of persistent larval muscle fibers split longitud inally to form templates for the six pairs of DLMs. By examining secti ons at various times during pupal development, we found that splitting of the larval muscles does not occur in twist mutants, indicating tha t Twist function is required to induce major changes in the larval tem plates prior to differentiation. The function of Twist in larval muscl e splitting is likely mediated by myocyte enhancer factor-2 (MEF2) sin ce in Mef2 hypomorphic mutants splitting is also reduced and Mef2 expr ession is dependent upon Twist. Our findings define specific roles for Twist and MEF2 during pupal myogenesis and demonstrate that these tra nscription factors function in adult muscle precursor cells to regulat e downstream factors controlling muscle cell splitting and morphogenes is. (C) 1998 Academic Press.