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 flight muscles (DLMs), whereas when Twist function is reduced, only three pairs of DLMs are formed. Although twist is expressed in precursors of adult muscles throughout the larval and early pupal stages, we demonstrate that Twist function is required only during the late larval stage for DLM patterning. In wild-type flies, this is just prior to the time when three pairs of persistent larval muscle fibers split longitudinally to form templates for the six pairs of DLMs. By examining sections at various times during pupal development, we found that splitting of the larval muscles does not occur in twist mutants, indicating that Twist function is required to induce major changes in the larval templates prior to differentiation. The function of Twist in larval muscle splitting is likely mediated by myocyte enhancer factor-2 (MEF2) since in Mef2 hypomorphic mutants splitting is also reduced and Mef2 expression is dependent upon Twist. Our findings define specific roles for Twist and MEF2 during pupal myogenesis and demonstrate that these transcription factors function in adult muscle precursor cells to regulate downstream factors controlling muscle cell splitting and morphogenesis.