DROSOPHILA MEF2, A TRANSCRIPTION FACTOR THAT IS ESSENTIAL FOR MYOGENESIS

mef2 encodes the only apparent Drosophila homolog of the vertebrate my ocyte-specific enhancer factor 2 (MEF2). We show herein that the Droso phila MEF2 protein is expressed throughout the mesoderm following gast rulation. Later in embryogenesis, its expression is maintained in prec ursors and differentiated cells of the somatic and visceral musculatur e, as well as the heart. We have characterized genetic deficiencies an d EMS-induced point mutations that result in complete loss of MEF2 pro tein in homozygous mutant embryos. These embryos exhibit a dramatic ab sence of myosin heavy chain (MHC)-expressing myoblasts and lack differ entiated muscle fibers. Examination of earlier events of muscle develo pment indicates that the specification and early differentiation of so matic muscle precursors are not affected because even-skipped-, nautil us-, and beta 3-tubulin-expressing myoblasts are present. However, the se partially differentiated cells are unable to undergo further differ entiation to form muscle fibers in the absence of mef2. The later aspe cts of differentiation of the visceral mesoderm and the heart are also disrupted in mef2 mutant embryos, although the specification and earl y development of these tissues appear unaffected. Midgut morphogenesis is disrupted in the mutant embryos, presumably as a consequence of ab normal development of the visceral mesoderm. In the heart, the cardial cells do not express MHC. These results indicate that MEF2 is require d for later aspects of differentiation of the three major types of mus culature, which include body wall muscles, gut musculature, and the he art, in the Drosophila embryo.