MISEXPRESSION OF NAUTILUS INDUCES MYOGENESIS IN CARDIOBLASTS AND ALTERS THE PATTERN OF SOMATIC MUSCLE-FIBERS

nautilus (nau), one member of the myogenic regulatory family of bHLH-e ncoding genes, is expressed in a subset of muscle precursors and diffe rentiated fibers in the Drosophila embryo. To elucidate the role of na utilus in myogenesis, we have misexpressed it using the GAL4-targeted system. We find that ectopic expression results in lethality throughou t Drosophila development. We analyzed the effects of embryonic express ion in mesodermal tissues that include the cardioblasts of the dorsal vessel as well most, if not all, of the presumptive somatic muscle pre cursors. Immunohistochemical staining for muscle MHC revealed abnormal ities that include an absence of cardial cells, coincident with the ap pearance of novel muscle fibers adjacent to the dorsal vessel. Moreove r, many cardioblasts express increased levels of muscle-specific genes such as myosin, actin 57B, and Mlp60A, a protein that is restricted t o the somatic, visceral, and pharyngeal muscles. These data suggest th at the missing cardial cells have been transformed into cells with pro perties similar to those of the somatic muscles. In addition, ubiquito us expression of nautilus in somatic muscle cells of these embryos res ulted in muscle pattern defects. Specifically, muscles that do not nor mally express nautilus were frequently absent, and novel fibers were o bserved in positions reminiscent of nau-expressing muscles. These data imply that nautilus can alter the developmental program of muscle pre cursors. In summary, we suggest that nautilus induces myogenic differe ntiation in vivo when ectopically expressed in developing cardioblasts and may affect the myogenic differentiation program of specific muscl e fibers. (C) 1997 Academic Press.