Misexpression ofnautilusInduces Myogenesis in Cardioblasts and Alters the Pattern of Somatic Muscle Fibers

Abstract

nautilus(nau), one member of the myogenic regulatory family of bHLH-encoding genes, is expressed in a subset of muscle precursors and differentiated fibers in theDrosophilaembryo. To elucidate the role ofnautilusin myogenesis, we have misexpressed it using the GAL4-targeted system. We find that ectopic expression results in lethality throughoutDrosophiladevelopment. We analyzed the effects of embryonic expression in mesodermal tissues that include the cardioblasts of the dorsal vessel as well most, if not all, of the presumptive somatic muscle precursors. Immunohistochemical staining for muscle MHC revealed abnormalities that include an absence of cardial cells, coincident with the appearance of novel muscle fibers adjacent to the dorsal vessel. Moreover, many cardioblasts express increased levels of muscle-specific genes such as myosin, actin 57B, and Mlp60A, a protein that is restricted to the somatic, visceral, and pharyngeal muscles. These data suggest that the missing cardial cells have been transformed into cells with properties similar to those of the somatic muscles. In addition, ubiquitous expression ofnautilusin somatic muscle cells of these embryos resulted in muscle pattern defects. Specifically, muscles that do not normally expressnautiluswere frequently absent, and novel fibers were observed in positions reminiscent ofnau-expressing muscles. These data imply thatnautiluscan alter the developmental program of muscle precursors. In summary, we suggest thatnautilusinduces myogenic differentiationin vivowhen ectopically expressed in developing cardioblasts and may affect the myogenic differentiation program of specific muscle fibers.