Autonomous and nonautonomous Notch functions for embryonic muscle and epidermis development in Drosophila.

The Notch (N) gene encodes a cell signaling protein that mediates neuronal and epidermal determination in Drosophila embryos. N also regulates several aspects of myogenic development; embryos lacking N function have too many muscle founder cells and fail to properly differentiate somatic muscle. To identify cell-autonomous requirements for Notch function during muscle development, we expressed a Notch minigene in the mesoderm, but not in the ectoderm, of amorphic N-embryos. In these embryos, muscle founder hypertrophy is rescued, indicating that Notch is autonomously required by mesoderm cells to regulate the proper number of muscle founders. However, somatic muscle differentiation is only partially normalized, suggesting that Notch is also required in the ectoderm for proper muscle development. Additionally, mesodermal expression of Notch partially rescues epidermal development in overlying neurogenic ectoderm. This is unexpected, since previous studies suggest that Notch is autonomously required by proneural ectoderm cells for epidermal development. Mesodermal expression of a truncated Notch protein lacking the extracellular domain does not rescue ventral epidermis, suggesting that the extra-cellular domain of Notch can non-autonomously rescue epidermal development across germ layers.