MSH MAY PLAY A CONSERVED ROLE IN DORSOVENTRAL PATTERNING OF THE NEUROECTODERM AND MESODERM

Many of the mechanisms that govern the patterning of the Drosophila ne uroectoderm and mesoderm are still unknown. Here we report the sequenc e, expression, and regulation of the homeobox gene msh, which is likel y to play an important role in the early patterning events of these tw o tissue primordia. msh expression is first observed in late blastoder m embryos and occurs in longitudinal bands of cells that are fated to become lateral neuroectoderm This expression is under the control of d orsoventral axis-determination genes and depends on dpp-mediated repre ssion in the dorsal half of the embryo and on flb-(EGF) mediated repre ssion ventrally. The bands of msh expression define the cells that wil l form the lateral columns of proneural gene expression and give rise to the lateral row of SI neuroblasts. This suggests that msh may be on e of the upstream regulators of the achaete-scute (AS-C) genes and may play a role that is analogous to that of the homeobox gene vnd/NK2 in the medial sector of the neuroectoderm. During neuroblast segregation , msh expression is maintained in a subset of neuroblasts, indicating that msh, like vnd/NK2, could function in both dorsoventral patterning of the neuroectoderm and neuroblast specification. The later phase of msh expression that occurs after the first wave of neuroblast segrega tion in defined ectodermal and mesodermal clusters of cells points to similar roles of msh in patterning and cell fate specification of the peripheral nervous system, dorsal musculature, and the fat body. A com parison of the expression patterns of the vertebrate homologs of msh, vnd/NK2, and AS-C genes reveals striking similarities in dorsoventral patterning of the Drosophila and vertebrate neuroectoderm and indicate s that genetic circuitries in neural patterning are evolutionarily con served.