Drosophila neuroblasts and epithelial cells in the procephalic neurogenic region divide perpendicular to the surface, and segregate the proteins Numb and Prospero into the basal daughter cell. We demonstrate here that orientation of the mitotic spindle and correct localization of Numb and Prospero in these cells require the inscuteable gene. Moreover, ectopic expression of inscuteable in other epithelial cells leads to spindle reorientation. The Inscuteable protein localizes to the apical cell cortex before mitosis, suggesting that Inscuteable functions in establishing polarity for asymmetric cell division.
The periodic, seven-stripe pattern of the primary pair-rule gene even-skipped (eve) is initiated by crude, overlapping gradients of maternal and gap gene proteins in the early Drosophila embryo. Previous genetic studies suggest that one of the stripes, stripe 2, is initiated by the maternal morphogen bicoid (bcd} and the gap protein hunchback (hb), while the borders of the stripe are formed by selective repression, involving the gap protein giant (gt) in anterior regions and the Kriippel (Kr) protein in posterior regions. Here, we present several lines of evidence that are consistent with this model for stripe 2 expression, including in vitro DNA-binding experiments and transient cotransfection assays in cultured cells. These experiments suggest that repression involves a competition or short-range quenching mechanism, whereby the binding of gt and Kr interferes with the binding or activity of bcd and hb activators at overlapping or neighboring sites within the eve stripe 2 promoter element. Such short-range repression could reflect a general property of promoters composed of multiple, but autonomous regulatory elements.
Dynamic epithelial reorganization is essential for morphogenesis of various organs. In Drosophila embryos, for example, the Malpighian tubule is generated by cellular rearrangement of a preexisting epithelium and the tracheal network is formed by outgrowth, branching, and fusion of epithelial vesicles. Here we report that the previously identified locus shotgun (shg) encodes DE-cadherin, an epithelial cell-cell adhesion molecule of the classic cadherin type and that zygotic shg mutations rather specifically impair processes of the dynamic epithelial morphogenesis. In the mutants, the Malpighian tubule disintegrated into small spherical structures, and the tracheal network formation was blocked in selected steps. The malformation of these organs could be rescued by overexpression of DE-cadherin cDNA under a heat shock promoter. Unexpectedly, the zygotic null condition did not severely affect general epithelial organization; most epithelial tissues maintained not only their cell-cell associations but also their apicobasal polarity in the mutants. The zygotic null mutant retained a certain level of maternally derived DE-cadherin molecules until the end of embryogenesis. These results suggest that zygotic DE-cadherin expression is critical for the rearrangement processes of epithelial cells, whereas the maternally derived DE-cadherin may serve only for the maintenance of the static architecture of the epithelia. [Key Words: DE-cadherin; tubulogenesis; cell rearrangement; Drosophila]Received November 20, 1995; revised version accepted January 29, 1996.Epithelial cells can reposition themselves without breaking the cell layer. This type of cellular rearrangement plays an important role in the production of tissues or organs of diverse morphology (for review, see Gumbiner 1992). A well-known example is amphibian gastrulation, in which a process of intercellular movement, called convergent extension, causes a dramatic elongation of the mesodermal tissue (Keller et al. 1992). In Drosophila embryos, a similar mechanism operates for morphogenesis of tubular organs. These include Malpighian tubules (MTs) and probably tracheal trees as well, both of which are of ectodermal origin and simple epithelial monolayers. MTs arise from the posterior region of the hindgut {for review, see Skaer 1992Skaer , 1993. Primordia of the tubules first grow out by cell divisions, and when the proliferation is complete, 12-14 cells encircle the lumen. Subsequently the cells start shifting 4Corresponding author. 5Present address:
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