Adherens junction-dependent and -independent processes regulate epithelial polarity and structure during early Drosophila development. T.J. Harris, F. Wang, M. Peifer. Dept Biol, Univ North Carolina, Chapel Hill, NC.

   Armadillo (Arm) functions as a core component of adherens junctions (AJs), linking DE-cadherin to -catenin, which in turn binds actin. Previous results from our and other labs suggest that AJs play a key role in polarity establishment, but the relationships between AJs and other regulators of cell polarity are unclear. We have examined maternal and zygotic (M/Z) arm mutants to help discriminate between AJ-dependent and AJ-independent processes during early epithelial development. We demonstrate that epithelial polarity is initially established by an AJ-independent mechanism during cellularization. In wild type embryos, Bazooka accumulates in apical complexes along cellularization furrows. In M/Z arm mutants, Bazooka accumulates at these same sites despite the absence of detectable AJ components. Since M/Z arm mutants undergo widespread cell dissociation at gastrulation, we investigated whether specific parts of the embryo might maintain epithelial cell polarity. In gastrulation stage mutants, early embryonic axis specification occurs, but epithelial reorganizations are disrupted. Along the ventral surface, a ventral plate is formed, but the ventral furrow fails to invaginate, leaving mitotic mesodermal and neuronal cells over the ventral surface. Although the ventral surface loses epithelial structure, cell polarity is maintained on the dorsal side of M/Z arm mutants. Transverse furrows form and show polarized distributions of Bazooka and Discs Large in the absence of detectable AJ components. Later, these cells elongate and concomitantly accumulate DE-cadherin and truncated Arm, but fail to recruit -catenin. Thus, cell polarity can be established without AJs, and partially functional AJs can maintain epithelial polarity and cell shape. Understanding the mechanisms by which this occurs will be important for our understanding of the establishment and maintenance of epithelial cell polarity.