ARMADILLO, BAZOOKA, AND STARDUST ARE CRITICAL FOR EARLY STAGES IN FORMATION OF THE ZONULA ADHERENS AND MAINTENANCE OF THE POLARIZED BLASTODERM EPITHELIUM IN DROSOPHILA

Cellularization of the Drosophila embryo results in the formation of a cell monolayer with many characteristics of a polarized epithelium. W e have used antibodies specific to cellular junctions and nascent plas ma membranes to study the formation of the zonula adherens (ZA) in rel ation to the establishment of basolateral membrane polarity. The same approach was then used as a test system to identify X-linked zygotical ly active genes required for ZA formation. We show that ZA formation b egins during cellularization and that the basolateral membrane domain is established at mid-gastrulation, By creating deficiences for define d regions of the X chromosome, we have identified genes that are requi red for the formation of the ZA and the generation of basolateral memb rane polarity. We show that embryos mutant for both stardust (sdt) and bazooka (baz) fail to form a ZA. In addition to the failure to establ ish the ZA, the formation of the monolayered epithelium is disrupted a fter cellularization, resulting in formation of a multilayered cell sh eet by mid-gastrulation. SEM analysis of mutant embryos revealed a con version of cells exhibiting epithelial characteristics into cells exhi biting mesenchymal characteristics. To investigate how mutations that affect an integral component of the ZA itself influence ZA formation, we examined embryos with reduced maternal and zygotic supply of wild-t ype Arm protein. These embryos, like embryos mutant for both sdt and b at, exhibit an early disruption of ZA formation. These results suggest that early stages in the assembly of the ZA are critical for the stab ility of the polarized blastoderm epithelium.