The proneural genes achaete (ac) and scute (sc) confer to Drosophila epidermal cells the ability to become sensory mother cells (SMCs). In imaginal discs, ac-sc are expressed in groups of cells, the proneural clusters, which are thought to delimit the areas where SMCs arise. We have visualized with the resolution of single cells the initial stages of sensory organ development by following the evolving pattern of proneural clusters and the emergence of SMCs. At reproducible positions within clusters, a small number of cells accumulate increased amounts of ac-sc protein. Subsequently, one of these cells, the SMC, accumulates the highest amount. Later, at least some SMCs become surrounded by cells with reduced ac-sc expression, a phenomenon probably related to lateral inhibition. Genetic mosaic analyses of cells with different doses of ac-sc genes, the sc expression in sc mutants, and the above findings show that the levels of ac-sc products are most important for SMC singling-out and SMC state maintenance. These products do not intervene in the differentiation of SMC descendants. The extramacrochaetae gene, an antagonist of proneural genes, negatively regulates sc expression, probably by interfering with activators of this gene.

Both in Drosophila and vertebrate epithelial cells, the establishment of apicobasal polarity requires the apically localized, membrane-associated Par-3–Par-6–aPKC protein complex. In Drosophila, this complex colocalizes with the Crumbs–Stardust (Sdt)–Pals1-associated TJ protein (Patj) complex. Genetic and molecular analyses suggest a functional relationship between them. We show, by overexpression of a kinase-dead Drosophila atypical PKC (DaPKC), the requirement for the kinase activity of DaPKC to maintain the position of apical determinants and to restrict the localization of basolateral ones. We demonstrate a novel physical interaction between the apical complexes, via direct binding of DaPKC to both Crb and Patj, and identify Crumbs as a phosphorylation target of DaPKC. This phosphorylation of Crumbs is functionally significant. Thus, a nonphosphorylatable Crumbs protein behaves in vivo as a dominant negative. Moreover, the phenotypic effect of overexpressing wild-type Crumbs is suppressed by reducing DaPKC activity. These results provide a mechanistic framework for the functional interaction between the Par-3–Par-6–aPKC and Crumbs–Sdt–Patj complexes based in the posttranslational modification of Crb by DaPKC.

The pattern of cell proliferation in the Drosophila imaginal wing primordium is spatially and temporally heterogeneous. Direct visualization of cells in S, G2, and mitosis phases of the cell cycle reveals several features invariant throughout development. The fraction of cells in the disc in the different cell cycle stages is constant, the majority remaining in G1. Cells in the different phases of the cell cycle mainly appear in small synchronic clusters that are nonclonally derived but result from changing local cell-cell interactions. Cluster synchronization occurs before S and in the G2/M phases. Rates of cell division are neither constant nor clonal features. Cell cycle progression is linear rather than concentric. Clusters appear throughout the disc but with symmetries related to presumptive wing patterns, compartment boundaries, and vein clonal restrictions.Morphogenesis in multicellular organisms is associated with patterned cell proliferation. As a rule, the size of an organ and its shape are species specific. Specific forms then result from local cell behavior reflecting the active genome of the proliferating cells.