Pattern formation in a secondary field: a hierarchy of regulatory genes subdivides the developing Drosophila wing disc into discrete subregions

The Serrate locus of Drosophila and its role in morphogenesis of the wing imaginal discs: control of cell proliferation

The Drosophila gene Serrate encodes a transmembrane protein with 14 EGF-like repeats in its extracellular domain. Here we show that loss-of-function mutations in this gene lead to larval lethality. Homozygous mutant larvae fail to differentiate the anterior spiracles, exhibit poorly developed mouth-hooks and show a severe reduction in the size of the wing and haltere primordia, which is not due to cell death. The few homozygous mutant escapers that pupariate develop into pharate adults that almost completely lack wings and halteres. Clonal analysis in the adult epidermis demonstrates a requirement for Serrate during wing and haltere development. Targeted ectopic expression of Serrate in the imaginal discs using the yeast transcriptional activator Gal4 results in regionally restricted induction of cell proliferation, e.g. the ventral tissues in the case of the wings and halteres. The results suggest that the wild-type function of Serrate is required for the control of position-specific cell proliferation during development of meso- and metathoracic dorsal discs, which in turn exerts a direct effect on morphogenesis.

The wingless signalling pathway and the patterning of the wing margin in Drosophila

The margin of the wing of Drosophila is defined and patterned from a stripe of cells expressing the wingless (wg) gene that is established during the third larval instar in the developing wing blade. The expression of the genes cut and achaete in a small domain in the prospective wing margin region reflects the activity of wg and probably mediate its function. Our results indicate that, in the wing margin, the wingless signal requires the activity of at least three genes: armadillo (arm), dishevelled (dsh) and shaggy (sgg) and that the functional relationship between these genes and wg is the same as that which exist during the patterning of the larval epidermis. These observations indicate that arm, dsh and sgg encode elements of a unique ‘wingless signalling pathway’ that is used several times throughout development.

dishevelled is required during wingless signaling to establish both cell polarity and cell identity

The dishevelled gene of Drosophila is required to establish coherent arrays of polarized cells and is also required to establish segments in the embryo. Here, we show that loss of dishevelled function in clones, in double heterozygotes with wingless mutants and in flies bearing a weak dishevelled transgene leads to patterning defects which phenocopy defects observed in wingless mutants alone. Further, polarized cells in all body segments require dishevelled function to establish planar cell polarity, and some wingless alleles and dishevelled; wingless double heterozygotes exhibit bristle polarity defects identical to those seen in dishevelled alone. The requirement for dishevelled in establishing polarity in cell autonomous. The dishevelled gene encodes a novel intracellular protein that shares an amino acid motif with several other proteins that are found associated with cell junctions. Clonal analysis of dishevelled in leg discs provides a unique opportunity to test the hypothesis that the wingless dishevelled interaction species at least one of the circumferential positional values predicted by the polar coordinate model. We propose that dishevelled encodes an intracellular protein required to respond to a wingless signal and that this interaction is essential for establishing both cell polarity and cell identity.

Mechanisms of compartment formation: evidence that non-proliferating cells do not play a critical role in defining the D/V lineage restriction in the developing wing of Drosophila

The dorsoventral (D/V) lineage boundary in the developing wing disc of Drosophila restricts growing cells to the prospective dorsal or ventral compartments of the wing blade. This restriction appears along the prospective margin of the wing some time during the middle to late stages of wing disc growth. It has been proposed that the restriction is established and maintained by the formation of a zone of non-proliferating cells that acts as a barrier between cells in the dorsal and ventral compartments (O'Brochta and Bryant, Nature 313, 138–141, 1985). In the adult, however, no group of barrier cells has been identified between the compartments. This study will show the following. (1) A group of cells does exist that lies between the dorsal and ventral rows of margin bristle precursors; these cells, which express cut in the late third instar wing disc, are thus in an ideal position to act as barrier cells. (2) This cut-expressing region is split into dorsal and ventral regions by the expression of the dorsal-specific gene apterous. (3) The D/V lineage restriction defined by marked dorsal and ventral clones lies in the middle of the cut-expressing region and is exactly congruent with the boundary of apterous expression. (4) No group of barrier cells is observed between dorsal and ventral clones. (5) Clones often run along the boundary for long distances, suggesting that they can grow along the D/V boundary without crossing it. These results thus do not support the existence of a groups of cells acting as a barrier between dorsal and ventral compartments. Nor do they support a critical role for division rates near the D/V boundary in establishing or maintaining the lineage restriction.

Expanded: a gene involved in the control of cell proliferation in imaginal discs

The expanded gene was first identified by a spontaneous mutation that causes broad wings. We have identified an enhancer-trap insertion within expanded and used it to generate additional mutations, including one null allele. expanded is an essential gene, necessary for proper growth control of imaginal discs and, when mutant, causes either hyperplasia or degeneration depending on the disc. Wing overgrowth in expanded hypermorphs is limited to specific regions along the anterior-posterior and dorsal-ventral axis. expanded encodes a novel 1429 amino acid protein that is localized to the apical surface of disc cells and contains three potential SH3-binding sites. Together, these observations suggest that the Expanded protein engages in protein-protein interactions regulating cell proliferation in discs.