Cell-fate choice and boundary formation by combined action of Notch and engrailed in the Drosophila hindgut

The Drosophila large intestine is initially subdivided into dorsal and ventral domains with distinct cell types, and a one-cell-wide strand of boundary cells is induced between them. Here we show that cell identity and localization of the boundary cells are determined by the combined action of Delta, Notch, and engrailed genes. The prospective dorsal domain of the hindgut primordium expresses engrailed. Engrailed represses Delta, which is ubiquitously expressed throughout the prospective hindgut region in early blastodermal stages, in the dorsal domain, and thus generates a Delta-positive/negative prepattern. Expression of Engrailed protein determines the dorsal domain, while an Engrailed-negative (Delta-positive) region is differentiated into the ventral domain. Delta-positive ventral cells activate a Notch cascade in abutting dorsal cells, and thus induce their differentiation into boundary cells. Mis-expression of a constitutively active Notch intracellular domain causes the entire large intestine to develop as boundary cells. It was also found that the transducing activity of a transmembrane form of activated Notch, which requires further proteolytic processing to generate intracellular fragments, is suppressed in the Delta-positive domain. Delta acts in two distinct ways: it activates the Notch signaling pathway in adjacent Delta-negative cells, and, at the same time, autonomously blocks Notch signaling in Delta-positive cells by affecting Notch processing.     

Expression of en and wg in the embryonic head and brain of Drosophila indicates a refolded band of seven segment remnants.

Based on the expression pattern of the segment polarity genes engrailed and wingless during the embryonic development of the larval head, we found evidence that the head of Drosophila consists of remnants of seven segments (4 pregnathal and 3 gnathal) all of which contribute cells to neuromeres in the central nervous system. Until completion of germ band retraction, the four pregnathal segment remnants and their corresponding neuromeres become arranged in an S-shape. We discuss published evidence for seven head segments and morphogenetic movements during head formation in various insects (and crustaceans).     

Conservation of the expression of Dll, en, and wg in the eye-antennal imaginal disc of stalk-eyed flies

SUMMARY We studied the developmental basis of exaggerated eye span in two species of stalk-eyed flies ( Cyrtodiopsis dalmanni and Sphyracephala beccarri ). These flies have eyes laterally displaced at the end of eyestalks, and males have greatly exaggerated eye span, which they use as a sexual display. To investigate eye span development we have compared eye-antennal disc morphology and the expression of three key regulator genes of Drosophila head development, Distal-less ( Dll ) , engrailed ( en ), and wingless ( wg ), in the stalk-eyed flies and Drosophila . We found great similarity in the basic division of the disc into anterior-antennal and posterior-eye portions and in the general patterning of Dll, en , and wg . Unexpectedly, our results showed that although the eye and antenna are adjacent in adult stalk-eyed flies, their primordia are physically separated by the presence of an intervening region between the anterior and posterior portions of the disc. This region is absent from Drosophila eye-antennal discs. We chose two stalk-eyed fly species that differed in the degree of eyestalk exaggeration but surprisingly we found no corresponding difference in the size of the en-wg expression domains that mark the boundaries of the dorsal head capsule primordia. In summary, our expression data establish the regional identity of the eye-antennal disc and provide a framework from which to address the developmental genetics of hypercephaly.     

Early pattern formation in the developing Drosophila eye

The formation of complex cellular arrays from unpatterned epithelia is a widespread developmental phenomenon. Insights into the mechanisms regulating this transformation have come from studying the development of the Drosophila compound eye. Pattern formation in the eye primordium is a highly ordered process in which the onset of differentiation is coordinated with synchronization of cell cycle progression. Recent studies have identified a number of genes that are required for early patterning events, and provide a link between the regulation of proliferation and pattern formation.     

Dorsal ventral polarity and pattern formation in the Drosophila embryo

The establishment of polarity along the dorsal-ventral axis of the Drosophila embryo requires the graded distribution of the dorsal morphogen. Several maternal genes are responsible for the formation of the gradient and their products act in an ordered series of events that begins during oogenesis and involves two different cell types, the oocyte and the follicle cells. The last step in the series results in selective nuclear localization of dorsal proteins, dorsal is thought to regulate the expression of zygotic genes in a concentration dependent way. The zygotic genes determine cell fates in specific regions of the embryo and direct other genes involved in the processes of differentiation.     

sog and dpp exert opposing maternal functions to modify toll signaling and pattern the dorsoventral axis of the Drosophila embryo

The short gastrulation (sog) and decapentaplegic (dpp) genes function antagonistically in the early Drosophila zygote to pattern the dorsoventral (DV) axis of the embryo. This interplay between sog and dpp determines the extent of the neuroectoderm and subdivides the dorsal ectoderm into two territories. Here, we present evidence that sog and dpp also play opposing roles during oogenesis in patterning the DV axis of the embryo. We show that maternally produced Dpp increases levels of the I(kappa)B-related protein Cactus and reduces the magnitude of the nuclear concentration gradient of the NF(kappa)B-related Dorsal protein, and that Sog limits this effect. We present evidence suggesting that Dpp signaling increases Cactus levels by reducing a signal-independent component of Cactus degradation. Epistasis experiments reveal that sog and dpp act downstream of, or in parallel to, the Toll receptor to reduce translocation of Dorsal protein into the nucleus. These results broaden the role previously defined for sog and dpp in establishing the embryonic DV axis and reveal a novel form of crossregulation between the NF(kappa)B and TGF(beta) signaling pathways in pattern formation.     

hedgehog and engrailed: pattern formation and polarity in the Drosophila abdomen

Like the Drosophila embryo, the abdomen of the adult consists of alternating anterior (A) and posterior (P) compartments. However the wing is made by only part of one A and part of one P compartment. The abdomen therefore offers an opportunity to compare two compartment borders (A/P is within the segment and P/A intervenes between two segments), and ask if they act differently in pattern formation. In the embryo, abdomen and wing P compartment cells express the selector gene engrailed and secrete Hedgehog protein whilst A compartment cells need the patched and smoothened genes in order to respond to Hedgehog. We made clones of cells with altered activities of the engrailed, patched and smoothened genes. Our results confirm (1) that the state of engrailed, whether 'off' or 'on', determines whether a cell is of A or P type and (2) that Hedgehog signalling, coming from the adjacent P compartments across both A/P and P/A boundaries, organises the pattern of all the A cells. We have uncovered four new aspects of compartments and engrailed in the abdomen. First, we show that engrailed acts in the A compartment: Hedgehog leaves the P cells and crosses the A/P boundary where it induces engrailed in a narrow band of A cells. engrailed causes these cells to form a special type of cuticle. No similar effect occurs when Hedgehog crosses the P/A border. Second, we look at the polarity changes induced by the clones, and build a working hypothesis that polarity is organised, in both compartments, by molecule(s) emanating from the A/P but not the P/A boundaries. Third, we show that both the A and P compartments are each divided into anterior and posterior subdomains. This additional stratification makes the A/P and the P/A boundaries fundamentally distinct from each other. Finally, we find that when engrailed is removed from P cells (of, say, segment A5) they transform not into A cells of the same segment, but into A cells of the same parasegment (segment A6).     

Divide and conquer: pattern formation in Drosophila embryonic epidermis

The pattern of differentiated cell types within tissues and organs is often established by organizers, the localized sources of secreted ligands. Although the mechanisms underlying organizer function have been extensively studied, only in a few cases is it clear how an organizer ultimately controls each individual cell's fate across a field of progenitor cells. One of these cases involves the establishment of a precise pattern of cell differentiation across the embryonic epidermis in Drosophila. Here, we review several recent reports that help to elucidate the regulatory principles used to control this pattern. Because organizers are conserved, the same fundamental principles might operate in other organizers.     

Genetic and developmental analyses of chaetae pattern formation in Drosophila tergites

Summary The role of the achaete-scute complex and extramacrochaetae, Notch, Delta, Enhancer of split and Hairless genes in chaeta patterning in Drosophila tergites was studied in genetic mosaics and in mutant combinations. The mutant phenotypes of different alleles of each gene can be ordered in characteristic topographical seriations. These seriations are related to the pattern of proliferation of histoblasts and the time of singularization of sensory organ mother cells from surrounding epidermal cells. Genetic mosaics of lethal alleles show that these genes are fundamentally involved in this singularization and subsequent differentiation. The study of mutant combinations of alleles of these genes reveals specific relationships of epistasis and synergism between them. The results suggest that spatial and temporal variations in achaete-scute complex functional products in cells, modulated by the activity of other genes involved in signal transduction, define the patterned differentiation of sensory organs in tergites. Offprint requests to: A. García-Bellido     

Modeling pattern formation: counting to two in the Drosophila egg

The EGF receptor pathway patterns the Drosophila egg and specifies the position of its dorsal appendages. A new mathematical analysis of this patterning network has highlighted its crucial features and provided novel insights into the spatial and temporal kinetics controlling patterning.     

Effect of the wingless (wg1) mutation on wing and haltere development in Drosophila melanogaster

Genetic and developmental studies of wingless (wg¹), a new second chromosome recessive mutation in Drosophila melanogaster, have shown that it affects not only wing and haltere development (giving rise to wingless and/or halterless flies), but also results in various abnormalities of the mesothorax. The larvae destined to develop into wingless and/or halterless flies possessed underdeveloped mesothoracic and/or metathoracic imaginal discs.     

Divergence of the Regulation of alpha-Amylase Activity in Drosophila melanogaster, Drosophila funebris, and Drosophila saltans

The regulation of amylase activity in threeDrosophila species, D. melanogaster,D. funebris and D. saltans, wasanalyzed by measuring the specific activity levels infour dietary environments, cornmeal, glucose, 5% starch, and 10% starch, at threedevelopmental stages, i.e., the third-instar larval,pupal, and 2-day-old adult stages. The developmentalprofiles of amylase activity for the threeDrosophila species showed that the level of activity washigh at the larval and adult stages but substantiallylow at the pupal stage, suggesting thatDrosophila does not utilize starch at the pupalstage. Divergence in the regulation of amylase was observed amongthe three Drosophila species on the followingpoints. (1) The order of amylase specific activity wasD. melanogaster > D. funebris >D. saltans. (2) The response pattern to the dietary environment varied amongthe species and changed during development. (3) Thetiming of the switch in the response pattern to thedietary environment during development was before pupation in D. funebris and D.saltans but after pupation in D.melanogaster. The significance of the divergence inthe regulation of amylase activity for adaptation to astarch environment in Drosophila is discussed.     

A combinatorial enhancer recognized by Mad, TCF and Brinker first activates then represses dpp expression in the posterior spiracles of Drosophila

A previous genetic analysis of a reporter gene carrying a 375-bp region from a dpp intron (dppMX-lacZ) revealed that the Wingless and Dpp pathways are required to activate dpp expression in posterior spiracle formation. Here we report that within the dppMX region there is an enhancer with binding sites for TCF and Mad that are essential for activating dppMX expression in posterior spiracles. There is also a binding site for Brinker likely employed to repress dppMX expression. This combinatorial enhancer may be the first identified with the ability to integrate temporally distinct positive (TCF and Mad) and negative (Brinker) inputs in the same cells. Cuticle studies on a unique dpp mutant lacking this enhancer showed that it is required for viability and that the Filzkorper are U-shaped rather than straight. Together with gene expression data from these mutants and from brk mutants, our results suggest that there are two rounds of Dpp signaling in posterior spiracle development. The first round is associated with dorsal-ventral patterning and is necessary for designating the posterior spiracle field. The second is governed by the combinatorial enhancer and begins during germ band retraction. The second round appears necessary for proper spiracle internal morphology and fusion with the remainder of the tracheal system. Intriguingly, several aspects of dpp posterior spiracle expression and function are similar to demonstrated roles for Wnt and BMP signaling in proximal-distal outgrowth of the mammalian embryonic lung.     

Defective proventriculus is required for pattern formation along the proximodistal axis,cell proliferation and formation of veins in the Drosophila wing

Many genes have been identified that are required for the establishment of the dorsoventral (DV) and anteroposterior (AP) axes of the Drosophila wing. By contrast, little is known about the genes and mechanisms that pattern the proximodistal (PD) axis. Vestigial (Vg) is instrumental in patterning this axis, but the genes that mediate its effects and the mechanisms that operate during PD patterning are not known. We show that the gene defective proventriculus (dve) is required for a region of the PD axis encompassing the distal region of the proximal wing (PW) and a small part of the adjacent wing pouch. Loss-of-function of dve results in the deletion of this region and, consequently, shortening of the PD axis. dve expression is activated by Vg in a non-autonomous manner, and is repressed at the DV boundary through the combined activity of Nubbin and Wg. Besides its role in the establishment of the distal part of the PW, dve is also required for the formation of the wing veins 2 and 5, and the proliferation of wing pouch cells, especially in regions anterior to wing vein 3 and posterior to wing vein 4. The study of the regulation of dve expression provides information about the strategies employed to subdivide and pattern the PD axis, and reveals the importance of vg during this process.