G2 arrest of cell cycle ensures a determination process of sensory mother cell formation in Drosophila

 In Drosophila, the sensory mother cells of macrochaetes are chosen from among the mitotically quiescent clusters of cells in wing imaginal discs, where other cells are proliferating. The pattern of cyclin A, one of the G2 cyclins, reveals that mitotically quiescent clusters of cells are arrested in G2. When precocious mitoses are induced during sensory mother cell determination by the ectopic expression of string, a known G2/M transition regulator, the formation of sensory mother cells is disturbed, resulting in the loss of macrochaetes in the adult notum. This suggests that G2 arrest of the cell cycle ensures the proper determination of sensory mother cells. Received: 16 December 1996 / Accepted: 14 March 1997     

Movement of bristle precursors contributes to the spacing pattern in Drosophila

In Drosophila melanogaster, microchaetes (small bristles) are regularly spaced and form five straight rows in the acrostichal region of the adult notum. Microchaetes develop from sensory organ precursors that arise as single, evenly-spaced cells during pupal development. In this article we address the question of how the precursor cells remain aligned throughout pupal development, in spite of continued division of the intervening epidermal cells. Using in vivo imaging we show that bristle precursors move about continuously throughout development, covering distances of up to one or two cell diameters. During this process, they remain aligned in wild-type flies, suggesting that the movement may be regulated. Flies mutant for scabrous (sca) have a disorganised pattern of bristles with little or no alignment. In vivo observations of sca mutants indicated that the precursor cells move around more than in the wild type, but that, in spite of this the precursor cells and resulting bristles never become well aligned. They appear to follow a more complex path, suggesting that the movement is not co-ordinated. Moreover, analysis of the alignment of precursor cells in vivo in wild-type and sca mutant flies indicate that mutant animals are not able to maintain the pattern of precursor cells during development. Analysis of mosaic flies confirmed the time-lapse observations and showed furthermore that bristles preferentially move towards high levels of Scabrous. We suggest that, by altering the properties of epithelial cells in a graded fashion, Scabrous may provide cues that allow the precursors to remain evenly spaced after they have segregated.     

Cullin-3 regulates pattern formation, external sensory organ development and cell survival during Drosophila development

Ubiquitin-mediated proteolysis regulates the steady-state abundance of proteins and controls cellular homoeostasis by abrupt elimination of key effector proteins. A multienzyme system targets proteins for destruction through the covalent attachment of a multiubiquitin chain. The specificity and timing of protein ubiquitination is controlled by ubiquitin ligases, such as the Skp1-Cullin-F box protein complex. Cullins are major components of SCF complexes, and have been implicated in degradation of key regulatory molecules including Cyclin E, beta-catenin and Cubitus interruptus. Here, we describe the genetic identification and molecular characterisation of the Drosophila Cullin-3 homologue. Perturbation of Cullin-3 function has pleiotropic effects during development, including defects in external sensory organ development, pattern formation and cell growth and survival. Loss or overexpression of Cullin-3 causes an increase or decrease, respectively, in external sensory organ formation, implicating Cullin-3 function in regulating the commitment of cells to the neural fate. We also find that Cullin-3 function modulates Hedgehog signalling by regulating the stability of full-length Cubitus interruptus (Ci155). Loss of Cullin-3 function in eye discs but not other imaginal discs promotes cell-autonomous accumulation of Ci155. Conversely, overexpression of Cullin-3 results in a cell-autonomous stabilisation of Ci155 in wing, haltere and leg, but not eye, imaginal discs suggesting tissue-specific regulation of Cullin-3 function. The diverse nature of Cullin-3 phenotypes highlights the importance of targeted proteolysis during Drosophila development.     

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.     

The formation of sense organs in Drosophila: a logical approach

The genetic analysis of development has revealed the importance of small sets of interacting genes in most morphogenetic processes. The results of gene interactions have so far been examined intuitively. This approach is largely sufficient when one deals with simple interactions, a feedback circuit for example. As more components become involved, however, it is difficult to make sure that the intuitive approach gives a comprehensive view of the behaviour of the system. In this paper, we illustrate the use of a logical approach to describe the genetic circuit that underlies the singling out of sense organ precursor cells in Drosophila. We show how to apply logical modelling to a realistic problem, and how this approach allows an easy assessment of the dynamic properties of the system, i.e., of its possible evolutions and of its reactions to fluctuations and perturbations.     

IFT52 plays an essential role in sensory cilia formation and neuronal sensory function in Drosophila

Cilia are microtubule-based, hair-like organelles involved in sensory function or motility, playing critical roles in many physiological processes such as reproduction, organ development, and sensory perception. In insects, cilia are restricted to certain sensory neurons and sperms, being important for chemical and mechanical sensing, and fertility. Although great progress has been made regarding the mechanism of cilia assembly, the formation of insect cilia remains poorly understand, even in the insect model organism Drosophila. Intraflagellar transport (IFT) is a cilia-specific complex that traffics protein cargos bidirectionally along the ciliary axoneme and is essential for most cilia. Here we investigated the role of IFT52, a core component of IFT-B, in cilia/flagellar formation in Drosophila. We show that Drosophila IFT52 is distributed along the sensory neuronal cilia, and is essential for sensory cilia formation. Deletion of Ift52 results in severe defects in cilia-related sensory behaviors. It should be noted that IFT52 is not detected in spermatocyte cilia or sperm flagella of Drosophila. Accordingly, ift52 mutants can produce sperms with normal motility, supporting a dispensable role of IFT in Drosophila sperm flagella formation. Altogether, IFT52 is a conserved protein essential for sensory cilia formation and sensory neuronal function in insects.     

Drosophila Chitinase 2 is expressed in chitin producing organs for cuticle formation

The architecture of the outer body wall cuticle is fundamental to protect arthropods against invading pathogens and numerous other harmful stresses. Such robust cuticles are formed by parallel running chitin microfibrils. Molting and also local wounding leads to dynamic assembly and disassembly of the chitin-matrix throughout development. However, the underlying molecular mechanisms that organize proper chitin-matrix formation are poorly known. Recently we identified a key region for cuticle thickening at the apical cell surface, the cuticle assembly zone, where Obstructor-A (Obst-A) coordinates the formation of the chitin-matrix. Obst-A binds chitin and the deacetylase Serpentine (Serp) in a core complex, which is required for chitin-matrix maturation and preservation. Here we present evidence that Chitinase 2 (Cht2) could be essential for this molecular machinery. We show that Cht2 is expressed in the chitin-matrix of epidermis, trachea, and the digestive system. There, Cht2 is enriched at the apical cell surface and the dense chitin-matrix. We further show that in Cht2 knockdown larvae the assembly zone is rudimentary, preventing normal cuticle formation and pore canal organization. As sequence similarities of Cht2 and the core complex proteins indicate evolutionarily conserved molecular mechanisms, our findings suggest that Cht2 is involved in chitin formation also in other insects.     

Competence to develop sensory organs is temporally and spatially regulated in Drosophila epidermal primordia.

The Drosophila adult cuticle displays a stereotyped pattern of sensory organs (SOs). Its deployment requires the expression of the achaete (ac) and scute (sc) genes. Their products confer to cells of epidermal primordia (imaginal discs and histoblasts) the ability to become SO precursors (SOPs). In imaginal discs, ac and sc expression is spatially restricted to cell clusters within which one or a few cells become SOP(s). With the help of ubiquitous sc expression provided at different developmental times by a heat shock-sc (HSSC) chimeric gene, we have analyzed the response of epidermal primordia to the proneural action of the sc product, and have tested whether the patterned distribution of ac/sc products is necessary to position SOs correctly within the epidermis. Each primordium responds to HSSC expression by developing SOs only during a characteristic developmental period. In the absence of the endogenous ac and sc genes, most SOs induced by HSSC are of the correct type and are located in wild type positions. These results indicate that the capacity of primordia to respond to sc is temporally and spatially regulated, that specification of the type of SO does not depend on ac/sc, and that SO positioning utilizes topological information independent of the spatially restricted distribution of ac/sc products.     

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.     

Fine structure of the sensory organs of Drosophila melanogaster Meigen larva (Diptera : Drosophilidae)

Fine structure of the prominent external and internal sensory organs of Drosophila melaogaster Meigan (Diptera : Drosophilidae) larva was determined by transmission electron microscopy (TEM). The external sensory organs, namely, antennal, maxillary, ventral and labial organs, dorsal pits, sensory cones on the 8th and 9th abdominal segments, and the sensory hairs on the body of the larva, were studied. A new knob in pit (KIP) sensillum innervated by 3 dendrites was found on the dorsolateral surface on either side. Four tufts of hairs at the posterior end of each great lateral tracheal trunk were found to be sensory with a dendrite at the base of each tuft.Internally in the pharynx of the larva, 3 groups of symmetrically located sensory organs, namely, the anteroventral, dorsal, and posteroventral groups were found. In all they contain 10 pairs of sensilla. Amongst them, 2 pairs of sensilla were found to be of the compound type; one having 9 dendrites arranged in 3 groups of 4, 3 and 2; while the other had 6 dendrites grouped as 2 and 4. In addition, 2 groups of sensilla were found on the internal dorsal fold on either side.Similarities were observed in the dendritic organisation of sensilla in the internal mouthparts of the Drosophila larva and the adult. Unlike nerves of the adult, the larval nerves connected with the dorsal and maxillary organs have a significantly thick layer of glial cells ensheathing the bundle of axons.     

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.     

Gain-of-function screen for genes that affect Drosophila muscle pattern formation

This article reports the production of an EP-element insertion library with more than 3,700 unique target sites within the Drosophila melanogaster genome and its use to systematically identify genes that affect embryonic muscle pattern formation. We designed a UAS/GAL4 system to drive GAL4-responsive expression of the EP-targeted genes in developing apodeme cells to which migrating myotubes finally attach and in an intrasegmental pattern of cells that serve myotubes as a migration substrate on their way towards the apodemes. The results suggest that misexpression of more than 1.5% of the Drosophila genes can interfere with proper myotube guidance and/or muscle attachment. In addition to factors already known to participate in these processes, we identified a number of enzymes that participate in the synthesis or modification of protein carbohydrate side chains and in Ubiquitin modifications and/or the Ubiquitin-dependent degradation of proteins, suggesting that these processes are relevant for muscle pattern formation.     

Numb and alpha-Adaptin regulate Sanpodo endocytosis to specify cell fate in Drosophila external sensory organs

During asymmetric cell division in Drosophila sensory organ precursors (SOPs), the Numb protein segregates into one of the two daughter cells, in which it inhibits Notch signalling to specify pIIb cell fate. We show here that Numb acts in SOP cells by inducing the endocytosis of Sanpodo, a four-pass transmembrane protein that has previously been shown to regulate Notch signalling in the central nervous system. In sanpodo mutants, SOP cells divide symmetrically into two pIIb cells. We show that Sanpodo is cortical in pIIa, but colocalizes with Notch and Delta in Rab5- and Rab7-positive endocytic vesicles in pIIb. Sanpodo endocytosis requires alpha-Adaptin, a Numb-binding partner involved in clathrin-mediated endocytosis. In numb or alpha-adaptin mutants, Sanpodo is not endocytosed. Surprisingly, this defect is observed already before and during mitosis, which suggests that Numb not only acts in pIIb, but also regulates endocytosis throughout the cell cycle. Numb binds to Sanpodo by means of its phosphotyrosine-binding domain, a region that is essential for Numb function. Our results establish numb- and alpha-adaptin-dependent endocytosis of Sanpodo as the mechanism by which Notch is regulated during external sensory organ development.     

A tissue specific cytochrome P450 required for the structure and function of Drosophila sensory organs

Cytochrome P450s have generally been acknowledged as broadly tuned detoxifying enzymes. However, emerging evidence argues P450s have an integral role in cell signaling and developmental processes, via their metabolism of retinoic acid, arachidonic acid, steroids, and other cellular ligands. To study the morphogenesis of Drosophila sensory organs, we examined mutants with impaired mechanosensation and discovered one, nompH, encodes the cytochrome P450 CYP303a1. We now report the characterization of nompH, a mutant defective in the function of peripheral chemo- and mechanoreceptor cells, and demonstrate CYP303a1 is essential for the development and structure of external sensory organs which mediate the reception of vital mechanosensory and chemosensory stimuli. Notably this P450 is expressed only in sensory bristles, localizing in the apical region of the socket cell. The wide diversity of the P450 family and the growing number of P450s with developmental phenotypes suggests the exquisite tissue and subcellular specificity of CYP303a1 illustrates an important aspect of P450 function; namely, a strategy to process critical developmental signals in a tissue- and cell-specific manner.     

Patterning of Drosophila leg sensory organs through combinatorial signaling by hedgehog, decapentaplegic and wingless.

During development, global patterning events initiate signal transduction cascades which gradually establish an array of individual cell fates. Many of the genes which pattern Drosophila are expressed throughout development and specify diverse cell types by creating unique local environments which establish the expression of locally acting genes. This process is exemplified by the patterning of leg microchaete rows. hairy (h) is expressed in a spatially restricted manner in the leg imaginal disc and functions to position adult leg bristle rows by negatively regulating the proneural gene achaete, which specifies sensory cell fates. While much is known about the events that partition the leg imaginal disc and about sensory cell differentiation, the mechanisms that refine early patterning events to the level of individual cell fate specification are not well understood. We have investigated the regulation of h expression along the dorsal/ventral (D/V) axis of the leg adjacent to the anterior/posterior (A/P) compartment boundary and have found that it requires input from both D/V and A/P patterning mechanisms. Expression of the D/V axis h stripe (D/V-h) is controlled by dorsal- and ventral-specific enhancer elements which are targets of Decapentaplegic (Dpp) and Wingless (Wg) signaling, respectively, but which are also dependent on Hedgehog (Hh) signaling for activation. D/V-h expression is lost in smoothened mutant clones and is specifically activated by exogenously supplied Cubitus interruptus (Ci). D/V-h expression is also lost in clones deficient for Dpp and Wg signaling, but ectopic activation of D/V-h by Dpp and Wg is limited to the A/P compartment boundary where endogenous levels of full-length Ci are high. We propose that D/V-h expression is regulated in a non-linear pathway in which Ci plays a dual role. In addition to serving as an upstream activator of Dpp and Wg, Ci acts combinatorially with them to activate D/V-h expression.