The development of normal and ectopic sensilla in the wings of hairy and Hairy wing mutants of Drosophila

Wg signaling via Zw3 and mad restricts self-renewal of sensory organ precursor cells in Drosophila

It is well known that the Dpp signal transducer Mad is activated by phosphorylation at its carboxy-terminus. The role of phosphorylation on other regions of Mad is not as well understood. Here we report that the phosphorylation of Mad in the linker region by the Wg antagonist Zw3 (homolog of vertebrate Gsk3-β) regulates the development of sensory organs in the anterior-dorsal quadrant of the wing. Proneural expression of Mad-RNA interference (RNAi) or a Mad transgene with its Zw3/Gsk3-β phosphorylation sites mutated (MGM) generated wings with ectopic sensilla and chemosensory bristle duplications. Studies with pMad-Gsk (an antibody specific to Zw3/Gsk3-β-phosphorylated Mad) in larval wing disks revealed that this phosphorylation event is Wg dependent (via an unconventional mechanism), is restricted to anterior-dorsal sensory organ precursors (SOP) expressing Senseless (Sens), and is always co-expressed with the mitotic marker phospho-histone3. Quantitative analysis in both Mad-RNAi and MGM larval wing disks revealed a significant increase in the number of Sens SOP. We conclude that the phosphorylation of Mad by Zw3 functions to prevent the self-renewal of Sens SOP, perhaps facilitating their differentiation via asymmetric division. The conservation of Zw3/Gsk3-β phosphorylation sites in vertebrate homologs of Mad (Smads) suggests that this pathway, the first transforming growth factor β-independent role for any Smad protein, may be widely utilized for regulating mitosis during development.

Wing vein patterning in Drosophila and the analysis of intercellular signaling

The positioning and elaboration of ectodermal veins in the wing of Drosophila melanogaster rely on widely utilized developmental signals, including those mediated by EGF, BMP, Hedgehog, Notch, and Wnt. Analysis of vein patterning mutants, using the molecular and genetic mosaic techniques available in Drosophila, has provided important insights into how a combination of short-range and long-range signaling can pattern a simple epidermal tissue. Moreover, venation has become a powerful system for isolating and analyzing novel components in these signaling pathways. I here review the basic events of vein patterning and give examples of how changes in venation have been used to identify important features of cell signaling pathways.

Time-lapse and cell ablation reveal the role of cell interactions in fly glia migration and proliferation

Migration and proliferation have been mostly explored in culture systems or fixed preparations. We present a simple genetic model, the chains of glia moving along fly wing nerves, to follow such dynamic processes by time-lapse in the whole animal. We show that glia undergo extensive cytoskeleton and mitotic apparatus rearrangements during division and migration. Single cell labelling identifies different glia: pioneers with high filopodial,exploratory, activity and, less active followers. In combination with time-lapse, altering this cellular environment by genetic means or cell ablation has allowed to us define the role of specific cell-cell interactions. First, neurone-glia interactions are not necessary for glia motility but do affect the direction of migration. Second, repulsive interactions between glia control the extent of movement. Finally, autonomous cues control proliferation.

neuralized functions cell-autonomously to regulate a subset of notch-dependent processes during adult Drosophila development

neuralized (neu) represents one of the strong neurogenic mutants in Drosophila. Mutants of this class display, among other phenotypes, a strong overcommitment to neural fates at the expense of epidermal fates. We analyzed the role of neu during adult development by using mutant clonal analysis, misexpression of wild-type and truncated forms of Neu, and examination of genetic interactions with N-pathway mutations. We find that neu is required cell-autonomously for lateral inhibition during peripheral neurogenesis and for multiple asymmetric cell divisions in the sensory lineage. In contrast, neu is apparently dispensable for other N-mediated processes, including lateral inhibition during wing vein development and wing margin induction. Misexpression of wild-type Neu causes defects in both peripheral neurogenesis and wing vein development, while a truncated form lacking the RING finger is further capable of inhibiting formation of the wing margin. In addition, the phenotypes produced by misexpression of wild-type and truncated Neu proteins are sensitive to the dosage of several N-pathway components. Finally, using epitope-tagged Neu proteins, we localize Neu to the plasma membrane and reveal a novel morphology to the sensory organ precursor cells of wing imaginal discs. Collectively, these data indicate a key role for neu in the reception of the lateral inhibitory signal during peripheral neurogenesis.

Wasp, the Drosophila Wiskott-Aldrich syndrome gene homologue, is required for cell fate decisions mediated by Notch signaling

Wiskott-Aldrich syndrome proteins, encoded by the Wiskott-Aldrich syndrome gene family, bridge signal transduction pathways and the microfilament-based cytoskeleton. Mutations in the Drosophila homologue, Wasp (Wsp), reveal an essential requirement for this gene in implementation of cell fate decisions during adult and embryonic sensory organ development. Phenotypic analysis of Wsp mutant animals demonstrates a bias towards neuronal differentiation, at the expense of other cell types, resulting from improper execution of the program of asymmetric cell divisions which underlie sensory organ development. Generation of two similar daughter cells after division of the sensory organ precursor cell constitutes a prominent defect in the Wsp sensory organ lineage. The asymmetric segregation of key elements such as Numb is unaffected during this division, despite the misassignment of cell fates. The requirement for Wsp extends to additional cell fate decisions in lineages of the embryonic central nervous system and mesoderm. The nature of the Wsp mutant phenotypes, coupled with genetic interaction studies, identifies an essential role for Wsp in lineage decisions mediated by the Notch signaling pathway.

Some fly sensory organs are gliogenic and require glide/gcm in a precursor that divides symmetrically and produces glial cells

In flies, the choice between neuronal and glial fates depends on the asymmetric division of multipotent precursors, the neuroglioblast of the central nervous system and the IIb precursor of the sensory organ lineage. In the central nervous system, the choice between the two fates requires asymmetric distribution of the glial cell deficient/glial cell missing (glide/gcm) RNA in the neuroglioblast. Preferential accumulation of the transcript in one of the daughter cells results in the activation of the glial fate in that cell, which becomes a glial precursor. Here we show that glide/gcm is necessary to induce glial differentiation in the peripheral nervous system. We also present evidence that glide/gcm RNA is not necessary to induce the fate choice in the peripheral multipotent precursor. Indeed, glide/gcm RNA and protein are first detected in one daughter of IIb but not in IIb itself. Thus, glide/gcm is required in both central and peripheral glial cells, but its regulation is context dependent. Strikingly, we have found that only subsets of sensory organs are gliogenic and express glide/gcm. The ability to produce glial cells depends on fixed, lineage related, cues and not on stochastic decisions. Finally, we show that after glide/gcm expression has ceased, the IIb daughter migrates and divides symmetrically to produce several mature glial cells. Thus, the glide/gcm-expressing cell, also called the fifth cell of the sensory organ, is indeed a glial precursor. This is the first reported case of symmetric division in the sensory organ lineage. These data indicate that the organization of the fly peripheral nervous system is more complex than previously thought.

Cellular mechanisms of wingless/Wnt signal transduction

Wg/Wnt signaling regulates cell proliferation and differentiation in species as divergent as nematodes, flies, frogs, and humans. Many components of this highly conserved process have been characterized and work from a number of laboratories is beginning to elucidate the mechanism by which this class of secreted growth factor triggers cellular decisions. The Wg/Wnt ligand apparently binds to Frizzled family receptor molecules to initiate a signal transduction cascade involving the novel cytosolic protein Dishevelled and the serine/threonine kinase Zeste-white 3/GSK3. Antagonism of Zw3 activity leads to stabilization of Armadillo/beta-catenin, which provides a transactivation domain when complexed with the HMG box transcription factor dTCF/LEF-1 and thereby activates expression of Wg/Wnt-responsive genes. The Wg/Wnt ligands pass through the secretory pathway and associate with extracellular matrix components; recent work shows that sulfated glycosaminoglycans are essential for proper transduction of the signal. Mutant forms of Wg in Drosophila reveal separable aspects of Wg function and suggest that proper transport of the protein across cells is essential for cell fate specification. Complex interactions with the Notch and EGF/Ras signaling pathways also play a role in cell fate decisions during different phases of Drosophila development. These many facets of Wg/Wnt signaling have been elucidated through studies in a variety of species, each with powerful and unique experimental approaches. The remarkable conservation of this pathway suggests that Wg/Wnt signal transduction represents a fundamental mechanism for the generation of diverse cell fates in animal embryos.

klumpfuss, a Drosophila gene encoding a member of the EGR family of transcription factors, is involved in bristle and leg development

The klumpfuss (klu) transcription unit in Drosophila gives rise to two different transcripts of 4.5 and 4.9 kb, both of which encode a putative transcription factor with four zinc-finger motifs of the C2H2 class. Zinc-finger 2–4 are homologous to those of the proteins of the EGR transcription factor family. As in the case of the most divergent member of the family, the Wilms' tumor suppressor gene (WT-1), klu contains an additional zinc finger, which is only distantly related. Loss of klumpfuss function is semilethal and causes a variety of defects in bristles and legs of adults, as well as in mouth hooks and brains of larvae. Analysis of the mutants indicates that klumpfuss is required for proper specification and differentiation of a variety of cells, including the sensory organ mother cells and those of the distal parts of tarsal segments.

Polychaetoid is required to restrict segregation of sensory organ precursors from proneural clusters in Drosophila

Reduction of wild-type activity of the polychaetoid (pyd) gene results in formation of extra mechanosensory bristles on the head and notum of adult Drosophila. Loss of pyd function results in decreased ability to restrict sensory organ precursor (SOP) formation to a single cell per proneural cluster. Although the initial proneural cluster pattern of achaete expression is not altered in pyd mutants, extra cells within proneural clusters express the high levels of achaete characteristic of SOPs. This observation suggests that pyd+ functions as a negative regulator of achaete-scute complex expression within the proneural cluster. Synergistic interactions between pyd and Notch, Delta and extramacrochaetae mutations support this model. We also demonstrate that pyd is required for normal eye development.

Notch regulates wingless expression and is not required for reception of the paracrine wingless signal during wing margin neurogenesis in Drosophila

In the developing wing margin of Drosophila, wingless is normally expressed in a narrow stripe of cells adjacent to the proneural cells that form the sensory bristles of the margin. Previous work has shown that this wingless is required for the expression of the proneural achaete-scute complex genes and the subsequent formation of the sensory bristles along the margin; recently, it has been proposed that the proneural cells require the Notch protein to properly receive the wingless signal. We have used clonal analysis of a null allele of Notch to test this idea directly. We found that Notch was not required by prospective proneural margin cells for the expression of scute or the formation of sensory precursors, indicating Notch is not required for the reception of wingless signal. Loss of Notch from proneural cells produced cell-autonomous neurogenic phenotypes and precocious differentiation of sensory cells, as would be expected if Notch had a role in lateral inhibition within the proneural regions. However, loss of scute expression and of sensory precursors was observed if clones substantially included the normal region of wingless expression. These ‘anti-proneural’ phenotypes were associated with the loss of wingless expression; this loss may be partially or wholly responsible for the anti-proneural phenotype. Curiously, Notch- clones limited to the dorsal or ventral compartments could disrupt wingless expression and proneural development in the adjacent compartment. Analysis using the temperature-sensitive Notch allele indicated that the role of Notch in the regulation of wingless expression precedes the requirement for lateral inhibition in proneural cells. Furthermore, overexpression of wingless with a heat shock-wingless construct rescued the loss of sensory precursors associated with the early loss of Notch.

Cis-regulation of achaete and scute: shared enhancer-like elements drive their coexpression in proneural clusters of the imaginal discs

The pattern of bristles and other sensory organs on the adult cuticle of Drosophila is prefigured in the imaginal discs by the pattern of expression of the proneural achaete (ac) and scute (sc) genes, two members of the ac-sc complex (AS-C). These genes are simultaneously expressed by groups of cells (the proneural clusters) located at constant positions in discs. Their products (transcription factors of the basic-helix-loop-helix family) allow cells to become sensory organ mother cells (SMCs), a fate normally realized by only one or a few cells per cluster. Here we show that the highly complex pattern of proneural clusters is constructed piecemeal, by the action on ac and sc of site-specific, enhancer-like elements distributed along most of the AS-C (approximately 90 kb). Fragments of AS-C DNA containing these enhancers drive reporter lacZ genes in only one or a few proneural clusters. This expression is independent of the ac and sc endogenous genes, indicating that the enhancers respond to local combinations of factors (prepattern). We show further that the cross-activation between ac and sc, discovered by means of transgenes containing either ac or sc promoter fragments linked to lacZ and thought to explain the almost identical patterns of ac and sc expression, does not occur detectably between the endogenous ac and sc genes in most proneural clusters. Our data indicate that coexpression is accomplished by activation of both ac and sc by the same set of position-specific enhancers.

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.

Neuronal specificity and its development in the Drosophila wing disc and its derivatives

The imaginal wing disc of flies gives rise to the adult wing blade and dorsal thorax (notum). A great deal has been learned in recent years about the process of neurogenesis in this disc; a number of genes that play crucial roles in the formation of sensory mother cells and in the differentiation of the sensory organs have been identified and their roles defined. Given this extensive background of developmental genetics, it has seemed profitable to summarize what is known about the end-products of neural development, the adult sensory organs. Discussed are their physiological function and role in behavior, the pathways followed by their axons in the CNS, and both genes and epigenetic processes that might play some role in the later stages of neural development and in adult function. The highly individual characteristics of certain of the sensory organs is emphasized, both in the context of their adult roles and as a challenge for future studies in developmental genetics.

The spatial organization of epidermal structures: hairy establishes the geometrical pattern of Drosophila leg bristles by delimiting the domains of achaete expression

The spatial organization of Drosophila melanogaster epidermal structures in embryos and adults constitutes a classic model system for understanding how the two dimensional arrangement of particular cell types is generated. For example, the legs of the Drosophila melanogaster adult are covered with bristles, which in most segments are arranged in longitudinal rows. Here we elucidate the key roles of two regulatory genes, hairy and achaete, in setting up this periodic bristle pattern. We show that achaete is expressed during pupal leg development in a dynamic pattern which changes, by approximately 6 hours after puparium formation, into narrow longitudinal stripes of 3–4 cells in width, each of which represents a field of cells (proneural field) from which bristle precursor cells are selected. This pattern of gene expression foreshadows the adult bristle pattern and is established in part through the function of the hairy gene, which also functions in patterning other adult sense organs. In pupal legs, hairy is expressed in four longitudinal stripes, located between every other pair of achaete stripes. We show that in the absence of hairy function achaete expression expands into the interstripe regions that normally express hairy, fusing the two achaete stripes and resulting in extra-wide stripes of achaete expression. This misexpression of achaete, in turn, alters the fields of potential bristle precursor cells which leads to the misalignment of bristle rows in the adult. This function of hairy in patterning achaete expression is distinct from that in the wing in which hairy suppresses late expression of achaete but has no effect on the initial patterning of achaete expression. Thus, the leg bristle pattern is apparently regulated at two levels: a global regulation of the hairy and achaete expression patterns which partitions the leg epidermis into striped zones (this study) and a local regulation (inferred from other studies on the selection of neural precursor cells) that involves refinement steps which may control the alignment and spacing of bristle cells within these zones.

Development and organization of glial cells in the peripheral nervous system of Drosophila melanogaster

We have used enhancer trap lines as markers to recognize glial cells in the wing peripheral nervous system of Drosophila melanogaster. Their characterization has enabled us to define certain features of glial differentiation and organization. In order to ask whether glial cells originate within the disc or whether they migrate to the wing nerves from the central nervous system, we used two approaches. In cultured wing discs from glial-specific lines, peripheral glial precursors are already present within the imaginal tissue during the third larval stage. Glial cells differentiate on a wing nerve even in mutants in which that nerve does not connect to the central nervous system. To assess whether peripheral glial cells originate from ectoderm or from mesoderm, we cultured discs from which the mesodermally derived adepithelial cells had been removed. Our findings indicate that peripheral glial cells originate from ectodermally derived cells. As has already been shown for the embryonic central nervous system, gliogenesis in the periphery is an early event during adult development: glial cells, or their precursors, are already present at stages when neurons are still differentiating. Finally, our results also suggest that peripheral glial cells may not display a stereotyped arrangement.

Shaggy (zeste-white 3) and the formation of supernumerary bristle precursors in the developing wing blade of Drosophila

The development of supernumerary bristle precursors induced by the mutation shaggy (sgg; also known as zeste-white 3) was examined in the developing wing blade of imaginal and pupal Drosophila. sgg clones were induced by mitotic recombination; clones were marked using enhancer-trap flies which express beta-galactosidase ubiquitously in imaginal tissues, while bristle precursors were identified using sensillum and bristle-specific enhancer-trap lines. It was shown that the precursors of supernumerary sgg bristles in the wing blade mimicked the development of morphologically similar margin bristles, developing in a manner similar to that of anterior sensory bristles in anterior clones and posterior noninnervated bristles in posterior clones. Interestingly, supernumerary anterior sensory bristles appeared outside the normal regions of "proneural" gene activity as identified using anti-achaete. Moreover, sgg could induce the ectopic expression of achaete in anterior clones. Thus, in the anterior wing blade the sgg mutation leads to the formation of ectopic proneural regions.