Control of growth and patterning of the Drosophila wing imaginal disc by EGFR-mediated signaling

The effectiveness of EGFR knockdown by RNAi lines varies depending on the tissue

In Drosophila , the Epidermal growth factor receptor (EGFR) signaling pathway is known to be critically involved in multiple stages of development. We induced a loss of function phenotype in the eyes, wings, and somatic follicle cells using four EGFR RNAi lines: HMS05003 and JF02283, which produce short hairpin RNAs, as well as JF01368 and KK100051, which produce long hairpin RNAs. Using these four lines, we completed a systematic comparison of the ability of short hairpin vs long hairpin RNAi lines to produce loss-of-function phenotypes in the above-mentioned tissues. Tissue specific knockdown was achieved by using Gal4 drivers specific to the three tissues being studied. In the eyes and wings, the KK100051 line induced the most severe phenotype, while the JF01368 line was the least severe, but in the somatic follicle cells, the KK100051 line was the least effective, while the JF01368 and JF02283 lines were comparable with respect to phenotypic severity. We conclude that there is significant tissue specific variability exhibited by the different RNAi lines.

The wing imaginal disc

The Drosophila wing imaginal disc is a tissue of undifferentiated cells that are precursors of the wing and most of the notum of the adult fly. The wing disc first forms during embryogenesis from a cluster of ∼30 cells located in the second thoracic segment, which invaginate to form a sac-like structure. They undergo extensive proliferation during larval stages to form a mature larval wing disc of ∼35,000 cells. During this time, distinct cell fates are assigned to different regions, and the wing disc develops a complex morphology. Finally, during pupal stages the wing disc undergoes morphogenetic processes and then differentiates to form the adult wing and notum. While the bulk of the wing disc comprises epithelial cells, it also includes neurons and glia, and is associated with tracheal cells and muscle precursor cells. The relative simplicity and accessibility of the wing disc, combined with the wealth of genetic tools available in Drosophila, have combined to make it a premier system for identifying genes and deciphering systems that play crucial roles in animal development. Studies in wing imaginal discs have made key contributions to many areas of biology, including tissue patterning, signal transduction, growth control, regeneration, planar cell polarity, morphogenesis, and tissue mechanics.

RAL GTPases mediate EGFR-driven intestinal stem cell proliferation and tumourigenesis

RAS-like (RAL) GTPases function in Wnt signalling-dependent intestinal stem cell proliferation and regeneration. Whether RAL proteins work as canonical RAS effectors in the intestine and the mechanisms of how they contribute to tumourigenesis remain unclear. Here, we show that RAL GTPases are necessary and sufficient to activate EGFR/MAPK signalling in the intestine, via induction of EGFR internalisation. Knocking down Drosophila RalA from intestinal stem and progenitor cells leads to increased levels of plasma membrane-associated EGFR and decreased MAPK pathway activation. Importantly, in addition to influencing stem cell proliferation during damage-induced intestinal regeneration, this role of RAL GTPases impacts on EGFR-dependent tumourigenic growth in the intestine and in human mammary epithelium. However, the effect of oncogenic RAS in the intestine is independent from RAL function. Altogether, our results reveal previously unrecognised cellular and molecular contexts where RAL GTPases become essential mediators of adult tissue homeostasis and malignant transformation.

A unified mechanism for the control of Drosophila wing growth by the morphogens Decapentaplegic and Wingless

Development of the Drosophila wing-a paradigm of organ development-is governed by 2 morphogens, Decapentaplegic (Dpp, a BMP) and Wingless (Wg, a Wnt). Both proteins are produced by defined subpopulations of cells and spread outwards, forming gradients that control gene expression and cell pattern as a function of concentration. They also control growth, but how is unknown. Most studies have focused on Dpp and yielded disparate models in which cells throughout the wing grow at similar rates in response to the grade or temporal change in Dpp concentration or to the different amounts of Dpp "equalized" by molecular or mechanical feedbacks. In contrast, a model for Wg posits that growth is governed by a progressive expansion in morphogen range, via a mechanism in which a minimum threshold of Wg sustains the growth of cells within the wing and recruits surrounding "pre-wing" cells to grow and enter the wing. This mechanism depends on the capacity of Wg to fuel the autoregulation of vestigial (vg)-the selector gene that specifies the wing state-both to sustain vg expression in wing cells and by a feed-forward (FF) circuit of Fat (Ft)/Dachsous (Ds) protocadherin signaling to induce vg expression in neighboring pre-wing cells. Here, we have subjected Dpp to the same experimental tests used to elucidate the Wg model and find that it behaves indistinguishably. Hence, we posit that both morphogens act together, via a common mechanism, to control wing growth as a function of morphogen range.

Growth control in the Drosophila wing disk

The regulation of size and shape is a fundamental requirement of biological development and has been a subject of scientific study for centuries, but we still lack an understanding of how organisms know when to stop growing. Imaginal wing disks of the fruit fly Drosophila melanogaster, which are precursors of the adult wings, are an archetypal tissue for studying growth control. The growth of the disks is dependent on many inter- and intra-organ factors such as morphogens, mechanical forces, nutrient levels, and hormones that influence gene expression and cell growth. Extracellular signals are transduced into gene-control signals via complex signal transduction networks, and since cells typically receive many different signals, a mechanism for integrating the signals is needed. Our understanding of the effect of morphogens on tissue-level growth regulation via individual pathways has increased significantly in the last half century, but our understanding of how multiple biochemical and mechanical signals are integrated to determine whether or not a cell decides to divide is still rudimentary. Numerous fundamental questions are involved in understanding the decision-making process, and here we review the major biochemical and mechanical pathways involved in disk development with a view toward providing a basis for beginning to understand how multiple signals can be integrated at the cell level, and how this translates into growth control at the level of the imaginal disk. This article is categorized under: Analytical and Computational Methods > Computational Methods Biological Mechanisms > Cell Signaling Models of Systems Properties and Processes > Cellular Models.

Dynamic MAPK signaling activity underlies a transition from growth arrest to proliferation in Drosophila scribble mutant tumors

Human tumors exhibit plasticity and evolving capacity over time. It is difficult to study the mechanisms of how tumors change over time in human patients, in particular during the early stages when a few oncogenic cells are barely detectable. Here, we used a Drosophila tumor model caused by loss of scribble (scrib), a highly conserved apicobasal cell polarity gene, to investigate the spatial-temporal dynamics of early tumorigenesis events. The fly scrib mutant tumors have been successfully used to model many aspects of tumorigenesis processes. However, it is still unknown whether Drosophila scrib mutant tumors exhibit plasticity and evolvability along the temporal axis. We found that scrib mutant tumors displayed different growth rates and cell cycle profiles over time, indicative of a growth arrest-to-proliferation transition as the scrib mutant tumors progress. Longitudinal bulk and single-cell transcriptomic analysis of scrib mutant tumors revealed that the MAPK pathway, including JNK and ERK signaling activities, showed quantitative changes over time. We found that high JNK signaling activity caused G2/M cell cycle arrest in early scrib mutant tumors. In addition, JNK signaling activity displayed a radial polarity with the JNK^high cells located at the periphery of scrib mutant tumors, providing an inherent mechanism that leads to an overall decrease in JNK signaling activity over time. We also found that ERK signaling activity, in contrast to JNK activity, increased over time and promoted growth in late-stage scrib mutant tumors. Furthermore, high JNK signaling activity repressed ERK signaling activity in early scrib mutant tumors. Together, these data demonstrate that dynamic MAPK signaling activity, fueled by intratumor heterogeneity derived from tissue topological differences, drives a growth arrest-to-proliferation transition in scrib mutant tumors.

This article has an associated First Person interview with the joint first authors of the paper.

Summary: The authors provide evidence to show that a well-established Drosophila tumor model, caused by loss of apicobasal cell polarity, harbors a surprising degree of plasticity and evolvability along the temporal axis.

Mosaic Analysis in Drosophila

Since the founding of Drosophila genetics by Thomas Hunt Morgan and his colleagues over 100 years ago, the experimental induction of mosaicism has featured prominently in its recognition as an unsurpassed genetic model organism. The use of genetic mosaics has facilitated the discovery of a wide variety of developmental processes, identified specific cell lineages, allowed the study of recessive embryonic lethal mutations, and demonstrated the existence of cell competition. Here, we discuss how genetic mosaicism in Drosophila became an invaluable research tool that revolutionized developmental biology. We describe the prevailing methods used to produce mosaic animals, and highlight advantages and disadvantages of each genetic system. We cover methods ranging from simple "twin-spot" analysis to more sophisticated systems of multicolor labeling.

Specification and Patterning of Drosophila Appendages

Appendages are external projections of the body that serve the animal for locomotion, feeding, or environment exploration. The appendages of the fruit fly Drosophilamelanogaster are derived from the imaginal discs, epithelial sac-like structures specified in the embryo that grow and pattern during larva development. In the last decades, genetic and developmental studies in the fruit fly have provided extensive knowledge regarding the mechanisms that direct the formation of the appendages. Importantly, many of the signaling pathways and patterning genes identified and characterized in Drosophila have similar functions during vertebrate appendage development. In this review, we will summarize the genetic and molecular mechanisms that lead to the specification of appendage primordia in the embryo and their posterior patterning during imaginal disc development. The identification of the regulatory logic underlying appendage specification in Drosophila suggests that the evolutionary origin of the insect wing is, in part, related to the development of ventral appendages.

CHMP1B is a target of USP8/UBPY regulated by ubiquitin during endocytosis

Integration and down-regulation of cell growth and differentiation signals rely on plasma membrane receptor endocytosis and sorting towards either recycling vesicles or degradative lysosomes via multivesicular bodies (MVB). In this process, the endosomal sorting complex-III required for transport (ESCRT-III) controls membrane deformation and scission triggering intraluminal vesicle (ILV) formation at early endosomes. Here, we show that the ESCRT-III member CHMP1B can be ubiquitinated within a flexible loop known to undergo conformational changes during polymerization. We demonstrate further that CHMP1B is deubiquitinated by the ubiquitin specific protease USP8 (syn. UBPY) and found fully devoid of ubiquitin in a ~500 kDa large complex that also contains its ESCRT-III partner IST1. Moreover, EGF stimulation induces the rapid and transient accumulation of ubiquitinated forms of CHMP1B on cell membranes. Accordingly, CHMP1B ubiquitination is necessary for CHMP1B function in both EGF receptor trafficking in human cells and wing development in Drosophila. Based on these observations, we propose that CHMP1B is dynamically regulated by ubiquitination in response to EGF and that USP8 triggers CHMP1B deubiquitination possibly favoring its subsequent assembly into a membrane-associated ESCRT-III polymer.

In multicellular organisms, the interpretation and transmission of cell growth and differentiation signals strongly rely on plasma membrane receptors. Once activated by their ligands, these receptors activate downstream signaling cascades and are rapidly internalized into intracellular vesicles that fuse inside the cell to form the endosomal compartment. From there, the receptors are sorted towards either recycling vesicles or degradative lysosomes via multivesicular bodies. Receptors sorting therefore plays a crucial role in the integration and regulation of intracellular signals during development and numerous physio-pathological processes. It requires extensive membrane remodeling and scission events at the level of the endosomal compartment by so-called ESCRT proteins, including CHMP1B. In this study, we provide evidence for dynamic regulation of CHMP1B function and subcellular localization by ubiquitin linkage. We also show the contribution of the ubiquitin specific protease USP8 in this regulation, which is a known actor of intracellular trafficking and Cushing’s disease.

Ras/ERK-signalling promotes tRNA synthesis and growth via the RNA polymerase III repressor Maf1 in Drosophila

The small G-protein Ras is a conserved regulator of cell and tissue growth. These effects of Ras are mediated largely through activation of a canonical RAF-MEK-ERK kinase cascade. An important challenge is to identify how this Ras/ERK pathway alters cellular metabolism to drive growth. Here we report on stimulation of RNA polymerase III (Pol III)-mediated tRNA synthesis as a growth effector of Ras/ERK signalling in Drosophila. We find that activation of Ras/ERK signalling promotes tRNA synthesis both in vivo and in cultured Drosophila S2 cells. We also show that Pol III function is required for Ras/ERK signalling to drive proliferation in both epithelial and stem cells in Drosophila tissues. We find that the transcription factor Myc is required but not sufficient for Ras-mediated stimulation of tRNA synthesis. Instead we show that Ras signalling promotes Pol III function and tRNA synthesis by phosphorylating, and inhibiting the nuclear localization and function of the Pol III repressor Maf1. We propose that inhibition of Maf1 and stimulation of tRNA synthesis is one way by which Ras signalling enhances protein synthesis to promote cell and tissue growth.

Combining Time-of-Flight Secondary Ion Mass Spectrometry Imaging Mass Spectrometry and CARS Microspectroscopy Reveals Lipid Patterns Reminiscent of Gene Expression Patterns in the Wing Imaginal Disc of Drosophila melanogaster

Using label-free ToF-SIMS imaging mass spectrometry, we generated a map of small molecules differentially expressed in the Drosophila wing imaginal disc. The distributions of these moieties were in line with gene expression patterns observed during wing imaginal disc development. Combining ToF-SIMS imaging and coherent anti-Stokes Raman spectroscopy (CARS) microspectroscopy allowed us to locally identify acylglycerols as the main constituents of the pattern differentiating the future body wall tissue from the wing blade tissue. The findings presented herein clearly demonstrate that lipid localization patterns are strongly correlated with a developmental gene expression. From this correlation, we hypothesize that lipids play a so far unrecognized role in organ development.

Expanding the Iroquois genes repertoire: a non-transcriptional function in cell cycle progression

Drosophila Iroquois (Iro) proteins are components of the TALE homeodomain family of transcriptional regulators. They play key roles in territorial specification and pattern formation. A recent study has disclosed a novel developmental function of the Iro proteins. In the eye and wing imaginal discs, they can regulate the size of the territories that they specify. They do so by cell-autonomously controlling cell cycle progression. Indeed, Iro proteins down-regulate the activity of the CyclinE/Cdk2 complex by a transcription-independent mechanism. This novel function is executed mainly through 2 evolutionarily conserved domains of the Iro proteins: the Cyclin Binding Domain and the IRO-box, which mediate their binding to CyclinE-containing protein complexes. Here we discuss the functional implications of the control of the cell cycle by Iro proteins for development and oncogenesis.

Establishment of a Developmental Compartment Requires Interactions between Three Synergistic Cis-regulatory Modules

The subdivision of cell populations in compartments is a key event during animal development. In Drosophila, the gene apterous (ap) divides the wing imaginal disc in dorsal vs ventral cell lineages and is required for wing formation. ap function as a dorsal selector gene has been extensively studied. However, the regulation of its expression during wing development is poorly understood. In this study, we analyzed ap transcriptional regulation at the endogenous locus and identified three cis-regulatory modules (CRMs) essential for wing development. Only when the three CRMs are combined, robust ap expression is obtained. In addition, we genetically and molecularly analyzed the trans-factors that regulate these CRMs. Our results propose a three-step mechanism for the cell lineage compartment expression of ap that includes initial activation, positive autoregulation and Trithorax-mediated maintenance through separable CRMs.

The separation of cell populations into distinct functional units is essential for both vertebrate and invertebrate animal development. A classical paradigm for this phenomenon is the establishment of developmental compartments during Drosophila wing development. These compartments depend on the restricted expression of two selector genes, engrailed in the posterior compartment and apterous (ap) in the dorsal compartment. Yet, despite the central role these genes and their restricted expression patterns play in Drosophila development, we still do not understand how these patterns are established or maintained. Here, by dissecting the regulatory sequences required for ap expression, we solve this problem for this critical selector gene. We used a combination of experimental approaches to identify and functionally characterize the cis-regulatory modules (CRMs) that regulate ap expression during Drosophila wing development. For these analyses we implement a novel technique allowing us to study the function of these CRMs in vivo, at the native ap locus. We found three ap CRMs crucial for wing development: the Early (apE) and the D/V (apDV) enhancers and the ap PRE (apP). Only when all three regulatory elements are combined is a uniform and complete ap expression domain generated. In summary, our results indicate that ap is regulated in time and space by a three-step mechanism that generates a lineage compartment by integrating input from separate CRMs for the initiation, refinement and maintenance of its expression.

The Homeodomain Iroquois Proteins Control Cell Cycle Progression and Regulate the Size of Developmental Fields

During development, proper differentiation and final organ size rely on the control of territorial specification and cell proliferation. Although many regulators of these processes have been identified, how both are coordinated remains largely unknown. The homeodomain Iroquois/Irx proteins play a key, evolutionarily conserved, role in territorial specification. Here we show that in the imaginal discs, reduced function of Iroquois genes promotes cell proliferation by accelerating the G1 to S transition. Conversely, their increased expression causes cell-cycle arrest, down-regulating the activity of the Cyclin E/Cdk2 complex. We demonstrate that physical interaction of the Iroquois protein Caupolican with Cyclin E-containing protein complexes, through its IRO box and Cyclin-binding domains, underlies its activity in cell-cycle control. Thus, Drosophila Iroquois proteins are able to regulate cell-autonomously the growth of the territories they specify. Moreover, our results provide a molecular mechanism for a role of Iroquois/Irx genes as tumour suppressors.

The correct development of body organs, with their characteristic size and shape, requires the coordination of cell division and cell differentiation. Here we show that the Iroquois proteins (Irx in vertebrates) slow down cell division in the Drosophila imaginal discs, in addition to their well-known role in cell fate and territorial specification. In humans, inactivating mutations at the Irx genes are associated to several types of cancer, thus allowing their classification as tumour suppressor genes. We have observed that Drosophila Iroquois genes similarly behave as tumour suppressor genes. Iroquois proteins belong to a family of homeodomain-containing transcriptional regulators. However, our results indicate that they control cell division by a transcription independent mechanism based on their physical interaction with Cyclin E containing complexes, a key player in cell-cycle progression. We have identified two evolutionary conserved domains of Iroquois proteins, different from the homeodomain, involved in that interaction. This new function of Iroquois proteins places them in a key position to coordinate growth and differentiation during normal development. Our results further suggest a molecular mechanism for their role in tumour suppression. Future studies of Irx genes should help to determine if a similar mechanism could operate to help cancer progression when Irx activity is compromised.

The Drosophila melanogaster Mutants apblot and apXasta Affect an Essential apterous Wing Enhancer

The selector gene apterous (ap) plays a key role during the development of the Drosophila melanogaster wing because it governs the establishment of the dorsal-ventral (D-V) compartment boundary. The D-V compartment boundary is known to serve as an important signaling center that is essential for the growth of the wing. The role of Ap and its downstream effectors have been studied extensively. However, very little is known about the transcriptional regulation of ap during wing disc development. In this study, we present a first characterization of an essential wing-specific ap enhancer. First, we defined an 874-bp fragment about 10 kb upstream of the ap transcription start that faithfully recapitulates the expression pattern of ap in the wing imaginal disc. Analysis of deletions in the ap locus covering this element demonstrated that it is essential for proper regulation of ap and formation of the wing. Moreover, we showed that the mutations ap^blot and ap^Xasta directly affect the integrity of this enhancer, leading to characteristic wing phenotypes. Furthermore, we engineered an in situ rescue system at the endogenous ap gene locus, allowing us to investigate the role of enhancer fragments in their native environment. Using this system, we were able to demonstrate that the essential wing enhancer alone is not sufficient for normal wing development. The in situ rescue system will allow us to characterize the ap regulatory sequences in great detail at the endogenous locus.

TGF-α ligands can substitute for the neuregulin Vein in Drosophila development

ErbB receptors, including the epidermal growth factor receptor (Egfr), are activated by EGF ligands to govern cell proliferation, survival, migration and differentiation. The different EGF-induced cell responses in development are regulated by deployment of multiple ligands. These inputs, however, engage only a limited number of intracellular pathways and are thought to elicit specific responses by regulating the amplitude or duration of the intracellular signal. The single Drosophila Egfr has four ligands: three of the TGF-α-type and a single neuregulin-like called vein (vn). Here, we used mutant combinations and gene replacement to determine the constraints of ligand specificity in development. Mutant analysis revealed extensive ligand redundancy in embryogenesis and wing development. Surprisingly, we found that the essential role of vn in development could be largely replaced by expression of any TGF-α ligand, including spitz (spi), in the endogenous vn pattern. vn mutants die as white undifferentiated pupae, but the rescued individuals showed global differentiation of adult body parts. Spi is more potent than Vn, and the best morphological rescue occurred when Spi expression was reduced to achieve an intracellular signaling level comparable to that produced by Vn. Our results show that the developmental repertoire of a strong ligand like Spi is flexible and at the appropriate level can emulate the activity of a weak ligand like Vn. These findings align with a model whereby cells respond similarly to an equivalent quantitative level of an intracellular signal generated by two distinct ligands regardless of ligand identity.

Emergence of tissue sensitivity to Hox protein levels underlies the evolution of an adaptive morphological trait

Growth control scales morphological attributes and, therefore, provides a critical contribution to the evolution of adaptive traits. Yet, the genetic mechanisms underlying growth in the context of specific ecological adaptations are poorly understood. In water striders, adaptation to locomotion on the water surface is associated with allometric and functional changes in thoracic appendages, such that T2-legs, used as propelling oars, are longer than T3-legs, used as steering rudders. The Hox gene Ubx establishes this derived morphology by elongating T2-legs but shortening T3-legs. Using gene expression assays, RNAi knockdown, and comparative transcriptomics, we demonstrate that the evolution of water surface rowing as a novel means of locomotion is associated with the evolution of a dose-dependent promoting-repressing effect of Ubx on leg growth. In the water strider Limnoporus dissortis, T3-legs express six to seven times higher levels of Ubx compared to T2-legs. Ubx RNAi shortens T2-legs and the severity of this phenotype increases with increased depletion of Ubx protein. Conversely, Ubx RNAi lengthens T3-legs but this phenotype is partially rescued when Ubx protein is further depleted. This dose-dependent effect of Ubx on leg growth is absent in non-rowing relatives that retain the ancestral relative leg length. We also show that the spatial patterns of expression of dpp, wg, hh, egfr, dll, exd, hth, and dac are unchanged in Ubx RNAi treatments. This indicates that the dose-dependent opposite effect of Ubx on T2- and T3-legs operates without any apparent effect on the spatial expression of major leg patterning genes. Our data suggest that scaling of adaptive allometries can evolve through changes in the levels of expression of Hox proteins early during ontogeny, and in the sensitivity of the tissues that express them, without any major effects on pattern formation.

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Ubx is generally expressed at higher levels in T3- relative to T2-legs in semi-aquatic insects.

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It is only in the derived Gerridae where the high levels of Ubx result in reduced T3-leg length.

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In the Gerridae, the response of leg tissues to Ubx levels is bimodal.

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Changes in Ubx regulation and function have evolved in Limnoporus without disrupting patterning hierarchies.

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Changes in Hox protein levels and emergence of tissue sensitivity to these levels can shape adaptive morphological traits.

Tay bridge is a negative regulator of EGFR signalling and interacts with Erk and Mkp3 in the Drosophila melanogaster wing

The regulation of Extracellular regulated kinase (Erk) activity is a key aspect of signalling by pathways activated by extracellular ligands acting through tyrosine kinase transmembrane receptors. In this process, participate proteins with kinase activity that phosphorylate and activate Erk, as well as different phosphatases that inactivate Erk by de-phosphorylation. The state of Erk phosphorylation affects not only its activity, but also its subcellular localization, defining the repertoire of Erk target proteins, and consequently, the cellular response to Erk. In this work, we characterise Tay bridge as a novel component of the EGFR/Erk signalling pathway. Tay bridge is a large nuclear protein with a domain of homology with human AUTS2, and was previously identified due to the neuronal phenotypes displayed by loss-of-function mutations. We show that Tay bridge antagonizes EGFR signalling in the Drosophila melanogaster wing disc and other tissues, and that the protein interacts with both Erk and Mkp3. We suggest that Tay bridge constitutes a novel element involved in the regulation of Erk activity, acting as a nuclear docking for Erk that retains this protein in an inactive form in the nucleus.

Extracellular regulated kinases (Erk) mediate signalling by pathways activated by tyrosine kinase transmembrane receptors. The level of activated Erk depends on a highly regulated balance between cytoplasmic kinases and nuclear/cytoplasmic phosphatases, which determine the state of Erk phosphorylation. This affects Erk activity and its subcellular localization, defining the repertoire of Erk targets, and consequently, the cellular response to Erk. In this work, we use a genetic approach to characterise the gene tay bridge as a novel component of the EGFR/Erk signalling pathway. Tay bridge has a domain of homology with human AUTS2, and was previously identified due to the neuronal phenotypes displayed by loss-of-function mutations. We show that Tay bridge antagonizes EGFR signalling in the Drosophila melanogaster wing disc and other tissues, and that the protein interacts with both Erk and Mkp3. We suggest that Tay bridge constitutes a novel element involved in the regulation of Erk activity, acting as a nuclear docking for Erk that retains this protein in an inactive form in the nucleus. These results could provide important insights into the clinical consequences of AUTS2 mutations in humans, which are related to behavioural perturbations including autism, mental retardation, Attention Deficit Hyperactivity Disorder and alcohol drinking behaviour.

The Iroquois complex is required in the dorsal mesoderm to ensure normal heart development in Drosophila

Drosophila heart development is an invaluable system to study the orchestrated action of numerous factors that govern cardiogenesis. Cardiac progenitors arise within specific dorsal mesodermal regions that are under the influence of temporally coordinated actions of multiple signaling pathways. The Drosophila Iroquois complex (Iro-C) consists of the three homeobox transcription factors araucan (ara), caupolican (caup) and mirror (mirr). The Iro-C has been shown to be involved in tissue patterning leading to the differentiation of specific structures, such as the lateral notum and dorsal head structures and in establishing the dorsal-ventral border of the eye. A function for Iro-C in cardiogenesis has not been investigated yet. Our data demonstrate that loss of the whole Iro complex, as well as loss of either ara/caup or mirr only, affect heart development in Drosophila. Furthermore, the data indicate that the GATA factor Pannier requires the presence of Iro-C to function in cardiogenesis. Furthermore, a detailed expression pattern analysis of the members of the Iro-C revealed the presence of a possibly novel subpopulation of Even-skipped expressing pericardial cells and seven pairs of heart-associated cells that have not been described before. Taken together, this work introduces Iro-C as a new set of transcription factors that are required for normal development of the heart. As the members of the Iro-C may function, at least partly, as competence factors in the dorsal mesoderm, our results are fundamental for future studies aiming to decipher the regulatory interactions between factors that determine different cell fates in the dorsal mesoderm.

JAK/STAT signaling is required for hinge growth and patterning in the Drosophila wing disc

JAK/STAT signaling is localized to the wing hinge, but its function there is not known. Here we show that the Drosophila STAT Stat92E is downstream of Homothorax and is required for hinge development by cell-autonomously regulating hinge-specific factors. Within the hinge, Stat92E activity becomes restricted to gap domain cells that lack Nubbin and Teashirt. While gap domain cells lacking Stat92E have significantly reduced proliferation, increased JAK/STAT signaling there does not expand this domain. Thus, this pathway is necessary but not sufficient for gap domain growth. We show that reduced Wingless (Wg) signaling dominantly inhibits Stat92E activity in the hinge. However, ectopic JAK/STAT signaling does not perturb Wg expression in the hinge. We report negative interactions between Stat92E and the notum factor Araucan, resulting in restriction of JAK/STAT signaling from the notum. In addition, we find that the distal factor Nub represses the ligand unpaired as well as Stat92E activity. These data suggest that distal expansion of JAK/STAT signaling is deleterious to wing blade development. Indeed, mis-expression of Unpaired within the presumptive wing blade causes small, stunted adult wings. We conclude that JAK/STAT signaling is critical for hinge fate specification and growth of the gap domain and that its restriction to the hinge is required for proper wing development.

Epithelial neoplasia in Drosophila entails switch to primitive cell states

Only select cell types in an organ display neoplasia when targeted oncogenically. How developmental lineage hierarchies of these cells prefigure their neoplastic propensities is not yet well-understood. Here we show that neoplastic Drosophila epithelial cells reverse their developmental commitments and switch to primitive cell states. In a context of alleviated tissue surveillance, for example, loss of Lethal giant larvae (Lgl) tumor suppressor in the wing primordium induced epithelial neoplasia in its Homothorax (Hth)-expressing proximal domain. Transcriptional profile of proximally transformed mosaic wing epithelium and functional tests revealed tumor cooperation by multiple signaling pathways. In contrast, lgl(-) clones in the Vestigial (Vg)-expressing distal wing epithelium were eliminated by cell death. Distal lgl(-) clones, however, could transform when both tissue surveillance and cell death were compromised genetically and, alternatively, when the transcription cofactor of Hippo signaling pathway, Yorkie (Yki), was activated, or when Ras/EGFR signaling was up-regulated. Furthermore, transforming distal lgl(-) clones displayed loss of Vg, suggesting reversal of their terminal cell fate commitment. In contrast, reinforcing a distal (wing) cell fate commitment in lgl(-) clones by gaining Vg arrested their neoplasia and induced cell death. We also show that neoplasia in both distal and proximal lgl(-) clones could progress in the absence of Hth, revealing Hth-independent wing epithelial neoplasia. Likewise, neoplasia in the eye primordium resulted in loss of Elav, a retinal cell marker; these, however, switched to an Hth-dependent primitive cell state. These results suggest a general characteristic of "cells-of-origin" in epithelial cancers, namely their propensity for switch to primitive cell states.

Essential roles for stat92E in expanding and patterning the proximodistal axis of the Drosophila wing imaginal disc

The Drosophila wing imaginal disc is subdivided along the proximodistal axis into the distal pouch, the hinge, the surrounding pleura, and the notum. While the genetic pathways that specify the identity of each of these domains have been well studied, the mechanisms that coordinate the relative expansion of these domains are not well understood. Here we investigated the role of the stat92E signal transducer and activator of transcription in wing proximodistal development. We find that stat92E is active ubiquitously in early wing imaginal discs, where it acts to inhibit the induction of ectopic wing fields. Subsequently, stat92E activity is down regulated in the notum and distal pouch. These dynamics coincide with and contribute to the proportional subdivision and expansion of these primordia. As development proceeds, stat92E activity becomes restricted to the hinge, where it promotes normal expansion of the hinge, and restricts expansion of the notum. We also find that stat92E is required autonomously to specify dorsal pleura identity and inhibit notum identity to properly subdivide the body wall. Our data suggest that stat92E activity is regulated along the proximodistal axis to pattern this axis and control the relative expansion of the pouch, hinge, and notum.

odd-skipped genes and lines organize the notum anterior-posterior axis using autonomous and non-autonomous mechanisms

The growth and patterning of Drosophila wing and notum primordia depend on their subdivision into progressively smaller domains by secreted signals that emanate from localized sources termed organizers. While the mechanisms that organize the wing primordium have been studied extensively, those that organize the notum are incompletely understood. The genes odd-skipped (odd), drumstick (drm), sob, and bowl comprise the odd-skipped family of C(2)H(2) zinc finger genes, which has been implicated in notum growth and patterning. Here we show that drm, Bowl, and eyegone (eyg), a gene required for notum patterning, accumulate in nested domains in the anterior notum. Ectopic drm organized the nested expression of these anterior notum genes and downregulated the expression of posterior notum genes. The cell-autonomous induction of Bowl and Eyg required bowl, while the non-autonomous effects were independent of bowl. The homeodomain protein Bar is expressed along the anterior border of the notum adjacent to cells expressing the Notch (N) ligand Delta (Dl). bowl was required to promote Bar and repress Dl expression to pattern the anterior notum in a cell-autonomous manner, while lines acted antagonistically to bowl posterior to the Bowl domain. Our data suggest that the odd-skipped genes act at the anterior notum border to organize the notum anterior-posterior (AP) axis using both autonomous and non-autonomous mechanisms.

Mutual repression by bantam miRNA and Capicua links the EGFR/MAPK and Hippo pathways in growth control

BACKGROUND

The epidermal growth factor receptor (EGFR) and Hippo signaling pathways control cell proliferation and apoptosis to promote tissue growth during development. Misregulation of these pathways is implicated in cancer. Our understanding of the mechanisms that integrate the activity of these pathways remains fragmentary. This study identifies bantam microRNA as a common target of these pathways and suggests a mechanistic link between them.

RESULTS

The EGFR pathway acts through bantam to control tissue growth. bantam expression is regulated by the EGFR pathway, acting via repression of the transcriptional repressor Capicua. Thus EGFR signaling induces bantam expression by alleviating the effects of a repressor. bantam in turn acts in a negative feedback loop to limit Capicua expression.

CONCLUSIONS

bantam appears to be a transcriptional target of both the EGFR and Hippo growth control pathways. Feedback regulation by bantam on Capicua provides a means to link signal propagation by the EGFR pathway to activity of the Hippo pathway and may play an important role in integration of these two pathways in growth control.

Ligand-independent traffic of Notch buffers activated Armadillo in Drosophila

Full-length Notch receptor binds to the Wnt pathway effector β-catenin and mediates its endocytosis and degradation, demonstrating a novel mechanism by which Notch may modulate Wnt pathway activity.

Notch receptors act as ligand-dependent membrane-tethered transcription factors with a prominent role in binary cell fate decisions during development, which is conserved across species. In addition there is increasing evidence for other functions of Notch, particularly in connection with Wnt signalling: Notch is able to modulate the activity of Armadillo/ß-catenin, the effector of Wnt signalling, in a manner that is independent of its transcriptional activity. Here we explore the mechanism of this interaction in the epithelium of the Drosophila imaginal discs and find that it is mediated by the ligand-independent endocytosis and traffic of the Notch receptor. Our results show that Notch associates with Armadillo near the adherens junctions and that it is rapidly endocytosed promoting the traffic of an activated form of Armadillo into endosomal compartments, where it may be degraded. As Notch has the ability to interact with and downregulate activated forms of Armadillo, it is possible that in vivo Notch regulates the transcriptionally competent pool of Armadillo. These interactions reveal a previously unknown activity of Notch, which serves to buffer the function of activated Armadillo and might underlie some of its transcription-independent effects.

Establishment of the correct shape and pattern of tissues within an organism requires the integration of molecular information present in signalling and transcriptional networks and demands delicate exchanges and balances of their activities. A large body of experimental work has revealed close correlations in the activities of two pathways: Notch and Wnt, which suggest the existence of multiple links between them. Notch signalling relies in part upon the activity of the Notch protein, a membrane-bound receptor with a transcription factor domain that can be released from the membrane by proteolytic cleavage. On the other hand Wnt proteins are ligands that trigger changes in the activity of ß-catenin, which is called Armadillo in the fruit fly Drosophila melanogaster. In this study we uncover a previously unknown activity for Notch: endocytosis and trafficking of full length Notch, which targets Armadillo for degradation. This activity of Notch is independent of its ligands, Delta and Serrate, and of its downstream effector, the transcription factor Suppressor of Hairless. We further show that in the absence of Notch, which has been shown to act as a tumor suppressor in mammals, expression of an activated form of Armadillo causes tissue overgrowth and changes in the polarity of cells. Our results suggest that Drosophila Notch can promote the degradation of activated forms of Armadillo and may buffer cells against fluctuations in Wnt signalling activity.

Lines is required for normal operation of Wingless, Hedgehog and Notch pathways during wing development

The regulatory Lines/Drumstick/Bowl gene network is implicated in the integration of patterning information at several stages during development. Here, we show that during Drosophila wing development, Lines prevents Bowl accumulation in the wing primordium, confining its expression to the peripodial epithelium. In cells that lack lines or over-expressing Drumstick, Bowl stabilization is responsible for alterations such as dramatic overgrowths and cell identity changes in the proximodistal patterning owing to aberrant responses to signaling pathways. The complex phenotypes are explained by Bowl repressing the Wingless pathway, the earliest effect seen. In addition, Bowl sequesters the general co-repressor Groucho from repressor complexes functioning in the Notch pathway and in Hedgehog expression, leading to ectopic activity of their targets. Supporting this model, elimination of the Groucho interaction domain in Bowl prevents the activation of the Notch and Hedgehog pathways, although not the repression of the Wingless pathway. Similarly, the effects of ectopic Bowl are partially rescued by co-expression of either Hairless or Master of thickveins, co-repressors that act with Groucho in the Notch and Hedgehog pathways, respectively. We conclude that by preventing Bowl accumulation in the wing, primordial Lines permits the correct balance of nuclear co-repressors that control the activity of the Wingless, Notch and Hedgehog pathways.

Zinc-finger paralogues tsh and tio are functionally equivalent during imaginal development in Drosophila and maintain their expression levels through auto- and cross-negative feedback loops

teashirt (tsh) and tiptop (tio) are two Drosophila gene paralogues encoding zinc-finger transcription factors. While tsh is an important developmental regulator, tio null individuals are viable and fertile. Here, we show that tio and tsh have coincident expression domains in the imaginal discs, the precursors of the adult body, and that both genes show similar functional properties when expressed ectopically. Furthermore, tio is able to rescue the development of tsh mutants, indicating that both genes are functionally equivalent during imaginal development. Of interest, the transcriptional regulation of tio and tsh is linked by a negative feedback loop. This mechanism might be required to maintain a tight control on the total levels of tio/tsh and could help explaining why Drosophila keeps an apparently dispensable gene.

Spalt major controls the development of the notum and of wing hinge primordia of the Drosophila melanogaster wing imaginal disc

The Drosophila wing and the dorsal thorax develop from primordia within the wing imaginal disc. Here we show that spalt major (salm) is expressed within the presumptive dorsal body wall primordium early in wing disc development to specify notum and wing hinge tissue. Upon ectopic salm expression, dorsally located second leg disc cells develop notum and wing hinge tissue instead of sternopleural tissue. Similarly, by salm over-expression within the wing disc, wing blade formation is suppressed and a mirror-image duplication of the notum and wing hinge is formed. In large dorsal clones, which lack salm and its neighboring paralogue spalt related (salr), the cells of the notum primordium do not grow; these dorsal cells are not specified as notum, hence no notum outgrowth develops. These results suggest that the zinc finger factors encoded by the salm/salr complex play important roles in defining cells of the early wing disc as dorsal body wall cells, which develop into a large dorsal body wall territory and form mesonotum and some wing hinge tissue, and in delimiting the wing primordium. We also find that salm activity is down-regulated by its own product and by that of the Pax gene eyegone.

Reciprocal roles for bowl and lines in specifying the peripodial epithelium and the disc proper of the Drosophila wing primordium

Central to embryonic development is the generation of molecular asymmetries across fields of undifferentiated cells. The Drosophila wing imaginal disc provides a powerful system with which to understand how such asymmetries are generated and how they contribute to formation of a complex structure. Early in development, the wing primordium is subdivided into a thin layer of peripodial epithelium (PE) and an apposing thickened layer of pseudostratified columnar epithelium (CE), known as the disc proper (DP). The DP gives rise to the wing blade, hinge and dorsal mesothorax, whereas the PE makes only a minor contribution to the ventral hinge and pleura. The mechanisms that generate this major asymmetry and its contribution to wing development are poorly understood. The Lines protein destabilizes the nuclear protein Bowl in ectodermal structures. Here, we show that Bowl accumulates in the PE from early stages of wing development and is absent from the DP. Broad inhibition of Bowl in the PE resulted in the replacement of the PE with a mirror image duplication of the DP. The failure to generate the PE severely compromised wing growth and the formation of the notum. Conversely, the activation of bowl in the DP (by removal or inhibition of lines function) resulted in the transformation of the DP into PE. Thus, we provide evidence that bowl and lines act as a binary switch to subdivide the wing primordium into PE and DP, and assign crucial roles for this asymmetry in wing growth and patterning.

Apposition of iroquois expressing and non-expressing cells leads to cell sorting and fold formation in the Drosophila imaginal wing disc

Background

The organization of the different tissues of an animal requires mechanisms that regulate cell-cell adhesion to promote and maintain the physical separation of adjacent cell populations. In the Drosophila imaginal wing disc the iroquois homeobox genes are expressed in the notum anlage and contribute to the specification of notum identity. These genes are not expressed in the adjacent wing hinge territory. These territories are separated by an approximately straight boundary that in the mature disc is associated with an epithelial fold. The mechanism by which these two cell populations are kept separate is unclear.

Results

Here we show that the Iro-C genes participate in keeping the notum and wing cell populations separate. Indeed, within the notum anlage, cells not expressing Iro-C tend to join together and sort out from their Iro-C expressing neighbours. We also show that apposition of Iro-C expressing and non-expressing cells induces invagination and apico-basal shortening of the Iro-C^- cells. This effect probably underlies formation of the fold that separates the notum and wing hinge territories. In addition, cells overexpressing a member of the Iro-C contact one another and become organized in a network of thin strings that surrounds and isolates large groups of non-overexpressing cells. The strings appear to exert a pulling force along their longitudinal axis.

Conclusion

Apposition of cells expressing and non-expressing the Iro-C, as it occurs in the notum-wing hinge border of the Drosophila wing disc, influences cell behaviour. It leads to cell sorting, and cellular invagination and apical-basal shortening. These effects probably account for keeping the prospective notum and wing hinge cell populations separate and underlie epithelial fold formation. Cells that overexpress a member of the Iro-C and that confront non-expressing cells establish contacts between themselves and become organized in a network of thin strings. This is a complex and unique phenotype that might be important for the generation of a straight notum-wing hinge border.

Recruitment of cells into the Drosophila wing primordium by a feed-forward circuit of vestigial autoregulation

The Drosophila wing primordium is defined by expression of the selector gene vestigial (vg) in a discrete subpopulation of cells within the wing imaginal disc. Following the early segregation of the disc into dorsal (D) and ventral (V) compartments, vg expression is governed by signals generated along the boundary between the two compartments. Short-range DSL (Delta/Serrate/LAG-2)-Notch signaling between D and V cells drives vg expression in ;border' cells that flank the boundary. It also induces these same cells to secrete the long-range morphogen Wingless (Wg), which drives vg expression in surrounding cells up to 25-30 cell diameters away. Here, we show that Wg signaling is not sufficient to activate vg expression away from the D-V boundary. Instead, Wg must act in combination with a short-range signal produced by cells that already express vg. We present evidence that this vg-dependent, vg-inducing signal feeds forward from one cell to the next to entrain surrounding cells to join the growing wing primordium in response to Wg. We propose that Wg promotes the expansion of the wing primordium following the D-V segregation by fueling this non-autonomous autoregulatory mechanism.

Antagonistic and cooperative actions of the EGFR and Dpp pathways on the iroquois genes regulate Drosophila mesothorax specification and patterning

In Drosophila, restricted expression of the Iroquois complex (Iro-C) genes in the proximal region of the wing imaginal disc contributes to its territorial subdivision, specifying first the development of the notum versus the wing hinge, and subsequently, that of the lateral versus medial notum. Iro-C expression is under the control of the EGFR and Dpp signalling pathways. To analyze how both pathways cooperate in the regulation of Iro-C, we isolated several wing disc-specific cis-regulatory elements of the complex. One of these (IroRE(2)) integrates competing inputs of the EGFR and Dpp pathways, mediated by the transcription factors Pointed (downstream of EGFR pathway) and Pannier/U-shaped and Mothers against Dpp (Mad), in the case of Dpp. By contrast, a second element (IroRE(1)) mediates activation by both the EGFR and Dpp pathways, thus promoting expression of Iro-C in a region of elevated levels of Dpp signalling, the prospective lateral notum near the anterior-posterior compartment boundary. These results help define the molecular mechanisms of the interplay between the EGFR and Dpp pathways in the specification and patterning of the notum.

Ectopic expression of Tollo/Toll-8 antagonizes Dpp signaling and induces cell sorting in the Drosophila wing

The wing imaginal disc of Drosophila consists of the primordia for the adult wing and the body wall. The zinc-finger transcription factor Teashirt (Tsh) is expressed in the region proximal to the wing primordium and regulates the formation of the wing-body wall boundary. Here, we report that Tollo/Toll-8, a member of Toll family transmembrane proteins, is also expressed proximal to the wing domain. Ectopic expression of Decapentaplegic (Dpp), a morphogen for wing development, represses tollo expression in the proximal domain. Likewise, misexpression of Tollo in the presumptive wing strongly antagonizes the effects of Dpp signaling. The extracellular domain of Tollo containing the Leucine-Rich Repeats (LRR) is required for the inhibition of Dpp signaling in the wing. Furthermore, clones of cells with Tollo overexpression are sorted out from the surrounding wild-type cells, resulting in the formation of epithelial folds around the clone boundaries. Tsh is ectopically induced at the border of Tollo-expressing clones. Despite the strong effects of Tollo overexpression on Dpp signaling and cell sorting, loss-of-function tollo mutants are viable with normal external morphology. Our data suggest that Tollo function might be redundant but is sufficient to antagonize Dpp signaling and induce sorting of Tollo expressing cells from the wing cells to develop proximal cell fate.

Functional analysis of genes differentially expressed in the Drosophila wing disc: role of transcripts enriched in the wing region

Differential gene expression is the major mechanism underlying the development of specific body regions. Here we assessed the role of genes differentially expressed in the Drosophila wing imaginal disc, which gives rise to two distinct adult structures: the body wall and the wing. Reverse genetics was used to test the function of uncharacterized genes first identified in a microarray screen as having high levels of expression in the presumptive wing. Such genes could participate in elaborating the specific morphological characteristics of the wing. The activity of the genes was modulated using misexpression and RNAi-mediated silencing. Misexpression of eight of nine genes tested caused phenotypes. Of 12 genes tested, 10 showed effective silencing with RNAi transgenes, but only 3 of these had resulting phenotypes. The wing phenotypes resulting from RNAi suggest that CG8780 is involved in patterning the veins in the proximal region of the wing blade and that CG17278 and CG30069 are required for adhesion of wing surfaces. Venation and apposition of the wing surfaces are processes specific to wing development providing a correlation between the expression and function of these genes. The results show that a combination of expression profiling and tissue-specific gene silencing has the potential to identify new genes involved in wing development and hence to contribute to our understanding of this process. However, there are both technical and biological limitations to this approach, including the efficacy of RNAi and the role that gene redundancy may play in masking phenotypes.

Epidermal growth factor receptor and transforming growth factor-beta signaling contributes to variation for wing shape in Drosophila melanogaster

Wing development in Drosophila is a common model system for the dissection of genetic networks and their roles during development. In particular, the RTK and TGF-beta regulatory networks appear to be involved with numerous aspects of wing development, including patterning, cell determination, growth, proliferation, and survival in the developing imaginal wing disc. However, little is known as to how subtle changes in the function of these genes may contribute to quantitative variation for wing shape, per se. In this study 50 insertional mutations, representing 43 loci in the RTK, Hedgehog, TGF-beta pathways, and their genetically interacting factors were used to study the role of these networks on wing shape. To concurrently examine how genetic background modulates the effects of the mutation, each insertion was introgressed into two wild-type genetic backgrounds. Using geometric morphometric methods, it is shown that the majority of these mutations have profound effects on shape but not size of the wing when measured as heterozygotes. To examine the relationships between how each mutation affects wing shape hierarchical clustering was used. Unlike previous observations of environmental canalization, these mutations did not generally increase within-line variation relative to their wild-type counterparts. These results provide an entry point into the genetics of wing shape and are discussed within the framework of the dissection of complex phenotypes.

Palmitoylation of the EGFR Ligand Spitz by Rasp Increases Spitz Activity by Restricting Its Diffusion

Lipid modifications such as palmitoylation or myristoylation target intracellular proteins to cell membranes. Secreted ligands of the Hedgehog and Wnt families are also palmitoylated; this modification, which requires the related transmembrane acyltransferases Rasp and Porcupine, can enhance their secretion, transport, or activity. We show here that rasp is also essential for the developmental functions of Spitz, a ligand for the Drosophila epidermal growth factor receptor (EGFR). In cultured cells, Rasp promotes palmitate addition to the N-terminal cysteine residue of Spitz, and this cysteine is required for Spitz activity in vivo. Palmitoylation reduces Spitz secretion and enhances its plasma membrane association, but does not alter its ability to activate the EGFR in vitro. In vivo, overexpressed unpalmitoylated Spitz has an increased range of action but reduced activity. These data suggest a role for palmitoylation in restricting Spitz diffusion, allowing its local concentration to reach the threshold required for biological function.

Atrophin contributes to the negative regulation of epidermal growth factor receptor signaling in Drosophila

Dentato-rubral and pallido-luysian atrophy (DRPLA) is a dominant, progressive neurodegenerative disease caused by the expansion of polyglutamine repeats within the human Atrophin-1 protein. Drosophila Atrophin and its human orthologue are thought to function as transcriptional co-repressors. Here, we report that Drosophila Atrophin participates in the negative regulation of Epidermal Growth Factor Receptor (EGFR) signaling both in the wing and the eye imaginal discs. In the wing pouch, Atrophin loss of function clones induces cell autonomous expression of the EGFR target gene Delta, and the formation of extra vein tissue, while overexpression of Atrophin inhibits EGFR-dependent vein formation. In the eye, Atrophin cooperates with other negative regulators of the EGFR signaling to prevent the differentiation of surplus photoreceptor cells and to repress Delta expression. Overexpression of Atrophin in the eye reduces the EGFR-dependent recruitment of cone cells. In both the eye and wing, epistasis tests show that Atrophin acts downstream or in parallel to the MAP kinase rolled to modulate EGFR signaling outputs. We show that Atrophin genetically cooperates with the nuclear repressor Yan to inhibit the EGFR signaling activity. Finally, we have found that expression of pathogenic or normal forms of human Atrophin-1 in the wing promotes wing vein differentiation and acts as dominant negative proteins inhibiting endogenous fly Atrophin activity.

The egghead gene is required for compartmentalization in Drosophila optic lobe development

The correct targeting of photoreceptor neurons (R-cells) in the developing Drosophila visual system requires multiple guidance systems in the eye-brain complex as well as the precise organization of the target area. Here, we report that the egghead (egh) gene, encoding a glycosyltransferase, is required for a compartment boundary between lamina glia and lobula cortex, which is necessary for appropriate R1-R6 innervation of the lamina. In the absence of egh, R1-R6 axons form a disorganized lamina plexus and some R1-R6 axons project abnormally to the medulla instead of the lamina. Mosaic analysis demonstrates that this is not due to a loss of egh function in the eye or in the neurons and glia of the lamina. Rather, as indicated by clonal analysis and cell-specific genetic rescue experiments, egh is required in cells of the lobula complex primordium which transiently abuts the lamina and medulla in the developing larval brain. In the absence of egh, perturbation of sheath-like glial processes occurs at the boundary region delimiting lamina glia and lobula cortex, and inappropriate invasion of lobula cortex cells across this boundary region disrupts the pattern of lamina glia resulting in inappropriate R1-R6 innervation. This finding underscores the importance of the lamina/lobula compartment boundary in R1-R6 axon targeting.

Studies on epidermal growth factor receptor signaling in vertebrate limb patterning

The epidermal growth factor receptor (EGFR) regulates multiple patterning events in Drosophila limb development, but its role in vertebrate limb morphogenesis has received little attention. The EGFR and several of its ligands are expressed in developing vertebrate limbs in manners consistent with potential patterning roles. To gain insight into functions of EGFR signaling in vertebrate limb development, we expressed a constitutively active EGFR in developing chick limbs in ovo. Expression of activated EGFR causes pre- and postaxial polydactyly, including mirror-image-type digit duplication, likely due to induction of ectopic expression and/or modulation of genes involved in anterior-posterior (AP) patterning such as Sonic hedgehog (Shh), dHand, Patched (Ptc), Gli3, Hoxd13, Hoxd11, bone morphogenetic protein 2 (Bmp2), Gremlin, and FGF4. Activation of EGFR signaling dorsalizes the limb and alters expression of the dorsal-ventral (DV) patterning genes Wnt7a, Lmx, and En1. Ectopic and/or extended FGF8 expressing apical ectodermal ridges (AERs) are also seen. Interdigital regression is inhibited and the digits fail to separate, leading to syndactyly, likely due to antiapoptotic and pro-proliferative effects of activated EGFR signaling on limb mesoderm, and/or attenuation of interdigital Bmp4 expression. These findings suggest potential roles for EGFR signaling in AP and DV patterning, AER formation, and cell survival during limb morphogenesis.

Mutual repression between msh and Iro-C is an essential component of the boundary between body wall and wing in Drosophila

During development, the imaginal wing disc of Drosophila is subdivided into territories separated by developmental boundaries. The best characterized boundaries delimit compartments defined by cell-lineage restrictions. Here, we analyze the formation of a boundary that does not rely on such restrictions, namely, that which separates the notum (body wall) and the wing hinge (appendage). It is known that the homeobox genes of the Iroquois complex (Iro-C) define the notum territory and that the distal limit of the Iro-C expression domain demarks the boundary between the notum and the wing hinge. However, it is unclear how this boundary is established and maintained. We now find that msh, a homeobox gene of the Msx family, is strongly expressed in the territory of the hinge contiguous to the Iro-C domain. Loss- and gain-of-function analyses show that msh maintains Iro-C repressed in the hinge, while Iro-C prevents high level expression of msh in the notum. Thus, a mutual repression between msh and Iro-C is essential to set the limit between the contiguous domains of expression of these genes and therefore to establish and/or maintain the boundary between body wall and wing. In addition, we find that msh is necessary for proper growth of the hinge territory and the differentiation of hinge structures. msh also participates in the patterning of the notum, where it is expressed at low levels.

Species-specific color-pattern modifications of butterfly wings

We have previously shown that the systemic injection of sodium tungstate, a general protein-tyrosine phosphatase (PTPase) inhibitor, efficiently produces characteristic color-pattern modifications on the wings of the Painted Lady butterfly, Vanessa cardui. By using this method in the present study, we analyzed modification patterns of six species of Japanese butterflies. Whereas in Vanessa indica the black spots on the forewings reduced in size in response to the treatment, in Lycaena phlaeas the morphologically similar black spots enlarged in size. However, the metallic blue spots on the forewings of V. indica did enlarge in size, showing different behavior even within a single wing surface. The response patterns of Ypthima argus differed markedly from those of other species in that ectopic color-pattern elements were created. Colias erate showed minor modifications that coincidentally resembled the natural color-pattern of a closely related species, Colias palaeno. Through a comprehensive literature search, we confirmed the existence of naturally occurring aberrant color patterns with close similarities to the experimentally induced phenocopies in each of the modified species. Our results point out the possibility that a hypothetical transduction pathway with a PTPase for the scale-cell differentiation globally coordinates the wing-wide color-pattern development in butterflies.

Nucleotide variation in the Egfr locus of Drosophila melanogaster

The Epidermal growth factor receptor is an essential gene with diverse pleiotropic roles in development throughout the animal kingdom. Analysis of sequence diversity in 10.9 kb covering the complete coding region and 6.4 kb of potential regulatory regions in a sample of 250 alleles from three populations of Drosophila melanogaster suggests that the intensity of different population genetic forces varies along the locus. A total of 238 independent common SNPs and 20 indel polymorphisms were detected, with just six common replacements affecting >1475 amino acids, four of which are in the short alternate first exon. Sequence diversity is lowest in a 2-kb portion of intron 2, which is also highly conserved in comparison with D. simulans and D. pseudoobscura. Linkage disequilibrium decays to background levels within 500 bp of most sites, so haplotypes are generally restricted to up to 5 polymorphisms. The two North American samples from North Carolina and California have diverged in allele frequency at a handful of individual SNPs, but a Kenyan sample is both more divergent and more polymorphic. The effect of sample size on inference of the roles of population structure, uneven recombination, and weak selection in patterning nucleotide variation in the locus is discussed.

Conserved cross-interactions inDrosophilaandXenopusbetween Ras/MAPK signaling and the dual-specificity phosphatase MKP3

The extracellular signal-regulated kinase (ERK) is a key transducer of the epidermal growth factor receptor (EGFR) and fibroblast growth factor receptor (FGFR) signaling pathways, and its function is required in multiple processes during animal development. The activity of ERK depends on the phosphorylation state of conserved threonine and tyrosine residues, and this state is regulated by different kinases and phosphatases. A family of phosphatases with specificity toward both threonine and tyrosine residues in ERK (dual-specificity phosphatases) play a conserved role in its dephosphorylation and consequent inactivation. Here, we characterize the function of the dual-specificity phosphatase MKP3 in Drosophila EGFR and Xenopus FGFR signaling. The function of MKP3 is required during Drosophila wing vein formation and Xenopus anteroposterior neural patterning. We find that the expression of the MKP3 gene is localized in places of high EGFR and FGFR signaling. Furthermore, this restricted expression depends on ERK function both in Drosophila and Xenopus, suggesting that MKP3 constitutes a conserved negative feedback loop on the activity of the Ras/ERK signaling pathway.

Differing strategies for the establishment and maintenance ofteashirtandhomothoraxrepression in theDrosophilawing

Secreted signaling molecules such as Wingless (Wg) and Decapentaplegic(Dpp) organize positional information along the proximodistal (PD) axis of the Drosophila wing imaginal disc. Responding cells activate different downstream targets depending on the combination and level of these signals and other factors present at the time of signal transduction. Two such factors, teashirt (tsh) and homothorax (hth), are initially co-expressed throughout the entire wing disc, but are later repressed in distal cells, permitting the subsequent elaboration of distal fates. Control of tsh and hth repression is, therefore,crucial for wing development, and plays a role in shaping and sizing the adult appendage. Although both Wg and Dpp participate in this control, their specific contributions remain unclear. In this report, we analyze tshand hthregulation in the wing disc, and show that Wg and Dpp act independently as the primary signals for the repression of tsh and hth, respectively. In cells that receive low levels of Dpp, hth repression also requires Vestigial (Vg). Furthermore, although Dpp is required continuously for hth repression throughout development, Wg is only required for the initiation of tshrepression. Instead, the maintenance of tsh repression requires Polycomb group (PcG) mediated gene silencing, which is dispensable for hth repression. Thus, despite their overall similar expression patterns, tsh and hth repression in the wing disc is controlled by two very different mechanisms.

Color-pattern Modifications and Speciation in Butterflies of the Genus Vanessa and its Related Genera Cynthia and Bassaris

We have previously shown that the systemic injection of sodium tungstate, a protein-tyrosine phosphatase (PTPase) inhibitor, to pupae immediately after pupation efficiently produces characteristic color-pattern modifications on the wings of many species of butterflies including Vanessa indica and Cynthia cardui. In these species, the black spots reduced in size in response to the treatment. Similar modifications are occasionally seen in the field-caught aberrant individuals. Exceptionally, however, a C. cardui individual with enlarged black spots ("reversed" modification pattern) has been reported. Here we show that these modified patterns of V. indica and C. cardui are quite similar to the normal color-patterns of other Vanessa species. V. indica with tungstate-induced modifications resembled V. tameamea, V. samani, and Bassaris itea, whereas V. dilecta, V. atalanta, and V. dejeanii are similar to the "reversed" individual. Most features seen in the experimentally-modified V. indica were observed throughout the fore- and hindwings of V. samani. In contrast, the experimentally-induced color-patterns of C. cardui did not parallel variation of Cynthia butterflies. Since it has been proposed that a hypothetical transduction pathway with a PTPase for the scale-cell differentiation globally coordinates the wing-wide color-patterns, our findings suggest that spontaneous mutations in genes in this hypothetical pathway might have played a major role in creating new color-patterns and species in the Vanessa genus but not in the Cynthia genus. This evolutionary mechanism may probably be shared more widely in Lepidoptera, although this would not be a sole determinant for the color-pattern development and evolution.

Proximodistal subdivision of Drosophila legs and wings: the elbow-no ocelli gene complex

Appendages are thought to have arisen during evolution as outgrowths from the body wall of primitive bilateria. In Drosophila, subsets of body wall cells are set aside as appendage precursors through the action of secreted signaling proteins that direct localized expression of transcription factors. The Drosophila homeodomain protein Distal-less is expressed in the leg primordia and required for formation of legs, but not wings. The homeodomain protein Nubbin is expressed in the wing primordia and required for formation of wings, but not legs. Given that insect legs and wings have a common developmental and evolutionary origin, we sought to identify genes that underlie the specification of all appendage primordia. We present evidence that the zinc-finger proteins encoded by the elbow and no ocelli genes act in leg and wing primordia to repress body wall-specifying genes and thereby direct appendage formation.