Drawing lines in the Drosophila wing: initiation of wing vein development

Transcription factors underlying wing margin color patterns and pupal cuticle markings in butterflies

Background

The diversity of butterfly color patterns can be attributed to a relatively small number of pattern elements that are homologous across Lepidoptera. Although genes involved in patterning some of these elements have been identified, the development of several major elements remains poorly understood. To identify genes underlying wing pupal cuticle markings and wing margin color patterns, we examined expression of the candidate transcription factors Engrailed/Invected (En/Inv), Distal-less (Dll), Cubitus interruptus (Ci), and Spalt in two nymphalids: Junonia coenia and Bicyclus anynana.

Results

We found that En/Inv, Dll, and Ci mark domains on the J. coenia last-instar forewing disc that closely correspond to the position and shape of pupal cuticle markings. We also found that Spalt demarcates wing margin color patterns in both J. coenia and B. anynana, and that CRISPR/Cas9 deletions in the spalt gene result in reduction and loss of wing margin color patterns in J. coenia. These data demonstrate a role for spalt in promoting wing margin color patterning, in addition to its previously described role in eyespot patterning.

Conclusion

Our observations support the model that a core set of regulatory genes are redeployed multiple times, and in multiple roles, during butterfly wing pattern development. Of these genes, spalt is of special interest as it plays a dual role in both eyespot and margin color pattern development.

The weakest link: Haploid honey bees are more susceptible to neonicotinoid insecticides

Neonicotinoid insecticides are currently of major concern for the health of wild and managed insects that provide key ecosystem services like pollination. Even though sublethal effects of neonicotinoids are well known, there is surprisingly little information on how they possibly impact developmental stability, and to what extent genetics are involved. This holds especially true for haploid individuals because they are hemizygous at detoxification loci and may be more susceptible. Here we take advantage of haplodiploidy in Western honey bees, Apis mellifera, to show for the first time that neonicotinoids affect developmental stability in diploid females (workers), and that haploid males (drones) are even more susceptible. Phenotypic fore wing venation abnormalities and fluctuating wing asymmetry, as measures of developmental instability, were significantly increased under field-realistic neonicotinoid-exposure of colonies. The higher susceptibility of haploid drones suggests that heterozygosity can play a key role in the ability to buffer the sublethal effects of neonicotinoids. Aiming to improve conservation efforts, our findings highlight the urgent need to better understand the role that genetics plays at enabling non-target organisms to cope with insecticide exposure.

Molecular mechanisms underlying simplification of venation patterns in holometabolous insects

How mechanisms of pattern formation evolve has remained a central research theme in the field of evolutionary and developmental biology. The mechanism of wing vein differentiation in Drosophila is a classic text-book example of pattern formation using a system of positional-information, yet very little is known about how species with a different number of veins pattern their wings, and how insect venation patterns evolved. Here, we examine the expression pattern of genes previously implicated in vein differentiation in Drosophila in two butterfly species with more complex venation Bicyclus anynana and Pieris canidia. We also test the function of some of these genes in B. anynana. We identify both conserved as well as new domains of decapentaplegic, engrailed, invected, spalt, optix, wingless, armadillo, blistered, and rhomboid gene expression in butterflies, and propose how the simplified venation in Drosophila might have evolved via loss of decapentaplegic, spalt and optix gene expression domains, silencing of vein inducing programs at Spalt-expression boundaries, and changes in gene expression of vein maintenance genes.

A Rapid and Efficient Method to Dissect Pupal Wings of Drosophila Suitable for Immunodetections or PCR Assays

Wing development in Drosophila melanogaster is an ideal model for studying morphogenesis at tissue level. These appendages develop from a group of cells named wing imaginal discs formed during embryonic development. In the larval stages the imaginal discs grow, increasing its number of cells and forming monolayered epithelial structures. Inside the pupal case, the imaginal discs bud out and fold into bilayers along a line that becomes the future margin of the wing. During this process, the longitudinal primodia veins originate vein cells on the prospective dorsal and ventral surfaces of the wing. During the pupal stage the stripes of vein cells of each surface communicate in order to generate tight tubes; at the same time, the cross-veins begin their formation. With the help of appropriate molecular markers, it is possible to identify the major elements composing the wing during its development. For this reason, the ability to accurately detect transcripts or proteins in this structure is critical for studying their abundance and localization related to the development process of the wing. The procedure described here focuses on manipulating pupal wings, providing detailed instructions on how to dissect the wing during the pupal stage. The dissection of pupal tissue is more difficult to perform than their counterparts in third instar larvae. This is why this approach was developed, to obtain rapid and efficient high quality samples. Details of how to immunostain and mount these wing samples, to allow the visualization of proteins or cell components, are provided in the protocol. With little expertise it is possible to collect 8-10 high quality pupal wings in a short amount of time.

CRISPR/Cas9 and active genetics-based trans-species replacement of the endogenous Drosophila kni-L2 CRM reveals unexpected complexity

The knirps (kni) locus encodes transcription factors required for induction of the L2 wing vein in Drosophila. Here, we employ diverse CRISPR/Cas9 genome editing tools to generate a series of targeted lesions within the endogenous cis-regulatory module (CRM) required for kni expression in the L2 vein primordium. Phenotypic analysis of these 'in locus' mutations based on both expression of Kni protein and adult wing phenotypes, reveals novel unexpected features of L2-CRM function including evidence for a chromosome pairing-dependent process that promotes transcription. We also demonstrate that self-propagating active genetic elements (CopyCat elements) can efficiently delete and replace the L2-CRM with orthologous sequences from other divergent fly species. Wing vein phenotypes resulting from these trans-species enhancer replacements parallel features of the respective donor fly species. This highly sensitive phenotypic readout of enhancer function in a native genomic context reveals novel features of CRM function undetected by traditional reporter gene analysis.

What serial homologs can tell us about the origin of insect wings

Although the insect wing is a textbook example of morphological novelty, the origin of insect wings remains a mystery and is regarded as a chief conundrum in biology. Centuries of debates have culminated into two prominent hypotheses: the tergal origin hypothesis and the pleural origin hypothesis. However, between these two hypotheses, there is little consensus in regard to the origin tissue of the wing as well as the evolutionary route from the origin tissue to the functional flight device. Recent evolutionary developmental (evo-devo) studies have shed new light on the origin of insect wings. A key concept in these studies is “serial homology”. In this review, we discuss how the wing serial homologs identified in recent evo-devo studies have provided a new angle through which this century-old conundrum can be explored. We also review what we have learned so far from wing serial homologs and discuss what we can do to go beyond simply identifying wing serial homologs and delve further into the developmental and genetic mechanisms that have facilitated the evolution of insect wings.

The Deubiquitinase USP47 Stabilizes MAPK by Counteracting the Function of the N-end Rule ligase POE/UBR4 in Drosophila

RAS-induced MAPK signaling is a central driver of the cell proliferation apparatus. Disruption of this pathway is widely observed in cancer and other pathologies. Consequently, considerable effort has been devoted to understanding the mechanistic aspects of RAS-MAPK signal transmission and regulation. While much information has been garnered on the steps leading up to the activation and inactivation of core pathway components, comparatively little is known on the mechanisms controlling their expression and turnover. We recently identified several factors that dictate Drosophila MAPK levels. Here, we describe the function of one of these, the deubiquitinase (DUB) USP47. We found that USP47 acts post-translationally to counteract a proteasome-mediated event that reduces MAPK half-life and thereby dampens signaling output. Using an RNAi-based genetic interaction screening strategy, we identified UBC6, POE/UBR4, and UFD4, respectively, as E2 and E3 enzymes that oppose USP47 activity. Further characterization of POE-associated factors uncovered KCMF1 as another key component modulating MAPK levels. Together, these results identify a novel protein degradation module that governs MAPK levels. Given the role of UBR4 as an N-recognin ubiquitin ligase, our findings suggest that RAS-MAPK signaling in Drosophila is controlled by the N-end rule pathway and that USP47 counteracts its activity.

Spalt-mediated dve repression is a critical regulatory motif and coordinates with Iroquois complex in Drosophila vein formation

Veins are longitudinal cuticular structures that maintain shape of the wing. Drosophila melanogaster has six longitudinal veins (L1-L6) and two cross veins. The Zn-finger transcription factors of Spalt-complex (Sal) are required for positioning of the L2 and L5, and the homeodomain transcription factors of Iroquois complex (Iro-C) are required for formation of the L3 and L5 veins. The homeodomain transcriptional repressor Defective proventriculus (Dve) is uniformly expressed in the wing pouch of the larval imaginal disc. However, dve mutant wings showed loss of the L2 and L5, but not of the L3 and L4 veins. Temporal dve knockdown experiments indicate that the Dve activity is required for vein formation from late third larval instar to the prepupal stage. In the prepupal wing, Dve expression becomes nearly complementary to that of Sal through the Sal-mediated dve repression. Furthermore, coexpression of Dve and Iro-C relieved of Sal-mediated repression is required for the L5 formation in a dose-dependent manner. The relationship between Sal, Dve, and Iro-C in wing vein specification is quite similar to that in ommatidial cell-type specification. Our results provide information about the conserved function of dve regulatory motifs in cell differentiation.

Comparative developmental analysis of Drosophila and Tribolium reveals conserved and diverged roles of abrupt in insect wing evolution

Morphological innovation is a fundamental process in evolution, yet its molecular basis is still elusive. Acquisition of elytra, highly modified beetle forewings, is an important innovation that has driven the successful radiation of beetles. Our RNAi screening for candidate genes has identified abrupt (ab) as a potential key player in elytron evolution. In this study, we performed a series of RNA interference (RNAi) experiments in both Tribolium and Drosophila to understand the contributions of ab to the evolution of beetle elytra. We found that (i) ab is essential for proper wing vein patterning both in Tribolium and Drosophila, (ii) ab has gained a novel function in determining the unique elytron shape in the beetle lineage, (iii) unlike Hippo and Insulin, other shape determining pathways, the shape determining function of ab is specific to the elytron and not required in the hindwing, (iv) ab has a previously undescribed role in the Notch signal-associated wing formation processes, which appears to be conserved between beetles and flies. These data suggest that ab has gained a new function during elytron evolution in beetles without compromising the conserved wing-related functions. Gaining a new function without losing evolutionarily conserved functions may be a key theme in the evolution of morphologically novel structures.

Origin, development, and evolution of butterfly eyespots

This article reviews the latest developments in our understanding of the origin, development, and evolution of nymphalid butterfly eyespots. Recent contributions to this field include insights into the evolutionary and developmental origin of eyespots and their ancestral deployment on the wing, the evolution of eyespot number and eyespot sexual dimorphism, and the identification of genes affecting eyespot development and black pigmentation. I also compare features of old and more recently proposed models of eyespot development and propose a schematic for the genetic regulatory architecture of eyespots. Using this schematic I propose two hypotheses for why we observe limits to morphological diversity across these serially homologous 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.

Proteasome, but not autophagy, disruption results in severe eye and wing dysmorphia: a subunit- and regulator-dependent process in Drosophila

Proteasome-dependent and autophagy-mediated degradation of eukaryotic cellular proteins represent the two major proteostatic mechanisms that are critically implicated in a number of signaling pathways and cellular processes. Deregulation of functions engaged in protein elimination frequently leads to development of morbid states and diseases. In this context, and through the utilization of GAL4/UAS genetic tool, we herein examined the in vivo contribution of proteasome and autophagy systems in Drosophila eye and wing morphogenesis. By exploiting the ability of GAL4-ninaE. GMR and P{GawB}Bx^MS1096 genetic drivers to be strongly and preferentially expressed in the eye and wing discs, respectively, we proved that proteasomal integrity and ubiquitination proficiency essentially control fly’s eye and wing development. Indeed, subunit- and regulator-specific patterns of severe organ dysmorphia were obtained after the RNAi-induced downregulation of critical proteasome components (Rpn1, Rpn2, α5, β5 and β6) or distinct protein-ubiquitin conjugators (UbcD6, but not UbcD1 and UbcD4). Proteasome deficient eyes presented with either rough phenotypes or strongly dysmorphic shapes, while transgenic mutant wings were severely folded and carried blistered structures together with loss of vein differentiation. Moreover, transgenic fly eyes overexpressing the UBP2-yeast deubiquitinase enzyme were characterized by an eyeless-like phenotype. Therefore, the proteasome/ubiquitin proteolytic activities are undoubtedly required for the normal course of eye and wing development. In contrast, the RNAi-mediated downregulation of critical Atg (1, 4, 7, 9 and 18) autophagic proteins revealed their non-essential, or redundant, functional roles in Drosophila eye and wing formation under physiological growth conditions, since their reduced expression levels could only marginally disturb wing’s, but not eye’s, morphogenetic organization and architecture. However, Atg9 proved indispensable for the maintenance of structural integrity of adult wings in aged flies. In toto, our findings clearly demonstrate the gene-specific fundamental contribution of proteasome, but not autophagy, in invertebrate eye and wing organ development.

The Drosophila Arf GEF Steppke controls MAPK activation in EGFR signaling

Guanine nucleotide exchange factors (GEFs) of the cytohesin protein family are regulators of GDP/GTP exchange for members of the ADP ribosylation factor (Arf) of small GTPases. They have been identified as modulators of various receptor tyrosine kinase signaling pathways including the insulin, the vascular epidermal growth factor (VEGF) and the epidermal growth factor (EGF) pathways. These pathways control many cellular functions, including cell proliferation and differentiation, and their misregulation is often associated with cancerogenesis. In vivo studies on cytohesins using genetic loss of function alleles are lacking, however, since knockout mouse models are not available yet. We have recently identified mutants for the single cytohesin Steppke (Step) in Drosophila and we could demonstrate an essential role of Step in the insulin signaling cascade. In the present study, we provide in vivo evidence for a role of Step in EGFR signaling during wing and eye development. By analyzing step mutants, transgenic RNA interference (RNAi) and overexpression lines for tissue specific as well as clonal analysis, we found that Step acts downstream of the EGFR and is required for the activation of mitogen-activated protein kinase (MAPK) and the induction of EGFR target genes. We further demonstrate that step transcription is induced by EGFR signaling whereas it is negatively regulated by insulin signaling. Furthermore, genetic studies and biochemical analysis show that Step interacts with the Connector Enhancer of KSR (CNK). We propose that Step may be part of a larger signaling scaffold coordinating receptor tyrosine kinase-dependent MAPK activation.

Evolution of development: diversified dorsoventral patterning

Patterning of the dorsoventral axis by graded BMP signaling is conserved in the evolution of animals. However, this system has also proven to be highly adaptable, as is now highlighted by its short-range function in the leech Helobdella.

Drosophila as a model for epithelial tube formation

Epithelial tubular organs are essential for life in higher organisms and include the pancreas and other secretory organs that function as biological factories for the synthesis and delivery of secreted enzymes, hormones, and nutrients essential for tissue homeostasis and viability. The lungs, which are necessary for gas exchange, vocalization, and maintaining blood pH, are organized as highly branched tubular epithelia. Tubular organs include arteries, veins, and lymphatics, high-speed passageways for delivery and uptake of nutrients, liquids, gases, and immune cells. The kidneys and components of the reproductive system are also epithelial tubes. Both the heart and central nervous system of many vertebrates begin as epithelial tubes. Thus, it is not surprising that defects in tube formation and maintenance underlie many human diseases. Accordingly, a thorough understanding how tubes form and are maintained is essential to developing better diagnostics and therapeutics. Among the best-characterized tubular organs are the Drosophila salivary gland and trachea, organs whose relative simplicity have allowed for in depth analysis of gene function, yielding key mechanistic insight into tube initiation, remodeling and maintenance. Here, we review our current understanding of salivary gland and trachea formation - highlighting recent discoveries into how these organs attain their final form and function.

Genetic basis of wing morphogenesis in Drosophila: sexual dimorphism and non-allometric effects of shape variation

BACKGROUND

The Drosophila wing represents a particularly appropriate model to investigate the developmental control of phenotypic variation. Previous studies which aimed to identify candidate genes for wing morphology demonstrated that the genetic basis of wing shape variation in D. melanogaster is composed of numerous genetic factors causing small, additive effects. In this study, we analyzed wing shape in males and females from 191 lines of D. melanogaster, homozygous for a single P-element insertion, using geometric morphometrics techniques. The analysis allowed us to identify known and novel candidate genes that may contribute to the expression of wing shape in each sex separately and to compare them to candidate genes affecting wing size which have been identified previously using the same lines.

RESULTS

Our results indicate that more than 63% of induced mutations affected wing shape in one or both sexes, although only 33% showed significant differences in both males and females. The joint analysis of wing size and shape revealed that only 19% of the P-element insertions caused coincident effects on both components of wing form in one or both sexes. Further morphometrical analyses revealed that the intersection between veins showed the smallest displacements in the proximal region of the wing. Finally, we observed that mutations causing general deformations were more common than expected in both sexes whereas the opposite occurred with those generating local changes. For most of the 94 candidate genes identified, this seems to be the first record relating them with wing shape variation.

CONCLUSIONS

Our results support the idea that the genetic architecture of wing shape is complex with many different genes contributing to the trait in a sexually dimorphic manner. This polygenic basis, which is relatively independent from that of wing size, is composed of genes generally involved in development and/or metabolic functions, especially related to the regulation of different cellular processes such as motility, adhesion, communication and signal transduction. This study suggests that understanding the genetic basis of wing shape requires merging the regulation of vein patterning by signalling pathways with processes that occur during wing development at the cellular level.

Getting real with real-time qPCR: a case study of reference gene selection for morphological variation in Drosophila melanogaster wings

Accurate estimation of gene expression differences during development requires sensitive techniques combined with gold-standard normalization procedures. This is particularly true in the case of quantitative traits, where expression changes might be small. Nevertheless, systematic selection and validation of reference genes has been overlooked, even in Drosophila studies. Here, we tested the stability of six traditional reference genes across samples of imaginal wing disks from morphologically divergent strains of Drosophila melanogaster, in a two-class comparison: quantitative or qualitative variation in wing morphology. Overall, we identified and validated a pair of genes (RpL32 and Tbp) as being stably expressed in both experimental comparisons. These genes might be considered as a bona fide pair of reference genes for gene expression analyses of morphological divergence in D. melanogaster wings. They might also be taken as good candidates for experimental identification of stable reference genes in other morphological comparisons using Drosophila or other insect species. Besides, we found that some genes traditionally used as reference in qPCR experiments were not stably expressed in wing disks from the different fly strains. In fact, a significant bias was observed when the expression of three genes of interest, which are involved in the regulation of growth and patterning during imaginal wing development, was normalized with such putative reference genes. Our results demonstrate how inaccurate findings and opposite conclusions might be drawn if traditional reference genes are arbitrarily used for internal normalization without proper validation in the given experimental condition, a practice still common in qPCR experiments.

The MAP kinase ERK and its scaffold protein MP1 interact with the chromatin regulator Corto during Drosophila wing tissue development

Background

Mitogen-activated protein kinase (MAPK) cascades (p38, JNK, ERK pathways) are involved in cell fate acquisition during development. These kinase modules are associated with scaffold proteins that control their activity. In Drosophila, dMP1, that encodes an ERK scaffold protein, regulates ERK signaling during wing development and contributes to intervein and vein cell differentiation. Functional relationships during wing development between a chromatin regulator, the Enhancer of Trithorax and Polycomb Corto, ERK and its scaffold protein dMP1, are examined here.

Results

Genetic interactions show that corto and dMP1 act together to antagonize rolled (which encodes ERK) in the future intervein cells, thus promoting intervein fate. Although Corto, ERK and dMP1 are present in both cytoplasmic and nucleus compartments, they interact exclusively in nucleus extracts. Furthermore, Corto, ERK and dMP1 co-localize on several sites on polytene chromosomes, suggesting that they regulate gene expression directly on chromatin. Finally, Corto is phosphorylated. Interestingly, its phosphorylation pattern differs between cytoplasm and nucleus and changes upon ERK activation.

Conclusions

Our data therefore suggest that the Enhancer of Trithorax and Polycomb Corto could participate in regulating vein and intervein genes during wing tissue development in response to ERK signaling.

Spatial regulation of cell adhesion in the Drosophila wing is mediated by Delilah, a potent activator of βPS integrin expression

In spite of our conceptual view of how differential gene expression is used to define different cell identities, we still do not understand how different cell identities are translated into actual cell properties. The example discussed here is that of the fly wing, which is composed of two main cell types: vein and intervein cells. These two cell types differ in many features, including their adhesive properties. One of the major differences is that intervein cells express integrins, which are required for the attachment of the two wing layers to each other, whereas vein cells are devoid of integrin expression. The major signaling pathways that divide the wing to vein and intervein domains have been characterized. However, the genetic programs that execute these two alternative differentiation programs are still very roughly drawn. Here we identify the bHLH protein Delilah (Dei) as a mediator between signaling pathways that specify intervein cell-fate and one of the most significant realizators of this fate, βPS integrin. Dei's expression is restricted to intervein territories where it acts as a potent activator of βPS integrin expression. In the absence of normal Dei activity the level of βPS integrin is reduced, leading to a failure of adhesion between the dorsal and ventral wing layers and a consequent formation of wing blisters. The effect of Dei on βPS expression is not restricted to the wing, suggesting that Dei functions as a general genetic switch, which is turned on wherever a sticky cell-identity is determined and integrin-based adhesion is required.

Histone lysine demethylases function as co-repressors of SWI/SNF remodeling activities during Drosophila wing development

The conserved SWI/SNF chromatin remodeling complex uses the energy from ATP hydrolysis to alter local chromatin environments through disrupting DNA-histone contacts. These alterations influence transcription activation, as well as repression. The Drosophila SWI/SNF counterpart, known as the Brahma or Brm complex, has been shown to have an essential role in regulating the proper expression of many developmentally important genes, including those required for eye and wing tissue morphogenesis. A temperature sensitive mutation in one of the core complex subunits, SNR1 (SNF5/INI1/SMARCB1), results in reproducible wing patterning phenotypes that can be dominantly enhanced and suppressed by extragenic mutations. SNR1 functions as a regulatory subunit to modulate chromatin remodeling activities of the Brahma complex on target genes, including both activation and repression. To help identify gene targets and cofactors of the Brahma complex, we took advantage of the weak dominant nature of the snr1(E1) mutation to carry out an unbiased genetic modifier screen. Using a set of overlapping chromosomal deficiencies that removed the majority of the Drosophila genome, we looked for genes that when heterozygous would function to either enhance or suppress the snr1(E1) wing pattern phenotype. Among potential targets of the Brahma complex, we identified components of the Notch, EGFR and DPP signaling pathways important for wing development. Mutations in genes encoding histone demethylase enzymes were identified as cofactors of Brahma complex function. In addition, we found that the Lysine Specific Demethylase 1 gene (lsd1) was important for the proper cell type-specific development of wing patterning.

Individual carboxypeptidase D domains have both redundant and unique functions in Drosophila development and behavior

Metallocarboxypeptidase D (CPD) functions in protein and peptide processing. The Drosophila CPD svr gene undergoes alternative splicing, producing forms containing 1-3 active or inactive CP domains. To investigate the function of the various CP domains, we created transgenic flies expressing specific forms of CPD in the embryonic-lethal svr (PG33) mutant. All constructs containing an active CP domain rescued the lethality with varying degrees, and full viability required inactive CP domain-3. Transgenic flies overexpressing active CP domain-1 or -2 were similar to each other and to the viable svr mutants, with pointed wing shape, enhanced ethanol sensitivity, and decreased cold sensitivity. The transgenes fully compensated for a long-term memory deficit observed in the viable svr mutants. Overexpression of CP domain-1 or -2 reduced the levels of Lys/Arg-extended adipokinetic hormone intermediates. These findings suggest that CPD domains-1 and -2 have largely redundant functions in the processing of growth factors, hormones, and neuropeptides.

Identification of genes affecting wing patterning through a loss-of-function mutagenesis screen and characterization of med15 function during wing development

The development of the Drosophila melanogaster wing depends on the correct regulation of cell survival, growth, proliferation, differentiation, and pattern formation. These processes, and the genes controlling then, are common to the development of epithelia in many different organisms. To identify additional genes contributing to wing development we have carried out a genetic screen in mosaic wings carrying clones of homozygous mutant cells. We obtained 12 complementation groups corresponding to genes with a proven role in wing formation such as smoothened, thick veins, mothers against dpp, expanded, and fat and 71 new complementation groups affecting the pattern of veins and the size of wing. We mapped one of these groups to the mediator15 gene (med15), a component of the Mediator complex. We show that Med15 and other members of the Mediator complex are required, among other processes, for the transcription of decapentaplegic target genes.

Involvement of the MP1 scaffold protein in ERK signaling regulation during Drosophila wing development

Mitogen-activated protein kinase (MAPK) cascades are evolutionary conserved transduction pathways involved in many cellular processes. Kinase modules are associated with scaffold proteins that regulate signaling by providing critical spatial and temporal specificities. Some of these scaffold proteins have been shown to be conserved, both in sequence and function. In mouse, the scaffold MP1 (MEK Partner 1) forms a signaling complex with MEK1 and ERK1. In this work, we focus on Drosophila MP1 (dMP1). We show that dMP1 is expressed ubiquitously during embryonic and larval development. By in vitro and in vivo experiments, we show that dMP1 is located in the cytoplasm and the nuclei, and that it interacts with MEK and ERK. Genetic studies with transgenic Drosophila lines allowing either dMP1 over-expression or dMP1 down-regulation by RNA interference highlight dMP1 function in the control of cell differentiation during development of the Drosophila wing.

Genetic modifier screens reveal new components that interact with the Drosophila dystroglycan-dystrophin complex

The Dystroglycan-Dystrophin (Dg-Dys) complex has a capacity to transmit information from the extracellular matrix to the cytoskeleton inside the cell. It is proposed that this interaction is under tight regulation; however the signaling/regulatory components of Dg-Dys complex remain elusive. Understanding the regulation of the complex is critical since defects in this complex cause muscular dystrophy in humans. To reveal new regulators of the Dg-Dys complex, we used a model organism Drosophila melanogaster and performed genetic interaction screens to identify modifiers of Dg and Dys mutants in Drosophila wing veins. These mutant screens revealed that the Dg-Dys complex interacts with genes involved in muscle function and components of Notch, TGF-β and EGFR signaling pathways. In addition, components of pathways that are required for cellular and/or axonal migration through cytoskeletal regulation, such as Semaphorin-Plexin, Frazzled-Netrin and Slit-Robo pathways show interactions with Dys and/or Dg. These data suggest that the Dg-Dys complex and the other pathways regulating extracellular information transfer to the cytoskeletal dynamics are more intercalated than previously thought.

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.

The detached locus encodes Drosophila Dystrophin, which acts with other components of the Dystrophin Associated Protein Complex to influence intercellular signalling in developing wing veins

Dystrophin and Dystroglycan are the two central components of the multimeric Dystrophin Associated Protein Complex, or DAPC, that is thought to provide a mechanical link between the extracellular matrix and the actin cytoskeleton, disruption of which leads to muscular dystrophy in humans. We present the characterization of the Drosophila 'crossveinless' mutation detached (det), and show that the gene encodes the fly ortholog of Dystrophin. Our genetic analysis shows that, in flies, Dystrophin is a non-essential gene, and the sole overt morphological defect associated with null mutations in the locus is the variable loss of the posterior crossvein that has been described for alleles of det. Null mutations in Drosophila Dystroglycan (Dg) are similarly viable and exhibit this crossvein defect, indicating that both of the central DAPC components have been co-opted for this atypical function of the complex. In the developing wing, the Drosophila DAPC affects the intercellular signalling pathways involved in vein specification. In det and Dg mutant wings, the early BMP signalling that initiates crossvein specification is not maintained, particularly in the pro-vein territories adjacent to the longitudinal veins, and this results in the production of a crossvein fragment in the intervein between the two longitudinal veins. Genetic interaction studies suggest that the DAPC may exert this effect indirectly by down-regulating Notch signalling in pro-vein territories, leading to enhanced BMP signalling in the intervein by diffusion of BMP ligands from the longitudinal veins.

The costae presenting in high-temperature-induced vestigial wings of Drosophila: implications for anterior wing margin formation

It has long been noted that high temperature produces great variation in wing forms of the vestigial mutant of Drosophila. Most of the wings have defects in the wing blade and partially formed wing margin, which are the result of autonomous cell death in the presumptive wing blade or costal region of the wing disc. The vestigial gene (vg) and the interaction of Vg protein with other gene products are well understood. With this biochemical knowledge, reinvestigations of the high-temperature-induced vestigial wings and the elucidation of the molecular mechanism underlying the large-scale variation of the wing forms may provide insight into further understanding of development of the wing of Drosophila. As a first step of such explorations, I examined high-temperature-induced (29 degrees C) vestigial wings. In the first part of this paper, I provide evidences to show that the proximal and distal costae in these wings exhibit regular and continuous variation, which suggests different developmental processes for the proximal and distal costal sections. Judging by the costae presenting in the anterior wing margin, I propose that the proximal and distal costal sections are independent growth units. The genes that regulate formation of the distal costal section also strongly affect proliferation of cells nearby; however, the same phenomenon has not been found in the proximal costal section. The distal costal section seems to be an extension of the radius vein. vestigial, one of the most intensely researched temperature-sensitive mutations, is a good candidate for the study of marginal vein formation. In the second part of the paper, I regroup the wing forms of these wings, chiefly by comparison of venation among these wings, and try to elucidate the variation of the wing forms according to the results of previous work and the conclusions reached in the first part of this paper, and provide clues for further researches.

DrosophilaNemo antagonizes BMP signaling by phosphorylation of Mad and inhibition of its nuclear accumulation

Drosophila Nemo is the founding member of the Nemo-like kinase(Nlk) family of serine/threonine protein kinases that are involved in several Wnt signal transduction pathways. Here we report a novel function for Nemo in the inhibition of bone morphogenetic protein (BMP) signaling. Genetic interaction studies demonstrate that nemo can antagonize BMP signaling and can inhibit the expression of BMP target genes during wing development. Nemo can bind to and phosphorylate the BMP effector Mad. In cell culture, phosphorylation by Nemo blocks the nuclear accumulation of Mad by promoting export of Mad from the nucleus in a kinase-dependent manner. This is the first example of the inhibition of Drosophila BMP signaling by a MAPK and represents a novel mechanism of Smad inhibition through the phosphorylation of a conserved serine residue within the MH1 domain of Mad.

A gain-of-function screen identifying genes required for vein formation in the Drosophila melanogaster wing

The formation of the Drosophila wing involves developmental processes such as cell proliferation, pattern formation, and cell differentiation that are common to all multicellular organisms. The genes controlling these cellular behaviors are conserved throughout the animal kingdom, and the genetic analysis of wing development has been instrumental in their identification and functional characterization. The wing is a postembryonic structure, and most loss-of-function mutations are lethal in homozygous flies before metamorphosis. In this manner, loss-of-function genetic screens aiming to identify genes affecting wing formation have not been systematically utilized. As an alternative, a number of genetic searches have utilized the phenotypic consequences of gene gain-of-expression, as a method more efficient to search for genes required during imaginal development. Here we present the results of a gain-of-function screen designed to identify genes involved in the formation of the wing veins. We generated 13,000 P-GS insertions of a P element containing UAS sequences (P-GS) and combined them with a Gal4 driver expressed mainly in the developing pupal veins. We selected 500 P-GSs that, in combination with the Gal4 driver, result in modifications of the veins, changes in the morphology of the wing, or defects in the differentiation of the trichomes. The P-element insertion sites were mapped to the genomic sequence, identifying 373 gene candidates to participate in wing morphogenesis and vein formation.

Shaping BMP morphogen gradients in the Drosophila embryo and pupal wing

In the early Drosophila embryo, BMP-type ligands act as morphogens to suppress neural induction and to specify the formation of dorsal ectoderm and amnioserosa. Likewise, during pupal wing development, BMPs help to specify vein versus intervein cell fate. Here, we review recent data suggesting that these two processes use a related set of extracellular factors, positive feedback, and BMP heterodimer formation to achieve peak levels of signaling in spatially restricted patterns. Because these signaling pathway components are all conserved, these observations should shed light on how BMP signaling is modulated in vertebrate development.

Wnt signals mediate a fate decision between otic placode and epidermis

The otic placode, the anlagen of the inner ear, develops from an ectodermal field characterized by expression of the transcription factor Pax2. Previous fate mapping studies suggest that these Pax2+cells will give rise to both otic placode tissue and epidermis, but the signals that divide the Pax2+ field into placodal and epidermal territories are unknown. We report that Wnt signaling is normally activated in a subset of Pax2+ cells, and that conditional inactivation of β-catenin in these cells causes an expansion of epidermal markers at the expense of the otic placode. Conversely, conditional activation of β-catenin in Pax2+ cells causes an expansion of the otic placode at the expense of epidermis, and the resulting otic tissue expresses exclusively dorsal otocyst markers. Together, these results suggest that Wnt signaling acts instructively to direct Pax2+cells to an otic placodal, rather than an epidermal, fate and promotes dorsal cell identities in the otocyst.

Regulation of decapentaplegic expression during Drosophila wing veins pupal development

The differentiation of veins in the Drosophila wing relies on localised expression of decapentaplegic (dpp) in pro-vein territories during pupal development. The expression of dpp in the pupal veins requires the integrity of the shortvein region (shv), localised 5' to the coding region. It is likely that this DNA integrates positive and negative regulatory signals directing dpp transcription during pupal development. Here, we identify a minimal 0.9 kb fragment giving localised expression in the vein L5 and a 0.5 kb fragment giving expression in all longitudinal veins. Using a combination of in vivo expression of reporter genes regulated by shv sequences, in vitro binding assays and sequence comparisons between the shv region of different Drosophila species, we found binding sites for the vein-specific transciption factors Araucan, Knirps and Ventral veinless, as well as binding sites for the Dpp pathway effectors Mad and Med. We conclude that conserved vein-specific enhancers regulated by transcription factors expressed in individual veins collaborate with general vein and intervein regulators to establish and maintain the expression of dpp confined to the veins during pupal development.

MAP kinase subcellular localization controls both pattern and proliferation in the developing Drosophila wing

Mitogen-activated protein kinases (MAPKs) phosphorylate target proteins in both the cytoplasm and nucleus, and a strong correlation exists between the subcellular localization of MAPK and resulting cellular responses. It was thought that MAPK phosphorylation was always followed by rapid nuclear translocation. However, we and others have found that MAPK phosphorylation is not always sufficient for nuclear translocation in vivo. In the developing Drosophila wing, MAPK-mediated signaling is required both for patterning and for cell proliferation, although the mechanism of this differential control is not fully understood. Here, we show that phosphorylated MAPK (pMAPK) is held in the cytoplasm in differentiating larval and pupal wing vein cells, and we show that this cytoplasmic hold is required for vein cell fate. At the same time, we show that MAPK does move into the nucleus of other wing cells where it promotes cell proliferation. We propose a novel Ras pathway bifurcation in Drosophila and our results suggest a mechanism by which MAPK phosphorylation can signal two different cellular outcomes (differentiation versus proliferation) based on the subcellular localization of MAPK.

Long-range Dpp signaling is regulated to restrict BMP signaling to a crossvein competent zone

The sensitivity of the crossveins of the Drosophila wing to reductions in BMP signaling provides a valuable system for characterizing members of this signaling pathway. We demonstrate here two reasons for that sensitivity. First, the initial stage of posterior crossvein development depends on BMP signaling but is independent of EGF signaling. This is the opposite of the longitudinal veins, which rely of EGF signaling for their initial specification. Second, BMP signaling in the posterior crossvein depends on Decapentaplegic (Dpp) at a stage when it is being produced in the longitudinal veins. Thus, the posterior crossvein will be especially vulnerable to reductions in the levels or range of Dpp signaling. We investigated the roles of the BMP receptor Thickveins (Tkv) and the BMP inhibitor Short gastrulation (Sog) in allowing this long-range signaling. Expression of both is downregulated in the developing posterior crossvein. The Tkv downregulation depends on BMP signaling and may provide a positive feedback by allowing the spread of Dpp. The Sog downregulation is independent of BMP signaling; Sog misexpression experiments indicate that this prepattern is essential for posterior crossvein development. However, this requirement can be overridden by co-misexpression of the BMP agonist Cv-2, indicating the presence of as yet unknown cues; we discuss possible candidates.

SNR1 (INI1/SNF5) mediates important cell growth functions of the Drosophila Brahma (SWI/SNF) chromatin remodeling complex

SNR1 is an essential subunit of the Drosophila Brahma (Brm) ATP-dependent chromatin remodeling complex, with counterparts in yeast (SNF5) and mammals (INI1). Increased cell growth and wing patterning defects are associated with a conditional snr1 mutant, while loss of INI1 function is directly linked with aggressive cancers, suggesting important roles in development and growth control. The Brm complex is known to function during G1 phase, where it appears to assist in restricting entry into S phase. In Drosophila, the activity of DmcycE/CDK2 is rate limiting for entry into S phase and we previously found that the Brm complex can suppress a reduced growth phenotype associated with a hypomorphic DmcycE mutant. Our results reveal that SNR1 helps mediate associations between the Brm complex and DmcycE/CDK2 both in vitro and in vivo. Further, disrupting snr1 function suppressed DmcycEJP phenotypes, and increased cell growth defects associated with the conditional snr1E1 mutant were suppressed by reducing DmcycE levels. While the snr1E1-dependent increased cell growth did not appear to be directly associated with altered expression of G1 or G2 cyclins, transcription of the G2-M regulator string/cdc25 was reduced. Thus, in addition to important functions of the Brm complex in G1-S control, the complex also appears to be important for transcription of genes required for cell cycle progression.

Naturally segregating quantitative trait loci affecting wing shape of Drosophila melanogaster

Variation in vein position and wing shape of Drosophila melanogaster depends on many genes. In the following, we report the results of a QTL analysis of wing shape in D. melanogaster. We identified QTL responsible for natural variation for wing shape and analyzed their interactions with developmental genetic signaling pathways important for vein positioning. The QTL analysis indicated that the total number of QTL segregating in this population is likely to be very large. The locations of putative QTL identified in this study were compared to those identified in previous studies and, while there is more correspondence across studies than expected by chance on the third chromosome, the studies appear to have identified different QTL. Using a complementation design, we tested for interactions among these QTL with the Hedgehog and Decapentaplegic signaling pathways, which are important for the development and position of vein pairs L3-L4 and L2-L5. Three QTL showed strong interactions with these two pathways, supporting the hypothesis that these QTL are involved in these pathways. Naturally segregating variation can therefore act through known signaling pathways to produce variation in vein position.

Patterns in evolution: veins of the Drosophila wing

The development of the Drosophila wing is a classical model for studying the genetic control of tissue size, shape and patterning. A detailed picture of how positional information is interpreted by cells in the imaginal disc and translated into the adult wing vein pattern has recently emerged. It highlights the central role of dose-dependent activation of distinct cell transcription programs in response to the Hedgehog (Hh) and Decapentaplegic (Dpp) morphogens, as well as an early role of Notch signalling, in connecting the positioning of vein primordia and vein differentiation proper. The biochemical basis of the cross-talk that operates between these different signalling pathways is less well understood. New strategies made possible by the genome sequencing of several insect models should provide an important complement to the knowledge obtained from >60 years of genetic studies.

Activation and repression activities of ash2 in Drosophila wing imaginal discs

Polycomb (PcG) and trithorax (trxG) group genes are chromatin regulators involved in the maintenance of developmental decisions. Although their function as transcriptional regulators of homeotic genes has been well documented, little is known about their effect on other target genes or their role in other developmental processes. In this study, we have used the patterning of veins and interveins in the wing as a model with which to understand the function of the trxG gene ash2 (absent, small or homeotic discs 2). We show that ash2 is required to sustain the activation of the intervein-promoting genes net and blistered (bs) and to repress rhomboid (rho), a component of the EGF receptor (Egfr) pathway. Moreover, loss-of-function phenotypes of the Egfr pathway are suppressed by ash2 mutants, while gain-of-function phenotypes are enhanced. Our results also show that ash2 acts as a repressor of the vein L2-organising gene knirps (kni), whose expression is upregulated throughout the whole wing imaginal disc in ash2 mutants and mitotic clones. Furthermore, ash2-mediated inhibition of kni is independent of spalt-major and spalt-related. Together, these experiments indicate that ash2 plays a role in two processes during wing development: (1) maintaining intervein cell fate, either by activation of intervein genes or inhibition of vein differentiation genes; and (2) keeping kni in an off state in tissues beyond the L2 vein. We propose that the Ash2 complex provides a molecular framework for a mechanism required to maintain cellular identities in the wing development.

The Drosophila Brahma (SWI/SNF) chromatin remodeling complex exhibits cell-type specific activation and repression functions

The Brahma (Brm) complex of Drosophila melanogaster is a SWI/SNF-related chromatin remodeling complex required to correctly maintain proper states of gene expression through ATP-dependent effects on chromatin structure. The SWI/SNF complexes are comprised of 8-11 stable components, even though the SWI2/SNF2 (BRM, BRG1, hBRM) ATPase subunit alone is partially sufficient to carry out chromatin remodeling in vitro. The remaining subunits are required for stable complex assembly and/or proper promoter targeting in vivo. Our data reveals that SNR1 (SNF5-Related-1), a highly conserved subunit of the Brm complex, is required to restrict complex activity during the development of wing vein and intervein cells, illustrating a functional requirement for SNR1 in modifying whole complex activation functions. Specifically, we found that snr1 and brm exhibited opposite mutant phenotypes in the wing and differential misregulation of genes required for vein and intervein cell development, including rhomboid, decapentaplegic, thick veins, and blistered, suggesting possible regulatory targets for the Brm complex in vivo. Our genetic results suggest a novel mechanism for SWI/SNF-mediated gene repression that relies on the function of a 'core' subunit to block or shield BRM (SWI2/SNF2) activity in specific cells. The SNR1-mediated repression is dependent on cooperation with histone deacetylases (HDAC) and physical associations with NET, a localized vein repressor.

brinker and optomotor-blind act coordinately to initiate development of the L5 wing vein primordium in Drosophila

The stereotyped pattern of Drosophila wing veins is determined by the action of two morphogens, Hedgehog (Hh) and Decapentaplegic (Dpp), which act sequentially to organize growth and patterning along the anterior-posterior axis of the wing primordium. An important unresolved question is how positional information established by these morphogen gradients is translated into localized development of morphological structures such as wing veins in precise locations. In the current study, we examine the mechanism by which two broadly expressed Dpp signaling target genes, optomotor-blind (omb) and brinker (brk), collaborate to initiate formation of the fifth longitudinal (L5) wing vein. omb is broadly expressed at the center of the wing disc in a pattern complementary to that of brk, which is expressed in the lateral regions of the disc and represses omb expression. We show that a border between omb and brk expression domains is necessary and sufficient for inducing L5 development in the posterior regions. Mosaic analysis indicates that brk-expressing cells produce a short-range signal that can induce vein formation in adjacent omb-expressing cells. This induction of the L5 primordium is mediated by abrupt, which is expressed in a narrow stripe of cells along the brk/omb border and plays a key role in organizing gene expression in the L5 primordium. Similarly, in the anterior region of the wing, brk helps define the position of the L2 vein in combination with another Dpp target gene, spalt. The similar mechanisms responsible for the induction of L5 and L2 development reveal how boundaries set by dosage-sensitive responses to a long-range morphogen specify distinct vein fates at precise locations.

ThreeDrosophilaEXT genes shape morphogen gradients through synthesis of heparan sulfate proteoglycans

The signaling molecules Hedgehog (Hh), Decapentaplegic (Dpp) and Wingless(Wg) function as morphogens and organize wing patterning in Drosophila. In the screen for mutations that alter the morphogen activity, we identified novel mutants of two Drosophila genes, sister of tout-velu (sotv) and brother of tout-velu(botv), and new alleles of toutvelu (ttv). The encoded proteins of these genes belong to an EXT family of proteins that have or are closely related to glycosyltransferase activities required for biosynthesis of heparan sulfate proteoglycans (HSPGs). Mutation in any of these genes impaired biosynthesis of HSPGs in vivo, indicating that, despite their structural similarity, they are not redundant in the HSPG biosynthesis. Protein levels and signaling activities of Hh, Dpp and Wg were reduced in the cells mutant for any of these EXT genes to a various degree, Wg signaling being the least sensitive. Moreover, all three morphogens were accumulated in the front of EXT mutant cells, suggesting that these morphogens require HSPGs to move efficiently. In contrast to previous reports that ttv is involved exclusively in Hh signaling, we found that ttv mutations also affected Dpp and Wg. These data led us to conclude that each of three EXT genes studied contribute to Hh, Dpp and Wg morphogen signaling. We propose that HSPGs facilitate the spreading of morphogens and therefore, function to generate morphogen concentration gradients.