Altering the insertional specificity of a Drosophila transposable element

Lobe and Serrate are required for cell survival during early eye development in Drosophila

Organogenesis involves an initial surge of cell proliferation, leading to differentiation. This is followed by cell death in order to remove extra cells. During early development, there is little or no cell death. However, there is a lack of information concerning the genes required for survival during the early cell-proliferation phase. Here, we show that Lobe (L) and the Notch (N) ligand Serrate (Ser), which are both involved in ventral eye growth, are required for cell survival in the early eye disc. We observed that the loss-of-ventral-eye phenotype in L or Ser mutants is due to the induction of cell death and the upregulation of secreted Wingless (Wg). This loss-of-ventral-eye phenotype can be rescued by (i) increasing the levels of cell death inhibitors, (ii) reducing the levels of Hid-Reaper-Grim complex, or (iii) reducing canonical Wg signaling components. Blocking Jun-N-terminal kinase (JNK) signaling, which can induce caspase-independent cell death, significantly rescued ventral eye loss in L or Ser mutants. However, blocking both caspase-dependent cell death and JNK signaling together showed stronger rescues of the L- or Ser-mutant eye at a 1.5-fold higher frequency. This suggests that L or Ser loss-of-function triggers both caspase-dependent and -independent cell death. Our studies thus identify a mechanism responsible for cell survival in the early eye.

Transgenic Wnt/TCF pathway reporters: all you need is Lef?

The Wnt signaling pathway controls a large and diverse set of cell fate decisions in embryonic development, adult organ maintenance and disease. At the transcriptional level, Wnt/beta-catenin signaling is primarily mediated by the T-cell factor (TCF)/Lef-1 family of transcription factors, referred to here as TCFs. In order to track Wnt pathway activity during animal development, several laboratories have built transgenic reporter constructs containing multimerized TCF binding sites. Most of these reporters are active at multiple known sites of Wnt signaling, and several act as faithful reporters of pathway activity in specific contexts. However, multimerized TCF reporters should not be assumed to give a complete or definitive readout of Wnt signaling in vivo. Direct comparisons reveal discrepancies among reporters; in addition, there is good reason to expect that some important types of pathway activity, including target gene de-repression and TCF-independent Wnt or beta-catenin signaling, will not be accurately reported by such constructs. This review will discuss various transgenic Wnt/beta-catenin/TCF reporters, address the fidelity and completeness of their Wnt responsiveness, and contrast their in vivo transcriptional responses with those of natural Wnt target genes. Finally, three caveats to the interpretation of multimerized TCF reporter expression patterns will be proposed.

Tissue-specific laminin expression facilitates integrin-dependent association of the embryonic wing disc with the trachea in Drosophila

The interaction of heterologous tissues involves cell adhesion mediated by the extracellular matrix and its receptor integrins. The Drosophila wing disc is an ectodermal invagination that contacts specific tracheal branches at the basolateral cell surface. We show that an alpha subunit of laminin, encoded by wing blister (wb), is essential for the establishment of the interaction between the wing and trachea. During embryogenesis, wing disc cells present Wb at their basolateral surface and extend posteriorly, expanding their association to more posteriorly located tracheal branches. These migratory processes are impaired in the absence of the trachea, Wb, or integrins. Time-lapse and transmission electron microscopy analyses suggest that Wb facilitates integrin-dependent contact over a large surface and controls the cellular behavior of the wing cells, including their exploratory filopodial activity. Our data identify Wb laminin as an extracellular matrix ligand that is essential for integrin-dependent cellular migration in Drosophila.

JAK/STAT signaling promotes regional specification by negatively regulating wingless expression in Drosophila

During development, a small number of conserved signaling molecules regulate regional specification, in which uniform populations of cells acquire differences and ultimately give rise to distinct organs. In the Drosophila eye imaginal disc, Wingless (Wg) signaling defines the region that gives rise to head tissue. JAK/STAT signaling was thought to regulate growth of the eye disc but not pattern formation. However, we show that the JAK/STAT pathway plays an important role in patterning the eye disc: it promotes formation of the eye field through repression of the wg gene. Overexpression of the JAK/STAT activating ligand Unpaired in the eye leads to loss of wg expression and ectopic morphogenetic furrow initiation from the lateral margins. Conversely, tissue lacking stat92E, which cannot transduce JAK/STAT signals, is transformed from retinal tissue into head cuticle, a phenotype that is also observed with ectopic Wg signaling. Consistent with this, cells lacking stat92E exhibit ectopic wg expression. Conversely, wg is autonomously repressed in cells with hyperactivated Stat92E. Furthermore, we show that the JAK/STAT pathway regulates a small enhancer in the wg 3' cis genomic region. As this enhancer is devoid of Stat92E-binding elements, we conclude that Stat92E represses wg through another, as yet unidentified factor that is probably a direct target of Stat92E. Taken together, our study is the first to demonstrate a role for the JAK/STAT pathway in regional specification by acting antagonistically to wg.

Differential Delta expression underlies the diversity of sensory organ patterns among the legs of the Drosophila adult

Many studies have shown that morphological diversity among homologous animal structures is generated by the homeotic (Hox) genes. However, the mechanisms through which Hox genes specify particular morphological features are not fully understood. We have addressed this issue by investigating how diverse sensory organ patterns are formed among the legs of the Drosophila melanogaster adult. The Drosophila adult has one pair of legs on each of its three thoracic segments (the T1-T3 segments). Although homologous, legs from different segments have distinct morphological features. Our focus is on the formation of diverse patterns of small mechanosensory bristles or microchaetae (mCs) among the legs. On T2 legs, the mCs are organized into a series of longitudinal rows (L-rows) precisely positioned along the leg circumference. The L-rows are observed on all three pairs of legs, but additional and novel pattern elements are found on T1 and T3 legs. For example, at specific positions on T1 and T3 legs, some mCs are organized into transverse rows (T-rows). Our studies indicate that the T-rows on T1 and T3 legs are established as a result of Hox gene modulation of the pathway for patterning the L-row mC bristles. Our findings suggest that the Hox genes, Sex combs reduced (Scr) and Ultrabithorax (Ubx), establish differential expression of the proneural gene achaete (ac) by modifying expression of the ac prepattern regulator, Delta (Dl), in T1 and T3 legs, respectively. This study identifies Dl as a potential link between Hox genes and the sensory organ patterning hierarchy, providing insight into the connection between Hox gene function and the formation of specific morphological features.

A 3' cis-regulatory region controls wingless expression in the Drosophila eye and leg primordia

The precise regulation of wingless (wg) expression in the Drosophila eye disc is key to control the anteroposterior and dorsoventral patterning of this disc. Here, we identify an eye disc-specific wg cis-regulatory element that functions as a regulatory rheostat. Pannier (Pnr), a transcription factor previously proposed to act as an upstream activator of wg, is sufficient to activate the eye disc enhancer but required for wg expression only in the peripodial epithelium of the disc. We propose that this regulation of wg by Pnr appeared associated to the development of the peripodial epithelium in higher dipterans and was added to an existing mechanism regulating the deployment of wingless in the dorsal region of the eye primordium. In addition, our analysis identifies a separate ventral disc enhancer that lies adjacent to the eye-specific one, and thus altogether, they define a 1-kb genomic region where disc-specific enhancers of the wg gene are located.

Retrotransposons and regulatory suites

Cellular differentiation and multicellular development require the programmed expression of coregulated suites of genetic loci dispersed throughout the genome. How do functionally diverse loci come to share common regulatory motifs? A new paper finds that retrotransposons (RTEs) may play a role in providing common regulation to a group of functions expressed during the development of oocytes and preimplantation embryos. Examining cDNA libraries, Peaston et al. find that 13% of all processed transcripts in full-grown mouse oocytes contain RTE sequences, mostly from the MT family of retroviral-like elements. Smaller but still significant percentages of RTE sequences are found in cDNA libraries from 2-cell embryos and blastocysts. A quarter of these RTE sequences are at the 5' ends of chimeric transcripts that also contain exons from endogenous mouse loci. These chimeric transcripts display restricted expression in oocytes and preimplantation embryos and presumably originate from developmentally regulated LTR promoters. Some, but not all, chimeric transcripts encode novel protein products.

Activation of Rac1 or Cdc42 during early morphogenesis of eye discs induces ectopic antennae in Drosophila

The Rho family small guanosine triphosphatases (GTPases) play important roles in many cellular processes, especially in regulation of cytoskeletal organization. In this study, I examined the functions of Rac1 and Cdc42 for disc morphogenesis in Drosophila. I expressed either a constitutively active form or a dominant negative form of each protein during early morphogenesis of eye discs. Inactivation of Rac1 or Cdc42 resulted in small eye phenotypes. On the other hand, I found that activation of either Rac1 or Cdc42 induces ectopic antennae. In some cases, an almost complete antenna was observed instead of an eye, which was possibly transformation from an eye to an antenna. As a molecular evidence for the ectopic antennae, I found that the Distal-less protein, which is essential for the distalization process, was ectopically induced in the eye discs. I also found that the Decapentaplegic and Wingless proteins, which are upstream morphogenetic signaling proteins of the distalization process, could be ectopically induced by activation of Rac1 or Cdc42. My observations suggest novel functions of Rac1 and Cdc42 for disc morphogenesis.

A 21st century view of evolution: genome system architecture, repetitive DNA, and natural genetic engineering

The last 50 years of molecular genetics have produced an abundance of new discoveries and data that make it useful to revisit some basic concepts and assumptions in our thinking about genomes and evolution. Chief among these observations are the complex modularity of genome organization, the biological ubiquity of mobile and repetitive DNA sequences, and the fundamental importance of DNA rearrangements in the evolution of sequenced genomes. This review will take a broad overview of these developments and suggest some new ways of thinking about genomes as sophisticated informatic storage systems and about evolution as a systems engineering process.

Dynamics and function of intron sequences of the wingless gene during the evolution of the Drosophila genus

To understand the function and evolution of genes with complex patterns of expression, such as the Drosophila wingless gene, it is essential to know how their transcription is regulated. However, extracting the relevant regulatory information from a genome is still a complex task. We used a combination of comparative genomics and functional approaches to identify putative regulatory sequences in two introns (1 and 3) of the wingless gene and to infer their evolution. Comparison of the sequences obtained from several Drosophila species revealed colinear and well-conserved sequence blocks in both introns. Drosophila willistoni showed a rate of evolution, in both introns, faster than expected from its phylogenetic position. Intron 3 appeared to be composed of two separate modules, one of them lost in the willistoni group. We tested whether sequence conservation in noncoding regions is a reliable indicator of regulatory function and, if this function is conserved, by analyzing D. melanogaster transgenic reporter lines harboring intron 3 sequences from D. melanogaster (Sophophora subgenus) and the species from the Drosophila subgenus presenting the most divergent sequence, D. americana. The analysis indicated that intron 3 contains pupal enhancers conserved during the evolution of the genus, despite the fact that only 30% of the D. melanogaster intron 3 sequences lie in conserved blocks. Additional analysis of D. melanogaster transgenic reporter lines harboring intron 3 sequences from D. willistoni revealed the absence of an abdomen-specific expression pattern, probably due to the above-mentioned loss of a regulatory module in this species.

The Wingless morphogen gradient is established by the cooperative action of Frizzled and Heparan Sulfate Proteoglycan receptors

We have examined the respective contribution of Heparan Sulfate Proteoglycans (HSPGs) and Frizzled (Fz) proteins in the establishment of the Wingless (Wg) morphogen gradient. From the analysis of mutant clones of sulfateless/N-deacetylase-sulphotransferase in the wing imaginal disc, we find that lack of Heparan Sulfate (HS) causes a dramatic reduction of both extracellular and intracellular Wg in receiving cells. Our studies, together with others [Kirkpatrick, C.A., Dimitroff, B.D., Rawson, J.M., Selleck, S.B., 2004. Spatial regulation of Wingless morphogen distribution and signalling by Dally-like protein. Dev. Cell (in press)], reveals that the Glypican molecule Dally-like Protein (Dlp) is associated with both negative and positive roles in Wg short- and long-range signaling, respectively. In addition, analyses of the two Fz proteins indicate that the Fz and DFz2 receptors, in addition to transducing the signal, modulate the slope of the Wg gradient by regulating the amount of extracellular Wg. Taken together, our analysis illustrates how the coordinated activities of HSPGs and Fz/DFz2 shape the Wg morphogen gradient.

DrosophilaDeltex mediates Suppressor of Hairless-independent and late-endosomal activation of Notch signaling

Notch (N) signaling is an evolutionarily conserved mechanism that regulates many cell-fate decisions. deltex (dx) encodes an E3-ubiquitin ligase that binds to the intracellular domain of N and positively regulates N signaling. However, the precise mechanism of Dx action is unknown. Here, we found that Dx was required and sufficient to activate the expression of gene targets of the canonical Su(H)-dependent N signaling pathway. Although Dx required N and a cis-acting element that overlaps with the Su(H)-binding site, Dx activated a target enhancer of N signaling, the dorsoventral compartment boundary enhancer of vestigial (vgBE), in a manner that was independent of the Delta (Dl)/Serrate (Ser) ligands- or Su(H). Dx caused N to be moved from the apical cell surface into the late-endosome, where it accumulated stably and co-localized with Dx. Consistent with this, the dx gene was required for the presence of N in the endocytic vesicles. Finally, blocking the N transportation from the plasma membrane to the late-endosome by a dominant-negative form of Rab5 inhibited the Dx-mediated activation of N signaling, suggesting that the accumulation of N in the late-endosome was required for the Dx-mediated Su(H)-independent N signaling.

Hedgehog and Decapentaplegic instruct polarized growth of cell extensions in theDrosophilatrachea

The migration of cellular extensions is guided by signals from tissues with which they contact. Many axon guidance molecules regulate growth cone migration by directly regulating actin cytoskeletal dynamics. Secreted morphogens control global patterns of cell fate decisions during organogenesis through transcriptional regulation, and constitute another class of guidance molecules. We have investigated the guidance roles of the morphogens Hedgehog and Decapentaplegic during directed outgrowth of cytoplasmic extensions in the Drosophila trachea. A subset of tracheal terminal cells adheres to the internal surface of the epidermis and elongates cytoplasmic processes called terminal branches. Hedgehog promotes terminal branch spreading and its extension over the posterior compartment of the epidermis. Decapentaplegic,which is expressed at the onset of terminal branching, restricts dorsal extension of the terminal branch and ensures its monopolar growth. Orthogonal expression of Hedgehog and Decapentaplegic in the epidermis instructs monopolar extension of the terminal branch along the posterior compartment,thereby matching the pattern of airway growth with that of the epidermis.

Enhancer trapping in zebrafish using the Sleeping Beauty transposon

Background

Among functional elements of a metazoan gene, enhancers are particularly difficult to find and annotate. Pioneering experiments in Drosophila have demonstrated the value of enhancer "trapping" using an invertebrate to address this functional genomics problem.

Results

We modulated a Sleeping Beauty transposon-based transgenesis cassette to establish an enhancer trapping technique for use in a vertebrate model system, zebrafish Danio rerio. We established 9 lines of zebrafish with distinct tissue- or organ-specific GFP expression patterns from 90 founders that produced GFP-expressing progeny. We have molecularly characterized these lines and show that in each line, a specific GFP expression pattern is due to a single transposition event. Many of the insertions are into introns of zebrafish genes predicted in the current genome assembly. We have identified both previously characterized as well as novel expression patterns from this screen. For example, the ET7 line harbors a transposon insertion near the mkp3 locus and expresses GFP in the midbrain-hindbrain boundary, forebrain and the ventricle, matching a subset of the known FGF8-dependent mkp3 expression domain. The ET2 line, in contrast, expresses GFP specifically in caudal primary motoneurons due to an insertion into the poly(ADP-ribose) glycohydrolase (PARG) locus. This surprising expression pattern was confirmed using in situ hybridization techniques for the endogenous PARG mRNA, indicating the enhancer trap has replicated this unexpected and highly localized PARG expression with good fidelity. Finally, we show that it is possible to excise a Sleeping Beauty transposon from a genomic location in the zebrafish germline.

Conclusions

This genomics tool offers the opportunity for large-scale biological approaches combining both expression and genomic-level sequence analysis using as a template an entire vertebrate genome.

An axon scaffold induced by retinal axons directs glia to destinations in the Drosophila optic lobe

In the developing Drosophila visual system, glia migrate into stereotyped positions within the photoreceptor axon target fields and provide positional information for photoreceptor axon guidance. Glial migration conversely depends on photoreceptor axons, as glia precursors stall in their progenitor zones when retinal innervation is eliminated. Our results support the view that this requirement for retinal innervation reflects a role of photoreceptor axons in the establishment of an axonal scaffold that guides glial cell migration. Optic lobe cortical axons extend from dorsal and ventral positions towards incoming photoreceptor axons and establish at least four separate pathways that direct glia to proper destinations in the optic lobe neuropiles. Photoreceptor axons induce the outgrowth of these scaffold axons. Most glia do not migrate when the scaffold axons are missing. Moreover, glia follow the aberrant pathways of scaffold axons that project aberrantly, as occurs in the mutant dachsous. The local absence of glia is accompanied by extensive apoptosis of optic lobe cortical neurons. These observations reveal a mechanism for coordinating photoreceptor axon arrival in the brain with the distribution of glia to multiple target destinations, where they are required for axon guidance and neuronal survival.

Propagation of silencing; recruitment and repression of naive chromatin in trans by polycomb repressed chromatin

The Polycomb group (PcG) proteins maintain stable and heritable repression of homeotic genes. Typically, Polycomb response elements (PRE) that direct PcG repression are located at great distances (10s of kb) from the promoters of PcG-repressed genes, and it is not known how these PREs can communicate with promoters over such distances. Using Class II mouse PRC core complexes (mPCCs) assembled from recombinant subunits, we investigated how PcG complexes might bridge distant chromosomal regions. Like native and recombinant Drosophila Class II complexes, mPCC represses chromatin remodeling and transcription. Interestingly, mPCC bound to one polynucleosome template can recruit a second template from solution and renders it refractory to transcription and chromatin remodeling. A Drosophila PRC core complex (dPCC) also is able to recruit a second template. Posterior sex combs (PSC), a subunit of dPCC, inhibits chromatin remodeling and transcription efficiently but requires assembly with dRING1 to recruit chromatin. Thus, repression and template bridging require different subunits of PcG complexes, suggesting that long-range effects may be mechanistically distinct from repression.

Genetics of P-Element Transposition Into Drosophila melanogaster Centric Heterochromatin

Heterochromatin is a major component of higher eukaryotic genomes, but progress in understanding the molecular structure and composition of heterochromatin has lagged behind the production of relatively complete euchromatic genome sequences. The introduction of single-copy molecular-genetic entry points can greatly facilitate structure and sequence analysis of heterochromatic regions that are rich in repeated DNA. In this study, we report the isolation of 502 new P-element insertions into Drosophila melanogaster centric heterochromatin, generated in nine different genetic screens that relied on mosaic silencing (position-effect variegation, or PEV) of the yellow gene present in the transposon. The highest frequencies of recovery of variegating insertions were observed when centric insertions were used as the source for mobilization. We propose that the increased recovery of variegating insertions from heterochromatic starting sites may result from the physical proximity of different heterochromatic regions in germline nuclei or from the association of mobilizing elements with heterochromatin proteins. High frequencies of variegating insertions were also recovered when a potent suppressor of PEV (an extra Y chromosome) was present in both the mobilization and selection generations, presumably due to the effects of chromatin structure on P-element mobilization, insertion, and phenotypic selection. Finally, fewer variegating insertions were recovered after mobilization in females, in comparison to males, which may reflect differences in heterochromatin structure in the female and male germlines. FISH localization of a subset of the insertions confirmed that 98% of the variegating lines contain heterochromatic insertions and that these schemes produce a broader distribution of insertion sites. The results of these schemes have identified the most efficient methods for generating centric heterochromatin P insertions. In addition, the large collection of insertions produced by these screens provides molecular-genetic entry points for mapping, sequencing, and functional analysis of Drosophila heterochromatin.

Embryonic enhancers in the dpp disk region regulate a second round of Dpp signaling from the dorsal ectoderm to the mesoderm that represses Zfh-1 expression in a subset of pericardial cells

During germ band elongation, widespread decapentaplegic (dpp) expression in the dorsal ectoderm patterns the underlying mesoderm. These Dpp signals specify cardial and pericardial cell fates in the developing heart. At maximum germ band extension, dpp dorsal ectoderm expression becomes restricted to the dorsal-most or leading edge cells (LE). A second round of Dpp signaling then specifies cell shape changes in ectodermal cells leading to dorsal closure. Here we show that a third round of dpp dorsal ectoderm expression initiates during germ band retraction. This round of dpp expression is also restricted to LE cells but Dpp signaling specifies the repression of the transcription factor Zfh-1 in a subset of pericardial cells in the underlying mesoderm. Surprisingly, we found that cis-regulatory sequences that activate the third round of dpp dorsal ectoderm expression are found in the dpp disk region. We also show that the activation of this round of dpp expression is dependent upon prior Dpp signals, the signal transducer Medea, and possibly release from dTCF-mediated repression. Our results demonstrate that a second round of Dpp signaling from the dorsal ectoderm to the mesoderm is required to pattern the developing heart and that this round of dpp expression may be activated by combinatorial interactions between Dpp and Wingless.

The role of Wg signaling in the patterning of embryonic leg primordium in Drosophila

Cellular interaction between the proximal and distal domains of the limb plays key roles in proximal-distal patterning. In Drosophila, these domains are established in the embryonic leg imaginal disc as a proximal domain expressing escargot, surrounding the Distal-less expressing distal domain in a circular pattern. The leg imaginal disc is derived from the limb primordium that also gives rise to the wing imaginal disc. We describe here essential roles of Wingless in patterning the leg imaginal disc. Firstly, Wingless signaling is essential for the recruitment of dorsal-proximal, distal, and ventral-proximal leg cells. Wingless requirement in the proximal leg domain appears to be unique to the embryo, since it was previously shown that Wingless signal transduction is not active in the proximal leg domain in larvae. Secondly, downregulation of Wingless signaling in wing disc is essential for its development, suggesting that Wg activity must be downregulated to separate wing and leg discs. In addition, we provide evidence that Dll restricts expression of a proximal leg-specific gene expression. We propose that those embryo-specific functions of Wingless signaling reflect its multiple roles in restricting competence of ectodermal cells to adopt the fate of thoracic appendages.

Enhancer-promoter communication mediated by Chip during Pannier-driven proneural patterning is regulated by Osa

The GATA factor Pannier activates proneural achaete/scute (ac/sc) expression during development of the sensory organs of Drosophila through enhancer binding. Chip bridges Pannier with the (Ac/Sc)-Daughterless heterodimers bound to the promoter and facilitates the enhancer-promoter communication required for proneural development. We show here that this communication is regulated by Osa, which is recruited by Pannier and Chip. Osa belongs to Brahma chromatin remodeling complexes and we show that Osa negatively regulates ac/sc. Consequently, Pannier and Chip also play an essential role during repression of proneural gene expression. Our study suggests that altering chromatin structure is essential for regulation of enhancer-promoter communication.

Patterns of Hermes transposition in Drosophila melanogaster

Transposable elements are being developed as tools for genomics and for the manipulation of insect genotypes for the purposes of biological control. An understanding of their transposition behavior will facilitate the use of these elements. The behavior of an autonomous Hermes transposable element from Musca domestica in the soma and germ-line of Drosophila melanogaster was investigated using the method of transposon display. In the germ-line, Hermes transposed at a rate of approximately 0.03 jumps per element per generation. Within the soma Hermes exhibited markedly non-random patterns of integration. Certain regions of the genome were distinctly preferred over others as integration targets, while other regions were underrepresented among the integration sites used. One particular site accounted for 4.4% of the transpositions recovered in this experiment, all of which were located within a 2.5-kb region of the actin5C promoter. This region was also present within the Hermes element itself, suggesting that this clustering is an example of transposable element "homing". Clusters of integration sites were also observed near the original donor sites; these represent examples of local hopping. The information content (sequence specificity) of the 8-bp target site was low, and the consensus target site resembles that determined from plasmid-based integration assays.

Temporally dynamic response to Wingless directs the sequential elaboration of the proximodistal axis of the Drosophila wing

The Drosophila wing imaginal disc gives rise to three main regions along the proximodistal axis of the dorsal mesothoracic segment: the notum, proximal wing, and wing blade. Development of the wing blade requires the Notch and wingless signalling pathways to activate vestigial at the dorsoventral boundary. However, in the proximal wing, Wingless activates a different subset of genes, e.g., homothorax. This raises the question of how the downstream response to Wingless signalling differentiates between proximal and distal fate specification. Here, we show that a temporally dynamic response to Wingless signalling sequentially elaborates the proximodistal axis. In the second instar, Wingless activates genes involved in proximal wing development; later in the third instar, Wingless acts to direct the differentiation of the distal wing blade. The expression of a novel marker for proximal wing fate, zfh-2, is initially activated by Wingless throughout the "wing primordium," but later is repressed by the activity of Vestigial and Nubbin, which together define a more distal domain. Thus, activation of a distal developmental program is antagonistic to previously established proximal fate. In addition, Wingless is required early to establish proximal fate, but later when Wingless activates distal differentiation, development of proximal fate becomes independent of Wingless signalling. Since P-element insertions in the zfh-2 gene result in a revertable proximal wing deletion phenotype, it appears that zfh-2 activity is required for correct proximal wing development. Our data are consistent with a model in which Wingless first establishes a proximal appendage fate over notum, then the downstream response changes to direct the differentiation of a more distal fate over proximal. Thus, the proximodistal domains are patterned in sequence and show a distal dominance.

Genetic requirements of vestigial in the regulation of Drosophila wing development

The gene vestigial has been proposed to act as a master gene because of its supposed capacity to initiate and drive wing development. We show that the ectopic expression of vestigial only induces ectopic outgrowths with wing cuticular differentiation and wing blade gene expression patterns in specific developmental and genetic contexts. In the process of transformation, wingless seems to be an essential but insufficient co-factor of vestigial. vestigial ectopic expression alone or vestigial plus wingless co-expression in clones differentiate 'mixed' cuticular patterns (they contain wing blade trichomes and chaetae characteristic of the endogenous surrounding tissue) and express wing blade genes only in patches of cells within the clones. In addition, we have found that these clones, in the wing imaginal disc, may cause autonomous as well as non-autonomous cuticular transformations and wing blade gene expression patterns. These non-autonomous effects in surrounding cells result from recruitment or 'inductive assimilation' of vestigial or wingless-vestigial overexpressing cells.

Genome organization and reorganization in evolution: formatting for computation and function

This volume deals with the role of epigenetics in life and evolution. The most dynamic forms of functional genome formatting involve DNA interacting with cellular complexes that do not alter sequence information. Such important epigenetic phenomena are the main subjects of other articles in this volume. This article focuses on the long-lived form of genome formatting that lies within the DNA sequence itself. I argue for a computational view of genome function as the long-term information storage organelle of each cell. Structural formatting consists of organizing various signals and coding sequences into computationally ready systems facilitating genome expression and genome transmission. The basic features of genome organization can be understood by examining the E. coli lac operon as a paradigmatic genomic system. Multiple systems are connected through distributed signals and repetitive DNA to form higher-order genome system architectures. Molecular discoveries about mechanisms of DNA restructuring show that cells possess the natural genetic engineering functions necessary for evolutionary change by rearranging genomic components and reorganizing system architectures. The concepts of cellular computation and decision-making, genome system architecture, and natural genetic engineering combine to provide a new way of framing evolutionary theories and understanding genome sequence information.

Different mechanisms initiate and maintain wingless expression in the Drosophila wing hinge

The Drosophila gene wingless encodes a secreted signalling molecule that is required for many patterning events in both embryonic and postembryonic development. In the wing wingless is expressed in a complex and dynamic pattern that is controlled by several different mechanisms. These involve the Hedgehog and Notch pathways and the nuclear proteins Pannier and U-shaped. In this report, we analyse the mechanisms that drive wingless expression in the wing hinge. We present evidence that wingless is initially activated by a secreted signal that requires the genes vestigial, rotund and nubbin. Later in development, wingless expression in the wing hinge is maintained by a different mechanism, which involves an autoregulatory loop and requires the genes homothorax and rotund. We discuss the role of wingless in patterning the wing hinge.

Combinatorial signaling by an unconventional Wg pathway and the Dpp pathway requires Nejire (CBP/p300) to regulate dpp expression in posterior tracheal branches

The decapentaplegic (dpp) gene influences many developmental events in Drosophila melanogaster. We have been analyzing dpp expression in two groups of dorsal ectoderm cells at the posterior end of the embryo, in abdominal segment 8 and the telson. These dpp-expressing cells become tracheal cells in the posterior-most branches of the tracheal system (Dorsal Branch10, Spiracular Branch10, and the Posterior Spiracle). These branches are not identified by reagents typically used in analyses of tracheal development, suggesting that dpp expression confers a distinct identity upon posterior tracheal cells. We have determined that dpp posterior ectoderm expression begins during germ band extension and continues throughout development. We have isolated the sequences responsible for these aspects of dpp expression in a reporter gene. We have determined that an unconventional form of Wingless (Wg) signaling, Dpp signaling, and the transcriptional coactivator Nejire (CBP/p300) are required for the initiation and maintenance of dpp expression in the posterior-most branches of the tracheal system. Our data suggest a model for the integration of Wg and Dpp signals that may be applicable to branching morphogenesis in other developmental systems.

Wingless blocks bristle formation and morphogenetic furrow progression in the eye through repression of Daughterless

In the developing eye, wingless activity represses proneural gene expression (and thus interommatidial bristle formation) and positions the morphogenetic furrow by blocking its initiation in the dorsal and ventral regions of the presumptive eye. We provide evidence that wingless mediates both effects, at least in part, through repression of the basic helix-loop-helix protein Daughterless. daughterless is required for high proneural gene expression and furrow progression. Ectopic expression of wingless blocks Daughterless expression in the proneural clusters. This repression, and that of furrow progression, can be mimicked by an activated form of armadillo and blocked by a dominant negative form of pangolin/TCF. Placing daughterless under the control of a heterologous promoter blocks the ability of ectopic wingless to inhibit bristle formation and furrow progression. hedgehog and decapentapleigic could not rescue the wingless furrow progression block, indicating that wingless acts downstream of these genes. In contrast, Atonal and Scute, which are thought to heterodimerize with Daughterless to promote furrow progression and bristle formation, respectively, can block ectopic wingless action. These results are summarized in a model where daughterless is a major, but probably not the only, target of wingless action in the eye.

The patterns of wingless, decapentaplegic, and tinman position the Drosophila heart

Two secreted signaling molecules, wingless (wg) and decapentaplegic (dpp), are required to specify the heart in Drosophila. wg and dpp are also required to specify other cell types within the mesoderm and in many other regions of the embryo. Because the spatial patterns of wg and dpp are dynamic, different populations of mesodermal cells are exposed to different combinations of wg and/or dpp at different times. To determine whether the patterns of wg and dpp expression provide unique positional information for the specification of heart precursors, we altered these patterns. Our data suggest that wg and dpp contribute progressively to the elaboration of the expression pattern of the mesoderm-specific homeobox-containing gene tinman (tin), and that the overlap of wg and dpp at an early stage (9) as well as at a later stage (11) in the presence of tin-expressing cells directs cardiac-specific differentiation. Furthermore, ectopic tin expression in the ectoderm at wg/dpp intersects (the primordia of the thoracic imaginal disks) also leads to cardiac-specific differentiation, suggesting that tin confers mesoderm-specificity to the wg/dpp response. We conclude that ectopic heart can be generated by altering the patterns of wg and dpp within the tin-expressing mesoderm, or by ectopic induction of tin within the wg- and dpp-expressing ectoderm.

An analysis using the hobo genetic system reveals that combinatorial signaling by the Dpp and Wg pathways regulates dpp expression in leading edge cells of the dorsal ectoderm in Drosophila melanogaster

Our laboratory has contributed to the development of a genetic system based upon the hobo transposable element in Drosophila melanogaster. We recently reported that hobo, like the better-known P element, is capable of local transposition. In that study, we mobilized a hobo enhancer trap vector and generated two unique alleles of decapentaplegic (dpp), a transforming growth factor-beta family member with numerous roles during development. Here we report a detailed study of one of those alleles (dpp(F11)). To our knowledge, this is the first application of the hobo genetic system to understanding developmental processes. First, we demonstrate that lacZ expression from the dpp(F11) enhancer trap accurately reflects dpp mRNA accumulation in leading edge cells of the dorsal ectoderm. Then we show that combinatorial signaling by the Wingless (Wg) pathway, the Dpp pathway, and the transcriptional coactivator Nejire (CBP/p300) regulates dpp(F11) expression in these cells. Our analysis of dpp(F11) suggests a model for the integration of Wg and Dpp signals that may be applicable to other developmental systems. Our analysis also illustrates several new features of the hobo genetic system and highlights the value of hobo, as an alternative to P, in addressing developmental questions.

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

The subdivision of the Drosophila wing imaginal disc into dorsoventral (DV) compartments and limb-body wall (wing-notum) primordia depends on Epidermal Growth Factor Receptor (EGFR) signaling, which heritably activates apterous (ap) in D compartment cells and maintains Iroquois Complex (Iro-C) gene expression in prospective notum cells. We examine the source, identity and mode of action of the EGFR ligand(s) that specify these subdivisions. Of the three known ligands for the Drosophila EGFR, only Vein (Vn), but not Spitz or Gurken, is required for wing disc development. We show that Vn activity is required specifically in the dorsoproximal region of the wing disc for ap and Iro-C gene expression. However, ectopic expression of Vn in other locations does not reorganize ap or Iro-C gene expression. Hence, Vn appears to play a permissive rather than an instructive role in organizing the DV and wing-notum segregations, implying the existance of other localized factors that control where Vn-EGFR signaling is effective. After ap is heritably activated, the level of EGFR activity declines in D compartment cells as they proliferate and move ventrally, away from the source of the instructive ligand. We present evidence that this reduction is necessary for D and V compartment cells to interact along the compartment boundary to induce signals, like Wingless (Wg), which organize the subsequent growth and differentiation of the wing primordium.

Dominant Drop mutants are gain-of-function alleles of the muscle segment homeobox gene (msh) whose overexpression leads to the arrest of eye development

Dominant Drop (Dr) mutations are nearly eyeless and have additional recessive phenotypes including lethality and patterning defects in eye and sensory bristles due to cis-regulatory lesions in the cell cycle regulator string (stg). Genetic analysis demonstrates that the dominant small eye phenotype is the result of separate gain-of-function mutations in the closely linked muscle segment homeobox (msh) gene, encoding a homeodomain transcription factor required for patterning of muscle and nervous system. Reversion of the Dr(Mio) allele was coincident with the generation of lethal loss-of-function mutations in msh in cis, suggesting that the dominant eye phenotype is the result of ectopic expression. Molecular genetic analysis revealed that two dominant Dr alleles contain lesions upstream of the msh transcription start site. In the Dr(Mio) mutant, a 3S18 retrotransposon insertion is the target of second-site mutations (P-element insertions or deletions) which suppress the dominant eye phenotype following reversion. The pattern of 3S18 expression and the absence of msh in eye imaginal discs suggest that transcriptional activation of the msh promoter accounts for ectopic expression. Dr dominant mutations arrest eye development by blocking the progression of the morphogenetic furrow leading to photoreceptor cell loss via apoptosis. Gal4-mediated ubiquitous expression of msh in third-instar larvae was sufficient to arrest the morphogenetic furrow in the eye imaginal disc and resulted in lethality prior to eclosion. Dominant mutations in the human msx2 gene, one of the vertebrate homologs of msh, are associated with craniosynostosis, a disease affecting cranial development. The Dr mutations are the first example of gain-of-function mutations in the msh/msx gene family identified in a genetically tractible model organism and may serve as a useful tool to identify additional genes that regulate this class of homeodomain proteins.

DWnt4 and wingless elicit similar cellular responses during imaginal development

Wnt genes encode evolutionarily conserved secreted proteins that provide critical functions during development. Although Wnt proteins share highly conserved features, they also show sequence divergence, which almost certainly contributes to the variety of their signaling activities. We previously reported that DWnt4 and wingless (wg), two divergent clustered Wnt genes, can have either antagonist or distinct functions during Drosophila embryogenesis. Here we provide evidence that both genes can elicit similar cellular responses during imaginal development. Ectopic expression of DWnt4 along the anterior/posterior (A/P) boundary of imaginal discs alters morphogenesis of adult appendages. In the wing disc, DWnt4 phenocopies ectopic Wg activity by inducing notum to wing transformation, suggesting similar signaling capabilities of both molecules. In support of this, we demonstrate that DWnt4 can rescue wg loss-of-function phenotypes in the antenna and haltere and is able to substitute for Wg in wing field specification. We also show that both genes are transcribed in overlapping domains in imaginal discs, suggesting that DWnt4 may cooperate with wg during limb patterning.

Developmental roles and molecular characterization of a Drosophila homologue of Arabidopsis Argonaute1, the founder of a novel gene superfamily

BACKGROUND

Arabidopsis Argonaute1 (AGO1) is the founder of a novel gene superfamily that is conserved from fission yeasts to humans. AGO1, and several other members of this superfamily are necessary for stem cell renewal or RNA interference. However, little has been reported about their roles in animal development or about the molecular activities of any of the members.

RESULTS

We have isolated a Drosophila homologue of AGO1, dAGO1, in our attempt to search genetically for regulators of Wingless (Wg) signal transduction. dAGO1 is broadly expressed in the embryo and the imaginal disc. dAGO1 over-expression at wing margins suggested that it behaves as a positive regulator in the genetic background employed. Loss-of-function mutations of dAGO1, unexpectedly, did not give typical segment polarity phenotypes of the wg class; instead, dAGO1 maternal and zygotic mutant embryos showed developmental defects, with malformation of the nervous system being the most prominent. The mutant decreased in the numbers of several types of neurones and glia examined. The dAGO1 protein was distributed in the cytoplasm and co-sedimented with poly(U)- or poly(A)-conjugated beads.

CONCLUSION

Our results suggest that the dAGO1 protein exerts its developmental functions by binding to RNA either directly or indirectly.

Drosophila homologues of the transcriptional coactivation complex subunits TRAP240 and TRAP230 are required for identical processes in eye-antennal disc development

We have identified mutations in two genes, blind spot and kohtalo, that encode Drosophila homologues of human TRAP240 and TRAP230, components of a large transcriptional coactivation complex homologous to the yeast Mediator complex. Loss of either blind spot or kohtalo has identical effects on the development of the eye-antennal disc. Eye disc cells mutant for either gene can express decapentaplegic and atonal in response to Hedgehog signaling, but they maintain inappropriate expression of these genes and fail to differentiate further. Mutant cells in the antennal disc lose expression of Distal-less and misexpress eyeless, suggesting a partial transformation towards the eye fate. blind spot and kohtalo are not required for cell proliferation or survival, and their absence cannot be rescued by activation of the Hedgehog or Notch signaling pathways. These novel and specific phenotypes suggest that TRAP240 and TRAP230 act in concert to mediate an unknown developmental signal or a combination of signals.

Combgap relays wingless signal reception to the determination of cortical cell fate in the Drosophila visual system

The dorsoventral axis of the Drosophila visual cortex is patterned by nonautonomous signals expressed at its dorsal and ventral margins. wingless (wg) expression at the margins induces decapentaplegic (dpp), optomotor blind (omb), and aristaless in adjacent domains. We show that Combgap, a zinc finger protein, represses Wg target gene expression in the visual cortex. Wg signal reception downregulates combgap expression and derepresses target gene transcription. Combgap participates in a Hedgehog-controlled circuit in the developing wing and leg by regulating the expression of Cubitus interruptus. Combgap is thus a tissue-specific relay between Wingless and its target genes for the determination of cell fate in the visual cortex.

Novel signaling from the peripodial membrane is essential for eye disc patterning in Drosophila

The Drosophila eye disc is a sac of single layer epithelium with two opposing sides, the peripodial membrane (PM) and the disc proper (DP). Retinal morphogenesis is organized by Notch signaling at the dorsoventral (DV) boundary in the DP. Functions of the PM in coordinating growth and patterning of the DP are unknown. We show that the secreted proteins, Hedgehog, Wingless, and Decapentaplegic, are expressed in the PM, yet they control DP expression of Notch ligands, Delta and Serrate. Peripodial clones expressing Hedgehog induce Serrate in the DP while loss of peripodial Hedgehog disrupts disc growth. Furthermore, PM cells extend cellular processes to the DP. Therefore, peripodial signaling is critical for eye pattern formation and may be mediated by peripodial processes.

Crossveinless 2 contains cysteine-rich domains and is required for high levels of BMP-like activity during the formation of the cross veins in Drosophila

The BMP-like signaling mediated by the ligands Dpp and Gbb is required to reinforce the development of most veins in the Drosophila wing. However, the formation of the cross veins is especially sensitive to reductions in BMP-like signaling. We show here that the formation of the definitive cross veins occurs after the initial specification of the longitudinal veins in a process that requires localized BMP-like activity. Since Dpp and Gbb levels are not detectably higher in the early phases of cross vein development, other factors apparently account for this localized activity. Our evidence suggests that the product of the crossveinless 2 gene is a novel member of the BMP-like signaling pathway required to potentiate Gbb of Dpp signaling in the cross veins. crossveinless 2 is expressed at higher levels in the developing cross veins and is necessary for local BMP-like activity. The Crossveinless 2 protein contains a putative signal or transmembrane sequence, and a partial Von Willebrand Factor D domain similar to those known to regulate the formation of intramolecular and intermolecular bonds. It also contains five cysteine-rich domains, similar to the cysteine-rich domains found in Chordin, Short Gastrulation and Procollagen that are known to bind BMP-like ligands. These features strongly suggest that Crossveinless 2 acts extracelluarly or in the secretory pathway to directly potentiate Dpp or Gbb signaling.

P element homing to the Drosophila bithorax complex

P elements containing a 7 kb DNA fragment from the middle of the Drosophila bithorax complex insert preferentially into the bithorax complex or into the adjacent chromosome regions. This ‘homing’ property is similar to that reported for the engrailed promoter (Hama, C., Ali, Z. and Kornberg, T. B. (1990) Genes Dev. 4, 1079–1093). The 7 kb fragment does not contain any known promoter, but it acts as a boundary element separating adjacent segmental domains. An enhancer-trap P element was constructed with the homing fragment and the selectable marker flanked by FRT sites. P insertions can be trimmed down by Flp-mediated recombination to just the lacZ reporter, so that the (beta)-galactosidase pattern is not influenced by sequences inside the P element. Twenty insertions into the bithorax complex express (beta)-galactosidase in segmentally limited patterns, reflecting the segmental domains of the bithorax complex where the elements reside. The mapping of segmental domains has now been revised, with enlargement of the abx/bx, bxd/pbx, and the iab-3 domains. The FRT sites in the P elements permit recombination between pairs of elements on opposite chromosomes, to generate duplications or deletions of the DNA between the two insertion sites. Using this technique, the length of the Ultrabithorax transcription unit was varied from 37 to 138 kb, but there was surprisingly little effect on Ultrabithorax function.

homothorax and iroquois-C genes are required for the establishment of territories within the developing eye disc

In Drosophila the eye-antennal disc gives rise to most adult structures of the fly's head. Yet the molecular basis for its regionalization during development is poorly understood. Here we show that homothorax is required early during development for normal eye development and is necessary for the formation of the ventral head capsule. In the ventral region of the disc only, homothorax and wingless are involved in a positive feedback loop necessary to restrict eye formation. homothorax is able to prevent the initiation and progression of the morphogenetic furrow without inducing wingless, which points to homothorax as a key negative regulator of eye development. In addition, we show that the iroquois-complex genes are required for dorsal head development antagonizing the function of homothorax in this region of the disc.

Drosophila SNS, a member of the immunoglobulin superfamily that is essential for myoblast fusion

The Drosophila sticks-and-stones (sns) locus was identified on the basis of its mutant phenotype, the complete absence of body wall muscles and corresponding presence of unfused myoblasts. The genetic location of the mutation responsible for this apparent defect in myoblast fusion was determined by recombination and deficiency mapping, and the corresponding wild-type gene was isolated in a molecular walk. Identification of the SNS coding sequence revealed a putative member of the immunoglobulin superfamily (IgSF) of cell adhesion molecules. As anticipated from this homology, SNS is enriched at the membrane and clusters at discrete sites, coincident with the occurrence of myoblast fusion. Both the sns transcript and the encoded protein are expressed in precursors of the somatic and visceral musculature of the embryo. Within the presumptive somatic musculature, SNS expression is restricted to the putative fusion-competent cells and is not detected in unfused founder cells. Thus, SNS represents the first known marker for this subgroup of myoblasts, and provides an opportunity to identify pathways specifying this cell type as well as transcriptional regulators of fusion-specific genes. To these ends, we demonstrate that the presence of SNS-expressing cells is absolutely dependent on Notch, and that expression of SNS does not require the myogenic regulatory protein MEF2.

Drosophila SNS, a member of the immunoglobulin superfamily that is essential for myoblast fusion

The Drosophila sticks-and-stones (sns) locus was identified on the basis of its mutant phenotype, the complete absence of body wall muscles and corresponding presence of unfused myoblasts. The genetic location of the mutation responsible for this apparent defect in myoblast fusion was determined by recombination and deficiency mapping, and the corresponding wild-type gene was isolated in a molecular walk. Identification of the SNS coding sequence revealed a putative member of the immunoglobulin superfamily (IgSF) of cell adhesion molecules. As anticipated from this homology, SNS is enriched at the membrane and clusters at discrete sites, coincident with the occurrence of myoblast fusion. Both the sns transcript and the encoded protein are expressed in precursors of the somatic and visceral musculature of the embryo. Within the presumptive somatic musculature, SNS expression is restricted to the putative fusion-competent cells and is not detected in unfused founder cells. Thus, SNS represents the first known marker for this subgroup of myoblasts, and provides an opportunity to identify pathways specifying this cell type as well as transcriptional regulators of fusion-specific genes. To these ends, we demonstrate that the presence of SNS-expressing cells is absolutely dependent on Notch, and that expression of SNS does not require the myogenic regulatory protein MEF2.

A dual role for homothorax in inhibiting wing blade development and specifying proximal wing identities in Drosophila

The Drosophila wing imaginal disc gives rise to three body parts along the proximo-distal (P-D) axis: the wing blade, the wing hinge and the mesonotum. Development of the wing blade initiates along part of the dorsal/ventral (D/V) compartment boundary and requires input from both the Notch and wingless (wg) signal transduction pathways. In the wing blade, wg activates the gene vestigial (vg), which is required for the wing blade to grow. wg is also required for hinge development, but wg does not activate vg in the hinge, raising the question of what target genes are activated by wg to generate hinge structures. Here we show that wg activates the gene homothorax (hth) in the hinge and that hth is necessary for hinge development. Further, we demonstrate that hth also limits where along the D/V compartment boundary wing blade development can initiate, thus helping to define the size and position of the wing blade within the disc epithelium. We also show that the gene teashirt (tsh), which is coexpressed with hth throughout most of wing disc development, collaborates with hth to repress vg and block wing blade development. Our results suggest that tsh and hth block wing blade development by repressing some of the activities of the Notch pathway at the D/V compartment boundary.

pannier acts upstream of wingless to direct dorsal eye disc development in Drosophila

The dorsoventral midline of the Drosophila eye disc is a source of signals that stimulate growth of the eye disc, define the point at which differentiation initiates, and direct ommatidial rotation in opposite directions in the two halves of the eye disc. This boundary region seems to be established by the genes of the iroquois complex, which are expressed in the dorsal half of the disc and inhibit fringe expression there. Fringe controls the activation of Notch and the expression of its ligands, with the result that Notch is activated only at the fringe expression boundary at the midline. The secreted protein Wingless activates the dorsal expression of the iroquois genes. We show here that pannier, which encodes a GATA family transcription factor expressed at the dorsal margin of the eye disc from embryonic stages on, acts upstream of wingless to control mirror and fringe expression and establish the dorsoventral boundary. Loss of pannier function leads to the formation of an ectopic eye field and the reorganization of ommatidial polarity, and ubiquitous pannier expression can abolish the eye field. Pannier is thus the most upstream element yet described in dorsoventral patterning of the eye disc.

The Drosophila sox gene, fish-hook, is required for postembryonic development

In Drosophila, the fish-hook (fish) gene encodes a Sox protein essential for embryonic segmentation and nervous system organization. In this study we examined potential functional roles of fish in postembryonic developmental processes, including those involved in adult appendage development. We show here that Fish protein is expressed in discrete patterns in the larval eye-antennal and leg imaginal discs, the central nervous system, the hindgut, and salivary glands. Genetic mosaic studies indicated that fish function is required for the growth or survival of imaginal cells, and the expression of engrailed and wingless. Ectopic expression of Fish protein resulted in severe disruption of adult structures; legs and antennae were truncated and eye formation was suppressed. These morphological defects were correlated with altered expression patterns of the wingless, decapentaplegic, and bric-a-brac genes. Finally, analysis of truncated versions of Fish protein indicated that the HMG domain was sufficient for Fish nuclear localization and that removal of the transcriptional activation domain did not eliminate Fish function. While Sox proteins have been shown to be important for eye and limb formation in vertebrates, these data provide the first evidence for Sox protein functions in appendage development in invertebrates.

Tissue-specific regulation of vein/EGF receptor signaling in Drosophila

Signaling by the Drosophila EGF receptor (DER) is modulated by four known EGF-like proteins: the agonists Vein (Vn), Spitz (Spi), and Gurken (Grk) and the antagonist Argos (Aos). DER is broadly expressed and thus tissue-specific regulation of ligand expression and activity is an important mechanism for controlling signaling. Here we investigate the tissue-specific regulation of Vn signaling by examining vn transcriptional control and Vn target gene activation in the embryo and the wing. The results show a complex temporal and spatial regulation of vn transcription involving multiple signaling pathways and tissue-specific activation of Vn target genes. In the embryo, vn is a target of Spi/DER signaling mediated by the ETS transcription factor PointedP1 (PntP1). This establishes a positive feedback loop in addition to the negative feedback loop involving Aos. The simultaneous production of Vn provides a mechanism for dampening Aos inhibition and thus fine-tunes signaling. In the larval wing pouch, vn is not a target of Spi/DER signaling but is expressed along the anterior-posterior boundary in response to Hedgehog (Hh) signaling. Repression by Wingless (Wg) signaling further refines the vn expression pattern by causing a discontinuity at the dorsal-ventral boundary. The potential for vn to activate DER target genes correlates with its roles in development: vn has a minor role in embryogenesis and does not induce DER target genes such as aos and pntP1 in the embryo. Conversely, vn has a major role in wing development and Vn/DER signaling is a potent inducer of DER target genes in the wing disc. Spi also has the potential to induce DER target genes in the wing disc. However, the ligands appear to evoke specific responses that result in different patterns of target gene expression. Finally, we show that other factors modulate the potential of Vn so that induction of Vn/DER target genes in the wing pouch is cell specific.

Different levels, but not different isoforms, of the Drosophila transcription factor DMEF2 affect distinct aspects of muscle differentiation

mef2 genes encode alternatively spliced transcription factor isoforms that function in muscle differentiation in both Drosophila and vertebrates. Drosophila mef2 (Dmef2) has been shown to be required for the differentiation of a variety of distinct muscle types. However, many possible aspects of its function in muscle remain unexplored. There has also been no analysis in vivo of the activity of different MEF2 isoforms in any species. Our investigation centred on the role of different levels of DMEF2 in the Drosophila embryo in regulating diverse events of muscle differentiation and on the functional significance of Dmef2 alternative splicing. We used the GAL4/UAS system to both misexpress and overexpress individual DMEF2 isoforms and to rescue the different aspects of the Dmef2 mutant phenotype. Ectopic ectodermal expression of DMEF2 activated muscle gene expression and inhibited epidermal differentiation. Overexpression of DMEF2 in the mesoderm disrupted differentiation of the somatic and visceral muscle and the heart. The use of different DMEF2 levels in the rescue experiments revealed an activity range compatible with differentiation of the different muscle types: the consequence of too little or too much DMEF2 activity was disrupted differentiation. These rescue experiments also revealed that distinct DMEF2 thresholds are required for different properties within a cell and also for different cells within a muscle type and for different muscle types. Finally, each isoform functioned equivalently in these experiments, including in the stringent test of rescue of the Dmef2 mutant phenotype.

Dfrizzled-3, a new Drosophila Wnt receptor, acting as an attenuator of Wingless signaling in wingless hypomorphic mutants

In Drosophila, two Frizzled proteins, Frizzled and Dfrizzled-2, have been reported to serve as receptors of Wingless. Here, we identified the third member of the Drosophila Frizzled family (Dfrizzled-3). In contrast to Dfrizzled-2, Dfrizzled-3 was transcriptionally upregulated by Wingless signaling. Although Dfrizzled-3 was capable of binding to Wingless in vitro, Wingless-dependent Armadillo/beta -catenin stabilization occurred much less effectively in Drosophila cells transfected with Dfrizzled-3 than in those with Dfrizzled-2. Flies lacking Dfrizzled-3 activity were viable and fertile, with few morphological defects. Genetic and immunochemical analysis indicated that the absence of Dfrizzled-3 activity suppresses the effects of hypomorphic wingless mutations such as failure of wing and antenna formation and restores target gene expression to the normal levels without change in wingless expression. Wingless signaling may thus be attenuated by Dfrizzled-3 at least in wingless hypomorphic mutants.

Mutual antagonism between signals secreted by adjacent wingless and engrailed cells leads to specification of complementary regions of the Drosophila parasegment

Specialized groups of cells known as organizers govern the establishment of cell type diversity across cellular fields. Segmental patterning within the Drosophila embryonic epidermis is one paradigm for organizer function. Here cells differentiate into smooth cuticle or distinct denticle types. At parasegment boundaries, cells expressing Wingless confront cells co-expressing Engrailed and Hedgehog. While Wingless is essential for smooth cell fates, the signals that establish denticle diversity are unknown. We show that wg mutants have residual mirror-symmetric pattern that is due to an Engrailed-dependent signal specifying anterior denticle fates. The Engrailed-dependent signal acts unidirectionally and Wg activity imposes this asymmetry. Reciprocally, the Engrailed/Hedgehog interface imposes asymmetry on Wg signaling. Thus, a bipartite organizer, with each signal acting essentially unidirectionally, specifies segmental pattern.