wingless is required for the formation of a subset of muscle founder cells during Drosophila embryogenesis

A Large Scale Systemic RNAi Screen in the Red Flour Beetle Tribolium castaneum Identifies Novel Genes Involved in Insect Muscle Development

Although muscle development has been widely studied in Drosophila melanogaster there are still many gaps in our knowledge, and it is not known to which extent this knowledge can be transferred to other insects. To help in closing these gaps we participated in a large-scale RNAi screen that used the red flour beetle, Tribolium castaneum, as a screening platform. The effects of systemic RNAi were screened upon double-stranded RNA injections into appropriate muscle-EGFP tester strains. Injections into pupae were followed by the analysis of the late embryonic/early larval muscle patterns, and injections into larvae by the analysis of the adult thoracic muscle patterns. Herein we describe the results of the first-pass screens with pupal and larval injections, which covered ∼8,500 and ∼5,000 genes, respectively, of a total of ∼16,500 genes of the Tribolium genome. Apart from many genes known from Drosophila as regulators of muscle development, a collection of genes previously unconnected to muscle development yielded phenotypes in larval body wall and leg muscles as well as in indirect flight muscles. We then present the main candidates from the pupal injection screen that remained after being processed through a series of verification and selection steps. Further, we discuss why distinct though overlapping sets of genes are revealed by the Drosophila and Tribolium screening approaches.

Wingless Signaling: A Genetic Journey from Morphogenesis to Metastasis

This FlyBook chapter summarizes the history and the current state of our understanding of the Wingless signaling pathway. Wingless, the fly homolog of the mammalian Wnt oncoproteins, plays a central role in pattern generation during development. Much of what we know about the pathway was learned from genetic and molecular experiments in Drosophila melanogaster, and the core pathway works the same way in vertebrates. Like most growth factor pathways, extracellular Wingless/Wnt binds to a cell surface complex to transduce signal across the plasma membrane, triggering a series of intracellular events that lead to transcriptional changes in the nucleus. Unlike most growth factor pathways, the intracellular events regulate the protein stability of a key effector molecule, in this case Armadillo/β-catenin. A number of mysteries remain about how the "destruction complex" destabilizes β-catenin and how this process is inactivated by the ligand-bound receptor complex, so this review of the field can only serve as a snapshot of the work in progress.

Myotube migration to cover and shape the testis of Drosophila depends on Heartless, Cadherin/Catenin, and myosin II

During Drosophila metamorphosis, nascent testis myotubes migrate from the prospective seminal vesicle of the genital disc onto pupal testes and then further to cover the testes with multinucleated smooth-like muscles. Here we show that DWnt2 is likely required for determination of testis-relevant myoblasts on the genital disc. Knock down of fibroblast growth factor receptor (FGFR) heartless by RNAi and a dominant-negative version revealed multiple functions of Heartless, namely regulation of the amount of myoblasts on the genital disc, connection of seminal vesicles and testes, and migration of muscles along the testes. Live imaging indicated that the downstream effector Stumps is required for migration of testis myotubes on the testis towards the apical tip. After myoblast fusion, myosin II is needed for migration of nascent testis myotubes, in which Thisbe-dependent fibroblast growth factor (FGF) signaling is activated. Cadherin-N is essential for connecting these single myofibers and for creating a firm testis muscle sheath that shapes and stabilizes the testis tubule. Based on these results, we propose a model for the migration of testis myotubes in which nascent testis myotubes migrate as a collective onto and along the testis, dependent on FGF-regulated expression of myosin II.

Summary:Drosophila testes and mammalian seminiferous tubules are surrounded by a muscle layer. Drosophila myotubes migrate towards testes in dependence of the FGF receptor Heartless, myosin II and Cadherin-N.

Wnt-Notch signalling crosstalk in development and disease

The Notch and Wnt pathways are two of only a handful of highly conserved signalling pathways that control cell-fate decisions during animal development (Pires-daSilva and Sommer in Nat Rev Genet 4: 39-49, 2003). These two pathways are required together to regulate many aspects of metazoan development, ranging from germ layer patterning in sea urchins (Peter and Davidson in Nature 474: 635-639, 2011) to the formation and patterning of the fly wing (Axelrod et al in Science 271:1826-1832, 1996; Micchelli et al in Development 124:1485-1495, 1997; Rulifson et al in Nature 384:72-74, 1996), the spacing of the ciliated cells in the epidermis of frog embryos (Collu et al in Development 139:4405-4415, 2012) and the maintenance and turnover of the skin, gut lining and mammary gland in mammals (Clayton et al in Nature 446:185-189, 2007; Clevers in Cell 154:274-284, 2013; Doupe et al in Dev Cell 18:317-323, 2010; Lim et al in Science 342:1226-1230, 2013; Lowell et al in Curr Biol 10:491-500, 2000; van et al in Nature 435:959-963, 2005; Yin et al in Nat Methods 11:106-112, 2013). In addition, many diseases, including several cancers, are caused by aberrant signalling through the two pathways (Bolós et al in Endocr Rev 28: 339-363, 2007; Clevers in Cell 127: 469-480, 2006). In this review, we will outline the two signalling pathways, describe the different points of interaction between them, and cover how these interactions influence development and disease.

Heart- and muscle-derived signaling system dependent on MED13 and Wingless controls obesity in Drosophila

Obesity develops in response to an imbalance of energy homeostasis and whole-body metabolism. Muscle plays a central role in the control of energy homeostasis through consumption of energy and signaling to adipose tissue. We reported previously that MED13, a subunit of the Mediator complex, acts in the heart to control obesity in mice. To further explore the generality and mechanistic basis of this observation, we investigated the potential influence of MED13 expression in heart and muscle on the susceptibility of Drosophila to obesity. Here, we show that heart/muscle-specific knockdown of MED13 or MED12, another Mediator subunit, increases susceptibility to obesity in adult flies. To identify possible muscle-secreted obesity regulators, we performed an RNAi-based genetic screen of 150 genes that encode secreted proteins and found that Wingless inhibition also caused obesity. Consistent with these findings, muscle-specific inhibition of Armadillo, the downstream transcriptional effector of the Wingless pathway, also evoked an obese phenotype in flies. Epistasis experiments further demonstrated that Wingless functions downstream of MED13 within a muscle-regulatory pathway. Together, these findings reveal an intertissue signaling system in which Wingless acts as an effector of MED13 in heart and muscle and suggest that Wingless-mediated cross-talk between striated muscle and adipose tissue controls obesity in Drosophila. This signaling system appears to represent an ancestral mechanism for the control of systemic energy homeostasis.

Discrete levels of Twist activity are required to direct distinct cell functions during gastrulation and somatic myogenesis

Twist (Twi), a conserved basic helix-loop-helix transcriptional regulator, directs the epithelial-to-mesenchymal transition (EMT), and regulates changes in cell fate, cell polarity, cell division and cell migration in organisms from flies to humans. Analogous to its role in EMT, Twist has been implicated in metastasis in numerous cancer types, including breast, pancreatic and prostate. In the Drosophila embryo, Twist is essential for discrete events in gastrulation and mesodermal patterning. In this study, we derive a twi allelic series by examining the various cellular events required for gastrulation in Drosophila. By genetically manipulating the levels of Twi activity during gastrulation, we find that coordination of cell division is the most sensitive cellular event, whereas changes in cell shape are the least sensitive. Strikingly, we show that by increasing levels of Snail expression in a severe twi hypomorphic allelic background, but not a twi null background, we can reconstitute gastrulation and produce viable adult flies. Our results demonstrate that the level of Twi activity determines whether the cellular events of ventral furrow formation, EMT, cell division and mesodermal migration occur.

Cadherin switching during the formation and differentiation of the Drosophila mesoderm - implications for epithelial-to-mesenchymal transitions

Epithelial-to-mesenchymal transition (EMT) is typically accompanied by downregulation of epithelial (E-) cadherin, and is often additionally accompanied by upregulation of a mesenchymal or neuronal (N-) cadherin. Snail represses transcription of the E-cadherin gene both during normal development and during tumour spreading. The formation of the mesodermal germ layer in Drosophila, considered a paradigm of a developmental EMT, is associated with Snail-mediated repression of E-cadherin and the upregulation of N-cadherin. By using genetic manipulation to remove or overexpress the cadherins, we show here that the complementarity of cadherin expression is not necessary for the segregation or the dispersal of the mesodermal germ layer in Drosophila. However, we discover different effects of E- and N-cadherin on the differentiation of subsets of mesodermal derivatives, which depend on Wingless signalling from the ectoderm, indicating differing abilities of E- and N-cadherin to bind to and sequester the common junctional and signalling effector β-catenin. These results suggest that the downregulation of E-cadherin in the mesoderm might be required to facilitate optimal levels of Wingless signalling.

A luminal glycoprotein drives dose-dependent diameter expansion of the Drosophila melanogaster hindgut tube

An important step in epithelial organ development is size maturation of the organ lumen to attain correct dimensions. Here we show that the regulated expression of Tenectin (Tnc) is critical to shape the Drosophila melanogaster hindgut tube. Tnc is a secreted protein that fills the embryonic hindgut lumen during tube diameter expansion. Inside the lumen, Tnc contributes to detectable O-Glycans and forms a dense striated matrix. Loss of tnc causes a narrow hindgut tube, while Tnc over-expression drives tube dilation in a dose-dependent manner. Cellular analyses show that luminal accumulation of Tnc causes an increase in inner and outer tube diameter, and cell flattening within the tube wall, similar to the effects of a hydrostatic pressure in other systems. When Tnc expression is induced only in cells at one side of the tube wall, Tnc fills the lumen and equally affects all cells at the lumen perimeter, arguing that Tnc acts non-cell-autonomously. Moreover, when Tnc expression is directed to a segment of a tube, its luminal accumulation is restricted to this segment and affects the surrounding cells to promote a corresponding local diameter expansion. These findings suggest that deposition of Tnc into the lumen might contribute to expansion of the lumen volume, and thereby to stretching of the tube wall. Consistent with such an idea, ectopic expression of Tnc in different developing epithelial tubes is sufficient to cause dilation, while epidermal Tnc expression has no effect on morphology. Together, the results show that epithelial tube diameter can be modelled by regulating the levels and pattern of expression of a single luminal glycoprotein.

Epithelial tubes constitute the functional units of vital organs, and they undergo highly regulated changes in size and shape during development to accommodate the three-dimensional configurations optimal for organ physiology. Through studies of Drosophila melanogaster, we show that epithelial tube diameter can be modelled simply by regulating the levels and pattern of expression of a single glycoprotein. The protein is secreted into the tubular lumen, where it forms a dense matrix and acts in a dose-dependent manner to drive diameter growth. We suggest that deposition of the protein into the lumen promotes local expansion of the lumen volume, and thereby stretching of the surrounding tube wall. Such a mechanism could represent a general means to adjust tube diameter during epithelial organ development.

Org-1, the Drosophila ortholog of Tbx1, is a direct activator of known identity genes during muscle specification

Members of the T-Box gene family of transcription factors are important players in regulatory circuits that generate myogenic and cardiogenic lineage diversities in vertebrates. We show that during somatic myogenesis in Drosophila, the single ortholog of vertebrate Tbx1, optomotor-blind-related-gene-1 (org-1), is expressed in a small subset of muscle progenitors, founder cells and adult muscle precursors, where it overlaps with the products of the muscle identity genes ladybird (lb) and slouch (slou). In addition, org-1 is expressed in the lineage of the heart-associated alary muscles. org-1 null mutant embryos lack Lb and Slou expression within the muscle lineages that normally co-express org-1. As a consequence, the respective muscle fibers and adult muscle precursors are either severely malformed or missing, as are the alary muscles. To address the mechanisms that mediate these regulatory interactions between Org-1, Lb and Slou, we characterized distinct enhancers associated with somatic muscle expression of lb and slou. We demonstrate that these lineage- and stage-specific cis-regulatory modules (CRMs) bind Org-1 in vivo, respond to org-1 genetically and require T-box domain binding sites for their activation. In summary, we propose that org-1 is a common and direct upstream regulator of slou and lb in the developmental pathway of these two neighboring muscle lineages. Cross-repression between slou and lb and combinatorial activation of lineage-specific targets by Org-1-Slou and Org-1-Lb, respectively, then leads to the distinction between the two lineages. These findings provide new insights into the regulatory circuits that control the proper pattering of the larval somatic musculature in Drosophila.

A machine learning approach for identifying novel cell type-specific transcriptional regulators of myogenesis

Transcriptional enhancers integrate the contributions of multiple classes of transcription factors (TFs) to orchestrate the myriad spatio-temporal gene expression programs that occur during development. A molecular understanding of enhancers with similar activities requires the identification of both their unique and their shared sequence features. To address this problem, we combined phylogenetic profiling with a DNA-based enhancer sequence classifier that analyzes the TF binding sites (TFBSs) governing the transcription of a co-expressed gene set. We first assembled a small number of enhancers that are active in Drosophila melanogaster muscle founder cells (FCs) and other mesodermal cell types. Using phylogenetic profiling, we increased the number of enhancers by incorporating orthologous but divergent sequences from other Drosophila species. Functional assays revealed that the diverged enhancer orthologs were active in largely similar patterns as their D. melanogaster counterparts, although there was extensive evolutionary shuffling of known TFBSs. We then built and trained a classifier using this enhancer set and identified additional related enhancers based on the presence or absence of known and putative TFBSs. Predicted FC enhancers were over-represented in proximity to known FC genes; and many of the TFBSs learned by the classifier were found to be critical for enhancer activity, including POU homeodomain, Myb, Ets, Forkhead, and T-box motifs. Empirical testing also revealed that the T-box TF encoded by org-1 is a previously uncharacterized regulator of muscle cell identity. Finally, we found extensive diversity in the composition of TFBSs within known FC enhancers, suggesting that motif combinatorics plays an essential role in the cellular specificity exhibited by such enhancers. In summary, machine learning combined with evolutionary sequence analysis is useful for recognizing novel TFBSs and for facilitating the identification of cognate TFs that coordinate cell type-specific developmental gene expression patterns.

Molecular mechanism underlying the regulatory specificity of a Drosophila homeodomain protein that specifies myoblast identity

A subfamily of Drosophila homeodomain (HD) transcription factors (TFs) controls the identities of individual muscle founder cells (FCs). However, the molecular mechanisms by which these TFs generate unique FC genetic programs remain unknown. To investigate this problem, we first applied genome-wide mRNA expression profiling to identify genes that are activated or repressed by the muscle HD TFs Slouch (Slou) and Muscle segment homeobox (Msh). Next, we used protein-binding microarrays to define the sequences that are bound by Slou, Msh and other HD TFs that have mesodermal expression. These studies revealed that a large class of HDs, including Slou and Msh, predominantly recognize TAAT core sequences but that each HD also binds to unique sites that deviate from this canonical motif. To understand better the regulatory specificity of an individual FC identity HD, we evaluated the functions of atypical binding sites that are preferentially bound by Slou relative to other HDs within muscle enhancers that are either activated or repressed by this TF. These studies showed that Slou regulates the activities of particular myoblast enhancers through Slou-preferred sequences, whereas swapping these sequences for sites that are capable of binding to multiple HD family members does not support the normal regulatory functions of Slou. Moreover, atypical Slou-binding sites are overrepresented in putative enhancers associated with additional Slou-responsive FC genes. Collectively, these studies provide new insights into the roles of individual HD TFs in determining cellular identity, and suggest that the diversity of HD binding preferences can confer regulatory specificity.

Analysis of skeletal muscle development in Drosophila

The Drosophila system has been invaluable in providing important insights into mesoderm specification, muscle specification, myoblast fusion, muscle differentiation, and myofibril assembly. Here, we present a series of Drosophila protocols that enable the researcher to visualize muscle precursors and differentiated muscles, at all stages of development. In doing so, we also highlight the variety of techniques that are used to create these findings. These protocols are directly used for the Drosophila system, and are provided with explanatory detail to enable the researcher to apply them to other systems.

Diversification of muscle types in Drosophila: upstream and downstream of identity genes

Understanding gene regulatory pathways underlying diversification of cell types during development is one of the major challenges in developmental biology. Progressive specification of mesodermal lineages that are at the origin of body wall muscles in Drosophila embryos has been extensively studied during past years, providing an attractive framework for dissecting cell type diversification processes. In particular, it has been found that muscle founder cells that are at the origin of individual muscles display specific expression of transcription factors that control diversification of muscle types. These factors, encoded by genes collectively called muscle identity genes, are activated in discrete subsets of muscle founders. As a result, each founder cell is thought to carry a unique combinatorial code of identity gene expression. Considering this, to define temporally and spatially restricted expression of identity genes, a set of coordinated upstream regulatory inputs is required. But also, to realize the identity program and to form specific muscle types with distinct properties, an efficient battery of downstream identity gene targets needs to be activated. Here we review how the specificity of expression and action of muscle identity genes is acquired.

Canonical Wnt signaling in the visceral muscle is required for left-right asymmetric development of the Drosophila midgut

Many animals develop left-right (LR) asymmetry in their internal organs. The mechanisms of LR asymmetric development are evolutionarily divergent, and are poorly understood in invertebrates. Therefore, we studied the genetic pathway of LR asymmetric development in Drosophila. Drosophila has several organs that show directional and stereotypic LR asymmetry, including the embryonic gut, which is the first organ to develop LR asymmetry during Drosophila development. In this study, we found that genes encoding components of the Wnt-signaling pathway are required for LR asymmetric development of the anterior part of the embryonic midgut (AMG). frizzled 2 (fz2) and Wnt4, which encode a receptor and ligand of Wnt signaling, respectively, were required for the LR asymmetric development of the AMG. arrow (arr), an ortholog of the mammalian gene encoding low-density lipoprotein receptor-related protein 5/6, which is a co-receptor of the Wnt-signaling pathway, was also essential for LR asymmetric development of the AMG. These results are the first demonstration that Wnt signaling contributes to LR asymmetric development in invertebrates, as it does in vertebrates. The AMG consists of visceral muscle and an epithelial tube. Our genetic analyses revealed that Wnt signaling in the visceral muscle but not the epithelium of the midgut is required for the AMG to develop its normal laterality. Furthermore, fz2 and Wnt4 were expressed in the visceral muscles of the midgut. Consistent with these results, we observed that the LR asymmetric rearrangement of the visceral muscle cells, the first visible asymmetry of the developing AMG, did not occur in embryos lacking Wnt4 expression. Our results also suggest that canonical Wnt/β-catenin signaling, but not non-canonical Wnt signaling, is responsible for the LR asymmetric development of the AMG. Canonical Wnt/β-catenin signaling is reported to have important roles in LR asymmetric development in zebrafish. Thus, the contribution of canonical Wnt/β-catenin signaling to LR asymmetric development may be an evolutionarily conserved feature between vertebrates and invertebrates.

Characterization of early steps in muscle morphogenesis in a Drosophila primary culture system

Myogenesis in Drosophila embryos requires fusion between Founder cells (FCs) and Fusion Competent myoblasts (FCMs) to form multinucleate myotubes. Myoblast fusion is well characterized in embryos, and many factors required for this process have been identified; however, a number of questions pertaining to the mechanisms of fusion remain and are challenging to answer in the embryo. We have developed a modified primary cell culture protocol to address these questions in vitro. Using this system, we determined the optimal time for examining fusion in culture and confirmed that known fusion proteins are expressed and localized as in embryos. Importantly, we disrupted the actin and microtubule networks with the drugs latrunculin B and nocodazole, respectively, confirming that actin is required for myoblast fusion and showing for the first time that microtubules are also required for this process in Drosophila. Finally, we show that myotubes in culture adopt and maintain specific muscle identities.

Genetic control of muscle development: learning from Drosophila

Muscle development involves a complex sequence of time and spatially regulated cellular events leading to the formation of highly specialised syncytial muscle cells displaying a common feature, the capacity of contraction. Analyses of mechanisms controlling muscle development reveals that the main steps of muscle formation including myogenic determination, diversification of muscle precursors, myoblast fusion and terminal differentiation involve the actions of evolutionarily conserved genes. Thus dissecting the genetic control of muscle development in simple model organisms appears to be an attractive way to get insights into core genetic cascade that orchestrate myogenesis. In this respect, particularly insightful have been data generated using Drosophila as a model system. Notably, the interplay between intrinsic and extrinsic cues that determine the early myogenic decisions leading to the specification of muscle progenitors and those controlling myoblasts fusion are much better characterised in Drosophila than in vertebrate species. Also, adult Drosophila myogenesis, which leads to the formation of vertebrate-like multi-fibre muscles, emerges as a particularly well-adapted system to study normal and aberrant muscle development.

Daughterless dictates Twist activity in a context-dependent manner during somatic myogenesis

Somatic myogenesis in Drosophila relies on the reiterative activity of the basic helix-loop-helix transcriptional regulator, Twist (Twi). How Twi directs multiple cell fate decisions over the course of mesoderm and muscle development is unclear. Previous work has shown that Twi is regulated by its dimerization partner: Twi homodimers activate genes necessary for somatic myogenesis, whereas Twi/Daughterless (Da) heterodimers lead to the repression of these genes. Here, we examine the nature of Twi/Da heterodimer repressive activity. Analysis of the Da protein structure revealed a Da repression (REP) domain, which is required for Twi/Da-mediated repression of myogenic genes, such as Dmef2, both in tissue culture and in vivo. This domain is crucial for the allocation of mesodermal cells to distinct fates, such as heart, gut and body wall muscle. By contrast, the REP domain is not required in vivo during later stages of myogenesis, even though Twi activity is required for muscles to achieve their final pattern and morphology. Taken together, we present evidence that the repressive activity of the Twi/Da dimer is dependent on the Da REP domain and that the activity of the REP domain is sensitive to tissue context and developmental timing.

Segmental expression of Pax3/7 and engrailed homologs in tardigrade development

How morphological diversity arises through evolution of gene sequence is a major question in biology. In Drosophila, the genetic basis for body patterning and morphological segmentation has been studied intensively. It is clear that some of the genes in the Drosophila segmentation program are functioning similarly in certain other taxa, although many questions remain about when these gene functions arose and which taxa use these genes similarly to establish diverse body plans. Tardigrades are an outgroup to arthropods in the Ecdysozoa and, as such, can provide insight into how gene functions have evolved among the arthropods and their close relatives. We developed immunostaining methods for tardigrade embryos, and we used cross-reactive antibodies to investigate the expression of homologs of the pair-rule gene paired (Pax3/7) and the segment polarity gene engrailed in the tardigrade Hypsibius dujardini. We find that in H. dujardini embryos, Pax3/7 protein localizes not in a pair-rule pattern but in a segmentally iterated pattern, after the segments are established, in regions of the embryo where neurons later arise. Engrailed protein localizes in the posterior ectoderm of each segment before ectodermal segmentation is apparent. Together with previous results from others, our data support the conclusions that the pair-rule function of Pax3/7 is specific to the arthropods, that some of the ancient functions of Pax3/7 and Engrailed in ancestral bilaterians may have been in neurogenesis, and that Engrailed may have a function in establishing morphological boundaries between segments that is conserved at least among the Panarthropoda.

Parcas, a regulator of non-receptor tyrosine kinase signaling, acts during anterior-posterior patterning and somatic muscle development in Drosophila melanogaster

We have isolated parcas (pcs) in a screen to identify novel regulators of muscle morphogenesis. Pcs is expressed in the ovary and oocyte during oogenesis and again in the embryo, specifically in the developing mesoderm, throughout muscle development. pcs is first required in the ovary during oogenesis for patterning and segmentation of the early Drosophila embryo due primarily to its role in the regulation of Oskar (Osk) levels. In addition to the general patterning defects observed in embryos lacking maternal contribution of pcs, these embryos show defects in Wingless (Wg) expression, causing losses of Wg-dependent cell types within the affected segment. pcs activity is required again later during embryogenesis in the developing mesoderm for muscle development. Loss and gain of function studies demonstrate that pcs is necessary at distinct times for muscle specification and morphogenesis. Pcs is predicted to be a novel regulator of non-receptor tyrosine kinase (NRTK) signaling. We have identified one target of Pcs regulation, the Drosophila Tec kinase Btk29A. While Btk29A appears to be regulated by Pcs during its early role in patterning and segmentation, it does not appear to be a major target of Pcs regulation during muscle development. We propose that Pcs fulfils its distinct roles during development by the regulation of multiple NRTKs.

D-six4 plays a key role in patterning cell identities deriving from the Drosophila mesoderm

Patterning of the Drosophila embryonic mesoderm requires the regulation of cell type-specific factors in response to dorsoventral and anteroposterior axis information. For the dorsoventral axis, the homeodomain gene, tinman, is a key patterning mediator for dorsal mesodermal fates like the heart. However, equivalent mediators for more ventral fates are unknown. We show that D-six4, which encodes a Six family transcription factor, is required for the appropriate development of most cell types deriving from the non-dorsal mesoderm - the fat body, somatic cells of the gonad, and a specific subset of somatic muscles. Misexpression analysis suggests that D-Six4 and its likely cofactor, Eyes absent, are sufficient to impose these fates on other mesodermal cells. At stage 10, the mesodermal expression patterns of D-six4 and tin are complementary, being restricted to the dorsal and non-dorsal regions respectively. Our data suggest that D-six4 is a key mesodermal patterning mediator at this stage that regulates a variety of cell-type-specific factors and hence plays an equivalent role to tin. At stage 9, however, D-six4 and tin are both expressed pan-mesodermally. At this stage, tin function is required for full D-six4 expression. This may explain the known requirement for tin in some non-dorsal cell types.

Filtering transcriptional noise during development: concepts and mechanisms

The assignation of cell fates during eukaryotic development relies on the coordinated and stable expression of cohorts of genes within cell populations. The precise and reproducible nature of this process is remarkable given that, at the single-cell level, the transcription of individual genes is associated with noise - random molecular fluctuations that create variability in the levels of gene expression within a cell population. Here we consider the implications of transcriptional noise for development and suggest the existence of molecular devices that are dedicated to filtering noise. On the basis of existing evidence, we propose that one such mechanism might depend on the Wnt signalling pathway.

Development of Drosophila motoneurons: specification and morphology

In the Drosophila ventral nerve cord, segmentally repeated sets of approximately 80 motoneurons are generated during embryogenesis. Within each hemisegment, each motoneuron is characterised by its axonal projection and innervation of a particular target muscle as well as its dendritic tree in the central nervous system. Codes of transcriptional regulators appear to specify in a hierarchical fashion the cell type, motoneuron sub-types and eventually unique cellular identities. Recent studies show that patterns of connectivity in the periphery are mirrored by patterns of dendritic arborisation centrally thereby providing a neuronal correlate of connectivity to the anatomy of the motor system in the periphery. While the principal mechanisms that underlie the development of the peripheral neuromuscular system have been studied in some detail, much less is known about how the dendrites and their patterns of connections develop in the CNS.

Delivery of wingless to the ventral mesoderm by the developing central nervous system ensures proper patterning of individual slouch-positive muscle progenitors

During the development of any organism, care must be given to properly pattern gene expression in temporally and spatially regulated manners. This process becomes more complex when the signals that regulate a target tissue are produced in an adjacent tissue and must travel to the target tissue to affect gene expression. We have used the developing somatic mesoderm in Drosophila as a system in which to examine this problem. Our investigation uncovered a novel mechanism by which Wingless (Wg) can travel from its source in the ectoderm to regulate the expression of the somatic muscle founder identity gene, slouch, in the ventral mesoderm. Delivery of Wg to the mesoderm by the developing Central Nervous System (CNS) exploits the stereotypic formation of this tissue to provide high Wg levels to Slouch founder cell cluster II in a temporally specific manner. Coordinated development of these tissues provides a reliable mechanism for delivering high Wg levels to a subset of mesodermal cells. It also provides a means for one signaling pathway to be used reiteratively throughout development to impart unique positional and character information within a target field.

Embryonic origins of a motor system: motor dendrites form a myotopic map in Drosophila

The organisational principles of locomotor networks are less well understood than those of many sensory systems, where in-growing axon terminals form a central map of peripheral characteristics. Using the neuromuscular system of the Drosophila embryo as a model and retrograde tracing and genetic methods, we have uncovered principles underlying the organisation of the motor system. We find that dendritic arbors of motor neurons, rather than their cell bodies, are partitioned into domains to form a myotopic map, which represents centrally the distribution of body wall muscles peripherally. While muscles are segmental, the myotopic map is parasegmental in organisation. It forms by an active process of dendritic growth independent of the presence of target muscles, proper differentiation of glial cells, or (in its initial partitioning) competitive interactions between adjacent dendritic domains. The arrangement of motor neuron dendrites into a myotopic map represents a first layer of organisation in the motor system. This is likely to be mirrored, at least in part, by endings of higher-order neurons from central pattern-generating circuits, which converge onto the motor neuron dendrites. These findings will greatly simplify the task of understanding how a locomotor system is assembled. Our results suggest that the cues that organise the myotopic map may be laid down early in development as the embryo subdivides into parasegmental units.

Wnts as morphogens? The view from the wing of Drosophila

Morphogens are diffusible signalling molecules that pattern cellular fields by setting up differential gene expression in a concentration-dependent manner. Members of the Wnt family of signalling molecules are generally considered to be classical morphogens. However, a close analysis of their activity indicates that they do not fulfil all of the critera that are associated with the classical definition.

Notching up another pathway

The Notch proteins play a vital role in cell fate decisions in both invertebrate and vertebrate development. Careful analysis of this role has led to a model of signalling downstream of these receptors, via the CSL (CBF1, Suppressor of Hairless, Lag-1) family of transcription factors. There have been suggestions, however, that Notch can signal through other pathways. In the current paper, Ramain et al. provide compelling evidence for Notch signalling through a CSL-independent pathway and they demonstrate that the cytoplasmic protein, Deltex, is required for this signal. In addition, they show that Wnt signalling may regulate this Deltex-dependent signal.

Direct flight muscles in Drosophila develop from cells with characteristics of founders and depend on DWnt-2 for their correct patterning

The direct flight muscles (DFMs) of Drosophila allow for the fine control of wing position necessary for flight. In DWnt-2 mutant flies, certain DFMs are either missing or fail to attach to the correct epithelial sites. Using a temperature-sensitive allele, we show that DWnt-2 activity is required only during pupation for correct DFM patterning. DWnt-2 is expressed in the epithelium of the wing hinge primordium during pupation. This expression is in the vicinity of the developing DFMs, as revealed by expression of the muscle founder cell-specific gene dumbfounded in DFM precursors. The observation that a gene necessary for embryonic founder cell function is expressed in the DFM precursors suggests that these cells may have a similar founder cell role. Although the expression pattern of DWnt-2 suggests that it could influence epithelial cells to differentiate into attachment sites for muscle, the expression of stripe, a transcription factor necessary for epithelial cells to adopt an attachment cell fate, is unaltered in the mutant. Ectopic expression of DWnt-2 in the wing hinge during pupation can also create defects in muscle patterning without alterations in stripe expression. We conclude that DWnt-2 promotes the correct patterning of DFMs through a mechanism that is independent of the attachment site differentiation initiated by stripe.

Invertebrate myogenesis: looking back to the future of muscle development

Recent studies in invertebrates have provided important mechanistic insights into several general aspects of muscle development. Two new genes have been identified that are involved in muscle fusion in Drosophila and a novel maternal component was shown to be responsible for myogenic determination in an ascidian. In addition, genetic analyses of nematode and Drosophila homologues of factors known to be myogenic regulators in other species yielded surprising findings about both the evolutionary conservation and divergence of these functions. Drosophila myogenesis has become a highly informative model for understanding the interplay between the signaling and transcriptional networks that underlie cell-fate specification during embryonic development.

Homeotic Complex (Hox) gene regulation and homeosis in the mesoderm of the Drosophila melanogaster embryo: the roles of signal transduction and cell autonomous regulation

In this paper we evaluate homeosis and Homeotic Complex (Hox) regulatory hierarchies in the somatic and visceral mesoderm. We demonstrate that both Hox control of signal transduction and cell autonomous regulation are critical for establishing normal Hox expression patterns and the specification of segmental identity and morphology. We present data identifying novel regulatory interactions associated with the segmental register shift in Hox expression domains between the epidermis/somatic mesoderm and visceral mesoderm. A proposed mechanism for the gap between the expression domains of Sex combs reduced (Scr) and Antennapedia (Antp) in the visceral mesoderm is provided. Previously, Hox gene interactions have been shown to occur on multiple levels: direct cross-regulation, competition for binding sites at downstream targets and through indirect feedback involving signal transduction. We find that extrinsic specification of cell fate by signaling can be overridden by Hox protein expression in mesodermal cells and propose the term autonomic dominance for this phenomenon.

Wnt signalling: a theme with nuclear variations

Wnt proteins are involved in a large number of events during development and disease. The crucial element in the transduction of the signal elicited by Wnt is the state and activity of beta-catenin. There are two pools of beta-catenin, one associated with cadherins at the cell surface and a soluble one in the cytolasm, whose state and concentration are critical for Wnt signalling. In the absence of Wnt, the cytoplasmic pool is low due to targetted degradation of beta-catenin. Upon Wnt signalling, beta-catenin is stabilized. As a consequence, it can access the nucleus where it interacts with members of the Tcf family of transcription factors to modulate the expression of defined targets. Recent reports indicate that, in addition to Tcfs, beta-catenin can interact with other nuclear proteins raising the possibility that Wnt signalling has a wider modulatory effect on transcription than is mediated by its interactions with Tcfs. BioEssays 23:311-318, 2001.

Hindgut visceral mesoderm requires an ectodermal template for normal development in Drosophila

During Drosophila embryogenesis, the development of the midgut endoderm depends on interactions with the overlying visceral mesoderm. Here we show that the development of the hindgut also depends on cellular interactions, in this case between the inner ectoderm and outer visceral mesoderm. In this section of the gut, the ectoderm is essential for the proper specification and differentiation of the mesoderm, whereas the mesoderm is not required for the normal development of the ectoderm. Wingless and the fibroblast growth factor receptor Heartless act over sequential but interdependent phases of hindgut visceral mesoderm development. Wingless is required to establish the primordium and to enhance Heartless expression. Later, Heartless is required to promote the proper differentiation of the hindgut visceral mesoderm itself.

Wingless effects mesoderm patterning and ectoderm segmentation events via induction of its downstream target sloppy paired

Inactivation of either the secreted protein Wingless (Wg) or the forkhead domain transcription factor Sloppy Paired (Slp) has been shown to produce similar effects in the developing Drosophila embryo. In the ectoderm, both gene products are required for the formation of the segmental portions marked by naked cuticle. In the mesoderm, Wg and Slp activities are crucial for the suppression of bagpipe (bap), and hence visceral mesoderm formation, and the promotion of somatic muscle and heart formation within the anterior portion of each parasegment. In this report, we show that, during these developmental processes, wg and slp act in a common pathway in which slp serves as a direct target of Wg signals that mediates Wg effects in both germ layers. We present evidence that the induction of slp by Wg involves binding of the Wg effector Pangolin (Drosophila Lef-1/TCF) to multiple binding sites within a Wg-responsive enhancer that is located in 5′ flanking regions of the slp1 gene. Based upon our genetic and molecular analysis, we conclude that Wg signaling induces striped expression of Slp in the mesoderm. Mesodermal Slp is then sufficient to abrogate the induction of bagpipe by Dpp/Tinman, which explains the periodic arrangement of trunk visceral mesoderm primordia in wild type embryos. Conversely, mesodermal Slp is positively required, although not sufficient, for the specification of somatic muscle and heart progenitors. We propose that Wg-induced slp provides striped mesodermal domains with the competence to respond to subsequent slp-independent Wg signals that induce somatic muscle and heart progenitors. We also propose that in wg-expressing ectodermal cells, slp is an integral component in an autocrine feedback loop of Wg signaling.

The informational content of gradients of Wnt proteins

This perspective tackles the issues facing developmental biologists and cell biologists regarding how the molecular mechanisms for specifying cell fate are defined. This perspective focuses on members of the Wnt family. The author proposes that Wnt proteins may act as stabilizing signals for earlier inductive events in certain systems, for example, in Caenorhabditis elegans during the migration of two neurons and in Drosophila melanogaster during the patterning of the wing.

Barbu: an E(spl) m4/m(alpha)-related gene that antagonizes Notch signaling and is required for the establishment of ommatidial polarity

The Notch signaling pathway is required, in concert with cell-type-specific transcriptional regulators and other signaling processes, for multiple cell fate decisions during mesodermal and ectodermal tissue development. In many instances, Notch signaling occurs initially in a bidirectional manner and then becomes unidirectional upon amplification of small inherent differences in signaling activity between neighboring cells. In addition to ligands and extracellular modulators of the Notch receptor, several intracellular proteins have been identified that can positively or negatively influence the activity of the Notch pathway during these dynamic processes. Here, we describe a new gene, Barbu, whose product can antagonize Notch signaling activity during Drosophila development. Barbu encodes a small and largely cytoplasmic protein with sequence similarity to the proteins encoded by the transcription units m4 and m(alpha) of the E(spl) complex. Ectopic expression studies with Barbu provide evidence that Barbu can antagonize Notch during lateral inhibition processes in the embryonic mesoderm, sensory organ specification in imaginal discs and cell type specification in developing ommatidia. Barbu loss-of-function mutations cause lethality and disrupt the establishment of planar polarity and photoreceptor specification in eye imaginal discs, which may also be a consequence of altered Notch signaling activities. Furthermore, in the embryonic neuroectoderm, Barbu expression is inducible by activated Notch. Taken together, we propose that Barbu functions in a negative feed-back loop, which may be important for the accurate adjustment of Notch signaling activity and the extinction of Notch activity between successive rounds of signaling events.

Ventral neuroblasts and the heartless FGF receptor are required for muscle founder cell specification in Drosophila

Muscle founder cells are uniquely specified cells that fuse with neighboring myoblasts to generate the complex pattern of body wall muscles in the Drosophila embryo. We have investigated the positional specification of founder cells for ventral oblique muscles, marked by the restricted expression of tinman RNA and the activity of a D-mef2 enhancer. The formation of these ventral myoblasts requires the function of the Heartless FGF receptor in the mesoderm and the presence of ventral neuroblasts in the central nervous system. Overproduction of ventral neuroblasts due to the forced expression of the homeodomain protein Vnd leads to increased numbers of founder cells. These results suggest the use of a neuroectoderm-to-mesoderm signaling pathway in the specification of ventral muscle precursors.

Controls in patterning and diversification of somatic muscles during Drosophila embryogenesis

Recent genetic studies in Drosophila have provided important insights into the pathways determining the formation and diversification of body wall muscles. These pathways control a progressive subdivision of the mesoderm, ultimately leading to the specification of individual cells, the muscle founders, which are endowed with genetic programs capable of generating distinct muscle fibers. A network of activities of transcriptional regulators, signaling pathways, and lineage genes is beginning to emerge which controls successive steps of this muscle patterning and differentiation process.

Wnt signalling: pathway or network?

Members of the Wnt family of secreted glycoproteins participate in many signalling events during development. Recent findings suggest that Wnt signals can sometimes play a permissive role during cell-fate assignment. Wnt proteins have been shown to interact with a number of extracellular and cell-surface proteins, whereas many intracellular components of the Wnt-signalling pathway are also involved in other cellular functions. The consequences of Wnt signalling can be affected by members of the MAP kinase family. These observations suggest that the future understanding of Wnt signalling may require models that are based on a signalling network rather than a single linear pathway.

The Drosophila homeobox genes zfh-1 and even-skipped are required for cardiac-specific differentiation of a numb-dependent lineage decision

A series of inductive signals are necessary to subdivide the mesoderm in order to allow the formation of the progenitor cells of the heart. Mesoderm-endogenous transcription factors, such as those encoded by twist and tinman, seem to cooperate with these signals to confer correct context and competence for a cardiac cell fate. Additional factors are likely to be required for the appropriate specification of individual cell types within the forming heart. Similar to tinman, the zinc finger- and homeobox-containing gene, zfh-1, is expressed in the early mesoderm and later in the forming heart, suggesting a possible role in heart development. Here, we show that zfh-1 is specifically required for formation of the even-skipped (eve)-expressing subset of pericardial cells (EPCs), without affecting the formation of their siblings, the founders of a dorsal body wall muscle (DA1). In addition to zfh-1, mesodermal eve itself appears to be needed for correct EPC differentiation, possibly as a direct target of zfh-1. Epistasis experiments show that zfh-1 specifies EPC development independently of numb, the lineage gene that controls DA1 founder versus EPC cell fate. We discuss the combinatorial control mechanisms that specify the EPC cell fate in a spatially precise pattern within the embryo.

Repression by Notch is required before Wingless signalling during muscle progenitor cell development in Drosophila

The larval muscles of Drosophila arise from the fusion of muscle founder cells, which give each individual muscle its identity, with myoblasts (reviewed in [1]). Muscle founder cells arise from the asymmetric division of muscle progenitor cells, each of which develops from a group of cells in the somatic mesoderm that express lethal of scute [2]. All the cells in a cluster can potentially form muscle progenitors, but owing to lateral inhibition, only one or two develop as such [2] [3] [4] [5]. Muscle progenitors, and the subsequent founder cells, then express transcription factors such as Krüppel, S59 and Even-skipped, which confer identity on the muscle [6] [7] [8]. Definition of some muscle progenitors, including three groups that express S59, depends on Wingless signalling [9]. Lateral inhibition requires Delta signalling through Notch and the transcription factor Suppressor of Hairless [3] [4] [5]. As the Wingless and lateral-inhibition signals are sequential [8], one might expect that muscle progenitors would fail to develop in the absence of Wingless signalling, regardless of the presence or absence of lateral-inhibition signalling. Here, we examine the development of the S59-expressing muscle progenitor cells in mutant backgrounds in which both Wingless signalling and lateral inhibition are disrupted. We show that progenitor cells failed to develop when both these processes were disrupted. Our analysis also reveals a repressive function of Notch, required before or concurrently with Wingless signalling, which is unrelated to its role in lateral inhibition.

Muscle pattern diversification in Drosophila: the story of imaginal myogenesis

There are two phases of somatic muscle formation in Drosophila. During embryonic development, one phase of myogenesis generates larval muscle elements that mediate the relatively simple behavioural repertoire of the larva. During pupal metamorphosis, a diverse pattern of muscle fibres are assembled, and these facilitate the more elaborate behavioural patterns of the adult fly. In this review, we discuss the current status of understanding of the cellular, genetic, and molecular mechanisms of pattern formation during the second phase, imaginal muscle development. We briefly compare aspects of embryonic and adult myogenesis in Drosophila and muscle development in vertebrates and highlight conserved themes and disparities between these diverse myogenic programmes.

The heterotrimeric protein Go is required for the formation of heart epithelium in Drosophila

The gene encoding the alpha subunit of the Drosophila Go protein is expressed early in embryogenesis in the precursor cells of the heart tube, of the visceral muscles, and of the nervous system. This early expression coincides with the onset of the mesenchymal-epithelial transition to which are subjected the cardial cells and the precursor cells of the visceral musculature. This gene constitutes an appropriate marker to follow this transition. In addition, a detailed analysis of its expression suggests that the cardioblasts originate from two subpopulations of cells in each parasegment of the dorsal mesoderm that might depend on the wingless and hedgehog signaling pathways for both their determination and specification. In the nervous system, the expression of Goalpha shortly precedes the beginning of axonogenesis. Mutants produced in the Goalpha gene harbor abnormalities in the three tissues in which the gene is expressed. In particular, the heart does not form properly and interruptions in the heart epithelium are repeatedly observed, henceforth the brokenheart (bkh) name. Furthermore, in the bkh mutant embryos, the epithelial polarity of cardial cells was not acquired (or maintained) in various places of the cardiac tube. We predict that bkh might be involved in vesicular traffic of membrane proteins that is responsible for the acquisition of polarity.

Requirement for the Drosophila COE transcription factor Collier in formation of an embryonic muscle: transcriptional response to notch signalling

During Drosophila embryogenesis, mesodermal cells are recruited to form a stereotyped pattern of about 30 different larval muscles per hemisegment. The formation of this pattern is initiated by the specification of a special class of myoblasts, called founder cells, that are uniquely able to fuse with neighbouring myoblasts. We report here the role of the COE transcription factor Collier in the formation of a single muscle, muscle DA3([A])(DA4([T])). Col expression is first observed in two promuscular clusters (in segments A1-A7), the two corresponding progenitors and their progeny founder cells, but its transcription is maintained in only one of these four founder cells, the founder of muscle DA3([A]). This lineage-specific restriction depends on the asymmetric segregation of Numb during the progenitor cell division and involves the repression of col transcription by Notch signalling. In col mutant embryos, the DA3([A]) founder cells form but do not maintain col transcription and are unable to fuse with neighbouring myoblasts, leading to a loss-of-muscle DA3([A]) phenotype. In wild-type embryos, each of the DA3([A])-recruited myoblasts turns on col transcription, indicating that the conversion, by the DA3([A]) founder cell, of ‘naive’ myoblasts to express its distinctive pattern of gene expression involves activation of col itself. We find that muscles DA3([A]) and DO5([A]) (DA4([T]) and DO5([T])) derive from a common progenitor cell. Ectopic expression of Col is not sufficient, however, to switch the DO5([A]) to a DA3([A]) fate. Together these results lead us to propose that specification of the DA3([A]) muscle lineage requires both Col and at least one other transcription factor, supporting the hypothesis of a combinatorial code of muscle-specific gene regulation controlling the formation and diversification of individual somatic muscles.

Mechanisms of Wnt signaling in development

Wnt genes encode a large family of secreted, cysteine-rich proteins that play key roles as intercellular signaling molecules in development. Genetic studies in Drosophila and Caenorhabditis elegans, ectopic gene expression in Xenopus, and gene knockouts in the mouse have demonstrated the involvement of Wnts in processes as diverse as segmentation, CNS patterning, and control of asymmetric cell divisions. The transduction of Wnt signals between cells proceeds in a complex series of events including post-translational modification and secretion of Wnts, binding to transmembrane receptors, activation of cytoplasmic effectors, and, finally, transcriptional regulation of target genes. Over the past two years our understanding of Wnt signaling has been substantially improved by the identification of Frizzled proteins as cell surface receptors for Wnts and by the finding that beta-catenin, a component downstream of the receptor, can translocate to the nucleus and function as a transcriptional activator. Here we review recent data that have started to unravel the mechanisms of Wnt signaling.

Cellular mechanisms of wingless/Wnt signal transduction

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

Drosophila mef2 expression during mesoderm development is controlled by a complex array of cis-acting regulatory modules

The function of the Drosophila mef2 gene, a member of the MADS box supergene family of transcription factors, is critical for terminal differentiation of the three major muscle cell types, namely somatic, visceral, and cardiac. During embryogenesis, mef2 undergoes multiple phases of expression, which are characterized by initial broad mesodermal expression, followed by restricted expression in the dorsal mesoderm, specific expression in muscle progenitors, and sustained expression in the differentiated musculatures. In this study, evidence is presented that temporally and spatially specific mef2 expression is controlled by a complex array of cis-acting regulatory modules that are responsive to different genetic signals. Functional testing of approximately 12 kb of 5' flanking region of the mef2 gene showed that the initial widespread mesodermal expression is achieved through a 280-bp twist-dependent enhancer. The subsequent dorsal mesoderm-restricted mef2 expression is mediated through a 460-bp dpp-responsive regulatory module, which involves the function of the Smad4 homolog Medea and contains several binding sites for Medea and Mad. The analysis also showed that regulated mef2 expression in the caudal and trunk visceral mesoderm, which give rise to longitudinal and circular gut musculatures, respectively, is under the control of distinct enhancer elements. In addition, mef2 expression in the cardioblasts of the heart is dependent upon at least two distinct enhancers, which are active at different periods during embryogenesis. Moreover, multiple regulatory elements are differentially activated for specific expression in presumptive muscle founders, prefusion myoblasts, and differentiated muscle fibers. Taken together, the presented data suggest that specific expression of the mef2 gene in myogenic lineages in the Drosophila embryo is the result of multiple genetic inputs that act in an additive manner upon distinct enhancers in the 5' flanking region.

Combinatorial signaling codes for the progressive determination of cell fates in the Drosophila embryonic mesoderm

Mesodermal progenitors arise in the Drosophila embryo from discrete clusters of lethal of scute (l'sc)-expressing cells. Using both genetic loss-of-function and targeted ectopic expression approaches, we demonstrate here that individual progenitors are specified by the sequential deployment of unique combinations of intercellular signals. Initially, the intersection between the Wingless (Wg) and Decapentaplegic (Dpp) expression domains demarcate an ectodermal prepattern that is imprinted on the adjacent mesoderm in the form of a L'sc precluster. All mesodermal cells within this precluster are competent to respond to a subsequent instructive signal mediated by two receptor tyrosine kinases (RTKs), the Drosophila epidermal growth factor receptor (DER) and the Heartless (Htl) fibroblast growth factor receptor. By monitoring the expression of the diphosphorylated form of mitogen-associated protein kinase (MAPK), we found that these RTKs are activated in small clusters of cells within the original competence domain. Each cluster represents an equivalence group because all members initially resemble progenitors in their expression of both L'sc and mesodermal identity genes. Thus, localized RTK activity induces the formation of mesodermal equivalence groups. The RTKs remain active in the single progenitor that emerges from each cluster under the subsequent inhibitory influence of the neurogenic genes. Moreover, DER and Htl are differentially involved in the specification of particular progenitors. We conclude that distinct cellular identity codes are generated by the combinatorial activities of Wg, Dpp, EGF, and FGF signals in the progressive determination of embryonic mesodermal cells.

Transcriptional repression due to high levels of Wingless signalling

Extracellular signals can act at different threshold levels to elicit distinct transcriptional and cellular responses. Here, we examine the transcriptional regulation of the Wingless target gene Ultrabithorax (Ubx) in the embryonic midgut of Drosophila. Our previous work showed that Ubx transcription is stimulated in this tissue by Dpp and by low levels of Wingless signalling. We now find that high levels of Wingless signalling can repress Ubx transcription. The response sequence within the Ubx midgut enhancer required for this repression coincides with a motif required for transcriptional stimulation of Dpp, namely a tandem of binding sites for the Dpp-transducing protein, Mad. Indeed, Wingless-mediated repression depends on low levels of Dpp, although apparently not on Mad itself. In contrast, high levels of Dpp signalling antagonize Wingless-mediated repression. This suggests that transcriptional activation of Ubx is subject to competition between Dpp-activated Mad and another Smad whose function as a transcriptional repressor depends on high Wg signalling. Finally, we show that Wingless can repress its own expression via an autorepressive feedback loop that results in a change of the Wingless signalling profile during development.