Reciprocal effects of hyper- and hypoactivity mutations in the Drosophila pattern gene torso

Temporal integration of inductive cues on the way to gastrulation

Markers for the endoderm and mesoderm germ layers are commonly expressed together in the early embryo, potentially reflecting cells' ability to explore potential fates before fully committing. It remains unclear when commitment to a single-germ layer is reached and how it is impacted by external signals. Here, we address this important question in Drosophila, a convenient model system in which mesodermal and endodermal fates are associated with distinct cellular movements during gastrulation. Systematically applying endoderm-inducing extracellular signal-regulated kinase (ERK) signals to the ventral medial embryo-which normally only receives a mesoderm-inducing cue-reveals a critical time window during which mesodermal cell movements and gene expression are suppressed by proendoderm signaling. We identify the ERK target gene huckebein (hkb) as the main cause of the ventral furrow suppression and use computational modeling to show that Hkb repression of the mesoderm-associated gene snail is sufficient to account for a broad range of transcriptional and morphogenetic effects. Our approach, pairing precise signaling perturbations with observation of transcriptional dynamics and cell movements, provides a general framework for dissecting the complexities of combinatorial tissue patterning.

Antagonism between germ cell-less and Torso receptor regulates transcriptional quiescence underlying germline/soma distinction

Transcriptional quiescence, an evolutionarily conserved trait, distinguishes the embryonic primordial germ cells (PGCs) from their somatic neighbors. In Drosophila melanogaster, PGCs from embryos maternally compromised for germ cell-less (gcl) misexpress somatic genes, possibly resulting in PGC loss. Recent studies documented a requirement for Gcl during proteolytic degradation of the terminal patterning determinant, Torso receptor. Here we demonstrate that the somatic determinant of female fate, Sex-lethal (Sxl), is a biologically relevant transcriptional target of Gcl. Underscoring the significance of transcriptional silencing mediated by Gcl, ectopic expression of a degradation-resistant form of Torso (torso^Deg) can activate Sxl transcription in PGCs, whereas simultaneous loss of torso-like (tsl) reinstates the quiescent status of gcl PGCs. Intriguingly, like gcl mutants, embryos derived from mothers expressing torso^Deg in the germline display aberrant spreading of pole plasm RNAs, suggesting that mutual antagonism between Gcl and Torso ensures the controlled release of germ-plasm underlying the germline/soma distinction.

Receptor Tyrosine Kinases in Development: Insights from Drosophila

Cell-to-cell communication mediates a plethora of cellular decisions and behaviors that are crucial for the correct and robust development of multicellular organisms. Many of these signals are encoded in secreted hormones or growth factors that bind to and activate cell surface receptors, to transmit the cue intracellularly. One of the major superfamilies of cell surface receptors are the receptor tyrosine kinases (RTKs). For nearly half a century RTKs have been the focus of intensive study due to their ability to alter fundamental aspects of cell biology, such as cell proliferation, growth, and shape, and because of their central importance in diseases such as cancer. Studies in model organisms such a Drosophila melanogaster have proved invaluable for identifying new conserved RTK pathway components, delineating their contributions, and for the discovery of conserved mechanisms that control RTK-signaling events. Here we provide a brief overview of the RTK superfamily and the general mechanisms used in their regulation. We further highlight the functions of several RTKs that govern distinct cell-fate decisions in Drosophila and explore how their activities are developmentally controlled.

Signaling between somatic follicle cells and the germline patterns the egg and embryo of Drosophila

In Drosophila, specification of the embryonic body axes requires signaling between the germline and the somatic follicle cells. These signaling events are necessary to properly localize embryonic patterning determinants in the egg or eggshell during oogenesis. There are three maternal patterning systems that specify the anterior-posterior axis, and one that establishes the dorsal-ventral axis. We will first review oogenesis, focusing on the establishment of the oocyte and nurse cells and patterning of the follicle cells into different subpopulations. We then describe how two coordinated signaling events between the oocyte and follicle cells establish polarity of the oocyte and localize the anterior determinant bicoid, the posterior determinant oskar, and Gurken/epidermal growth factor (EGF), which breaks symmetry to initiate dorsal-ventral axis establishment. Next, we review how dorsal-ventral asymmetry of the follicle cells is transmitted to the embryo. This process also involves Gurken-EGF receptor (EGFR) signaling between the oocyte and follicle cells, leading to ventrally-restricted expression of the sulfotransferase Pipe. These events promote the ventral processing of Spaetzle, a ligand for Toll, which ultimately sets up the embryonic dorsal-ventral axis. We then describe the activation of the terminal patterning system by specialized polar follicle cells. Finally, we present open questions regarding soma-germline signaling during Drosophila oogenesis required for cell identity and embryonic axis formation.

The torso-like gene functions to maintain the structure of the vitelline membrane in Nasonia vitripennis, implying its co-option into Drosophila axis formation

Axis specification is a fundamental developmental process. Despite this, the mechanisms by which it is controlled across insect taxa are strikingly different. An excellent example of this is terminal patterning, which in Diptera such as Drosophila melanogaster occurs via the localized activation of the receptor tyrosine kinase Torso. In Hymenoptera, however, the same process appears to be achieved via localized mRNA. How these mechanisms evolved and what they evolved from remains largely unexplored. Here, we show that torso-like, known for its role in Drosophila terminal patterning, is instead required for the integrity of the vitelline membrane in the hymenopteran wasp Nasonia vitripennis. We find that other genes known to be involved in Drosophila terminal patterning, such as torso and Ptth, also do not function in Nasonia embryonic development. These findings extended to orthologues of Drosophila vitelline membrane proteins known to play a role in localizing Torso-like in Drosophila; in Nasonia these are instead required for dorso–ventral patterning, gastrulation and potentially terminal patterning. Our data underscore the importance of the vitelline membrane in insect development, and implies phenotypes caused by knockdown of torso-like must be interpreted in light of its function in the vitelline membrane. In addition, our data imply that the signalling components of the Drosophila terminal patterning systems were co-opted from roles in regulating moulting, and co-option into terminal patterning involved the evolution of a novel interaction with the vitelline membrane protein Torso-like.

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

Summary: In the parasitic wasp Nasonia, Tsl, a key component of the process that defines the termini of the embryo of Drosophila, has a function in the structure of the vitelline membrane.

A quantitative model of developmental RTK signaling

Receptor tyrosine kinases (RTKs) control a wide range of developmental processes, from the first stages of embryogenesis to postnatal growth and neurocognitive development in the adult. A significant share of our knowledge about RTKs comes from genetic screens in model organisms, which provided numerous examples demonstrating how specific cell fates and morphologies are abolished when RTK activation is either abrogated or significantly reduced. Aberrant activation of such pathways has also been recognized in many forms of cancer. More recently, studies of human developmental syndromes established that excessive activation of RTKs and their downstream signaling effectors, most notably the Ras signaling pathway, can also lead to structural and functional defects. Given that both insufficient and excessive pathway activation can lead to abnormalities, mechanistic analysis of developmental RTK signaling must address quantitative questions about its regulation and function. Patterning events controlled by the RTK Torso in the early Drosophila embryo are well-suited for this purpose. This mini review summarizes current state of knowledge about Torso-dependent Ras activation and discusses its potential to serve as a quantitative model for studying the general principles of Ras signaling in development and disease.

Divergent effects of intrinsically active MEK variants on developmental Ras signaling

Germline mutations in Ras pathway components are associated with a large class of human developmental abnormalities, known as RASopathies, that are characterized by a range of structural and functional phenotypes, including cardiac defects and neurocognitive delays. Although it is generally believed that RASopathies are caused by altered levels of pathway activation, the signaling changes in developing tissues remain largely unknown. We used assays with spatiotemporal resolution in Drosophila melanogaster (fruit fly) and Danio rerio (zebrafish) to quantify signaling changes caused by mutations in MAP2K1 (encoding MEK), a core component of the Ras pathway that is mutated in both RASopathies and cancers in humans. Surprisingly, we discovered that intrinsically active MEK variants can both increase and reduce the levels of pathway activation in vivo. The sign of the effect depends on cellular context, implying that some of the emerging phenotypes in RASopathies may be caused by increased, as well as attenuated, levels of Ras signaling.

Capturing embryonic development from metamorphosis: how did the terminal patterning signalling pathway of Drosophila evolve?

The Torso receptor tyrosine kinase has two crucial roles in Drosophila melanogaster development. One is in the control of insect moulting, which is regulated by the neuropeptide hormone PTTH (prothoracicotropic hormone). PTTH activates ERK signalling via Torso in the prothoracic gland to stimulate ecdysone secretion. Torso also has a role in control of one of the earliest events in embryogenesis in Drosophila; patterning of the embryonic termini. Here Torso is activated by a different, but related, peptide called Trunk. During terminal patterning another protein, Torso-like, has a key role in mediating activation of Torso by Trunk. Torso-like is also expressed in the prothoracic gland and null-mutants have defective developmental timing in Drosophila. This function, however, has been recently shown to be independent of Torso and PTTH. We refer to these proteins, Trunk, PTTH, Torso and Torso-like, as the Torso-activation module. Outside Drosophila we see that the genes encoding the Torso-activation module have a complex phylogenetic history, with different origins and multiple losses of components of this signalling pathway during arthropod evolution. This, together with expression and functional data in a range of insects, leads us to propose that the terminal patterning pathway in Drosophila and Tribolium arose through co-option of PTTH/Trunk and Torso, which has a role in developmental timing, into a new context, and that Torso-like was recruited specifically in the ovary to modulate the specificity of this pathway.

Receptor tyrosine kinases in Drosophila development

Tyrosine phosphorylation plays a significant role in a wide range of cellular processes. The Drosophila genome encodes more than 20 receptor tyrosine kinases and extensive studies in the past 20 years have illustrated their diverse roles and complex signaling mechanisms. Although some receptor tyrosine kinases have highly specific functions, others strikingly are used in rather ubiquitous manners. Receptor tyrosine kinases regulate a broad expanse of processes, ranging from cell survival and proliferation to differentiation and patterning. Remarkably, different receptor tyrosine kinases share many of the same effectors and their hierarchical organization is retained in disparate biological contexts. In this comprehensive review, we summarize what is known regarding each receptor tyrosine kinase during Drosophila development. Astonishingly, very little is known for approximately half of all Drosophila receptor tyrosine kinases.

Lack of tailless leads to an increase in expression variability in Drosophila embryos

Developmental processes are robust, or canalised: dynamic patterns of gene expression across space and time are regulated reliably and precisely in the presence of genetic and environmental perturbations. It remains unclear whether canalisation relies on specific regulatory factors (such as heat-shock proteins), or whether it is based on more general redundancy and distributed robustness at the network level. The latter explanation implies that mutations in many regulatory factors should exhibit loss of canalisation. Here, we present a quantitative characterisation of segmentation gene expression patterns in mutants of the terminal gap gene tailless (tll) in Drosophila melanogaster. Our analysis provides new insights into the dynamic mechanisms underlying gap gene regulation, and reveals significantly increased variability of gene expression in the mutant compared to the wild-type background. We show that both position and timing of posterior segmentation gene expression domains vary strongly from embryo-to-embryo in tll mutants. This variability must be caused by a vulnerability in the regulatory system which is hidden or buffered in the wild-type, but becomes uncovered by the deletion of tll. Our analysis provides evidence that loss of canalisation in mutants could be more widespread than previously thought.

The gap gene network

Gap genes are involved in segment determination during the early development of the fruit fly Drosophila melanogaster as well as in other insects. This review attempts to synthesize the current knowledge of the gap gene network through a comprehensive survey of the experimental literature. I focus on genetic and molecular evidence, which provides us with an almost-complete picture of the regulatory interactions responsible for trunk gap gene expression. I discuss the regulatory mechanisms involved, and highlight the remaining ambiguities and gaps in the evidence. This is followed by a brief discussion of molecular regulatory mechanisms for transcriptional regulation, as well as precision and size-regulation provided by the system. Finally, I discuss evidence on the evolution of gap gene expression from species other than Drosophila. My survey concludes that studies of the gap gene system continue to reveal interesting and important new insights into the role of gene regulatory networks in development and evolution.

Drosophila Raf's N terminus contains a novel conserved region and can contribute to torso RTK signaling

Drosophila Raf (DRaf) contains an extended N terminus, in addition to three conserved regions (CR1-CR3); however, the function(s) of this N-terminal segment remains elusive. In this article, a novel region within Draf's N terminus that is conserved in BRaf proteins of vertebrates was identified and termed conserved region N-terminal (CRN). We show that the N-terminal segment can play a positive role(s) in the Torso receptor tyrosine kinase pathway in vivo, and its contribution to signaling appears to be dependent on the activity of Torso receptor, suggesting this N-terminal segment can function in signal transmission. Circular dichroism analysis indicates that DRaf's N terminus (amino acids 1-117) including CRN (amino acids 19-77) is folded in vitro and has a high content of helical secondary structure as predicted by proteomics tools. In yeast two-hybrid assays, stronger interactions between DRaf's Ras binding domain (RBD) and the small GTPase Ras1, as well as Rap1, were observed when CRN and RBD sequences were linked. Together, our studies suggest that DRaf's extended N terminus may assist in its association with the upstream activators (Ras1 and Rap1) through a CRN-mediated mechanism(s) in vivo.

Drosophila female sterile (1) homeotic is a multifunctional transcriptional regulator that is modulated by Ras signaling

The Drosophila (fs(1)h) gene encodes small (Fs(1)hS) and large (Fs(1)hL) chromatin-binding BET protein transcription factor isoforms. Zygotic mutations cause either lethality or female sterility, whereas maternal mutations cause segmental deletions and thoracic homeotic transformations. Here, we describe novel fs(1)h embryonic phenotypes: homeosis of the head in zygotic mutants and deletion of head and tail regions in maternal mutants, similar to those caused by dominant torso (tor(D)) alleles. tor activates transcription of tailless (tll) and hückebein (hkb) by means of a canonical Ras pathway, through inactivation of Groucho (Gro), Capicua (Cic) and, possibly, Grainy-head (Grh) repressors. Expression of both tailless and hückebein are de-repressed in fs(1)h maternal mutants, as in tor(D), gro, grh, and cic mutant animals, indicating fs(1)h is also necessary for tll and hkb repression. These data link Ras signaling with modulation of a chromatin-binding transcription factor, Fs(1)h, suggesting a novel mechanism by which Ras can modulate gene expression.

Spatially distinct downregulation of Capicua repression and tailless activation by the Torso RTK pathway in the Drosophila embryo

Specification of the terminal regions of the Drosophila embryo depends on the Torso RTK pathway, which triggers expression of the zygotic genes tailless and huckebein at the embryonic poles. However, it has been shown that the Torso signalling pathway does not directly activate expression of these zygotic genes; rather, it induces their expression by inactivating, at the embryonic poles, a uniformly distributed repressor activity. In particular, it has been shown that Torso signalling regulates accumulation of the Capicua transcriptional repressor: as a consequence of Torso signalling Capicua is downregulated specifically at the poles of blastoderm stage embryos. Extending the current model, we show that activation of the Torso pathway can trigger tailless expression without eliminating Capicua. In addition, analysis of gene activation by the Torso pathway and downregulation of Capicua unveil differences between the terminal and the central embryonic regions that are independent of Torso signalling, hitherto thought to be the only system responsible for confering terminal specificities. These data provide new insights into the mode of action of the Torso signalling pathway and on the events patterning the early Drosophila embryo.

Functions and mechanisms of receptor tyrosine kinase Torso signaling: lessons from Drosophila embryonic terminal development

The Torso receptor tyrosine kinase (RTK) is required for cell fate specification in the terminal regions (head and tail) of the early Drosophila embryo. Torso contains a split tyrosine kinase domain and belongs to the type III subgroup of the RTK superfamily that also includes the platelet-derived growth factor receptors, stem cell or steel factor receptor c-Kit proto-oncoprotein, colony-stimulating factor-1 receptor, and vascular endothelial growth factor receptor. The Torso pathway has been a model system for studying RTK signal transduction. Genetic and biochemical studies of Torso signaling have provided valuable insights into the biological functions and mechanisms of RTK signaling during early Drosophila embryogenesis.

Capicua integrates input from two maternal systems in Drosophila terminal patterning

In Drosophila, the maternal terminal system specifies cell fates at the embryonic poles via the localised stimulation of the Torso receptor tyrosine kinase (RTK). Signalling by the Torso pathway relieves repression mediated by the Capicua and Groucho repressors, allowing the restricted expression of the zygotic terminal gap genes tailless and huckebein. Here we report a novel positive input into tailless and huckebein transcription by maternal posterior group genes, previously implicated in abdomen and pole cell formation. We show that absence of a subset of posterior group genes, or their overactivation, leads to the spatial reduction or expansion of the tailless and huckebein posterior expression domains, respectively. We demonstrate that the terminal and posterior systems converge, and that exclusion of Capicua from the termini of posterior group mutants is ineffective, accounting for reduced terminal gap gene expression in these embryos. We propose that the terminal and posterior systems function coordinately to alleviate transcriptional silencing by Capicua, and that the posterior system fine-tunes Torso RTK signalling output, ensuring precise spatial domains of tailless and huckebein expression.

Pattern formation: the eggshell holds the cue

Recent data indicate that Torsolike, a spatial cue for patterning terminal structures of a Drosophila embryo, is stably anchored in the fruitfly eggshell; an as yet unidentified factor is required for the high activity of Torsolike at the embryo termini.

The Drosophila embryonic patterning determinant torsolike is a component of the eggshell

The development of the head and tail regions of the Drosophila embryo is dependent upon the localized polar activation of Torso (Tor), a receptor tyrosine kinase that is uniformly distributed in the membrane of the developing embryo. Trunk (Trk), the proposed ligand for Tor, is secreted as an inactive precursor into the perivitelline fluid that lies between the embryonic membrane and the vitelline membrane (VM), the inner layer of the eggshell. The spatial regulation of Trk processing is thought to be mediated by the secreted product of the torsolike (tsl) gene, which is expressed during oogenesis by a specialized population of follicle cells present at the two ends of the oocyte. We show here that Tsl protein is specifically localized to the polar regions of the VM in laid eggs. We further demonstrate that although Tsl can associate with nonpolar regions of the VM, the activity of polar-localized Tsl is enhanced, suggesting the existence of another spatially restricted factor acting in this pathway. The incorporation of Tsl into the VM provides a mechanism for the transfer of spatial information from the follicle cells to the developing embryo. To our knowledge, Tsl represents the first example of an embryonic patterning determinant that is a component of the eggshell.

Drosophila gain-of-function mutant RTK torso triggers ectopic Dpp and STAT signaling

Overactivation of receptor tyrosine kinases (RTKs) has been linked to tumorigenesis. To understand how a hyperactivated RTK functions differently from wild-type RTK, we conducted a genome-wide systematic survey for genes that are required for signaling by a gain-of-function mutant Drosophila RTK Torso (Tor). We screened chromosomal deficiencies for suppression of a gain-of-function mutation tor (tor(GOF)), which led to the identification of 26 genomic regions that, when in half dosage, suppressed the defects caused by tor(GOF). Testing of candidate genes in these regions revealed many genes known to be involved in Tor signaling (such as those encoding the Ras-MAPK cassette, adaptor and structural molecules of RTK signaling, and downstream target genes of Tor), confirming the specificity of this genetic screen. Importantly, this screen also identified components of the TGFbeta (Dpp) and JAK/STAT pathways as being required for Tor(GOF) signaling. Specifically, we found that reducing the dosage of thickveins (tkv), Mothers against dpp (Mad), or STAT92E (aka marelle), respectively, suppressed tor(GOF) phenotypes. Furthermore, we demonstrate that in tor(GOF) embryos, dpp is ectopically expressed and thus may contribute to the patterning defects. These results demonstrate an essential requirement of noncanonical signaling pathways for a persistently activated RTK to cause pathological defects in an organism.

D-cbl, a negative regulator of the Egfr pathway, is required for dorsoventral patterning in Drosophila oogenesis

During Drosophila oogenesis, asymmetrically localized Gurken activates the EGF receptor (Egfr) and determines dorsal follicle cell fates. Using a mosaic follicle cell system we have identified a mutation in the D-cbl gene which causes hyperactivation of the Egfr pathway. Cbl proteins are known to downregulate activated receptors. We find that the abnormal Egfr activation is ligand dependent. Our results show that the precise regulation of Egfr activity necessary to establish different follicle cell fates requires two levels of control. The localized ligand Gurken activates Egfr to different levels in different follicle cells. In addition, Egfr activity has to be repressed through the activity of D-cbl to ensure the absence of signaling in the ventral most follicle cells.

Targeting gene expression to the head: the Drosophila orthodenticle gene is a direct target of the Bicoid morphogen

The Bicoid (Bcd) morphogen establishes the head and thorax of the Drosophila embryo. Bcd activates the transcription of identified target genes in the thoracic segments, but its mechanism of action in the head remains poorly understood. It has been proposed that Bcd directly activates the cephalic gap genes, which are the first zygotic genes to be expressed in the head primordium. It has also been suggested that the affinity of Bcd-binding sites in the promoters of Bcd target genes determines the posterior extent of their expression (the Gene X model). However, both these hypotheses remain untested. Here, we show that a small regulatory region upstream of the cephalic gap gene orthodenticle (otd) is sufficient to recapitulate early otd expression in the head primordium. This region contains two control elements, each capable of driving otd-like expression. The first element has consensus Bcd target sites that bind Bcd in vitro and are necessary for head-specific expression. As predicted by the Gene X model, this element has a relatively low affinity for Bcd. Surprisingly, the second regulatory element has no Bcd sites. Instead, it contains a repeated sequence motif similar to a regulatory element found in the promoters of otd-related genes in vertebrates. Our study is the first demonstration that a cephalic gap gene is directly regulated by Bcd. However, it also shows that zygotic gene expression can be targeted to the head primordium without direct Bcd regulation.

Dissecting the mechanism of torso receptor activation

Regulated activation of receptor tyrosine kinases depends both on the presence of the receptors at the cell surface and on the availability of their ligands. In Drosophila the torso (tor) tyrosine kinase receptor is distributed along the surface of the embryo but it is only activated at the poles by a diffusible extracellular ligand generated at each pole which is trapped by the receptor, thereby impeding further diffusion. However, it is not well understood how this signal is generated, although it is known to depend on the activity of many genes such as torso-like (tsl) and trunk (trk). To further investigate the mechanism involved in the local activation of the tor receptor we have altered the normal expression of the tsl protein by generating females in which the tsl gene is expressed in the oocyte under the control of the tor promoter rather than in the ovarian follicle cells. Analysis of the phenotypes generated by this hybrid gene and its interactions with mutations in other genes in the pathway has enabled us to further dissect the mechanism of tor receptor activation and to define more precisely the role of the different genes acting in this process.

The Drosophila 14–3-3 protein Leonardo enhances Torso signaling through D-Raf in a Ras 1-dependent manner

14-3-3 proteins have been shown to interact with Raf-1 and cause its activation when overexpressed. However, their precise role in Raf-1 activation is still enigmatic, as they are ubiquitously present in cells and found to associate with Raf-1 in vivo regardless of its activation state. We have analyzed the function of the Drosophila 14–3-3 gene leonardo (leo) in the Torso (Tor) receptor tyrosine kinase (RTK) pathway. In the syncytial blastoderm embryo, activation of Tor triggers the Ras/Raf/MEK pathway that controls the transcription of tailless (tll). We find that, in the absence of Tor, overexpression of leo is sufficient to activate tll expression. The effect of leo requires D-Raf and Ras1 activities but not KSR or DOS, two recently identified essential components of Drosophila RTK signaling pathways. Tor signaling is impaired in embryos derived from females lacking maternal expression of leo. We propose that binding to 14–3-3 by Raf is necessary but not sufficient for the activation of Raf and that overexpressed Drosophila 14–3-3 requires Ras1 to activate D-Raf.

Control of germ-band retraction in Drosophila by the zinc-finger protein HINDSIGHT

Drosophila embryos lacking hindsight gene function have a normal body plan and undergo normal germ-band extension. However, they fail to retract their germ bands. hindsight encodes a large nuclear protein of 1920 amino acids that contains fourteen C2H2-type zinc fingers, and glutamine-rich and proline-rich domains, suggesting that it functions as a transcription factor. Initial embryonic expression of hindsight RNA and protein occurs in the endoderm (midgut) and extraembryonic membrane (amnioserosa) prior to germ-band extension and continues in these tissues beyond the completion of germ-band retraction. Expression also occurs in the developing tracheal system, central and peripheral nervous systems, and the ureter of the Malpighian tubules. Strikingly, hindsight is not expressed in the epidermal ectoderm which is the tissue that undergoes the cell shape changes and movements during germ-band retraction. The embryonic midgut can be eliminated without affecting germ-band retraction. However, elimination of the amnioserosa results in the failure of germ-band retraction, implicating amnioserosal expression of hindsight as crucial for this process. Ubiquitous expression of hindsight in the early embryo rescues germ-band retraction without producing dominant gain-of-function defects, suggesting that hindsight's role in germ-band retraction is permissive rather than instructive. Previous analyses have shown that hindsight is required for maintenance of the differentiated amnioserosa (Frank, L. C. and Rushlow, C. (1996) Development 122, 1343–1352). Two classes of models are consistent with the present data. First, hindsight's function in germ-band retraction may be limited to maintenance of the amnioserosa which then plays a physical role in the retraction process through contact with cells of the epidermal ectoderm. Second, hindsight might function both to maintain the amnioserosa and to regulate chemical signaling from the amnioserosa to the epidermal ectoderm, thus coordinating the cell shape changes and movements that drive germ-band retraction.

Orthodenticle regulation during embryonic head development in Drosophila

The homeobox gene orthodenticle (otd) specifies anterior head development in the Drosophila embryo, otd-related genes are also found in vertebrates, with expression patterns suggesting that they are important for the development of anterior regions of the head and brain. Here, we analyze the molecular mechanisms by which otd expression is activated within its normal domain in the head and repressed outside this region. We demonstrate that, contrary to early models of embryonic pattern formation, high levels of the bicoid morphogen are not required for otd activation or for the establishment of anterior head structures. We also show that the terminal system contributes to otd activation in the head primordium. Finally, we identify a novel pathway mediated by the gap gene huckebein through which three maternal systems cooperate to repress otd expression at the anterior terminus of the embryo.

Signal transduction in the early Drosophila embryo: when genetics meets biochemistry

An elegant combination of genetic and biochemical approaches has been used to investigate a variety of signal transduction pathways in developmental processes. Here, we describe the 'terminal' signaling system in the Drosophila embryo, which is responsible for pattern formation in the polar regions of the embryo. This pathway involves a membrane-bound receptor tyrosine kinase (RTK) that is similar to other Drosophila RTKs, such as sevenless, and the mammalian RTKs, such as the epidermal growth factor or platelet-derived growth factor receptors.

Three maternal coordinate systems cooperate in the patterning of the Drosophila head

In contrast to the segmentation of the embryonic trunk region which has been extensively studied, relatively little is known about the development and segmentation of the Drosophila head. Proper development of the cephalic region requires the informational input of three of the four maternal coordinate systems. Head-specific gene expression is set up in response to a complex interaction between the maternally provided gene products and zygotically expressed genes. Several zygotic genes involved in head development have recently been characterized. A genetic analysis suggests that the segmentation of the head may use a mechanism different from the one acting in the trunk. The two genes of the sloppy paired locus (slp1 and slp2) are also expressed in the embryonic head. slp1 plays a predominant role in head formation while slp2 is largely dispensible. A detailed analysis of the slp head phenotype suggests that slp is important for the development of the mandibular segment as well as two adjacent pregnathal segments (antennal and ocular). Our analysis of regulatory interactions of slp with maternal and zygotic genes suggests that it behaves like a gap gene. Thus, phenotype and regulation of slp support the view that slp acts as a head-specific gap gene in addition to its function as a pair-rule and segment polarity gene in the trunk. We show that all three maternal systems active in the cephalic region are required for proper slp expression and that the different systems cooperate in the patterning of the head. The terminal and anterior patterning system appear to be closely linked. This cooperation is likely to involve a direct interaction between the bcd morphogen and the terminal system. Low levels of terminal system activity seem to potentiate bcd as an activator of slp, whereas high levels down-regulate bcd rendering it inactive. Our analysis suggests that dorsal, the morphogen of the dorsoventral system, and the head-specific gap gene empty spiracles act as repressor and corepressor in the regulation of slp. We discuss how positional information established independently along two axes can act in concert to control gene regulation in two dimensions.

An oncogenic form of human raf can specify terminal body pattern in Drosophila

Terminal portions of the Drosophila body pattern are specified by an extracellular ligand generated at each end of the early syncytial embryo. This ligand triggers the localized transcription of two gap segmentation genes, tailles (tll) and huckebein (hkb) through a signal transduction cascade involving the receptor tyrosine kinase torso (tor) and homologues of ras, raf, and mek (map kinae kinase). In contrast to the ligand, these signal transducing components are expressed ubiquitously. Here, we show that a constitutively active form of human raf1 protein can trigger tll and hkb transcription in Drosophila embryos and specify elements of the terminal body pattern. This result indicates a strong functional conservation between Drosophila and mammalian raf proteins and argues that the localized activity of Drosophila raf (D-raf) normally carries spatial information specifying the end portions of the body.

Genetic analysis of the X-chromosomal region 1E-2A of Drosophila melanogaster

Reversion mutagenesis of three single P elements located in the cytogenetic interval 1E-2A at the tip of the X chromosome of Drosophila melanogaster was used to recover new deletions in this chromosomal region. The deletions obtained include small aberrations within region 2A and larger lesions extending from 2A into 1E and 1B. All three screens also yielded terminal deficiencies. The new deficiencies, together with previously characterized rearrangements, were analyzed for their complementation behaviour with the maternal effect locus fs(1) Nasrat and lethal loci in the region. These analyses provide an overall genetic map of the interval 1E-2A. In addition, the smaller deletions were physically mapped within cloned genomic DNA of the 2A region.

Torso, a receptor tyrosine kinase required for embryonic pattern formation, shares substrates with the sevenless and EGF-R pathways in Drosophila

The maternally expressed Drosophila gene torso (tor) is a receptor tyrosine kinase that, when activated, initiates a signal transduction cascade that is responsible for the proper differentiation of the terminal, nonsegmented regions of the embryo. l(1)pole hole, the Drosophila raf-1 serine-threonine kinase homolog, and corkscrew, a tyrosine phosphatase, have been shown previously to function in this signal transduction pathway. We have identified other products in this pathway by carrying out a mutagenesis screen for dominant suppressors of a tor gain-of-function allele. More than 40 mutations, some of which fall into seven complementation groups, have been characterized genetically. Two of these correspond to mutations in ras-1 and Son of sevenless (Sos), which also function in the sevenless and EGF receptor (Der) tyrosine kinase pathways. The phenotypes of several other Su(tor) mutations suggest that they also function in other receptor tyrosine kinase-activated pathways at different times during Drosophila development.

Control of cell fate determination by p21ras/Ras1, an essential component of torso signaling in Drosophila

Determination of cell fate at the posterior termini of the Drosophila embryo is specified by the activation of the torso (tor) receptor tyrosine kinase. This signaling pathway is mediated by the serine/threonine kinase D-raf and a protein tyrosine phosphatase corkscrew (csw). We found that expression of an activated form of Ras1 during oogenesis resulted in embryos with tor gain-of-function phenotypes. To demonstrate that p21ras/Ras1 mediates tor signaling, we injected mammalian p21ras variants into early Drosophila embryos. We found that the injection of activated p21v-ras rescued the maternal-effect phenotypes of both tor and csw null mutations. These rescuing effects of p21v-ras are dependent on the presence of maternally derived D-raf activity. In addition, wild-type embryos show a terminal-class phenotype resembling csw when injected with p21rasN17, a dominant-negative form of p21ras. Furthermore, we have analyzed the maternal-effect phenotype of Son of sevenless (Sos), a positive regulator of Ras1, and showed that embryos derived from germ cells lacking Sos+ activity exhibit a terminal-class phenotype. Our study demonstrates that the Drosophila p21ras, encoded by Ras1, is an intrinsic component of the tor signaling pathway, where it is both necessary and sufficient in specifying posterior terminal cell fates. p21ras/Ras1 operates upstream of the D-raf kinase in this signaling pathway.

The torso receptor localizes as well as transduces the spatial signal specifying terminal body pattern in Drosophila

Specification of the end portions of the Drosophila body depends on the torso (tor) protein, a receptor tyrosine kinase that accumulates uniformly along the entire surface of the embryo but is activated only in the vicinity of the poles. Several genes are normally required for activating tor and appear to define a system in which a gene product tethered to the extracellular vitelline membrane at each end of the egg provides a local source for an extracellular tor ligand. This ligand would have to diffuse from the membrane to the cell surface of the embryo without losing its spatial localization. Here we report that the failure to accumulate tor protein at one or both poles leads to spatially inappropriate activity of more centrally located receptor. This ectopic activity depends on the same gene functions normally required for activating tor; thus we infer that it reflects inappropriate diffusion of the ligand to more central regions of the body. We conclude that the receptor not only transduces the spatial signal imparted by the tor ligand, but also ensures its correct localization by sequestering the ligand. Ligand trapping by receptor may also localize spatial signals in other patterning systems, including specification of the dorsal-ventral axis in Drosophila and of vulval cell fates in Caenorhabditis elegans.

Biochemical analysis of torso and D-raf during Drosophila embryogenesis: implications for terminal signal transduction

Determination of anterior and posterior terminal structures of Drosophila embryos requires activation of two genes encoding putative protein kinases, torso and D-raf. In this study, we demonstrate that Torso has intrinsic tyrosine kinase activity and show that it is transiently tyrosine phosphorylated (activated) at syncytial blastoderm stages. Torso proteins causing a gain-of-function phenotype are constitutively tyrosine phosphorylated, while Torso proteins causing a loss-of-function phenotype lack tyrosine kinase activity. The D-raf gene product, which is required for Torso function, is identified as a 90-kDa protein with intrinsic serine/threonine kinase activity. D-Raf is expressed throughout embryogenesis; however, the phosphorylation state of the protein changes during development. In wild-type embryos, D-Raf is hyperphosphorylated at 1 to 2 h after egg laying, and thereafter only the most highly phosphorylated form is detected. Embryos lacking Torso activity, however, show significant reductions in D-Raf protein expression rather than major alterations in the protein's phosphorylation state. This report provides the first biochemical analysis of the terminal signal transduction pathway in Drosophila embryos.

A protein kinase similar to MAP kinase activator acts downstream of the raf kinase in Drosophila

D-raf, a Drosophila homolog of Raf-1, plays key roles in multiple signal transduction pathways. Dsor1, a putative factor downstream of D-raf, was genetically identified by screening of dominant suppressors of D-raf. Dsor1Su1 mapped on X chromosome significantly suppressed the D-raf mutant phenotypes, and the loss-of-function mutations of Dsor1 showed phenotypes similar to those of the D-raf null mutations. Dsor1Su1 also significantly suppressed the mutations of other terminal class genes acting further upstream of D-raf. Molecular cloning of Dsor1 revealed its product with striking similarity to the microtubule-associated protein (MAP) kinase activator and yeast PBS2, STE7, and byr1. Our genetic results demonstrate the connection between raf and the highly conserved protein kinase cascade involving MAP kinase in vivo.

Biochemical analysis of torso and D-raf during Drosophila embryogenesis: implications for terminal signal transduction

Determination of anterior and posterior terminal structures of Drosophila embryos requires activation of two genes encoding putative protein kinases, torso and D-raf. In this study, we demonstrate that Torso has intrinsic tyrosine kinase activity and show that it is transiently tyrosine phosphorylated (activated) at syncytial blastoderm stages. Torso proteins causing a gain-of-function phenotype are constitutively tyrosine phosphorylated, while Torso proteins causing a loss-of-function phenotype lack tyrosine kinase activity. The D-raf gene product, which is required for Torso function, is identified as a 90-kDa protein with intrinsic serine/threonine kinase activity. D-Raf is expressed throughout embryogenesis; however, the phosphorylation state of the protein changes during development. In wild-type embryos, D-Raf is hyperphosphorylated at 1 to 2 h after egg laying, and thereafter only the most highly phosphorylated form is detected. Embryos lacking Torso activity, however, show significant reductions in D-Raf protein expression rather than major alterations in the protein's phosphorylation state. This report provides the first biochemical analysis of the terminal signal transduction pathway in Drosophila embryos.

Torso receptor activity is regulated by a diffusible ligand produced at the extracellular terminal regions of the Drosophila egg

torso encodes a receptor tyrosine kinase (torso) required for anterior and posterior terminal development of the Drosophila embryo. Injecting eggs with in vitro synthesized torso mRNAs revealed that torso activation is governed by an extracellular molecule, presumably the torso ligand, produced at terminal regions of the egg during early embryogenesis. In the absence of torso, the ligand shows no apparent localization, indicating that it is diffusible and normally bound by an excess of torso receptor at the egg poles. Mutant ligand-binding torso proteins can suppress telson formation in a dominant negative manner, suggesting that the ligand is limited in amount. Analysis of torso mutations indicates that torso functions as a tyrosine kinase and that gain-of-function mutations causing ligand-independent activation are located in the extracellular domain.

corkscrew encodes a putative protein tyrosine phosphatase that functions to transduce the terminal signal from the receptor tyrosine kinase torso

We describe the characterization of the Drosophila gene, corkscrew (csw), which is maternally required for normal determination of cell fates at the termini of the embryo. Determination of terminal cell fates is mediated by a signal transduction pathway that involves a receptor tyrosine kinase, torso, a serine/threonine kinase, D-raf, and the transcription factors, tailless and huckebein. Double mutant and cellular analyses between csw, torso, D-raf, and tailless indicate that csw acts downstream of torso and in concert with D-raf to positively transduce the torso signal via tailless, to downstream terminal genes. The csw gene encodes a putative nonreceptor protein tyrosine phosphatase covalently linked to two N-terminal SH2 domains, which is similar to the mammalian PTP1C protein.

Genetic control of cell fate in the termini of the Drosophila embryo

Cell fates in the anterior and posterior termini of the Drosophila embryo are programmed by multiple zygotic genes that are regulated in response to a maternally encoded signal transduction pathway. These genes specify terminal as distinct from central cell fates, program pattern along the anteroposterior and dorsoventral axes of the termini, and also control endoderm specification and terminal morphogenetic movements. Here, we use a genetic interaction test to dissect the zygotic components of the terminal genetic hierarchy. We show that two genes, lines and empty spiracles, act downstream of tailless to repress central and promote terminal cell fates along the anteroposterior axis of the termini. Genes that control dorsoventral pattern in the termini and genes that program terminal morphogenesis act in distinct branches of the genetic hierarchy that are independent of tailless.

Axis specification in the Drosophila embryo

Three genetic hierarchies control cell-fate specification in largely distinct regions of the antero-posterior axis of the Drosophila embryo, whereas a single hierarchy specifies dorso-ventral cell fates. Molecular genetic analysis of these hierarchies is leading to increased understanding of the nature of the regulatory circuitry that controls regional cell-fate specification.

Specification of cell fate in the developing eye of Drosophila

Determination of cell fate in the developing eye of Drosophila depends on a precise sequence of cellular interactions which generate the stereotypic array of ommatidia. In the eye imaginal disc, an initially unpatterned epithelial sheath of cells, the first step in this process may be the specification of R8 photoreceptor cells at regular intervals. Genes such as Notch and scabrous, known to be involved in bristle development, also participate in this process, suggesting that the specification of ommatidial founder cells and the formation of sensory organs in the adult epidermis may involve a similar mechanism, that of lateral inhibition. The subsequent steps of ommatidial assembly, following R8 assignment, involve a different mechanism: Undetermined cells read their position based on the contacts they make with neighbors that have already begun to differentiate. The development of the R7 photoreceptor cell, one of the eight photoreceptor cells in the ommatidium, is best understood. An important role seems to be played by sevenless, a receptor tyrosine kinase on the surface of the R7 precursor. It transmits the positional information--most likely encoded by the boss protein on the neighboring R8 cell membrane--into the cell via its tyrosine kinase, which activates a signal transduction cascade. Constitutive activation of the sevenless kinase by overexpression of an N-terminally truncated form results in the diversion of other ommatidial cells into the R7 pathway suggesting that activation of the sevenless signalling pathway is sufficient to specify R7 development. Genetic dissection of this pathway should therefore identify components of a signalling cascade activated by a tyrosine kinase.

Interaction between torso and dorsal, two elements of different transduction pathways in the Drosophila embryo

Early development of the Drosophila embryo is under the control of some maternal genes responsible for the establishment of its general pattern. Three sets of genes determine the anteroposterior pattern; two distinct systems specify anterior and posterior development and a third one, the terminal system, is responsible for the development of the poles of the embryo. A different set of genes specifies dorsoventral polarity, which is established by the graded activity of the dorsal gene product. Here I analyze the effect of the terminal system on the expression of two zygotic genes involved in dorsoventral pattern, snail and decapentaplegic, and I show that this effect is mediated by a reduction on dorsal activity by the terminal system. Due to the interaction of these two systems, both of which use transmembrane signalling mechanisms, the poles adopt a more dorsalized fate than their counterparts in the middle of the embryo.

Dual role of the Drosophila pattern gene tailless in embryonic termini

One of the first zygotically active genes required for formation of the terminal domains of the Drosophila embryo is tailless (tll). Expression of the tll gene is activated ectopically in gain-of-function mutants of the maternal terminal gene torso (tor); this suggests that tor normally activates the tll gene in the termini. Ectopic expression of tll under the control of an inducible promoter results in differentiation of ectopic terminal-specific structures, the Filzkörper, and leads to the activation of at least one gene, hunchback, that is required to form these structures. Ectopic expression of the tll gene also represses segmentation by repressing the gap genes Krüppel and knirps and probably also pair rule genes.

Drosophila headlines

Recent genetic and molecular data from Drosophila support the long-standing observations of morphology in suggesting that segmentation of the insect embryo proceeds in two phases. Organization of the cephalic segments uses a mechanism distinct from the familiar bierarchical cascade of segmentation genes that subdivides the trunk of the embryo.

Establishment of dorsal-ventral and terminal pattern in the Drosophila embryo

Dorsal-ventral and terminal pattern formation in the Drosophila embryo is mediated via inductive signals originating during oogenesis from the somatic follicle cells that ensheath the developing oocyte. This somatically expressed spatial information controls dorsal-ventral development by defining the polarity of a signal transduction pathway that results in the graded nuclear concentration of the dorsal gene product, a transcription factor.

oskar mRNA is localized to the posterior pole of the Drosophila oocyte

Mutants of the maternal posterior-group genes of Drosophila lack posterior body pattern elements and germ cells, both of which form through the action of determinants localized to the posterior pole of the oocyte. We report that transcripts of one of these genes, oskar, become localized to the posterior pole of oocytes shortly after the oocyte begins to differentiate visibly. Analysis of various posterior-group mutants reveals that localization of oskar mRNA is an early step in the posterior localization pathway. In addition, we find that nonsense oskar mutations disrupt osk mRNA localization, while missense oskar mutations do not.