The argos gene encodes a diffusible factor that regulates cell fate decisions in the Drosophila eye

Patterning of the Drosophila retina by the morphogenetic furrow

Pattern formation is the process by which cells within a homogeneous epithelial sheet acquire distinctive fates depending upon their relative spatial position to each other. Several proposals, starting with Alan Turing's diffusion-reaction model, have been put forth over the last 70 years to describe how periodic patterns like those of vertebrate somites and skin hairs, mammalian molars, fish scales, and avian feather buds emerge during development. One of the best experimental systems for testing said models and identifying the gene regulatory networks that control pattern formation is the compound eye of the fruit fly, Drosophila melanogaster. Its cellular morphogenesis has been extensively studied for more than a century and hundreds of mutants that affect its development have been isolated. In this review we will focus on the morphogenetic furrow, a wave of differentiation that takes an initially homogeneous sheet of cells and converts it into an ordered array of unit eyes or ommatidia. Since the discovery of the furrow in 1976, positive and negative acting morphogens have been thought to be solely responsible for propagating the movement of the furrow across a motionless field of cells. However, a recent study has challenged this model and instead proposed that mechanical driven cell flow also contributes to retinal pattern formation. We will discuss both models and their impact on patterning.

Single cell RNA sequencing of the adult Drosophila eye reveals distinct clusters and novel marker genes for all major cell types

The adult Drosophila eye is a powerful model system for phototransduction and neurodegeneration research. However, single cell resolution transcriptomic data are lacking for this tissue. We present single cell RNA-seq data on 1-day male and female, 3-day and 7-day old male adult eyes, covering early to mature adult eyes. All major cell types, including photoreceptors, cone and pigment cells in the adult eye were captured and identified. Our data sets identified novel cell type specific marker genes, some of which were validated in vivo. R7 and R8 photoreceptors form clusters that reflect their specific Rhodopsin expression and the specific Rhodopsin expression by each R7 and R8 cluster is the major determinant to their clustering. The transcriptomic data presented in this report will facilitate a deeper mechanistic understanding of the adult fly eye as a model system.

Differentiation signals from glia are fine-tuned to set neuronal numbers during development

Neural circuit formation and function require that diverse neurons are specified in appropriate numbers. Known strategies for controlling neuronal numbers involve regulating either cell proliferation or survival. We used the Drosophila visual system to probe how neuronal numbers are set. Photoreceptors from the eye-disc induce their target field, the lamina, such that for every unit eye there is a corresponding lamina unit (column). Although each column initially contains ~6 post-mitotic lamina precursors, only 5 differentiate into neurons, called L1-L5; the 'extra' precursor, which is invariantly positioned above the L5 neuron in each column, undergoes apoptosis. Here, we showed that a glial population called the outer chiasm giant glia (xg^O), which resides below the lamina, secretes multiple ligands to induce L5 differentiation in response to EGF from photoreceptors. By forcing neuronal differentiation in the lamina, we uncovered that though fated to die, the 'extra' precursor is specified as an L5. Therefore, two precursors are specified as L5s but only one differentiates during normal development. We found that the row of precursors nearest to xg^O differentiate into L5s and, in turn, antagonise differentiation signalling to prevent the 'extra' precursors from differentiating, resulting in their death. Thus, an intricate interplay of glial signals and feedback from differentiating neurons defines an invariant and stereotyped pattern of neuronal differentiation and programmed cell death to ensure that lamina columns each contain exactly one L5 neuron.

EyeVolve, a modular PYTHON based model for simulating developmental eye type diversification

Vision is among the oldest and arguably most important sensory modalities for animals to interact with their external environment. Although many different eye types exist within the animal kingdom, mounting evidence indicates that the genetic networks required for visual system formation and function are relatively well conserved between species. This raises the question as to how common developmental programs are modified in functionally different eye types. Here, we approached this issue through EyeVolve, an open-source PYTHON-based model that recapitulates eye development based on developmental principles originally identified in Drosophila melanogaster. Proof-of-principle experiments showed that this program’s animated timeline successfully simulates early eye tissue expansion, neurogenesis, and pigment cell formation, sequentially transitioning from a disorganized pool of progenitor cells to a highly organized lattice of photoreceptor clusters wrapped with support cells. Further, tweaking just five parameters (precursor pool size, founder cell distance and placement from edge, photoreceptor subtype number, and cell death decisions) predicted a multitude of visual system layouts, reminiscent of the varied eye types found in larval and adult arthropods. This suggests that there are universal underlying mechanisms that can explain much of the existing arthropod eye diversity. Thus, EyeVolve sheds light on common principles of eye development and provides a new computational system for generating specific testable predictions about how development gives rise to diverse visual systems from a commonly specified neuroepithelial ground plan.

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.

The mir-279/996 cluster represses receptor tyrosine kinase signaling to determine cell fates in the Drosophila eye

Photoreceptors in the crystalline Drosophila eye are recruited by receptor tyrosine kinase (RTK)/Ras signaling mediated by Epidermal growth factor receptor (EGFR) and the Sevenless (Sev) receptor. Analyses of an allelic deletion series of the mir-279/996 locus, along with a panel of modified genomic rescue transgenes, show that Drosophila eye patterning depends on both miRNAs. Transcriptional reporter and activity sensor transgenes reveal expression and function of miR-279/996 in non-neural cells of the developing eye. Moreover, mir-279/996 mutants exhibit substantial numbers of ectopic photoreceptors, particularly of R7, and cone cell loss. These miRNAs restrict RTK signaling in the eye, since mir-279/996 nulls are dominantly suppressed by positive components of the EGFR pathway and enhanced by heterozygosity for an EGFR repressor. miR-279/996 limit photoreceptor recruitment by targeting multiple positive RTK/Ras signaling components that promote photoreceptor/R7 specification. Strikingly, deletion of mir-279/996 sufficiently derepresses RTK/Ras signaling so as to rescue a population of R7 cells in R7-specific RTK null mutants boss and sev, which otherwise completely lack this cell fate. Altogether, we reveal a rare setting of developmental cell specification that involves substantial miRNA control.

Genetic regulation and function of epidermal growth factor receptor signalling in patterning of the embryonic Drosophila brain

The specification of distinct neural cell types in central nervous system development crucially depends on positional cues conferred to neural stem cells in the neuroectoderm. Here, we investigate the regulation and function of the epidermal growth factor receptor (EGFR) signalling pathway in early development of the Drosophila brain. We find that localized EGFR signalling in the brain neuroectoderm relies on a neuromere-specific deployment of activating (Spitz, Vein) and inhibiting (Argos) ligands. Activated EGFR controls the spatially restricted expression of all dorsoventral (DV) patterning genes in a gene- and neuromere-specific manner. Further, we reveal a novel role of DV genes—ventral nervous system defective (vnd), intermediate neuroblast defective (ind), Nkx6—in regulating the expression of vein and argos, which feed back on EGFR, indicating that EGFR signalling stands not strictly atop the DV patterning genes. Within this network of genetic interactions, Vnd acts as a positive EGFR feedback regulator. Further, we show that EGFR signalling becomes dependent on single-minded-expressing midline cells in the posterior brain (tritocerebrum), but remains midline-independent in the anterior brain (deuto- and protocerebrum). Finally, we demonstrate that activated EGFR controls the proper formation of brain neuroblasts by regulating the number, survival and proneural gene expression of neuroectodermal progenitor cells. These data demonstrate that EGFR signalling is crucially important for patterning and early neurogenesis of the brain.

Negative Regulation of Receptor Tyrosine Kinase (RTK) Signaling: A Developing Field

Trophic factors control cellular physiology by activating specific receptor tyrosine kinases (RTKs). While the over activation of RTK signaling pathways is associated with cell growth and cancer, recent findings support the concept that impaired down-regulation or deactivation of RTKs may also be a mechanism involved in tumor formation. Under this perspective, the molecular determinants of RTK signaling inhibition may act as tumor-suppressor genes and have a potential role as tumor markers to monitor and predict disease progression. Here, we review the current understanding of the physiological mechanisms that attenuate RTK signaling and discuss evidence that implicates deregulation of these events in cancer.

Lighting Up ERK Activity

Activation of extracellular signal regulated kinase (ERK) is used by many signaling pathways to control tissue patterning in a broad range of multicellular organisms. In this issue of Developmental Cell, Johnson et al. (2017) provide an optogenetic approach to manipulate this pathway with high precision and explore its signaling code.

Developmental roles of Rhomboid proteases

Rhomboid proteins have emerged as one of the most tantalizing and diverse families of proteases. Gene duplication events and structural alterations have sculpted the varied roles of this protein family, maintaining a conserved structural core throughout the bacterial, plant and animal kingdoms. Unresolved questions pop up at many junctions. This review will focus on a distinct class of Rhomboid proteins that plays an essential role in development. It will outline the diverse mechanisms by which these proteins are regulated, and the implications on the biological processes they control. While most of the review will deal with Rhomboids in Drosophila, a system that has been studied in the greatest detail, it will also explore parallels and differences in the function of Rhomboids in the flour beetle T. casteneum and the worm C. elegans.

Regulatory mechanisms of EGFR signalling during Drosophila eye development

EGFR signalling is a well-conserved signalling pathway playing major roles during development and cancers. This review explores what studying the EGFR pathway during Drosophila eye development has taught us in terms of the diversity of its regulatory mechanisms. This model system has allowed the identification of numerous positive and negative regulators acting at specific time and place, thus participating to the tight control of signalling. EGFR signalling regulation is achieved by a variety of mechanisms, including the control of ligand processing, the availability of the receptor itself and the transduction of the cascade in the cytoplasm. Ultimately, the transcriptional responses contribute to the establishment of positive and negative feedback loops. The combination of these multiple mechanisms employed to regulate the EGFR pathway leads to specific cellular outcomes involved in functions as diverse as the acquisition of cell fate, proliferation, survival, adherens junction remodelling and morphogenesis.

The EGFR signaling pathway controls gut progenitor differentiation during planarian regeneration and homeostasis

The planarian Schmidtea mediterranea maintains and regenerates all its adult tissues through the proliferation and differentiation of a single population of pluripotent adult stem cells (ASCs) called neoblasts. Despite recent advances, the mechanisms regulating ASC differentiation into mature cell types are poorly understood. Here, we show that silencing of the planarian EGF receptor egfr-1 by RNA interference (RNAi) impairs gut progenitor differentiation into mature cells, compromising gut regeneration and maintenance. We identify a new putative EGF ligand, nrg-1, the silencing of which phenocopies the defects observed in egfr-1(RNAi) animals. These findings indicate that egfr-1 and nrg-1 promote gut progenitor differentiation, and are thus essential for normal cell turnover and regeneration in the planarian gut. Our study demonstrates that the EGFR signaling pathway is an important regulator of ASC differentiation in planarians.

Unique and Overlapping Functions of Formins Frl and DAAM During Ommatidial Rotation and Neuronal Development in Drosophila

The noncanonical Frizzled/planar cell polarity (PCP) pathway regulates establishment of polarity within the plane of an epithelium to generate diversity of cell fates, asymmetric, but highly aligned structures, or to orchestrate the directional migration of cells during convergent extension during vertebrate gastrulation. In Drosophila, PCP signaling is essential to orient actin wing hairs and to align ommatidia in the eye, in part by coordinating the movement of groups of photoreceptor cells during ommatidial rotation. Importantly, the coordination of PCP signaling with changes in the cytoskeleton is essential for proper epithelial polarity. Formins polymerize linear actin filaments and are key regulators of the actin cytoskeleton. Here, we show that the diaphanous-related formin, Frl, the single fly member of the FMNL (formin related in leukocytes/formin-like) formin subfamily affects ommatidial rotation in the Drosophila eye and is controlled by the Rho family GTPase Cdc42. Interestingly, we also found that frl mutants exhibit an axon growth phenotype in the mushroom body, a center for olfactory learning in the Drosophila brain, which is also affected in a subset of PCP genes. Significantly, Frl cooperates with Cdc42 and another formin, DAAM, during mushroom body formation. This study thus suggests that different formins can cooperate or act independently in distinct tissues, likely integrating various signaling inputs with the regulation of the cytoskeleton. It furthermore highlights the importance and complexity of formin-dependent cytoskeletal regulation in multiple organs and developmental contexts.

Square Cell Packing in the Drosophila Embryo through Spatiotemporally Regulated EGF Receptor Signaling

Cells display dynamic and diverse morphologies during development, but the strategies by which differentiated tissues achieve precise shapes and patterns are not well understood. Here we identify a developmental program that generates a highly ordered square cell grid in the Drosophila embryo through sequential and spatially regulated cell alignment, oriented cell division, and apicobasal cell elongation. The basic leucine zipper transcriptional regulator Cnc is necessary and sufficient to produce a square cell grid in the presence of a midline signal provided by the EGF receptor ligand Spitz. Spitz orients cell divisions through a Pins/LGN-dependent spindle-positioning mechanism and controls cell shape and alignment through a transcriptional pathway that requires the Pointed ETS domain protein. These results identify a strategy for producing ordered square cell packing configurations in epithelia and reveal a molecular mechanism by which organized tissue structure is generated through spatiotemporally regulated responses to EGF receptor activation.

gone early, a novel germline factor, ensures the proper size of the stem cell precursor pool in the Drosophila ovary

In order to sustain lifelong production of gametes, many animals have evolved a stem cell-based gametogenic program. In the Drosophila ovary, germline stem cells (GSCs) arise from a pool of primordial germ cells (PGCs) that remain undifferentiated even after gametogenesis has initiated. The decision of PGCs to differentiate or remain undifferentiated is regulated by somatic stromal cells: specifically, epidermal growth factor receptor (EGFR) signaling activated in the stromal cells determines the fraction of germ cells that remain undifferentiated by shaping a Decapentaplegic (Dpp) gradient that represses PGC differentiation. However, little is known about the contribution of germ cells to this process. Here we show that a novel germline factor, Gone early (Goe), limits the fraction of PGCs that initiate gametogenesis. goe encodes a non-peptidase homologue of the Neprilysin family metalloendopeptidases. At the onset of gametogenesis, Goe was localized on the germ cell membrane in the ovary, suggesting that it functions in a peptidase-independent manner in cell-cell communication at the cell surface. Overexpression of Goe in the germline decreased the number of PGCs that enter the gametogenic pathway, thereby increasing the proportion of undifferentiated PGCs. Inversely, depletion of Goe increased the number of PGCs initiating differentiation. Excess PGC differentiation in the goe mutant was augmented by halving the dose of argos, a somatically expressed inhibitor of EGFR signaling. This increase in PGC differentiation resulted in a massive decrease in the number of undifferentiated PGCs, and ultimately led to insufficient formation of GSCs. Thus, acting cooperatively with a somatic regulator of EGFR signaling, the germline factor goe plays a critical role in securing the proper size of the GSC precursor pool. Because goe can suppress EGFR signaling activity and is expressed in EGF-producing cells in various tissues, goe may function by attenuating EGFR signaling, and thereby affecting the stromal environment.

Conserved miR-8/miR-200 defines a glial niche that controls neuroepithelial expansion and neuroblast transition

Neuroepithelial cell proliferation must be carefully balanced with the transition to neuroblast (neural stem cell) to control neurogenesis. Here, we show that loss of the Drosophila microRNA mir-8 (the homolog of vertebrate miR-200 family) results in both excess proliferation and ectopic neuroblast transition. Unexpectedly, mir-8 is expressed in a subpopulation of optic-lobe-associated cortex glia that extend processes that ensheath the neuroepithelium, suggesting that glia cells communicate with the neuroepithelium. We provide evidence that miR-8-positive glia express Spitz, a transforming growth factor α (TGF-α)-like ligand that triggers epidermal growth factor receptor (EGFR) activation to promote neuroepithelial proliferation and neuroblast formation. Further, our experiments suggest that miR-8 ensures both a correct glial architecture and the spatiotemporal control of Spitz protein synthesis via direct binding to Spitz 3' UTR. Together, these results establish glial-derived cues as key regulatory elements in the control of neuroepithelial cell proliferation and the neuroblast transition.

Sequential activation of ETS proteins provides a sustained transcriptional response to EGFR signaling

How signal transduction, which is dynamic and fluctuating by nature, is converted into a stable trancriptional response, is an unanswered question in developmental biology. Two ETS-domain transcription factors encoded by the pointed (pnt) locus, PntP1 and PntP2, are universal downstream mediators of EGFR-based signaling in Drosophila. Full disruption of pnt function in developing eye imaginal discs reveals a photoreceptor recruitment phenotype, in which only the R8 photoreceptor cell type is specified within ommatidia. Specific disruption of either pntP1 or pntP2 resulted in the same R8-only phenotype, demonstrating that both Pnt isoforms are essential for photoreceptor recruitment. We show that the two Pnt protein forms are activated in a sequential manner within the EGFR signaling pathway: MAPK phosphorylates and activates PntP2, which in turn induces pntP1 transcription. Once expressed, PntP1 is constitutively active and sufficient to induce target genes essential for photoreceptor development. Pulse-chase experiments indicate that PntP1 is stable for several hours in the eye disc. Sequential ETS-protein recruitment therefore allows sustained induction of target genes, beyond the transient activation of EGFR.

Trafficking of the EGFR ligand Spitz regulates its signaling activity in polarized tissues

Epidermal growth factor receptor (EGFR) ligands undergo a complex series of processing events during their maturation to active signaling proteins. Like its mammalian homologs, the predominant Drosophila EGFR ligand Spitz is produced as a transmembrane pro-protein. In the secretory pathway, Spitz is cleaved within its transmembrane domain to release the extracellular signaling domain. This domain is modified with an N-terminal palmitate group that tethers it to the plasma membrane. We found that the pro-protein can reach the cell surface in the absence of proteolysis, but that it fails to activate the EGFR. To address why the transmembrane pro-protein is inactive, whereas membrane association through the palmitate group promotes activity, we generated a panel of chimeric constructs containing the Spitz extracellular region fused to exogenous transmembrane proteins. Although the orientation of the EGF domain and its distance from the plasma membrane varies in these chimeras, they are all active in vivo. Thus, tethering Spitz to the membrane via a transmembrane domain at either terminus does not prevent activity. Conversely, removing the N-terminal palmitate group from the C-terminally tethered pro-protein does not render it active. Furthermore, we show that the Spitz transmembrane pro-protein can activate the EGFR in a tissue culture assay, indicating that its failure to signal in vivo is not due to structural features. In polarized imaginal disc cells, unprocessed Spitz pro-protein localizes to apical puncta, whereas the active chimeric Spitz constructs are basolaterally localized. Taken together, our data support the model that localized trafficking of the pro-protein restricts its ability to activate the receptor in polarized tissues.

Nemo regulates cell dynamics and represses the expression of miple, a midkine/pleiotrophin cytokine, during ommatidial rotation

Ommatidial rotation is one of the most important events for correct patterning of the Drosophila eye. Although several signaling pathways are involved in this process, few genes have been shown to specifically affect it. One of them is nemo (nmo), which encodes a MAP-like protein kinase that regulates the rate of rotation throughout the entire process, and serves as a link between core planar cell polarity (PCP) factors and the E-cadherin-β-catenin complex. To determine more precisely the role of nmo in ommatidial rotation, live-imaging analyses in nmo mutant and wild-type early pupal eye discs were performed. We demonstrate that ommatidial rotation is not a continuous process, and that rotating and non-rotating interommatidial cells are very dynamic. Our in vivo analyses also show that nmo regulates the speed of rotation and is required in cone cells for correct ommatidial rotation, and that these cells as well as interommatidial cells are less dynamic in nmo mutants. Furthermore, microarray analyses of nmo and wild-type larval eye discs led us to identify new genes and signaling pathways related to nmo function during this process. One of them, miple, encodes the Drosophila ortholog of the midkine/pleiotrophin secreted cytokines that are involved in cell migration processes. miple is highly up-regulated in nmo mutant discs. Indeed, phenotypic analyses reveal that miple overexpression leads to ommatidial rotation defects. Genetic interaction assays suggest that miple is signaling through Ptp99A, the Drosophila ortholog of the vertebrate midkine/pleiotrophin PTPζ receptor. Accordingly, we propose that one of the roles of Nmo during ommatial rotation is to repress miple expression, which may in turn affect the dynamics in E-cadherin-β-catenin complexes.

The Capicua repressor--a general sensor of RTK signaling in development and disease

Receptor tyrosine kinase (RTK) signaling pathways control multiple cellular decisions in metazoans, often by regulating the expression of downstream genes. In Drosophila melanogaster and other systems, E-twenty-six (ETS) transcription factors are considered to be the predominant nuclear effectors of RTK pathways. Here, we highlight recent progress in identifying the HMG-box protein Capicua (CIC) as a key sensor of RTK signaling in both Drosophila and mammals. Several studies have shown that CIC functions as a repressor of RTK-responsive genes, keeping them silent in the absence of signaling. Following the activation of RTK signaling, CIC repression is relieved, and this allows the expression of the targeted gene in response to local or ubiquitous activators. This regulatory switch is essential for several RTK responses in Drosophila, from the determination of cell fate to cell proliferation. Furthermore, increasing evidence supports the notion that this mechanism is conserved in mammals, where CIC has been implicated in cancer and neurodegeneration. In addition to summarizing our current knowledge on CIC, we also discuss the implications of these findings for our understanding of RTK signaling specificity in different biological processes.

Drosophila LSD1-CoREST demethylase complex regulates DPP/TGFβ signaling during wing development

The choice and timing of specific developmental pathways in organogenesis are determined by tissue-specific temporal and spatial cues that are acted upon to impart unique cellular and compartmental identities. A consequence of cellular signaling is the rapid transcriptional reprogramming of a wide variety of target genes. To overcome intrinsic epigenetic chromatin barriers to transcription modulation, histone modifying and remodeling complexes are employed. The deposition or erasure of specific covalent histone modifications, including acetylation, methylation, and ubiquitination are essential features of gene activation and repression. We have found that the activity of a specific class of histone demethylation enzymes is required for the specification of vein cell fates during Drosophila wing development. Genetic tests revealed that the Drosophila LSD1-CoREST complex is required for proper cell specification through regulation of the DPP/TGFβ pathway. An important finding from this analysis is that LSD1-CoREST functions through control of rhomboid expression in an EGFR-independent pathway.

Building an ommatidium one cell at a time

Since the discovery of a single white-eyed male in a population of red eyed flies over 100 years ago (Morgan, 1910), the compound eye of the fruit fly, Drosophila melanogaster, has been a favorite experimental system for identifying genes that regulate various aspects of development. For example, a fair amount of what we know today about enzymatic pathways and vesicular transport is due to the discovery and subsequent characterization of eye color mutants such as white. Likewise, our present day understanding of organogenesis has been aided considerably by studies of mutations, such as eyeless, that either reduce or eliminate the compound eyes. But by far the phenotype that has provided levers into the greatest number of experimental fields has been the humble "rough" eye. The fly eye is composed of several hundred unit-eyes that are also called ommatidia. These unit eyes are packed into a hexagonal array of remarkable precision. The structure of the eye is so precise that it has been compared with that of a crystal (Ready et al., 1976). Even the slightest perturbations to the structure of the ommatidium can be visually detected by light or electron microscopy. The cause for this is two-fold: (1) any defect that affects the hexagonal geometry of a single ommatidium can and will disrupt the positioning of surrounding unit eyes thereby propagating structural flaws and (2) disruptions in genes that govern the development of even a single cell within an ommatidium will affect all unit eyes. In both cases, the effect is the visual magnification of even the smallest imperfection. Studies of rough eye mutants have provided key insights into the areas of cell fate specification, lateral inhibition, signal transduction, transcription factor networks, planar cell polarity, cell proliferation, and programmed cell death just to name a few. This review will attempt to summarize the key steps that are required to assemble each ommatidium.

My what big eyes you have: how the Drosophila retina grows

The compound eye of the fruit fly, Drosophila melanogaster, has for decades been used extensively to study a number of critical developmental processes including tissue development, pattern formation, cell fate specification, and planar cell polarity. To a lesser degree it has been used to examine the cell cycle and tissue proliferation. Discovering the mechanisms that balance tissue growth and cell death in developing epithelia has traditionally been the realm of those using the wing disc. However, over the last decade a series of observations has demonstrated that the eye is a suitable and maybe even preferable tissue for studying tissue growth. This review will focus on how growth of the retina is controlled by the genes and pathways that govern the specification of tissue fate, the division of the epithelium into dorsal-ventral compartments, the initiation, and progression of the morphogenetic furrow and the second mitotic wave.

A gain-of-function germline mutation in Drosophila ras1 affects apoptosis and cell fate during development

The RAS/MAPK signal transduction pathway is an intracellular signaling cascade that transmits environmental signals from activated receptor tyrosine kinases (RTKs) on the cell surface and other endomembranes to transcription factors in the nucleus, thereby linking extracellular stimuli to changes in gene expression. Largely as a consequence of its role in oncogenesis, RAS signaling has been the subject of intense research efforts for many years. More recently, it has been shown that milder perturbations in Ras signaling during embryogenesis also contribute to the etiology of a group of human diseases. Here we report the identification and characterization of the first gain-of-function germline mutation in Drosophila ras1 (ras85D), the Drosophila homolog of human K-ras, N-ras and H-ras. A single amino acid substitution (R68Q) in the highly conserved switch II region of Ras causes a defective protein with reduced intrinsic GTPase activity, but with normal sensitivity to GAP stimulation. The ras1^R68Q mutant is homozygous viable but causes various developmental defects associated with elevated Ras signaling, including cell fate changes and ectopic survival of cells in the nervous system. These biochemical and functional properties are reminiscent of germline Ras mutants found in patients afflicted with Noonan, Costello or cardio-facio-cutaneous syndromes. Finally, we used ras1^R68Q to identify novel genes that interact with Ras and suppress cell death.

Planar cell polarity in Drosophila

In all multicellular organisms, epithelial cells are not only polarized along the apical-basal axis, but also within the epithelial plane, giving cells a sense of direction. Planar cell polarity (PCP) signaling regulates establishment of polarity within the plane of an epithelium. The outcomes of PCP signaling are diverse and include the determination of cell fates, the generation of asymmetric but highly aligned structures, such as the stereocilia in the human inner ear or the hairs on a fly wing, or the directional migration of cells during convergence and extension during vertebrate gastrulation. In humans, aberrant PCP signaling can result in severe developmental defects, such as open neural tubes (spina bifida), and can cause cystic kidneys. In this review, we discuss the basic mechanism and more recent findings of PCP signaling focusing on Drosophila melanogaster, the model organism in which most key PCP components were initially identified.

Regulation of epidermal growth factor receptor signalling by inducible feedback inhibitors

Signalling by the epidermal growth factor receptor (EGFR) controls morphogenesis and/or homeostasis of several tissues from worms to mammals. The correct execution of these programmes requires the generation of EGFR signals of appropriate strength and duration. This is obtained through a complex circuitry of positive and negative feedback regulation. Feedback inhibitory mechanisms restrain EGFR activity in time and space, which is key to ensuring that receptor outputs are commensurate to the cell and tissue needs. Here, we focus on the emerging field of inducible negative feedback regulation of the EGFR in mammals. In mammalian cells, four EGFR inducible feedback inhibitors (IFIs), namely LRIG1, RALT (also known as MIG6 and ERRFI1), SOCS4 and SOCS5, have been discovered recently. EGFR IFIs are expressed de novo in the context of early or delayed transcriptional responses triggered by EGFR activation. They all bind to the EGFR and suppress receptor signalling through several mechanisms, including catalytic inhibition and receptor downregulation. Here, we review the mechanistic basis of IFI signalling and rationalise the function of IFIs in light of gene-knockout studies that assign LRIG1 and RALT an essential role in restricting cell proliferation. Finally, we discuss how IFIs might participate in system control of EGFR signalling and highlight the emerging roles for IFIs in the suppression of EGFR-driven tumorigenesis.

Capicua DNA-binding sites are general response elements for RTK signaling in Drosophila

RTK/Ras/MAPK signaling pathways play key functions in metazoan development, but how they control expression of downstream genes is not well understood. In Drosophila, it is generally assumed that most transcriptional responses to RTK signal activation depend on binding of Ets-family proteins to specific cis-acting sites in target enhancers. Here, we show that several Drosophila RTK pathways control expression of downstream genes through common octameric elements that are binding sites for the HMG-box factor Capicua, a transcriptional repressor that is downregulated by RTK signaling in different contexts. We show that Torso RTK-dependent regulation of terminal gap gene expression in the early embryo critically depends on Capicua octameric sites, and that binding of Capicua to these sites is essential for recruitment of the Groucho co-repressor to the huckebein enhancer in vivo. We then show that subsequent activation of the EGFR RTK pathway in the neuroectodermal region of the embryo controls dorsal-ventral gene expression by downregulating the Capicua protein, and that this control also depends on Capicua octameric motifs. Thus, a similar mechanism of RTK regulation operates during subdivision of the anterior-posterior and dorsal-ventral embryonic axes. We also find that identical DNA octamers mediate Capicua-dependent regulation of another EGFR target in the developing wing. Remarkably, a simple combination of activator-binding sites and Capicua motifs is sufficient to establish complex patterns of gene expression in response to both Torso and EGFR activation in different tissues. We conclude that Capicua octamers are general response elements for RTK signaling in Drosophila.

Coordinated sequential action of EGFR and Notch signaling pathways regulates proneural wave progression in the Drosophila optic lobe

During neurogenesis in the medulla of the Drosophila optic lobe, neuroepithelial cells are programmed to differentiate into neuroblasts at the medial edge of the developing optic lobe. The wave of differentiation progresses synchronously in a row of cells from medial to the lateral regions of the optic lobe, sweeping across the entire neuroepithelial sheet; it is preceded by the transient expression of the proneural gene lethal of scute [l(1)sc] and is thus called the proneural wave. We found that the epidermal growth factor receptor (EGFR) signaling pathway promotes proneural wave progression. EGFR signaling is activated in neuroepithelial cells and induces l(1)sc expression. EGFR activation is regulated by transient expression of Rhomboid (Rho), which is required for the maturation of the EGF ligand Spitz. Rho expression is also regulated by the EGFR signal. The transient and spatially restricted expression of Rho generates sequential activation of EGFR signaling and assures the directional progression of the differentiation wave. This study also provides new insights into the role of Notch signaling. Expression of the Notch ligand Delta is induced by EGFR, and Notch signaling prolongs the proneural state. Notch signaling activity is downregulated by its own feedback mechanism that permits cells at proneural states to subsequently develop into neuroblasts. Thus, coordinated sequential action of the EGFR and Notch signaling pathways causes the proneural wave to progress and induce neuroblast formation in a precisely ordered manner.

Regulation of EGFR and Notch signaling by distinct isoforms of D-cbl during Drosophila development

Cells receive and interpret extracellular signals to regulate cellular responses such as proliferation, cell survival and differentiation. However, proper inactivation of these signals is critical for appropriate homeostasis. Cbl proteins are E3-ubiquitin ligases that restrict receptor tyrosine kinase (RTK) signaling, most notably EGFR (Epidermal Growth Factor Receptor), via the endocytic pathway. Consistently, many mutant phenotypes of Drosophila cbl (D-cbl) are due to inappropriate activation of EGFR signaling. However, not all D-cbl phenotypes can be explained by increased EGFR activity. Here, we report that D-Cbl also negatively regulates Notch activity during eye and wing development. D-cbl produces two isoforms by alternative splicing. The long isoform, D-CblL, regulates the EGFR. We found that the short isoform, D-CblS, preferentially restricts Notch signaling. Specifically, our data imply that D-CblS controls the activity of the Notch ligand Delta. Taken together, these data suggest that D-Cbl controls the EGFR and Notch/Delta signaling pathways through production of two alternatively spliced isoforms during development in Drosophila.

The archipelago tumor suppressor gene limits rb/e2f-regulated apoptosis in developing Drosophila tissues

BACKGROUND

The Drosophila archipelago gene (ago) encodes the specificity component of a ubiquitin ligase that targets the cyclin E and dMyc proteins for degradation. Its human ortholog, Fbw7, is commonly lost in cancers, suggesting that failure to degrade ago/Fbw7 targets drives excess tissue growth.

RESULTS

We find that ago loss induces hyperplasia of some organs but paradoxically reduces the size of the adult eye. This reflects a requirement for ago to restrict apoptotic activity of the rbf1/de2f1 pathway adjacent to the eye-specific morphogenetic furrow (MF): ago mutant cells display elevated de2f1 activity, express the prodeath dE2f1 targets hid and rpr, and undergo high rates of apoptosis. These phenotypes are dependent on rbf1, de2f1, hid, and the rbf1/de2f1 regulators cyclin E and dacapo but are independent of dp53. A transactivation-deficient de2f1 allele blocks MF-associated apoptosis of ago mutant cells but does not retard their clonal overgrowth, indicating that intact de2f1 function is required for the death but not overproliferation of ago cells. Epidermal growth factor receptor (EGFR) and wingless (wg) alleles also modify the ago apoptotic phenotype, indicating that these pathways may modulate the underlying sensitivity of ago mutant cells to apoptotic signals.

CONCLUSIONS

These data show that ago loss requires a collaborating block in cell death to efficiently drive tissue overgrowth and that this conditional phenotype reflects a role for ago in restricting apoptotic output of the rbf1/de2f1 pathway. Moreover, the susceptibility of ago mutant cells to succumb to this apoptotic program appears to depend on local variations in extracellular signaling that could thus determine tissue-specific fates of ago mutant cells.

Pattern formation in the Drosophila eye disc

Differentiation of the Drosophila compound eye from the eye imaginal disc is a progressive process: columns of cells successively differentiate in a posterior to anterior sequence, clusters of cells form at regularly spaced intervals within each column, and individual photoreceptors differentiate in a defined order within each cluster. The progression of differentiation across the eye disc is driven by a positive autoregulatory loop of expression of the secreted molecule Hedgehog, which is temporally delayed by the intercalation of a second signal, Spitz. Hedgehog refines the spatial position at which each column initiates its differentiation by inducing secondary signals that act over different ranges to control the expression of positive and negative regulators. The position of clusters within each column is controlled by secreted inhibitory signals from clusters in the preceding column, and a single founder neuron, R8, is singled out within each cluster by Notch-mediated lateral inhibition. R8 then sequentially recruits surrounding cells to differentiate by producing a short-range signal, Spitz, which induces a secondary short-range signal, Delta. Intrinsic transcription factors act in combination with these two signals to produce cell-type diversity within the ommatidium. The Hedgehog and Spitz signals are transported along the photoreceptor axons and reused within the brain as long-range and local cues to trigger the differentiation and assembly of target neurons.

Feedback control of the EGFR signaling gradient: superposition of domain-splitting events in Drosophila oogenesis

The morphogenesis of structures with repeated functional units, such as body segments and appendages, depends on multi-domain patterns of cell signaling and gene expression. We demonstrate that during Drosophila oogenesis, the two-domain expression pattern of Broad, a transcription factor essential for the formation of the two respiratory eggshell appendages, is established by a single gradient of EGFR activation that induces both Broad and Pointed, which mediates repression of Broad. Two negative-feedback loops provided by the intracellular inhibitors of EGFR signaling, Kekkon-1 and Sprouty, control the number and position of Broad-expressing cells and in this way influence eggshell morphology. Later in oogenesis, the gradient of EGFR activation is split into two smaller domains in a process that depends on Argos, a secreted antagonist of EGFR signaling. In contrast to the previously proposed model of eggshell patterning, we show that the two-domain pattern of EGFR signaling is not essential for specifying the number of appendages. Thus, the processes that define the two-domain patterns of Broad and EGFR activation are distinct; their actions are separated in time and have different effects on eggshell morphology.

Fasciclin 2, the Drosophila orthologue of neural cell-adhesion molecule, inhibits EGF receptor signalling

Adhesion proteins not only control the degree to which cells adhere to each other but are increasingly recognised as regulators of intercellular signalling. Using genetic screening in Drosophila, we have identified Fasciclin 2 (Fas2), the Drosophila orthologue of neural cell adhesion molecule (NCAM), as a physiologically significant and specific inhibitor of epidermal growth factor receptor (EGFR) signalling in development. We find that loss of fas2 genetically interacts with multiple genetic conditions that perturb EGFR signalling. Fas2 is expressed in dynamic patterns during imaginal disc development, and in the eye we have shown that this depends on EGFR activity, implying participation in a negative-feedback loop. Loss of fas2 causes characteristic EGFR hyperactivity phenotypes in the eye, notum and wing, and also leads to downregulation of Yan, a transcriptional repressor targeted for degradation by EGFR activity. No significant genetic interactions were detected with the Notch, Wingless, Hedgehog or Dpp pathways, nor did Fas2 inhibit the FGF receptor or Torso, indicating specificity in the inhibitory role of Fas2 in EGFR signalling. Our results introduce a new regulatory interaction between an adhesion protein and a Drosophila signalling pathway and highlight the extent to which the EGFR pathway must be regulated at multiple levels.

Cardiac gene regulatory networks in Drosophila

The Drosophila system has proven a powerful tool to help unlock the regulatory processes that occur during specification and differentiation of the embryonic heart. In this review, we focus upon a temporal analysis of the molecular events that result in heart formation in Drosophila, with a particular emphasis upon how genomic and other cutting-edge approaches are being brought to bear upon the subject. We anticipate that systems-level approaches will contribute greatly to our comprehension of heart development and disease in the animal kingdom.

Expression of the Drosophila secreted cuticle protein 73 (dsc73) requires Shavenbaby

Low stringency genomic library screens with genomic fragments from the sex determination gene doublesex identified the Drosophila secreted cuticle protein 73 (dsc73) gene, which encodes an 852-residue protein with an N-terminal signal sequence. In embryos, dsc73 RNA and protein are expressed to high levels in the epidermal cells that secrete the larval cuticle as well as in other cuticle-secreting tissues such as the trachea and salivary duct. Embryonic expression of dsc73 requires Shavenbaby, a transcription factor regulating cuticle formation. Double-labeling experiments with alphaCrb and alphaSAS reveal that, as with chitin and other known cuticle proteins, Dsc73 is secreted apically. Zygotic loss of dsc73 results in larval lethality but loss does not result in overt patterning defects or overt morphological defects in the embryonic tissues in which it is expressed. Thus, dsc73 encodes a novel secreted protein, and it is conserved within the Drosophila group. dsc73 may serve as a useful embryonic marker for cuticular patterning.

Distinct activation patterns of EGF receptor signaling in the homoplastic evolution of eggshell morphology in genus Drosophila

Homoplasy is a phenomenon in which organisms in different phylogenetic groups independently acquire similar traits. However, it is largely unknown how developmental mechanisms are altered to give rise to homoplasy. In the genus Drosophila, all species of the subgenus Sophophora, including Drosophila (D.) melanogaster, have eggshells with two dorsal appendages (DAs); most species in the subgenus Drosophila, including D. virilis, and in the subgenus Dorsilopha, have four-DAs. D. melanica belongs to the Drosophila subgenus, but has two-DAs, and phylogenetic analyses suggest that it acquired this characteristic independently. The patterning of the DAs is tightly regulated by epidermal growth factor receptor (EGFR) signaling in D. melanogaster. Previous studies suggested that a change in the EGFR signal activation pattern could have led to the divergence in DA number between D. melanogaster and D. virilis. Here, we compared the patterns of EGFR signal activation across the Drosophila subgenera by immunostaining for anti-activated MAP kinase (MAPK). Our analysis revealed distinct patterns of EGFR signal activation in each subgenus that was consistent with their phylogenetic relationship. In addition, the number of DAs always corresponded to the number of EGFR signaling activation domains in two, three, and four-DA species. Despite their common two-DA characteristic, the EGFR signaling activation pattern in D. melanica diverged significantly from that of species in the subgenus Sophophora. Our results suggest that acquisition of the homoplastic two-DA characteristic could be explained by modifications of the EGFR signaling system in the genus Drosophila that occurred independently and at least twice during evolution.

Downregulation of vertebrate Tel (ETV6) and Drosophila Yan is facilitated by an evolutionarily conserved mechanism of F-box-mediated ubiquitination

The vertebrate Ets transcriptional repressor Tel (ETV6) and its invertebrate orthologue, Yan, are both indispensable for development, and they orchestrate cell growth and differentiation by binding to DNA, thus inhibiting gene expression. To trigger cell differentiation, these barriers to transcriptional activation must be relieved, and it is established that posttranslational modifications, such as phosphorylation and sumoylation, can specifically impair the repressive functions of Tel and Yan and are crucial for modulating their transcriptional activity. To date, however, relatively little is known about the control of Tel and Yan protein degradation. In recent years, there has been a concentrated effort to assign functions to the large number of F-box proteins encoded by both vertebrate and invertebrate genomes. Here, we report the identification and characterization of a previously unreported, evolutionarily conserved F-box protein named Fbl6. We isolated both human and Drosophila melanogaster fbl6 cDNA and show that the encoded Fbl6 protein binds to both Tel and Yan via their SAM domains. We demonstrate that both Tel and Yan are ubiquitinated, a process which is stimulated by Fbl6 and leads to proteasomal degradation. We recently established that the sumoylation of Tel on lysine 11 negatively regulates its repressive function and that the sumoylation of Tel monomers, but not that of Tel oligomers, may sensitize Tel for proteasomal degradation. Here, we found that Fbl6 regulates Tel/Yan protein stability and allows appropriate spatiotemporal control of gene expression by these repressors.

Regulation of neurogenesis and epidermal growth factor receptor signaling by the insulin receptor/target of rapamycin pathway in Drosophila

Determining how growth and differentiation are coordinated is key to understanding normal development, as well as disease states such as cancer, where that control is lost. We have previously shown that growth and neuronal differentiation are coordinated by the insulin receptor/target of rapamycin (TOR) kinase (InR/TOR) pathway. Here we show that the control of growth and differentiation diverge downstream of TOR. TOR regulates growth by controlling the activity of S6 kinase (S6K) and eIF4E. Loss of s6k delays differentiation, and is epistatic to the loss of tsc2, indicating that S6K acts downstream or in parallel to TOR in differentiation as in growth. However, loss of eIF4E inhibits growth but does not affect the timing of differentiation. We also show, for the first time in Drosophila, that there is crosstalk between the InR/TOR pathway and epidermal growth factor receptor (EGFR) signaling. InR/TOR signaling regulates the expression of several EGFR pathway components including pointedP2 (pntP2). In addition, reduction of EGFR signaling levels phenocopies inhibition of the InR/TOR pathway in the regulation of differentiation. Together these data suggest that InR/TOR signaling regulates the timing of differentiation through modulation of EGFR target genes in developing photoreceptors.

Structural basis for EGFR ligand sequestration by Argos

Members of the epidermal growth factor receptor (EGFR) or ErbB/HER family and their activating ligands are essential regulators of diverse developmental processes. Inappropriate activation of these receptors is a key feature of many human cancers, and its reversal is an important clinical goal. A natural secreted antagonist of EGFR signalling, called Argos, was identified in Drosophila. We showed previously that Argos functions by directly binding (and sequestering) growth factor ligands that activate EGFR. Here we describe the 1.6-A resolution crystal structure of Argos bound to an EGFR ligand. Contrary to expectations, Argos contains no EGF-like domain. Instead, a trio of closely related domains (resembling a three-finger toxin fold) form a clamp-like structure around the bound EGF ligand. Although structurally unrelated to the receptor, Argos mimics EGFR by using a bipartite binding surface to entrap EGF. The individual Argos domains share unexpected structural similarities with the extracellular ligand-binding regions of transforming growth factor-beta family receptors. The three-domain clamp of Argos also resembles the urokinase-type plasminogen activator (uPA) receptor, which uses a similar mechanism to engulf the EGF-like module of uPA. Our results indicate that undiscovered mammalian counterparts of Argos may exist among other poorly characterized structural homologues. In addition, the structures presented here define requirements for the design of artificial EGF-sequestering proteins that would be valuable anti-cancer therapeutics.

Drosophila cbl is essential for control of cell death and cell differentiation during eye development

Background

Activation of cell surface receptors transduces extracellular signals into cellular responses such as proliferation, differentiation and survival. However, as important as the activation of these receptors is their appropriate spatial and temporal down-regulation for normal development and tissue homeostasis. The Cbl family of E3-ubiquitin ligases plays a major role for the ligand-dependent inactivation of receptor tyrosine kinases (RTKs), most notably the Epidermal Growth Factor Receptor (EGFR) through ubiquitin-mediated endocytosis and lysosomal degradation.

Methodology/Principal Findings

Here, we report the mutant phenotypes of Drosophila cbl (D-cbl) during eye development. D-cbl mutants display overgrowth, inhibition of apoptosis, differentiation defects and increased ommatidial spacing. Using genetic interaction and molecular markers, we show that most of these phenotypes are caused by increased activity of the Drosophila EGFR. Our genetic data also indicate a critical role of ubiquitination for D-cbl function, consistent with biochemical models.

Conclusions/Significance

These data may provide a mechanistic model for the understanding of the oncogenic activity of mammalian cbl genes.

Nemo is required in a subset of photoreceptors to regulate the speed of ommatidial rotation

Both dramatic and subtle morphogenetic movements are of paramount importance in molding cells and tissues into functional form. Cells move either independently or as populations and the distance traversed by cells varies greatly, but in all cases, the output is common: to organize cells into or within organs and epithelia. In the developing Drosophila eye, a highly specialized, 90 degrees rotational movement of subsets of cells imposes order by polarizing the retinal epithelium across its dorsoventral axis. This process was proposed to take place in two 45 degrees steps, with the second under control of the gene nemo (nmo), a serine/threonine kinase. While our analysis confirms that these subsets of cells, the ommatidial precursors, do stall at 45 degrees , we demonstrate that nmo is also required through most of the first 45 degrees of rotation to regulate the speed at which the ommatidial precursors move. In addition, although the precursors reach only the halfway point by the end of larval life, this work demonstrates that patterning events that occur during pupal life move the ommatidial units an additional 15 degrees . A re-analysis of nmo mosaic clones indicates that nmo is required in photoreceptors R1, R6 and R7 for normal orientation. This work also demonstrates that two major isoforms of nmo rescue the nmo(P1) phenotype. Finally, a dominant modifier screen of a nmo misexpression background identified genomic regions that potentially regulate rotation. The results presented here suggest a model in which a motor for rotation is established in a nemo-dependent fashion in a subset of cells.

Drosophila Myosin II, Zipper, is essential for ommatidial rotation

The adult Drosophila retina is a highly polarized epithelium derived from a precursor tissue that is initially symmetric across its dorsoventral axis. Specialized 90 degrees rotational movements of subsets of cells, the ommatidial precursors, establish mirror symmetry in the retinal epithelium. Myosin II, or Zipper (Zip), a motor protein, regulates the rate at which ommatidia rotate: in zip mutants, the rate of rotation is significantly slowed. Zip is concentrated in the cells that we show to be at the likely interface between rotating and non-rotating cells: the boundary between differentiated and undifferentiated cells. Zip is also robust in newly added ommatidial cells, consistent with our model that the machinery that drives rotation should shift to newly recruited cells as they are added to the growing ommatidium. Finally, cell death genes and canonical Wnt signaling pathway members genetically modify the zip phenotype.

The EGFR ligands Spitz and Keren act cooperatively in the Drosophila eye

The EGFR signalling cascade is responsible for coordinating a wide variety of events during Drosophila eye development. It remains something of a mystery how it is that cells are able to interpret the signal so as to choose the appropriate response from the battery of possibilities: division, differentiation, cell shape change and so on. Since the cascade is essentially linear below the receptor, different cellular responses cannot be regulated by alternative signal transduction pathways. The main diversity lies upstream, in the multiple activating ligands. Spitz, Gurken and Vein have been long studied, but little is known about the physiological functions of the fourth ligand, Keren, although various roles have been predicted based on the differences between mutants in the known ligands and those of the receptor. Here, we have isolated a mutant in the keren gene, and demonstrate that Keren does indeed participate in EGFR signalling in the eye, where it acts redundantly with Spitz to control R8 spacing, cell clustering and survival. Thus, specificity cannot be determined by ligand choice, and must instead be a consequence of cell-intrinsic factors, although we speculate that there may be some quantitative differences in signalling elicited by the two ligands.

Signal integration during development: mechanisms of EGFR and Notch pathway function and cross-talk

Metazoan development relies on a highly regulated network of interactions between conserved signal transduction pathways to coordinate all aspects of cell fate specification, differentiation, and growth. In this review, we discuss the intricate interplay between the epidermal growth factor receptor (EGFR; Drosophila EGFR/DER) and the Notch signaling pathways as a paradigm for signal integration during development. First, we describe the current state of understanding of the molecular architecture of the EGFR and Notch signaling pathways that has resulted from synergistic studies in vertebrate, invertebrate, and cultured cell model systems. Then, focusing specifically on the Drosophila eye, we discuss how cooperative, sequential, and antagonistic relationships between these pathways mediate the spatially and temporally regulated processes that generate this sensory organ. The common themes underlying the coordination of the EGFR and Notch pathways appear to be broadly conserved and should, therefore, be directly applicable to elucidating mechanisms of information integration and signaling specificity in vertebrate systems.