Ommatidial development in Drosophila eye disc fragments

Dachshund and C-terminal Binding Protein bind directly during Drosophila eye development

The transcription factor Dachshund (Dac) and the transcriptional co-regulator C-terminal Binding Protein (CtBP) were identified as the retinal determination factors during Drosophila eye development . A previous study established that Dac and CtBP interact genetically during eye development. Co-immunoprecipitation assays suggested that both molecules interact in the Drosophila larval eye-antennal disc. Our present study shows that Dac and CtBP bind each other directly, as determined by GST pull-down assays. Thus, our results demonstrate the molecular mechanism of Dac and CtBP interaction and suggest the direct binding of these two transcription regulators in the cells of the eye disc promotes the Drosophila eye specification.

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.

The early history of the eye-antennal disc of Drosophila melanogaster

A pair of eye-antennal imaginal discs give rise to nearly all external structures of the adult Drosophila head including the compound eyes, ocelli, antennae, maxillary palps, head epidermis, and bristles. In the earliest days of Drosophila research, investigators would examine thousands of adult flies in search of viable mutants whose appearance deviated from the norm. The compound eyes are dispensable for viability and perturbations to their structure are easy to detect. As such, the adult compound eye and the developing eye-antennal disc emerged as focal points for studies of genetics and developmental biology. Since few tools were available at the time, early researchers put an enormous amount of thought into models that would explain their experimental observations-many of these hypotheses remain to be tested. However, these "ancient" studies have been lost to time and are no longer read or incorporated into today's literature despite the abundance of field-defining discoveries that are contained therein. In this FlyBook chapter, I will bring these forgotten classics together and draw connections between them and modern studies of tissue specification and patterning. In doing so, I hope to bring a larger appreciation of the contributions that the eye-antennal disc has made to our understanding of development as well as draw the readers' attention to the earliest studies of this important imaginal disc. Armed with the today's toolkit of sophisticated genetic and molecular methods and using the old papers as a guide, we can use the eye-antennal disc to unravel the mysteries of development.

Integrative genomic analysis reveals novel regulatory mechanisms of eyeless during Drosophila eye development

Eyeless (ey) is one of the most critical transcription factors for initiating the entire eye development in Drosophila. However, the molecular mechanisms through which Ey regulates target genes and pathways have not been characterized at the genomic level. Using ChIP-Seq, we generated an endogenous Ey-binding profile in Drosophila developing eyes. We found that Ey binding occurred more frequently at promoter compared to non-promoter regions. Ey promoter binding was correlated with the active transcription of genes involved in development and transcription regulation. An integrative analysis revealed that Ey directly regulated a broad and highly connected genetic network, including many essential patterning pathways, and known and novel eye genes. Interestingly, we observed that Ey could target multiple components of the same pathway, which might enhance its control of these pathways during eye development. In addition to protein-coding genes, we discovered Ey also targeted non-coding RNAs, which represents a new regulatory mechanism employed by Ey. These findings suggest that Ey could use multiple molecular mechanisms to regulate target gene expression and pathway function, which might enable Ey to exhibit a greater flexibility in controlling different processes during eye development.

Dynamics and heterogeneity of a fate determinant during transition towards cell differentiation

Yan is an ETS-domain transcription factor responsible for maintaining Drosophila eye cells in a multipotent state. Yan is at the core of a regulatory network that determines the time and place in which cells transit from multipotency to one of several differentiated lineages. Using a fluorescent reporter for Yan expression, we observed a biphasic distribution of Yan in multipotent cells, with a rapid inductive phase and slow decay phase. Transitions to various differentiated states occurred over the course of this dynamic process, suggesting that Yan expression level does not strongly determine cell potential. Consistent with this conclusion, perturbing Yan expression by varying gene dosage had no effect on cell fate transitions. However, we observed that as cells transited to differentiation, Yan expression became highly heterogeneous and this heterogeneity was transient. Signals received via the EGF Receptor were necessary for the transience in Yan noise since genetic loss caused sustained noise. Since these signals are essential for eye cells to differentiate, we suggest that dynamic heterogeneity of Yan is a necessary element of the transition process, and cell states are stabilized through noise reduction.

Hedgehog and extramacrochaetae in the Drosophila eye: an irresistible force meets an immovable object

During the third and final larval instar stage, thousands of pluripotent cells within the Drosophila eye imaginal disc are transformed into a near perfect neurocrystalline lattice of 800 unit eyes called ommatidia. This transformation begins with the initiation of the morphogenetic furrow at the posterior margin of the eye field. The furrow, which marks the leading edge of a wave of differentiation, passes across the epithelium transforming unpatterned and undifferentiated cells into rows of periodically spaced clusters of photoreceptor neurons. As cells enter and exit the furrow they undergo dramatic alterations in cellular architecture and gene expression, many of which are required to propel the furrow forward and for proper cell fate specification. The Decapentaplegic (Dpp) and Hedgehog (Hh) signaling pathways are required for the initiation and progression of the furrow, respectively. Consistent with a role in furrow progression, the loss of Hh pathway activity results in a "furrow stop" phenotype. In contrast, reductions in levels of the helix-loop-helix transcription factor, Extramacrochaetae (Emc), lead to the polar opposite phenotype--the furrow accelerates. Recently, we demonstrated that the furrow stop and furrow acceleration phenotypes are molecularly connected. Emc appears to serve as a brake on the furrow by dampening the activity of the Hh pathway. Loss of Emc leads to an upsurge in Hh pathway activity and a faster moving furrow. The acceleration of the furrow appears to be due to an increase in levels of the full-length isoform of Cubitus Interruptus (Ci (155)) and Suppressor of Fused [Su(fu)]. Here we will briefly review the mechanisms by which Hh drives and Emc impedes the progression of the furrow across the developing retina.

Competition among gene regulatory networks imposes order within the eye-antennal disc of Drosophila

The eye-antennal disc of Drosophila gives rise to numerous adult tissues, including the compound eyes, ocelli, antennae, maxillary palps and surrounding head capsule. The fate of each tissue is governed by the activity of unique gene regulatory networks (GRNs). The fate of the eye, for example, is controlled by a set of fourteen interlocking genes called the retinal determination (RD) network. Mutations within network members lead to replacement of the eyes with head capsule. Several studies have suggested that in these instances all retinal progenitor and precursor cells are eliminated via apoptosis and as a result the surrounding head capsule proliferates to compensate for retinal tissue loss. This model implies that the sole responsibility of the RD network is to promote the fate of the eye. We have re-analyzed eyes absent mutant discs and propose an alternative model. Our data suggests that in addition to promoting an eye fate the RD network simultaneously functions to actively repress GRNs that are responsible for directing antennal and head capsule fates. Compromising the RD network leads to the inappropriate expression of several head capsule selector genes such as cut, Lim1 and wingless. Instead of undergoing apoptosis, a population of mutant retinal progenitors and precursor cells adopt a head capsule fate. This transformation is accompanied by an adjustment of cell proliferation rates such that just enough head capsule is generated to produce an intact adult head. We propose that GRNs simultaneously promote primary fates, inhibit alternative fates and establish cell proliferation states.

Ligand-independent activation of the Hedgehog pathway displays non-cell autonomous proliferation during eye development in Drosophila

Deregulation of the Hedgehog (Hh) signaling pathway is associated with the development of human cancer including medullobastoma and basal cell carcinoma. Loss of Patched or activation of Smoothened in mouse models increases the occurrence of tumors. Likewise, in a Drosophila eye model, deregulated Hedgehog signaling causes overgrowth of eye and head tissues. Surprisingly, we show that cells with deregulated Hh signaling do not or only little contribute to the tissue overgrowth. Instead, they become more sensitive to apoptosis and may eventually be eliminated. Nevertheless, these mutant cells increase proliferation in the adjacent wild-type tissue, i.e., in a non-cell autonomous manner. This non-cell autonomous effect is position-dependent and restricted to mutant cells in the anterior portion of the eye. We also observe precocious non-cell autonomous differentiation in genetic mosaics with deregulated Hh signaling. Together, these non-cell autonomous growth and differentiation phenotypes in the Drosophila eye model reveal another strategy by which oncogenes may generate a supportive micro-environment for tumor growth.

Retinal determination the beginning of eye development

The road to producing an eye begins with the decision to commit a population of cells to adopting an eye tissue fate, the process of retinal determination. Over the past decade and a half, a network of transcription factors has been found to mediate this process in all seeing animals. This retinal determination network is known to regulate not only tissue fate but also cell proliferation, pattern formation, compartment boundary establishment, and even retinal cell specification. The compound eye of the fruit fly, Drosophila melanogaster, has proven to be an excellent experimental system to study the mechanisms by which this network regulates organogenesis and tissue patterning. In fact the founding members of most of the gene families that make up this network were first isolated in Drosophila based on loss-of-function phenotypes that affect the eye. This chapter will highlight the history of discovery of the retinal determination network and will draw attention to the molecular and biochemical mechanisms that underlie our understanding of how the fate of the retina is determined.

The molecular circuitry governing retinal determination

The developing eye of the fruit fly, Drosophila melanogaster, has become a premier model system for studying the genetic and molecular mechanisms that govern tissue determination. Over the last fifteen years a regulatory circuit consisting of the members of the Pax, Six, Eya and Dach gene families has been identified and shown to govern the specification of a wide range of tissues including the retina of both insects and mammals. These genes are not organized in a simple developmental pathway or cascade in which there is a unidirectional flow of information. Rather, there are multiple feedback loops built into the system rendering its appearance and functionality more in line with the workings of a network. In this review I will attempt to describe the genetic, molecular and biochemical interactions that govern the specification of the Drosophila compound eye. In particular, the primary focus will be on the interactions that have been experimentally verified at the molecular and biochemical levels. During the course of this description I will also attempt to place each discovery in its own historical context. While a number of signaling pathways play significant roles in early eye development this review will focus on the network of nuclear factors that promote retinal determination.

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.

Direct control of the proneural gene atonal by retinal determination factors during Drosophila eye development

The determination of neuronal identity in Drosophila cells depends on the accurate expression of proneural genes. The proneural gene atonal (ato) encodes a basic-HLH protein required for photoreceptor and chordotonal organ formation. The initial expression of ato in imaginal discs is regulated by sequences that lie 3' to its open reading frame. In this report, we show that the initial ato transcription in different imaginal discs is regulated by distinct 3' cis-regulatory sequences. The eye-specific ato 3' cis-regulatory sequence consists of two distinct elements we term 2.8 PB and 3.6 BP that regulate ato transcription during different stages of eye development. The 2.8 PB enhancer contains a highly conserved consensus binding site for the retinal determination (RD) factor Sine oculis (So). Mutation of this So binding site abolishes 2.8 PB enhancer activity. Furthermore the RD factors So and Eyes absent (Eya) are required for 2.8 PB enhancer activity and can induce ectopic 2.8 PB reporter expression. In contrast, ectopic Dpp signaling is not sufficient to induce ato 3' enhancer activation but can induce increased levels of RD factor Dachshund (Dac) and synergize with So and Eya to increase ato 3' enhancer activity. These results demonstrate a direct mechanism by which the RD factors regulate ato expression and suggest an important role of Dpp in the activation of ato 3' enhancer is to regulate the levels of RD factors.

Neuronal differentiation in Drosophila ommatidium

Using monoclonal and polyclonal antibodies as differentiation markers, we have found that the eight photoreceptors of the Drosophila ommatidium differentiate in a fixed sequence. The foundation photoreceptor, R8, expresses neural antigens first. The paired photoreceptors R2/5 are next to express, followed by the pair R3/4, followed by the pair R1/6; R7 is the final photoreceptor to differentiate. From previous studies it is known that Drosophila photoreceptors use local, positional cues to select their identities. Together with the morphological picture of ommatidial development, the sequential order of photoreceptor differentiation demonstrated here suggests that these cues may be encoded in the particular combination of cells an undetermined cell finds itself in contact with.

Signal integration during development: insights from the Drosophila eye

The Drosophila eye is a highly ordered epithelial tissue composed of approximately 750 subunits called ommatidia arranged in a reiterated hexagonal pattern. At higher resolution, observation of the constituent photoreceptors, cone cells, and pigment cells of the eye reveals a highly ordered mosaic of amazing regularity. This relatively simple organization belies the repeated requirement for spatially and temporally coordinated inputs from the Hedgehog (Hh), Wingless (Wg), Decapentaplegic (Dpp), JAK-STAT, Notch, and receptor tyrosine kinase (RTK) signaling pathways. This review will discuss how signaling inputs from the Notch and RTK pathways, superimposed on the developmental history of a cell, facilitate context-specific and appropriate cell fate specification decisions in the developing fly eye. Lessons learned from investigating the combinatorial signal integration strategies underlying Drosophila eye development will likely reveal cell-cell communication paradigms relevant to many aspects of invertebrate and mammalian development. Developmental Dynamics 229:162-175, 2004.

The role of Wingless signaling in establishing the anteroposterior and dorsoventral axes of the eye disc

The posteriorly expressed signaling molecules Hedgehog and Decapentaplegic drive photoreceptor differentiation in the Drosophila eye disc, while at the anterior lateral margins Wingless expression blocks ectopic differentiation. We show here that mutations in axin prevent photoreceptor differentiation and lead to tissue overgrowth and that both these effects are due to ectopic activation of the Wingless pathway. In addition, ectopic Wingless signaling causes posterior cells to take on an anterior identity, reorienting the direction of morphogenetic furrow progression in neighboring wild-type cells. We also show that signaling by Decapentaplegic and Hedgehog normally blocks the posterior expression of anterior markers such as Eyeless. Wingless signaling is not required to maintain anterior Eyeless expression and in combination with Decapentaplegic signaling can promote its downregulation, suggesting that additional molecules contribute to anterior identity. Along the dorsoventral axis of the eye disc, Wingless signaling is sufficient to promote dorsal expression of the Iroquois gene mirror, even in the absence of the upstream factor pannier. However, Wingless signaling does not lead to ventral mirror expression, implying the existence of ventral repressors.

The development of the pattern of retinal ganglion cells in the chick retina: mechanisms that control differentiation

Neurons in both vertebrate and invertebrate eyes are organized in regular arrays. Although much is known about the mechanisms involved in the formation of the regular arrays of neurons found in invertebrate eyes, much less is known about the mechanisms of formation of neuronal mosaics in the vertebrate eye. The purpose of these studies was to determine the cellular mechanisms that pattern the first neurons in vertebrate retina, the retinal ganglion cells. We have found that the ganglion cells in the chick retina develop as a patterned array that spreads from the central to peripheral retina as a wave front of differentiation. The onset of ganglion cell differentiation keeps pace with overall retinal growth; however, there is no clear cell cycle synchronization at the front of differentiation of the first ganglion cells. The differentiation of ganglion cells is not dependent on signals from previously formed ganglion cells, since isolation of the peripheral retina by as much as 400 μm from the front of ganglion cell differentiation does not prevent new ganglion cells from developing. Consistent with previous studies, blocking FGF receptor activation with a specific inhibitor to the FGFRs retards the movement of the front of ganglion cell differentiation, while application of exogenous FGF1 causes the precocious development of ganglion cells in peripheral retina. Our observations, taken together with those of previous studies, support a role for FGFs and FGF receptor activation in the initial development of retinal ganglion cells from the undifferentiated neuroepithelium peripheral to the expanding wave front of differentiation.

Transcriptional regulation of atonal during development of the Drosophila peripheral nervous system

atonal is a proneural gene for the development of Drosophila chordotonal organs and photoreceptor cells. We show here that atonal expression is controlled by modular enhancer elements located 5′ or 3′ to the atonal-coding sequences. During chordotonal organ development, the 3′ enhancer directs expression in proneural clusters; whereas successive modular enhancers located in the 5′ region drive tissue-specific expression in chordotonal organ precursors in the embryo and larval leg, wing and antennal imaginal discs. Similarly, in the eye disc, the 3′ enhancer directs initial expression in a stripe anterior to the morphogenetic furrow. These atonal-expressing cells are then patterned through a Notch-dependent process into initial clusters, representing the earliest patterning event yet identified during eye morphogenesis. A distinct 5′ enhancer drives expression in intermediate groups and R8 cells within and posterior to the morphogenetic furrow. Both enhancers are required for normal atonal function in the eye. The 5′ enhancer, but not the 3′ enhancer, depends on endogenous atonal function, suggesting a switch from regulation directed by other upstream genes to atonal autoregulation during the process of lateral inhibition. The regulatory regions identified in this study can thus account for atonal expression in every tissue and essentially in every stage of its expression during chordotonal organ and photoreceptor development.

Cell determination strategies in the Drosophila eye

Cells in the Drosophila eye are determined by inductive signalling. Here I describe a new model of eye development that explains how simple intercellular signals could specify the diverse cell types that constitute the ommatidium. This model arises from the recent observation that the Drosophila homologue of the EGF receptor (DER) is used reiteratively to trigger the differentiation of each of the cell types--successive rounds of DER activation recruit first the photoreceptors, then cone and finally pigment cells. It seems that a cell's identity is not determined by the specific signal that induces it, but is instead a function of the state of the cell when it receives the signal. DER signalling is activated by the ligand, Spitz, and inhibited by the secreted protein, Argos. Spitz is initially produced by the central cells in the ommatidium and diffuses over a small distance. Argos has a longer range, allowing it to block more distal cells from being activated by low levels of Spitz; I have termed this interplay between a short-range activator and a long-range inhibitor ‘remote inhibition’. Since inductive signalling is common in many organisms and its components have been conserved, it is possible that the logic of signalling may also be conserved.

Early decisions in Drosophila eye morphogenesis

Recent analyses have shed light on the roles of genes involved in early events of eye cell determination and the spatiotemporal control of differentiation within the eye field. These genes function at sequential steps in the programming, initiation, or progression of differentiation, highlighting an elegant orchestration of gene activities to achieve this striking developmental event. Progress has been made in the study of the coordination between cell cycle control and cell differentiation, as well as in the genetic control of morphogenetic movements within the developing eye disc.

Hairy and emc negatively regulate morphogenetic furrow progression in the Drosophila eye

The initial steps of pattern formation in the developing Drosophila eye involve the coordination of cell cycles, changes in cell shape, and the specification of the R8 photoreceptor cell. These events begin several cell rows ahead of the morphogenetic furrow and are positively regulated by secreted signaling proteins and the proneural HLH transcription factor atonal (ato). Two HLH regulatory proteins that function to suppress neuronal development in other tissues, extra macrochaetae (emc) and hairy (h), are expressed ahead of the morphogenetic furrow. While neither h nor emc is required for photoreceptor cell determination, in emc-h-clones the morphogenetic furrow and differentiated eye field advance up to eight ommatidial rows ahead of adjacent wild-type tissue. This indicates that morphogenetic furrow progression and neuronal differentiation are negatively regulated by a combination of anteriorly expressed HLH regulatory proteins.

Regulation of furrow progression in the Drosophila eye by cAMP-dependent protein kinase A

The earliest physical sign of differentiation in the Drosophila retina is the passage of the morphogenetic furrow across the epithelium of the eye disc. Secreted factors encoded by hedgehog (hh) and decapentaplegic (dpp) have been implicated in propagation of the furrow and the subsequent initiation of photoreceptor differentiation. The morphogenetic furrow initiates at the posterior edge of the third larval instar eye imaginal disc. Its continued progression towards the anterior is believed to depend upon secretion of Hh protein by the differentiating clusters of photoreceptors that emerge posterior to the moving furrow. This progression is marked by the initiation of expression of the transforming growth factor-beta homologue Dpp in cells entering the furrow anteriorly, and loss of dpp expression in cells emerging posteriorly. Although the transmembrane protein encoded by the patched gene has been genetically implicated as the Hh receptor, the intercellular signalling pathways involved in these inductive processes remain uncharacterized. Here we show that the catalytic subunit of cyclic AMP-dependent protein kinase A (Pka-C1) is required for the correct spatial regulation of dpp expression during eye development. Loss of Pka-C1 function is sufficient to produce an ectopic morphogenetic wave marked by premature ectopic photoreceptor differentiation and non-autonomous propagation of dpp expression. Our results indicate that Pka-C1 lies in a signalling pathway that controls the orderly temporal progression of differentiation across the eye imaginal disc.

Growth and differentiation in the Drosophila eye coordinated by hedgehog

Differentiation of the Drosophila retina is asynchronous: it starts at the posterior margin of the eye imaginal disc and progresses anteriorly over two days. During this time the disc continues to grow, increasing in size by approximately eightfold. An indentation in the epithelium, the morphogenetic furrow, marks the front edge of the differentiation wave. Anterior progression of the furrow is thought to be driven by signals emanating from differentiating photoreceptor cells in the posterior eye disc. A good candidate for such a signal is the product of the hedgehog (hh) gene; it is expressed, and presumably secreted, by differentiating photoreceptors and its function is required for continued furrow movement. Here we show that ectopic expression of hedgehog sets in motion ectopic furrows in the anterior eye disc. In addition to changes in cell shape, these ectopic furrows are associated with a tightly orchestrated series of events, including proliferation, cell cycle synchronization and pattern formation, that parallel normal furrow progression. We propose that the morphogenetic furrow coincides with a transient boundary that coordinates growth and differentiation of the eye disc, and that hedgehog is necessary and sufficient to propagate this boundary across the epithelium.

dachshund encodes a nuclear protein required for normal eye and leg development in Drosophila

Neural specification and differentiation in the Drosophila eye sweep across the unpatterned epithelial monolayer of the eye imaginal disc following a developmental wave termed the morphogenetic furrow. The furrow begins at the posterior margin of the eye imaginal disc and moves anteriorly as a linear front. Progression of the furrow requires the function of hedgehog, which encodes a secreted signaling protein. We characterize mutations in dachshund, a gene that encodes a novel nuclear protein required for normal cell-fate determination of imaginal disc cells. In the absence of dachshund function, cells at the posterior margin of the eye disc fail to follow a retinal differentiation pathway and appear to adopt a cuticle fate instead. These cells are therefore unable to respond to pattern propagation signals such as hedgehog and furrow initiation does not occur. In contrast, cells in more anterior portions of the eye disc are able to differentiate as retinal cells in the absence of dachshund activity and respond normally to patterning signals. These results suggest that posterior margin cells are distinct from other cells of the eye imaginal disc by early stages of development. dachshund is also necessary for proper differentiation of a subset of segments in the developing leg. Null mutations in dachshund result in flies with no eyes and shortened legs.

The segment polarity gene hedgehog is required for progression of the morphogenetic furrow in the developing Drosophila eye

Cell-type specification in the Drosophila compound eye begins at the morphogenetic furrow. The furrow sweeps across the developing eye epithelium and is coincident with four classes of cellular events: coordinated changes in cell shape, changes in gene expression, synchronization of the cell cycle, and the specification of a regular array of ommatidial founder cells. The molecular mechanisms that induce these events in the developing eye have hitherto been unknown. We identify here a gene specifically required for furrow progression, hedgehog (hh). We show that hh expression posterior to the morphogenetic furrow is continuously required for its progression. We propose that forward diffusion of hh protein induces anterior cells to enter the furrow.

The TGF beta homolog dpp and the segment polarity gene hedgehog are required for propagation of a morphogenetic wave in the Drosophila retina

Development of the Drosophila retina occurs asynchronously; differentiation, its front marked by the morphogenetic furrow, progresses across the eye disc epithelium over a 2 day period. We have investigated the mechanism by which this front advances, and our results suggest that developing retinal cells drive the progression of morphogenesis utilizing the products of the hedgehog (hh) and decapentaplegic (dpp) genes. Analysis of hh and dpp genetic mosaics indicates that the products of these genes act as diffusible signals in this process. Expression of dpp in the morphogenetic furrow is closely correlated with the progression of the furrow under a variety of conditions. We show that hh, synthesized by differentiating cells, induces the expression of dpp, which appears to be a primary mediator of furrow movement.

Neuronal development in the Drosophila retina

Nervous systems of higher organisms are comprised of a variety of cell types which are interconnected in a precise manner. The molecular mechanisms that lead to the specification of neuronal cell types are not well understood. The compound eye of the fruit fly Drosophila is an attractive experimental system to understand these mechanisms. The compound eye is a reiterated neural pattern with several hundred unit structures and is amenable to both classical and molecular genetic methods. During the development of the compound eye cell-cell interactions and positional information play a critical role in the determination of cell fate. Recent genetic and molecular studies have provided important clues regarding the nature of the molecules involved in cellular signalling and neuronal differentiation.

Isolation of temperature-sensitive mutations of the tyrosine kinase receptor sevenless (sev) in Drosophila and their use in determining its time of action

Loss-of-function mutations in the sevenless (sev) gene in Drosophila result in the failure to differentiate a specific photoreceptor cell type--namely, the R7 cell. The sev gene encodes a cell-surface receptor tyrosine kinase that functions in the presumptive R7 cell to transduce developmental cues from its neighbors, instructing it to differentiate along the R7 cell pathway. We have isolated temperature-sensitive alleles of sev and used them to show that Sev activity is required for several hours during the development of each R7 cell to specify R7 cell differentiation. Our data also suggest that the presumptive R7 cell remains for approximately 5 hr in an undetermined state in the absence of the Sev-mediated signal before committing to an alternative fate. We have determined the molecular lesions in four of the temperature-sensitive alleles. One of these mutations disrupts the Gly-Xaa-Gly-Xaa-Xaa-Gly consensus in the ATP-binding site of the kinase domain.

Spacing differentiation in the developing Drosophila eye: a fibrinogen-related lateral inhibitor encoded by scabrous

In the development of multicellular organisms a diversity of cell types differentiate at specific positions. Spacing patterns, in which an array of two or more cell types forms from a uniform field of cells, are a common feature of development. Identical precursor cells may adopt different fates because of competition and inhibition between them. Such a pattern in the developing Drosophila eye is the evenly spaced array of R8 cells, around which other cell types are subsequently recruited. Genetic studies suggest that the scabrous mutation disrupts a signal produced by R8 cells that inhibits other cells from also becoming R8 cells. The scabrous locus was cloned, and it appears to encode a secreted protein partly related to the beta and gamma chains of fibrinogen. It is proposed that the sca locus encodes a lateral inhibitor of R8 differentiation. The roles of the Drosophila EGF-receptor homologue (DER) and Notch genes in this process were also investigated.

Induction of cell fate in the Drosophila retina: the bride of sevenless protein is predicted to contain a large extracellular domain and seven transmembrane segments

Previous genetic mosaic studies established that expression of the Drosophila bride of sevenless (boss) gene is required in photoreceptor neuron R8 for the development of photoreceptor neuron R7. This led to the proposal that boss encodes or regulates an R7-specific inductive cue. We have identified the boss gene based on small deletions in mutant alleles and sequenced both cDNAs and corresponding genomic regions. One P element and three X-ray-induced boss alleles show different deletions in the gene ranging in size from 2 to 23 bp, each causing frameshifts leading to premature termination of translation. The boss gene encodes a protein of 896 amino acids with a putative amino-terminal signal sequence, a large extracellular region of 498 amino acids, and seven potential transmembrane domains followed by a carboxy-terminal cytoplasmic tail of 115 amino acids. The putative membrane localization of the boss protein is consistent with a model in which direct interaction between the boss and sevenless proteins specifies R7 cell fate. Another model in which the boss protein functions as a receptor is proposed based on its similarity to the G protein-linked family of membrane receptors.

The homeo domain protein rough is expressed in a subset of cells in the developing Drosophila eye where it can specify photoreceptor cell subtype

The Drosophila homeo box gene rough is required in photoreceptor cells R2 and R5 for normal eye development. We show here that rough protein expression is limited to a subset of cells in the developing retina where it is transiently expressed for 30-60 hr. The rough protein is first expressed broadly in the morphogenetic furrow but is rapidly restricted to the R2, R3, R4, and R5 precursor cells. Ubiquitous expression of rough under the control of the hsp70 promoter in third-instar larvae suppresses the initial steps of ommatidial assembly. Structures derived from other imaginal discs are not affected. Ectopic expression of rough in the R7 precursor, through the use of the sevenless promoter, causes this cell to develop into an R1-6 photoreceptor subtype; however, this cell still requires sevenless function for its neural differentiation. Taken together with previous analyses of the rough mutant phenotype, these results suggest that the normal role of rough is to establish the unique cell identity of photoreceptors R2 and R5.

The emergence of order in the Drosophila pupal retina

During pupation, long-range order is imposed on the autonomously developing ommatidia which compose the Drosophila eye. To accomplish this, eight additional cell types arise: the primary, secondary, and tertiary pigment cells, and the four cells that form the bristle. These cells form an interweaving lattice between ommatidia. The lattice is refined when excess cells are removed to bring neighboring ommatidia into register. Recent evidence suggests that in larval development, local contacts direct cell fate. The same appears to be true during pupal development: the contacts a cell makes predict the cell type it will become. Cells which contact the anterior or posterior cone cells in an ommatidium invariably become primary pigment cells. Cells which contact primary pigment cells from different ommatidia become secondary and tertiary pigment cells. Bristle development is in several ways distinct from ommatidial development. The four cells of each bristle group appear to be immediate descendents of a single founder cell. During their early differentiation, they do not make stereotyped contacts with surrounding ommatidial cells, but do make particular contacts within the bristle group. And unlike the surrounding ommatidia, differentiation of the bristles radiates from the center of the eye to the edges. As cells are removed during two stages of programmed cell death, the bristles are brought into their final position. When all cells in the lattice have achieved their final position, a second stage of retinal development begins as structures specific to each cell type are produced. This paper follows these various stages of pupal development, and suggests how local cell-cell contacts may produce the cells needed for a functional retina.

A multifaceted approach to neural development

The developing compound eye of the fruit fly, Drosophila offers notable advantages for a study of neural differentiation. It is a sensitive amplifier of a simple neural module; each eye is an approximately 700-fold repetition of the unit eye or ommatidium, which is a precise, stereotyped assembly of photoreceptors and accessory cells. The eye develops in a monolayer epithelium, which greatly reduces the complexities of cell-cell interactions often encountered in CNS development, and has permitted a detailed, cell-by-cell description of cell behavior during ommatidial development. Finally, the foundation of fly genetics permits a mutational analysis of eye development and the advanced molecular genetics of Drosophila allows close scrutiny of genes of interest. A recent convergence of cellular, genetic and molecular studies of ommatidial development suggests a model for neural differentiation in the fly eye.

Cell-cell interaction in the Drosophila retina: the bride of sevenless gene is required in photoreceptor cell R8 for R7 cell development

Genetic mosaic studies indicate that environmental cues play a critical role in photoreceptor cell (R-cell) development in the Drosophila compound eye. Recent analysis of the sevenless gene suggests that its product, a cell surface protein containing a putative intracellular domain homologous to tryosine kinases, is a receptor for a signal specifying an R7-specific pathway of cellular development. We report the identification of a second genetic locus, bride of sevenless (boss) required for the development of R7. Genetic mosaic analysis demonstrates that boss expression is required in the R8 cell for a neighboring cell to become R7. However, the development of R1-R6 and R8 is boss-independent. These data indicate a central role for R8 in directing ommatidial assembly and are consistent with boss encoding the ligand recognized by the sevenless receptor.

Localization of the sevenless protein, a putative receptor for positional information, in the eye imaginal disc of Drosophila

The Drosophila gene sevenless encodes a putative trans-membrane receptor required for the formation of one particular cell, the R7 photoreceptor, in each ommatidium of the compound eye. Mutations in this gene result in the cell normally destined to form the R7 cell forming a non-neuronal cell type instead. These observations have led to the proposal that the sevenless protein receives at least part of the positional information required for the R7 developmental pathway. We have generated antibodies specific for sevenless and have examined expression of the protein by light and electron microscopy. sevenless protein is present transiently at high levels in at least 9 cells in each developing ommatidium and is detectable several hours before any overt differentiation of R7. The protein is mostly localized at the apices of the cells, in microvilli, but is also found deeper in the tissue where certain cells contact the R8 cell. This finding suggests that R8 expresses a ligand for the sevenless protein.