The emergence of order in the Drosophila pupal retina

Cell recognition during synaptogenesis is revealed after temperature-shock-induced perturbations in the developing fly's optic lamina

Houseflies (Musca domestica) were exposed to pulses of heat (1 h) or cold (several hours) during early pupal life, and the effects were investigated on the development of the first optic neuropile, or lamina, of the visual system. The treatments were designed to perturb the cellular organization of the cartridges, the unit synaptic structures of the lamina, so as to provide novel synaptic opportunities among the normally fixed composition of these modules, thereby testing the preferences of their component cells during synaptogenesis. Various abnormalities were identified, but these were not always consistent between flies: retinal abnormalities included the loss and fusion of rhabdomeres, especially of the central cells of the ommatidium, whereas in the lamina low frequencies of abnormal cartridges were found. These included seven that were studied with serial sections, which instead of the normal pair of L1 and L2 monopolar interneurons had supernumerary cells of this type. The normal pairing of L1 and L2 at postsynaptic sites of receptor terminal tetrad synapses was preserved in these cases, the cells eschewing pairings of homologous L1/L2 or L2/L2 partners. This meant that more than one L1 could pair with a single L2 and vice versa, even at the same terminal, and appeared to do so opportunistically on the basis of proximity, with cells closer to each other pairing more frequently. Thus the cells behave during synaptogenesis as if they recognize other cells only as cell types (receptor, L1 or L2) and not as individual cells.

Expression of Drosophila glass protein and evidence for negative regulation of its activity in non-neuronal cells by another DNA-binding protein

The glass gene encodes a DNA-binding zinc-finger protein required for the development of Drosophila photoreceptor cells and which appears to regulate a number of genes specifically expressed in photoreceptors. We have generated monoclonal antibodies to Glass and used them to examine Glass distribution during development. Glass is expressed in all cell types of the developing eye and in all other organs that contain photoreceptor cells in Drosophila, including a small number of cells in the brain. We altered the normal pattern of glass expression by placing the gene under the control of the hsp70 promoter. Our results suggest that nonphotoreceptor cells are restricted in their response to Glass expression. In an effort to discover the mechanism of this restriction, we examined the expression of a number of reporter gene constructs. Our results suggest that nonsensory cells are unable to express certain reporter constructs in response to Glass expression because another DNA-binding factor represses Glass activity in nonsensory cells.

Identification of a Drosophila activin receptor

Activins are cytokines of the transforming growth factor beta superfamily that control various events during vertebrate embryo development and cell differentiation in the adult, and act through transmembrane receptors that contain a cytoplasmic protein-serine/threonine kinase domain. We describe the identification, deduced primary structure, and expression pattern of Atr-II, a receptor serine/threonine kinase found in Drosophila. With the exception of the spacing of 10 cysteine residues, the extracellular domain of Atr-II is very dissimilar from those of vertebrate activin receptors, yet it binds activin with high affinity and specificity. The kinase domain sequence of Atr-II is 60% identical to those of activin receptors from vertebrates, suggesting similarities in their signaling mechanisms. Maternal Atr-II transcript and its product are abundant in the oocyte. During development, the highest levels of Atr-II transcript and protein are observed in the mesoderm and gut. The possible role of an activin signaling system in Drosophila development is discussed.

The strawberry notch gene functions with Notch in common developmental pathways

Genetic and phenotypic analysis of strawberry notch suggests that its gene product is required during embryogenesis and oogenesis, and for the development of the eye, wing and leg. Several lines of evidence suggest that strawberry notch participates together with Notch in many common pathways. A number of strawberry notch mutant phenotypes are similar to those of Notch mutants and can be rescued by an extra copy of wild-type Notch. In addition, mutations in strawberry notch interact strongly with Notch mutants in a tissue-specific manner. Mutations in the strawberry notch and Notch loci also show very similar interactions with genes like Hairless, Delta, groucho, Serrate, and deltex that have all been proposed to participate in Notch related pathways. The genetic evidence presented here suggests that strawberry notch participates with members of the Notch pathway in facilitating developmentally relevant cell-cell communications.

Analyses of PS integrin functions during Drosophila development

The Drosophila position-specific (PS) antigens are homologues of the vertebrate integrins, a family of transmembrane proteins that function in cell-matrix and cell-cell adhesion. The common beta subunit of PS integrins (PS beta) is encoded by the lethal(l)myospheroid gene (mys) and is required during wing, eye and muscle development. By expressing PS beta protein at defined developmental periods, we have shown that PS integrins are required throughout pupation, but not earlier, for normal development of wings. In contrast, the key requirement for PS integrins in eye development occurs only in the late pupa. Furthermore, PS integrins are apparently not required for the differentiation of the ommatidial cells; only for their organization. These results are consistent with roles for PS integrins in the interactions between the wing epithelia during the two phases of pupal wing expansion and in maintaining the attachment of a fully formed fenestrated membrane to the basement membrane of the retina. We have also examined the functional significance of alternative splicing of the transcript of the mys gene using P element-mediated transformation to introduce transgenes producing only one of the two spliced forms of PS beta. We find that either form is sufficient to rescue postembryonic mys phenotypes in the wing, eye and muscle but that both of the two splice forms are necessary to rescue the mys embryonic defects. This result indicates a requirement for the alternative splicing of mys during embryogenesis. The location of the alternative exons suggests that the two forms of the PS beta integrin subunit may interact with alternative alpha subunits and/or ligands.

Genetic analysis of laminin A reveals diverse functions during morphogenesis in Drosophila

In order to dissect the functions of laminin A in vivo, we have undertaken a molecular and genetic characterization of the laminin A subunit (lamA) gene in Drosophila. Sequence analysis predicts a multidomain structure similar to mammalian homologs. We generated a series of complete and partial loss-of-function mutant alleles of the lamA gene; complete loss-of-function mutations lead to late embryonic lethality. Certain combinations of partial loss-of-function lamA alleles give rise to escaper adults, which have rough eyes associated with changes in cell fate and pattern, misshapen legs and defects in wing structure. These phenotypes suggest that laminin A has diverse functions during morphogenesis in Drosophila.

Genetic analysis of chromosomal region 97D2-9 of Drosophila melanogaster

The rough homeobox gene of D. melanogaster is required for the correct patterning of the developing eye. The locus maps to cytological location 97D2-5, a region which has not been extensively characterised. As part of our genetic and molecular characterization of rough we carried out an EMS mutagenesis to generate mutants that map to the surrounding region, 97D2-9 which is deleted in Df(3R)ro-XB3. We have generated 1 visible and 13 lethal mutations which, together with the previously described Toll and ms(3)K10 loci, and other unpublished lethals, define nine complementation groups--four lethal, three semi-lethal, one visible and one male-sterile. In addition to rough, one other locus within this region, 1(3)97De, was shown to be required for formation of the normal pattern of photoreceptor cells in the compound eye.

Gene expression during imaginal disc regeneration detected using enhancer-sensitive P-elements

When imaginal disc fragments from Drosophila are cultured in adult female hosts, they either duplicate the part of the pattern specified by the fate map, or regenerate to replace the missing part. The new tissue is added by proliferation of a small number of cells from the cut edge, brought together when the wound heals to form a regeneration blastema. Specification of the new pattern has been explained by assuming interactions among cells of different positional value in the regeneration blastema. In order to identify genes which might mediate these events, we screened over eight hundred independently isolated autosomal insertions of an enhancer-sensitive P-element, for altered lac-z expression in regenerating discs following cell death induced by a temperature-sensitive cell-lethal mutation. Two further screens divided the positive lines into four groups based on appropriate timing of the lac-z response in the cell-lethal mutant background and the expected response to an alternate source of cell death. Expression in wing disc fragments cultured in vivo was most frequent in the target class defined by the screens. In this direct test, lac-z expression was found in 23 lines and in most cases was spatially and temporally correlated with the formation of the regeneration blastema. Our results suggest a very substantial transcriptional response during the early stages of imaginal disc regeneration. lac-z expression in control imaginal discs, embryos and adult ovaries of the positive lines was also assayed. The selected insertions included: a small class expressed only in discs undergoing regeneration and apparently not at any other stage, possibly representing genes active exclusively in regeneration; a larger class expressed in the embryo or during oogenesis, but not normally in imaginal discs, as expected for functions recruited from earlier stages of the developmental program; and finally a class with spatially patterned expression in normal discs. This class included several insertions with expression associated with compartment boundaries, including one at the decapentaplegic (dpp), and one at the crumbs (crb) locus, a growth factor homologue, and an EGF-repeat gene respectively. Some of the expression patterns observed in cultured disc fragments provide evidence for cell communication in the regeneration blastema.

The role of the cut gene in the specification of central projections by sensory axons in Drosophila

Mutations in the cut gene transform sense organs in Drosophila embryos from external sensory (es) receptors to chordotonal (ch) organs. We have investigated whether their central axonal projections are also transformed. Following Lucifer yellow injection of the sensory neuron, wild-type es and ch organs show characteristic, different projection patterns in the CNS. Transformed es neurons in cut embryos are variable in their projection patterns: some resemble wild-type es neurons, others ch neurons, while yet others are unlike either of these. We conclude that the cut gene influences axonal projections, although its action as a simple modality switch is open to question. Additional genes could be involved in the specification of the central axonal projection of the transformed neurons.

Expression of the Drosophila optomotor-blind gene transcript in neuronal and glial cells of the developing nervous system

Mutations in the complex gene locus optomotor-blind (omb) can lead to defects in the development of both the optic lobes and external features of the adult fly. We describe here the expression of omb in the developing and adult nervous system using in situ hybridization. During embryogenesis, omb expression is first observed in the optic lobe anlagen. It later expands to a larger part of the developing larval brain and to the gnathal lobes. Cells in the ventral and peripheral nervous systems begin to express omb after completion of germ band extension. Later in embryonic development, expression declines and only persists in the antennomaxillary complex and in part of the brain hemispheres. During the larval and pupal stages, omb expression in the brain is confined to the developing optic lobes and contiguous regions of the central brain. At these stages, only a few cells show expression in the ventral ganglion. In the eye imaginal disc, transcript accumulation is most conspicuous in a group of presumptive glia precursor cells posterior to the morphogenetic furrow and in the optic stalk. In the adult brain, expression is prominent in several regions of the optic lobe cortex and along the border between central brain and optic lobes. In the mutation In(1)ombH31, 40 kb of regulatory DNA, downstream from the transcription unit, are removed from the omb gene. In(1)ombH31 is characterized by the lack of a set of giant interneurons from the lobula plate of the adult optic lobes. We find that, already during embryogenesis, there is a drastic difference between wild type and In(1)ombH31 in the level of the omb transcript in the optic lobe primordia. The adult mutant phenotype may thus be caused by omb misexpression during embryonic development.

Phagocyte recognition of cells undergoing apoptosis

A key feature of apoptosis is that cells undergoing this programmed form of death are recognized by phagocytes and ingested while still intact, protecting tissues from the potentially harmful consequences of exposure to the contents of the dying cells. This article reviews recent data which indicate that phagocyte recognition of apoptotic cells as 'senescent-self' involves at least three classes of receptors on the phagocyte surface, while apoptotic cells may display their 'edible' status in a number of different ways.

Implications of dynamic patterns of Delta and Notch expression for cellular interactions during Drosophila development

Delta and Notch function are required for cell fate specification in numerous tissues during embryonic and postembryonic Drosophila development. Delta is expressed by all members of interacting cell populations within which fates are being specified and is subsequently down-regulated as cells stably adopt particular fates. Multiphasic expression in the derivatives of many germ layers implies successive requirements for Delta function in a number of tissues. At the cellular level, Delta and Notch expression are generally coincident within developing tissues. At the subcellular level, Delta and Notch are localized in apparent endocytic vesicles during down-regulation from the surfaces of interacting cells, implying an interaction consistent with their proposed roles as signal and receptor in cellular interactions during development.

Regulation of Drosophila neural development by a putative secreted protein

The Drosophila strawberry (sty) locus was isolated by P-element insertion mutagenesis in a screen for mutations affecting eye development. Analysis of the mutant phenotype and the putative expression pattern of the sty gene suggested that it has multiple functions. Mutations in the sty gene lead to irregular spacing of ommatidia, an increase in the number of photoreceptor cells, as well as abnormal axonal projections to the lamina and disrupted structure of the optic lobes in the adult fly. The sty mutation also causes abnormal head involution, a change in a number of sensilla in the antennomaxillary complex in the embryonic stage and abnormal morphogenesis of the maxillary palp and wings in later stages. We examined the presumptive expression of the sty gene during development by histochemical staining for lacZ expression from enhancer trap elements inserted within the sty gene. During embryogenesis, expression of lacZ showed a segmental pattern in the ectoderm and in the nervous system. In the eye imaginal discs, lacZ was expressed in photoreceptor cells beginning a few rows posterior to the morphogenetic furrow. The lacZ was also expressed in the wing disc. In the adult, lacZ was expressed in the retina and lamina. We cloned the sty gene by P-element tagging and found that it encodes a putative secreted protein containing a cysteine-rich region similar to the epidermal growth factor (EGF) repeat. On the basis of the loss of functional phenotype, the expression pattern and the predicted structure of its product, we propose that sty encodes a diffusible protein acting as a signal involved in lateral inhibition within the developing nervous system and also as a factor involved either directly or indirectly in axonal guidance and optic lobe development.

Fine structure of a developing insect olfactory organ: morphogenesis of the silkmoth antenna

The olfactory organ of the silkmoth Antheraea polyphemus is the feathered antenna which carries about 70,000 olfactory sensilla in the male. It develops within 3 weeks from a leaf-shaped epidermal sac by means of segmental primary and secondary indentations which proceed from the periphery towards the centerline. During the first day post-apolysis, the antennal epidermis differentiates into segmentally arranged, alternating sensillogenic and non-sensillogenic regions. Within the first 2 days post-apolysis, the anlagen of olfactory sensilla arise from electron-dense mother cells in the sensillogenic epidermis. The axons of the developing sensilla begin to form the primary innervation pattern during the second day. The sensilla develop approximately within the first 10 days to their final shape, while the indentations are completed during the same period of time. The indentations are most probably driven by long basal extensions of epidermal cells, the epidermal feet. Primary indentations follow the course of segmentally arranged tracheal bundles and form the segments of the antenna. The secondary indentations follow the course of the primary segmental nerves which are reconstructed by this process. During the remaining time of development, the cuticle of the antenna and the sensory hairs is secreted by the epidermal and the hair-forming cells.

The involvement of the Notch locus in Drosophila oogenesis

The Notch gene in Drosophila encodes a transmembrane protein with homology to EGF that, in a variety of tissues, appears to mediate cell interactions necessary for cell fate choices. Here we demonstrate that oogenesis and spermatogenesis depend on Notch. We examine the phenotypes of the temperature-sensitive Notch allele, Nts1, and, using a monoclonal antibody, determine the cellular and subcellular distribution of Notch protein during oogenesis. We show that Nts1 is associated with a missense mutation in the extracellular, EGF homologous region of Notch and that at non-permissive temperatures oogenesis is blocked and the subcellular distribution of the protein is altered. In wild-type ovaries, Notch protein is found on the apical surface of somatically derived follicle cells, while in the germline-derived cells the protein is not polarized. These findings are discussed in view of the hypothesis that Notch acts as a multifunctional receptor to mediate developmentally important cell interactions.

mip causes hyperinnervation of a retinotopic map in Drosophila by excessive recruitment of R7 photoreceptor cells

The two central photoreceptor neurons of the Drosophila eye, R7 and R8, form a retinotopic map in the optic lobe of the fly brain. We have developed a technique that allows us to visualize the projections of these neurons with high resolution. Using this technique, we have identified a new mutant, mip (more inner photoreceptors), in which this map shows a striking hyperinnervation. The extra terminals in the brain derive from an excessive recruitment of sevenless-independent R7 photoreceptor cells during eye development. The original R7, however, remains sevenless responsive. The behavior of this gene suggests that recruitment to the R7 pathway, and possibly to multiple programs in ommatidial assembly, is partially regulated by inhibition.

Ellipse mutations in the Drosophila homologue of the EGF receptor affect pattern formation, cell division, and cell death in eye imaginal discs

Ellipse alleles are mutations of the EGF-receptor homologue that reduce the number of ommatidia in the eye imaginal disc. Cobalt sulfide staining, expression of hairy and scabrous proteins, and mosaic analysis indicated that Elp mutations affect ommatidial precluster formation in the morphogenetic furrow. BrdU incorporation studies suggest that cells diverted from precluster formation instead enter S-phase after the morphogenetic furrow. Genetic studies suggest that the DER has multiple functions during eye development and that several recessive hypomorphic alleles affect another aspect of DER function that is required after precluster formation. Elp mutations show genetic interactions with the neurogenic mutations Notch and Delta. The small number of ommatidia that differentiate in Elp/Elp are separated more than in wildtype and have been studied to investigate what aspects of ommatidium development are intrinsic to the ommatidium itself. It appears that each developing ommatidium cues the determination of photoreceptors, cone cells, and primary pigment cells, but that the secondary and tertiary pigment cells, and the mechanosensory bristles, can form independently. The normal rotation of ommatidia in the dorsal-ventral axis does not require the presence of the ommatidial array. A short-range signal from a nearby ommatidium is important for mitosis. Cells not close to an ommatidium do not go through mitosis and many die.

A putative Ras GTPase activating protein acts as a negative regulator of signaling by the Sevenless receptor tyrosine kinase

A Drosophila gene with similarity to the mammalian Ras GTPase activating protein has been isolated in screens for mutations that affect eye development. Inactivation of the locus, Gap1, mimics constitutive activation of the Sevenless receptor tyrosine kinase and eliminates the need for a functional Sevenless protein in the R7 cell. Our results suggest that Gap1 acts as a negative regulator of signaling by Sevenless by down-regulating the activity of the Ras1 protein, which has been shown to be a key element in signaling by Sevenless.

The scalloped gene encodes a novel, evolutionarily conserved transcription factor required for sensory organ differentiation in Drosophila

The scalloped (sd) gene of Drosophila melanogaster was initially characterized by mutants affecting structures on the wing of the adult fly. The sequence of a cDNA clone of the gene reveals a predicted protein sequence homologous to that of a human transcriptional enhancer factor, TEF-1 (68% identity). The homology includes a sequence motif, the TEA domain, that was shown previously to be a DNA-binding domain of TEF-1. An sd enhancer trap strain expresses the reporter gene in a subset of neuroblasts in the central nervous system and in the peripheral sense organs of the embryo. The reporter gene is later expressed in specific regions of the imaginal discs, including regions of the wing disc destined to become structures defective in viable sd mutants. Later still, expression in the adult brain is restricted to subsets of cells, some in regions involved in the processing of gustatory information. These observations indicate that the sd gene encodes a transcription factor that functions in the regulation of cell-specific gene expression during Drosophila development, particularly in the differentiation of the nervous system.

The presumptive R7 cell of the developing Drosophila eye receives positional information independent of sevenless, boss and sina

Studies on the development of the R7 photoreceptor in the Drosophila eye thus far have identified three genes that specifically affect this cell: sevenless, boss and sina. In each of these mutants the R7 precursor develops instead as the equatorial cone cell (EQC). We have isolated an enhancer trap line, H214, in which beta-galactosidase is primarily expressed in the R7 cell throughout its development. In mutations of sevenless, boss and sina, expression in H214 is initially reduced although still present in the R7 precursor and persists in the EQC into which it develops. The EQC in wild type never expresses lacZ in H214. This result is in contrast to that seen with other enhancer trap lines that display expression in R7, and indicates that some aspect of R7 differentiation is independent of the genetic pathway(s) involving sevenless, boss and sina.

Dual Bar homeo box genes of Drosophila required in two photoreceptor cells, R1 and R6, and primary pigment cells for normal eye development

In the Bar mutation of Drosophila, ommatidial differentiation is known to be suppressed in the anterior portion of the eye. Our structural analysis shows that the Bar region contains a pair of homeo box genes, BarH1 and BarH2. These genes encode polypeptides similar in size and sequence and share a common homeo domain that is identical in sequence except for putative trans-activator-binding sites. We also show, by mosaic analysis and immunostaining with anti-BarH1/BarH2 antibodies, that BarH1 and BarH2 are not only specifically coexpressed but also functionally required in R1/R6 prephotoreceptors and primary pigment cells in developing ommatidia. In R1/R6, the expression of BarH1 and BarH2 appears to be regulated by rough and glass gene products. BarH1 and BarH2 proteins are essential to normal lens formation, formation of three types of pigment cells, and elimination of excess cells from mature ommatidia. Taken together, our results suggest that Bar homeo domain proteins may play key roles in the fate-determination processes of pigment cells and cone cells.

Histamine-like immunoreactivity in the visual system and brain of Drosophila melanogaster

In this study, immunohistochemistry on cryostat sections is used to demonstrate anti-histamine immunoreactivity in the Drosophila brain. The results support earlier findings that histamine is probably a transmitter of insect photoreceptors. It is further shown that, in Drosophila, all imaginal photoreceptors including receptor type R7 are anti-histamine immunoreactive, whereas the larval photoreceptors do not seem to contain histamine. In addition to the photoreceptors, fibres in the antennal nerve and approximately 12 neurons in each brain hemisphere show strong histamine-like immunoreactivity. These cells arborize extensively in large parts of the central brain.

The role of transcription factors in the developing Drosophila eye

In the developing Drosophila compound eye, multipotent precursor cells are induced to develop into particular cell types through sequential induction. In the target cells, transcription factors may be modulated by the inductive signals to execute their instructions. Four recently isolated genes may encode such developmentally modulated transcription factors.

Interaction of bride of sevenless membrane-bound ligand and the sevenless tyrosine-kinase receptor

During development of the Drosophila retina, the R8 photoreceptor neuron induces a neighbouring cell to assume an R7 cell fate. Genetic data suggest that the induction is mediated by two transmembrane proteins encoded by bride of sevenless and sevenless. A direct interaction between these two proteins was demonstrated by the heterotypic aggregation of cell lines expressing them. In the developing eye the sevenless-dependent internalization of bride of sevenless by the R7 precursor cell provides evidence for a direct interaction between these two proteins in vivo.

Identification of a retinal protein in Drosophila with antibody to the alpha subunit of bovine brain G(o) protein

An antibody directed against the alpha(o) subunit of bovine brain G(o) (R4) was used to identify a Drosophila retinal protein which may be the analogue of vertebrate transducin. The immunoreactivity appears predominantly in the retinal and occellar rhabdomeres. On a Western blot, the antibody recognizes a 41 kDa protein that is present in the heads of yellow white flies, but not in the heads of eyeless mutant flies, eyes absent. This protein is not recognized by an antibody raised against Drosophila alpha(o). Antibody R4 intensely stains rhabdomeres and, to a lesser extent, the neuropil of the central nervous system in tissue sections of adult flies. Antibody to Drosophila alpha(o) stains the neuropil of the central nervous system, but does not stain rhabdomeres. In developing flies, faint immunoreactivity appears in the retinal rhabdomeres at about 70% of the time through pupal development and increases to its apparent adult maximal level about 1 day after eclosion. Tissue sections from a phototransduction mutant, norp A, have retinal immunoreactivity at normal levels up to about 1 week after eclosion, but by 2 weeks, immunoreactivity has largely disappeared. This disappearance parallels the degeneration of the retina in norp A mutants. In Drosophila and other invertebrates, light activates a phospholipase C in the retina. The identification of a protein in Drosophila rhabdomeres with an antibody raised against a mammalian G protein alpha subunit thought to be involved in phospholipase C activation suggests that there may be common structural features between the putative Drosophila transducin and alpha(o). The identification of regions common to mammalian alpha(o) and Drosophila transducin may then provide clues to the structural requirements for PLC activation.

Tissue specific effects of ommochrome pathway mutations in Drosophila melanogaster

The tissue-specific effects of 17 mutations affecting the synthesis of brown eye pigment (xanthommatin) have been investigated by combining them with chocolate and red cells, two mutations causing ectopic pigmentation of the Malpighian tubules and larval fat body (which normally only synthesize pigment precursors). The majority of mutations block the pigmentation of four organs; the normally pigmented eyes and ocelli, and ectopically pigmented tubules and fat body. They represent genes that would appear to be required for the normal operation of the pathway per se and are likely to encode structural proteins. Mutations at 5 loci affect pigmentation of a subset of organs: cd and po affect only the eyes and ocelli; kar affects the eyes, ocelli and fat body; car causes excretion of pigment from tubules; and z affects pigmentation of the eyes alone. Of these loci, only z has been shown to encode a regulatory protein and the role of the remaining four gene products is not clear. Two mutations affecting the red eye pigments (drosopterins), bw and mal, do not substantially perturb brown pigment synthesis in any of the four organs.

Ablation of Drosophila photoreceptor cells by conditional expression of a toxin gene

We have used toxin-mediated ablation to study some aspects of visual system development in Drosophila melanogaster. To devise a method that permits the conditional expression of a cellular toxin, we introduced an amber mutation into the diphtheria toxin-A-chain gene. In transgenic animals, this toxin gene can be activated by providing the gene for an amber suppressor tRNA. By coupling this toxin gene to the photoreceptor cell-specific promoter of the chaoptic gene, photoreceptor cells could be specifically ablated during development. Photoreceptor cell-specific markers normally activated during pupal development failed to appear after midpupation. Photoreceptor cells were absent from the retinas of adult flies at eclosion. We have assessed the consequences of photoreceptor cell ablation for eye and optic lobe development. We suggest that the larval photoreceptor nerve is not essential, in the late larval stages, for retinula photoreceptor cell axons to achieve their proper projection pattern in the brain. Moreover, while retinula photoreceptor innervation is initially required for the development of normal optic ganglia, the ablation of these cells in midpupation has no discernible effect. This approach to cell-specific ablation should be generally applicable to the study of cellular functions in development and behavior.

Complex cellular and subcellular regulation of notch expression during embryonic and imaginal development of Drosophila: implications for notch function

The Notch gene in Drosophila encodes a transmembrane protein with homology to EGF that appears to mediate cell-cell interactions necessary for proper epidermal vs. neural fate decisions. In this study, we examine Notch expression in detail throughout embryonic and imaginal development using confocal laser-scanning microscopy and specific mAb probes. We find that Notch is expressed in a tissue-specific manner as early as the cellular blastoderm stage, when cells of the presumptive mesoderm clearly express less Notch than adjacent ectodermal precursors. Notch is abundantly expressed during the initial determination of neuronal lineages, such as the embryonic neuroblasts and the precursors of sensory neurons in the imaginal disc epithelia, but expression quickly decreases during subsequent differentiation. These changing patterns of Notch expression do not correlate well with cell movements, and thus do not appear to support the notion that the major function of Notch is to maintain epithelial integrity via adhesive mechanisms. Our data suggest instead that Notch may act as a cell-surface receptor, perhaps functioning in the lateral inhibition mechanism that is necessary for proper spacing of neuronal precursors.

Star is required in a subset of photoreceptor cells in the developing Drosophila retina and displays dosage sensitive interactions with rough

We report that mutations at the Star locus act as dominant enhancers of the eye phenotype displayed by flies carrying a null allele of rough. Our analysis of double mutants at different stages of eye development suggests that this phenotype results from defects in the early stages of photoreceptor cell differentiation in the eye imaginal disc. Complete loss of Star function during retinal development, analyzed in mosaic animals, results in cell death, visible as scars in the adult eye. The requirement for wild-type Star function, however, is confined to only a subset of photoreceptor cells, R8, R2, and R5, which are the first three cells to differentiate neurally in the developing retina. These results suggest an essential role for the Star gene in the initial events of ommatidial cluster formation during the development of the Drosophila compound eye.

The segment polarity gene armadillo encodes a functionally modular protein that is the Drosophila homolog of human plakoglobin

The Drosophila segment polarity gene armadillo is required for pattern formation within embryonic segments and imaginal discs. We have found that armadillo is highly conserved during evolution; it is 63% identical to human plakoglobin, a protein found in adhesive junctions joining epithelial and other cells. We have examined arm protein localization in a number of larval tissues and found that arm protein accumulation within cells shares many features with the accumulation of plakoglobin. We have compared the phenotype and molecular lesions responsible for the different arm mutations. Surprisingly, severely truncated proteins retain some function; the degree of function is strictly correlated with the length of the truncated protein, suggesting that the internally repetitive arm protein is modular in function. We present a possible model for the cellular role of arm.

seven in absentia, a gene required for specification of R7 cell fate in the Drosophila eye

Signals from directly adjacent cells are critical for the development of the R7 photoreceptor cell in the Drosophila eye. We have identified a gene, seven in absentia (sina), for which loss of function results in the R7 precursor cell adopting the fate of a nonneuronal cell type. Function of the sina gene is required only in R7 for correct R7 cell development. The sina transcription unit is located within the intron of a gene that encodes an R7 cell-specific opsin. The sina protein, which has a potential metal binding domain, is localized in the nuclei of several ommatidial precursor cells including R7, and sina expression in R7 appears before R7 overtly begins to differentiate. These data indicate that the sina gene product is necessary at a stage in the determination of R7 cell fate when R7 receives and interprets developmental signals from neighboring cells, and possibly acts by regulating gene expression.

Notch is required for successive cell decisions in the developing Drosophila retina

Mutations in the Notch locus affect a variety of developmental decisions in Drosophila. In this paper, we examine the role of Notch in the developing retina. We reduced Notch activity at successive intervals during development of the retina, and then examined the effect on individual cells. When Notch activity was reduced, cells responded by selecting inappropriate developmental pathways. We found that all cell types appear to require Notch when establishing their fate. To examine further Notch's role in eye development, we examined two alleles of Notch--split and facet-glossy. split flies show defects in the initial clustering of photoreceptors, whereas the defects in facet-glossy flies are due to the misrouting of presumptive primary pigment cells into the secondary pigment cell pathway. Our results suggest that Notch plays a permissive role in the cell-cell interactions used to assemble the eye.