Mechanism of hedgehog signaling during Drosophila eye development

Although Hedgehog (Hh) signaling is essential for morphogenesis of the Drosophila eye, its exact link to the network of tissue-specific genes that regulate retinal determination has remained elusive. In this report, we demonstrate that the retinal determination gene eyes absent (eya) is the crucial link between the Hedgehog signaling pathway and photoreceptor differentiation. Specifically, we show that the mechanism by which Hh signaling controls initiation of photoreceptor differentiation is to alleviate repression of eya and decapentaplegic (dpp) expression by the zinc-finger transcription factor Cubitus interruptus (Ci(rep)). Furthermore, our results suggest that stabilized, full length Ci (Ci(act)) plays little or no role in Drosophila eye development. Moreover, while the effects of Hh are primarily concentration dependent in other tissues, hh signaling in the eye acts as a binary switch to initiate retinal morphogenesis by inducing expression of the tissue-specific factor Eya.

Two Pax genes, eye gone and eyeless, act cooperatively in promoting Drosophila eye development

We report the identification of a Drosophila Pax gene, eye gone (eyg), which is required for eye development. Loss-of-function eyg mutations cause reduction or absence of the eye. Similar to the Pax6 eyeless (ey) gene, ectopic expression of eyg induces extra eye formation, but at sites different from those induced by ey. Several lines of evidence suggest that eyg and ey act cooperatively: (1) eyg expression is not regulated by ey, nor does it regulate ey expression, (2) eyg-induced ectopic morphogenetic furrow formation does not require ey, nor does ey-induced ectopic eye production require eyg, (3) eyg and ey can partially substitute for the function of the other, and (4) coexpression of eyg and ey has a synergistic enhancement of ectopic eye formation. Our results also show that eyg has two major functions: to promote cell proliferation in the eye disc and to promote eye development through suppression of wg transcription.

Distinction between color photoreceptor cell fates is controlled by Prospero in Drosophila

The Drosophila compound eye consists of approximately 750 independently functioning ommatidia, each containing two photoreceptor subpopulations. The outer photoreceptors participate in motion detection, while the inner photoreceptors contribute to color vision. Although the inner photoreceptors, R7 and R8, terminally differentiate into functionally related cells, they differ in their molecular and morphological makeup. Our data indicates that several aspects of R7 versus R8 cell fate determination are regulated by the transcription factor Prospero (Pros). pros is specifically expressed in R7 cells, and R7 cells mutant for pros derepress R8 rhodopsins, lose R7 rhodopsins and acquire an R8-like morphology. This suggests that R7 inner photoreceptor cell fate is acquired from a default R8-like fate that is regulated, in part, via the direct transcriptional repression of R8 rhodopsins in R7 cells. Furthermore, this study provides transcriptional targets for pros that may lend insight into its role in regulating neuronal development in flies and vertebrates.

Distinct phospholipase C-gamma-dependent signaling pathways in the Drosophila eye and wing are revealed by a new small wing allele

The Drosophila genome contains a single phospholipase C-gamma (PLC-gamma) homolog, encoded by small wing (sl), that acts as an inhibitor of receptor tyrosine kinase (RTK) signaling during photoreceptor R7 development. Although the existing sl alleles behave genetically as nulls, they may still produce truncated Sl products that could in theory still provide limited PLC-gamma function. Both to identify a true null allele and to probe structure-function relationships in Sl, we carried out an F(1) screen for new sl mutations and identified seven new alleles. Flies homozygous for any of these alleles are viable, with the same short-wing phenotype described previously; however, two of the alleles differ from any of those previously isolated in the severity of the eye phenotype: sl(9) homozygotes have a slightly more extreme extra-R7 phenotype, whereas sl(7) homozygotes have an almost wild-type eye. We determined the mutant defect in all seven alleles, revealing that sl(9) is a molecular null due to a very early stop codon, while sl(7) has a missense mutation in the highly conserved Y catalytic domain. Together with in vitro mutagenesis of the residue affected by the sl(7) mutation, these results confirm the role of Sl in RTK signaling and provide evidence for two genetically separable PLC-gamma-dependent pathways affecting the development of the eye and the wing.

Requirement of the roughest gene for differentiation and time of death of interommatidial cells during pupal stages of Drosophila compound eye development

The roughest locus of Drosophila melanogaster encodes a transmembrane protein of the immunoglobulin superfamily required for several developmental processes, including axonal pathfinding in the developing optic lobe, mechanosensory bristle differentiation and myogenesis. In the compound eye, rst was previously shown to be required for establishing the correct number and spacing of secondary and tertiary pigment cells during the final steps of ommatidial assembly. We have further investigated its function in the developing pupal retina by performing a developmental and molecular analysis of a novel dominant rst allele, rst(D). In addition to showing evidence that rst(D) is a regulatory mutant, the results strongly suggest a previously unnoticed role of the rst gene in the differentiation of secondary/tertiary pigment cell fate as well as establishing the correct timing of surplus cell removal by programmed cell death in the compound eye.

Axon targeting in the Drosophila visual system

The neuronal wiring of the Drosophila melanogaster visual system is constructed through an intricate series of cell-cell interactions. Recent studies have identified some of the gene regulatory and cytoskeletal signaling pathways responsible for the layer-specific targeting of Drosophila photoreceptor axons. Target selection decisions of the R1-R6 subset of photoreceptor axons have been found to be influenced by the nuclear factors Brakeless and Runt, and target selection decisions of the R7 subset of axons have been found to require the cell-surface proteins Ptp69d, Lar and N-cadherin. A role for the visual system glia in orienting photoreceptor axon outgrowth and target selection has also been uncovered.

Photopigment gene expression and rhabdom formation in the crayfish (Procambarus clarkii)

This study examines the expression of the photopigment gene in the developing retina of the freshwater crayfish Procambarus clarkii(Crustacea, Malacostraca, Decapoda). Both sense and anti-sense RNA probes were used for in situ hybridization (ISH) of whole embryos collected at various stages during development. A characteristic of retinal development is the formation of screening pigment in the retinular cells of the retinal ommatidia. This pigmentation is seen as a band that begins at the lateral side of the retinal field and progresses medially. At hatching the retina is approximately 50% pigmented. ISH of whole embryos shows that expression of the photopigment gene by the retinular cells correlates with the extent of the screening pigment band in the retina and with the presence of rhabdoms within the ommatidia. Sections taken through embryos after being hybridized indicate that staining is localized in the cytoplasm of the retinular cells and in the axonal region below the basement membrane. No staining reaction was seen in the rhabdoms of older ommatidia. ISH staining was also seen at the anterior midline of the protocerebrum where extraretinal photoreceptors have been reported. The data presented here show a close correlation of opsin expression within the retinular cells of the ommatidia and the formation of the very early rhabdoms, similar to Drosophila. The results will be discussed in relation to recent studies in Drosophila that suggest rhodopsin plays a role in effecting the organization of the terminal web-like cytoskeleton at the base of the developing rhabdom microvilli.

Drosophila NAB (dNAB) is an orphan transcriptional co-repressor required for correct CNS and eye development

The mammalian NAB proteins have been identified previously as potent co-repressors of the EGR family of zinc finger transcription factors. Drosophila NAB (dNAB), like its mammalian counterparts, binds EGR1 and represses EGR1-mediated transcriptional activation from a synthetic promoter. In contrast, dNAB does not bind the Drosophila EGR-related protein klumpfuss. dnab RNA is expressed exclusively in a subset of neuroblasts in the embryonic and larval central nervous system (CNS), as well as in several larval imaginal disc tissues. Here, we describe the creation of targeted deletion mutations in the dnab gene and the identification of additional, EMS-induced dnab mutations by genetic complementation analysis. Null alleles in dnab cause larval locomotion defects and early larval lethality (L1-L2). A putative hypomorphic allele in dnab instead causes early adult lethality due to severe locomotion defects. In the dnab -/- CNS, axon outgrowth/guidance and glial development appear normal; however, a subset of eve+ neurons forms in reduced numbers. In addition, mosaic analysis in the eye reveals that dnab -/- clones are either very small or absent. Similarly, dNAB overexpression in the eye causes eyes to be very small with few ommatidia. These dramatic eye-specific phenotypes will prove useful for enhancer/suppressor screens to identify dnab-interacting genes.

Scabrous controls ommatidial rotation in the Drosophila compound eye

Establishment of planar polarity in the Drosophila compound eye requires precise 90 degrees rotation of the ommatidial clusters during development. We found that the morphogenetic furrow controls the stop of ommatidial rotation at 90 degrees by emitting signals to posterior ommatidial clusters. One such signal, Scabrous, is synthesized in the furrow cells and transported in vesicles to ommatidial row 6-8. Scabrous vesicles are transported through actin-based cellular extensions but not transcytosis. Scabrous functions nonautonomously to control the stop of ommatidial rotation by suppressing nemo activity in the second 45 degrees rotation. We propose that the morphogenetic furrow regulates precise ommatidial rotation by transporting Scabrous and perhaps other factors through actin-based cellular extensions.

Drosophila nemo is an essential gene involved in the regulation of programmed cell death

Nemo-like kinases define a novel family of serine/threonine kinases that are involved in integrating multiple signaling pathways. They are conserved regulators of Wnt/Wingless pathways, which may coordinate Wnt with TGFbeta-mediated signaling. Drosophila nemo was identified through its involvement in epithelial planar polarity, a process regulated by a non-canonical Wnt pathway. We have previously found that ectopic expression of Nemo using the Gal4-UAS system resulted in embryonic lethality associated with defects in patterning and head development. In this study we present our analyses of the phenotypes of germline clone-derived embryos. We observe lethality associated with head defects and reduction of programmed cell death and conclude that nmo is an essential gene. We also present data showing that nmo is involved in regulating apoptosis during eye development, based on both loss of function phenotypes and on genetic interactions with the pro-apoptotic gene reaper. Finally, we present genetic data from the adult wing that suggest the activity of ectopically expressed Nemo can be modulated by Jun N-terminal kinase (JNK) signaling. Such an observation supports the model that there is cross-talk between Wnt, TGFbeta and JNK signaling at multiple stages of development.

rugose (rg), a Drosophila A kinase anchor protein, is required for retinal pattern formation and interacts genetically with multiple signaling pathways

In the developing Drosophila eye, cell fate determination and pattern formation are directed by cell-cell interactions mediated by signal transduction cascades. Mutations at the rugose locus (rg) result in a rough eye phenotype due to a disorganized retina and aberrant cone cell differentiation, which leads to reduction or complete loss of cone cells. The cone cell phenotype is sensitive to the level of rugose gene function. Molecular analyses show that rugose encodes a Drosophila A kinase anchor protein (DAKAP 550). Genetic interaction studies show that rugose interacts with the components of the EGFR- and Notch-mediated signaling pathways. Our results suggest that rg is required for correct retinal pattern formation and may function in cell fate determination through its interactions with the EGFR and Notch signaling pathways.

A three-tiered mechanism for regulation of planar cell polarity

Some epithelial cells are polarized along an axis orthogonal to their apical-basal axes. Recent studies in Drosophila lead to the view that three classes of signaling molecules govern the planar cell polarity (PCP) pathway. The first class, or module, functions across whole tissues, providing directional information to individual cells. The second module, apparently shared by all planar polarized tissues, and related to the canonical Wnt signaling pathway, interprets the directional signal to produce subcellular asymmetries. The third modules are tissue specific, acting to translate subcellular asymmetry into the appropriate morphological manifestations in the different cell types.

Morgue mediates apoptosis in the Drosophila melanogaster retina by promoting degradation of DIAP1

Inhibitor of apoptosis proteins (IAPs) provide a critical barrier to inappropriate apoptotic cell death through direct binding and inhibition of caspases. We demonstrate that degradation of IAPs is an important mechanism for the initiation of apoptosis in vivo. Drosophila Morgue, a ubiquitin conjugase-related protein, promotes DIAP1 down-regulation in the developing retina to permit selective programmed cell death. Morgue complexes with DIAP1 in vitro and mediates DIAP1 degradation in a manner dependent on the Morgue UBC domain. Reaper (Rpr) and Grim, but not Hid, also promote the degradation of DIAP1 in vivo, suggesting that these proteins promote cell death through different mechanisms.

Ectopic expression of the Drosophila Cdk1 inhibitory kinases, Wee1 and Myt1, interferes with the second mitotic wave and disrupts pattern formation during eye development

Wee1 kinases catalyze inhibitory phosphorylation of the mitotic regulator Cdk1, preventing mitosis during S phase and delaying it in response to DNA damage or developmental signals during G2. Unlike yeast, metazoans have two distinct Wee1-like kinases, a nuclear protein (Wee1) and a cytoplasmic protein (Myt1). We have isolated the genes encoding Drosophila Wee1 and Myt1 and are using genetic approaches to dissect their functions during normal development. Overexpression of Dwee1 or Dmyt1 during eye development generates a rough adult eye phenotype. The phenotype can be modified by altering the gene dosage of known regulators of the G2/M transition, suggesting that we could use these transgenic strains in modifier screens to identify potential regulators of Wee1 and Myt1. To confirm this idea, we tested a collection of deletions for loci that can modify the eye overexpression phenotypes and identified several loci as dominant modifiers. Mutations affecting the Delta/Notch signaling pathway strongly enhance a GMR-Dmyt1 eye phenotype but do not affect a GMR-Dwee1 eye phenotype, suggesting that Myt1 is potentially a downstream target for Notch activity during eye development. We also observed interactions with p53, which suggest that Wee1 and Myt1 activity can block apoptosis.

Retinal development in Drosophila: specifying the first neuron

In vertebrates, a proneural basic helix-loop-helix transcription factor (Ath5, Atonal homolog 5) plays a crucial role in the specification of the first retinal neuron: the retinal ganglion cell (RGC). Math5 homozygous null mutant mice lack RGCs and have no optic nerve. Furthermore, the expression of the Ath5 protein is regulated to give a non-random dispersed pattern of RGCs. In Drosophila, retinal histogenesis is precisely coordinated and is associated with a progressive wave called the morphogenetic furrow. In the furrow, single precisely spaced cells are specified to become the first retinal neural cell type: the R8 photoreceptor cell. This Drosophila founder cell specification is coincident with and dependant upon the expression of the fly Ath5 ortholog: Atonal. Indeed, in both taxa, the process of founder cell specification may be viewed as the regulation of Atonal expression. It is now clear that, in flies, this regulation depends on the action of inductive and inhibitory signals. This review concentrates on the signaling mechanisms that produce this precise pattern of founder cells.

Asymmetric localization of frizzled and the determination of notch-dependent cell fate in the Drosophila eye

BACKGROUND

During patterning of the Drosophila eye, a critical step is the Notch-mediated cell fate decision that determines the identities of the R3/R4 photoreceptor pair in each ommatidium. Depending on the decision taken, the ommatidium adopts either the dorsal or ventral chiral form. This decision is directed by the activity of the planar polarity genes, and, in particular, higher activity of the receptor Frizzled confers R3 fate.

RESULTS

We present evidence that Frizzled does not modulate Notch activity via Rho GTPases and a JNK cascade as previously proposed. We find that the planar polarity proteins Frizzled, Dishevelled, Flamingo, and Strabismus adopt asymmetric protein localizations in the developing photoreceptors. These protein localizations correlate with the bias of Notch activity between R3/R4, suggesting that they are necessary to modulate Notch activity between these cells. Additional data support a mechanism for regulation of Notch activity that could involve direct interactions between Dishevelled and Notch at the cell cortex.

CONCLUSIONS

In the light of our findings, we conclude that Rho GTPases/JNK cascades are not major effectors of planar polarity in the Drosophila eye. We propose a new model for the control of R3/R4 photoreceptor fate by Frizzled, whereby asymmetric protein localization is likely to be a critical step in modulation of Notch activity. This modulation may occur via direct interactions between Notch and Dishevelled.

Membrane trafficking in Drosophila wing and eye development

It is clear that membrane transport is essential to the proper sorting and delivery of membrane bound receptors and ligands, and secreted signaling molecules. Molecular genetic studies in Drosophila are particularly well suited to studies of membrane transport in development. The conservation of cell signaling pathways and membrane transport molecules between Drosophila and other species makes the results obtained in these studies of general interest. In addition, the ability to generate gain- and loss-of-function genetic mutations of various strengths, and the ability to generate transgenic flies that direct protein expression to tissues during development are of particular advantage. Several recent papers suggest that interesting and novel roles for membrane transport processes will be uncovered by studying classically defined membrane transport proteins in developmental contexts. Together these studies suggest that regulation of membrane transport may represent an additional mechanism to regulate the strength of cell-cell signaling during development.

Larval optic nerve and adult extra-retinal photoreceptors sequentially associate with clock neurons during Drosophila brain development

The visual system is one of the input pathways for light into the circadian clock of the Drosophila brain. In particular, extra-retinal visual structures have been proposed to play a role in both larval and adult circadian photoreception. We have analyzed the interactions between extra-retinal structures of the visual system and the clock neurons during brain development. We first show that the larval optic nerve, or Bolwig nerve, already contacts clock cells (the lateral neurons) in the embryonic brain. Analysis of visual system-defective genotypes showed that the absence of the afferent Bolwig nerve resulted in a severe reduction of the lateral neurons dendritic arborization, and that the inhibition of nerve activity induced alterations of the dendritic morphology. During wild-type development, the loss of a functional Bolwig nerve in the early pupa was also accompanied by remodeling of the arborization of the lateral neurons. Approximately 1.5 days later, visual fibers that came from the Hofbauer-Buchner eyelet, a putative photoreceptive organ for the adult circadian clock, were seen contacting the lateral neurons. Both types of extra-retinal photoreceptors expressed rhodopsins RH5 and RH6, as well as the norpA-encoded phospholipase C. These data strongly suggest a role for RH5 and RH6, as well as NORPA, signaling in both larval and adult extra-retinal circadian photoreception. The Hofbauer-Buchner eyelet therefore does not appear to account for the previously described norpA-independent light input to the adult clock. This supports the existence of yet uncharacterized photoreceptive structures in Drosophila.

R8 development in theDrosophilaeye: a paradigm for neural selection and differentiation

The Drosophila eye is an outstanding model with which to decipher mechanisms of neural differentiation. Paramount to normal eye development is the organized selection and differentiation of a patterned array of R8 photoreceptors – the founding photoreceptor of each ommatidium that coordinates the incorporation of all other photoreceptors. R8 development is a complex process that requires the integration of transcription factors and signaling pathways, many of which are highly conserved and perform similar functions in other species. This article discusses the developmental control of the four key elements of R8 development: selection, spacing, differentiation and orchestration of later events. New questions that have surfaced because of recent advances in the field are addressed, and the unique characteristics of R8 development are highlighted through comparisons with neural specification in other Drosophila tissues and with ganglion cell development in the mammalian retina.

Crumbs, the Drosophila homologue of human CRB1/RP12, is essential for photoreceptor morphogenesis

The apical transmembrane protein Crumbs is a central regulator of epithelial apical-basal polarity in Drosophila. Loss-of-function mutations in the human homologue of Crumbs, CRB1 (RP12), cause recessive retinal dystrophies, including retinitis pigmentosa. Here we show that Crumbs and CRB1 localize to corresponding subdomains of the photoreceptor apical plasma membrane: the stalk of the Drosophila photoreceptor and the inner segment of mammalian photoreceptors. These subdomains support the morphogenesis and orientation of the photosensitive membrane organelles: rhabdomeres and outer segments, respectively. Drosophila Crumbs is required to maintain zonula adherens integrity during the rapid apical membrane expansion that builds the rhabdomere. Crumbs also regulates stalk development by stabilizing the membrane-associated spectrin cytoskeleton, a function mechanistically distinct from its role in epithelial apical-basal polarity. We propose that Crumbs is a central component of a molecular scaffold that controls zonula adherens assembly and defines the stalk as an apical membrane subdomain. Defects in such scaffolds may contribute to human CRB1-related retinal dystrophies.

Drosophila Crumbs is a positional cue in photoreceptor adherens junctions and rhabdomeres

Drosophila Crumbs (Crb) is required for apical-basal polarity and is an apical determinant in embryonic epithelia. Here, we describe properties of Crb that control the position and integrity of the photoreceptor adherens junction and photosensitive organ, or rhabdomere. In contrast to normal photoreceptor adherens junctions and rhabdomeres, which span the depth of the retina, adherens junctions and rhabdomeres of Crb-deficient photoreceptors initially accumulate at the top of the retina and fail to maintain their integrity as they stretch to the retinal floor. We show that Crb controls localization of the adherens junction through its intracellular domain containing a putative binding site for a protein 4.1 superfamily protein (FERM). Although loss of Crb or overexpression of the FERM binding domain causes mislocalization of adherens junctions, they do not result in a significant loss of photoreceptor polarity. Mutations in CRB1, a human homologue of crb, are associated with photoreceptor degeneration in retinitis pigmentosa 12 (RP12) and Leber congenital amaurosis (LCA). The intracellular domain of CRB1 behaves similarly to its Drosophila counterpart when overexpressed in the fly eye. Our studies may provide clues for mechanisms of photoreceptor degeneration in RP12 and LCA.

Drosophila JAB1/CSN5 acts in photoreceptor cells to induce glial cells

Different classes of photoreceptor neurons (R cells) in the Drosophila compound eye form connections in different optic ganglia. The R1-R6 subclass connects to the first optic ganglion, the lamina, and relies upon glial cells as intermediate targets. Conversely, R cells promote glial cell development including migration of glial cells into the target region. Here, we show that the JAB1/CSN5 subunit of the COP9 signalosome complex is expressed in R cells, accumulates in the developing optic lobe neuropil, and through the analysis of a unique set of missense mutations, is required in R cells to induce lamina glial cell migration. In these CSN5 alleles, R1-R6 targeting is disrupted. Genetic analysis of protein null alleles further revealed that the COP9 signalosome is required at an earlier stage of development for R cell differentiation.

Eyes closed, aDrosophilap47 homolog, is essential for photoreceptor morphogenesis

Starting with a mutation impacting photoreceptor morphogenesis, we identify here a Drosophila gene, eyes closed (eyc), as a fly homolog of p47, a protein co-factor of the p97 ATPase implicated in membrane fusion. Temporal misexpression of Eyc during rhabdomere extension early in pupal life results in inappropriate retention of normally transient adhesions between developing rhabdomeres. Later Eyc misexpression results in endoplasmic reticulum proliferation and inhibits rhodopsin transport to the developing photosensitive membrane. Loss of Eyc function results in a lethal failure of nuclear envelope assembly in early zygotic divisions. Phenotypes resulting from eyc mutations provide the first in vivo evidence for a role for p47 in membrane biogenesis.

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Perturbing nuclear transport in Drosophila eye imaginal discs causes specific cell adhesion and axon guidance defects

To study nucleocytoplasmic transport during multicellular development, we developed a sensitive nuclear protein import assay in living blastoderm embryos. We show that dominant negative truncations of the human nuclear transport receptor karyopherinbeta/Importinbeta (DNImpbeta) disrupt mRNA export and protein import in Drosophila. To test the sensitivity of different developmental processes to nuclear trafficking perturbations, we expressed DNImpbeta behind the morphogenetic furrow of the eye disc, at a time when photoreceptors are patterned and project their axons to the brain. DNImpbeta expression does not disrupt the correct specification of different photoreceptors, but causes a defect in cell adhesion that leads to some photoreceptors descending below the layer of ommatidia. The photoreceptors initially project their axons correctly to the posterior, but later their axons are unable to enter the optic stalk en route to the brain and continue to project an extensive network of misguided axons. The axon guidance and cell adhesion defects are both due to a disruption in the function of Ketel, the Drosophila ortholog of Importinbeta. We conclude that cell adhesion and axon guidance in the eye have specific requirements for nucleocytoplasmic transport, despite involving processes that occur primarily at the cell surface.

Developmental and transcriptional consequences of mutations in Drosophila TAF(II)60

In vitro, the TAF(II)60 component of the TFIID complex contributes to RNA polymerase II transcription initiation by serving as a coactivator that interacts with specific activator proteins and possibly as a promoter selectivity factor that interacts with the downstream promoter element. In vivo roles for TAF(II)60 in metazoan transcription are not as clear. Here we have investigated the developmental and transcriptional requirements for TAF(II)60 by analyzing four independent Drosophila melanogaster TAF(II)60 mutants. Loss-of-function mutations in Drosophila TAF(II)60 result in lethality, indicating that TAF(II)60 provides a nonredundant function in vivo. Molecular analysis of TAF(II)60 alleles revealed that essential TAF(II)60 functions are provided by two evolutionarily conserved regions located in the N-terminal half of the protein. TAF(II)60 is required at all stages of Drosophila development, in both germ cells and somatic cells. Expression of TAF(II)60 from a transgene rescued the lethality of TAF(II)60 mutants and exposed requirements for TAF(II)60 during imaginal development, spermatogenesis, and oogenesis. Phenotypes of rescued TAF(II)60 mutant flies implicate TAF(II)60 in transcriptional mechanisms that regulate cell growth and cell fate specification and suggest that TAF(II)60 is a limiting component of the machinery that regulates the transcription of dosage-sensitive genes. Finally, TAF(II)60 plays roles in developmental regulation of gene expression that are distinct from those of other TAF(II) proteins.

An enhanced mutant of red fluorescent protein DsRed for double labeling and developmental timer of neural fiber bundle formation

We developed a new variant of coral-derived red fluorescent protein, DsRed S197Y, which is brighter and essentially free from secondary fluorescence peak. This makes it an ideal reporter for double labeling with green fluorescent protein (GFP). Though purified protein shows only 20% stronger fluorescence emission, culture cells that express DsRed S197Y exhibit a 3-3.5 times higher level of fluorescence than the cells that express wild-type DsRed. The much slower fluorescence maturation of DsRed than that of GFP is a beneficial feature for a fluorescent developmental timer application. When GFP and DsRed S197Y are expressed simultaneously, emissions start at different latency. This provides information about the time after the onset of expression. It reflects the order of cell differentiation if the expression is activated upon differentiation of certain types of cells. We applied this system to the developing brain of Drosophila and visualized, for the first time, the formation order of neural fibers within a large bundle. Our results showed that newly extending fibers of the mushroom body neurons mainly run into the core rather than to the periphery of the existing bundle. DsRed-based timer thus presents an indispensable tool for developmental biology and genetics of model organisms.

The cyclope gene of Drosophila encodes a cytochrome c oxidase subunit VIc homolog

Cytochrome c oxidase is the terminal enzyme of the mitochondrial electron transfer chain. In eukaryotes, the enzyme is composed of 3 mitochondrial DNA-encoded subunits and 7-10 (in mammals) nuclear DNA-encoded subunits. This enzyme has been extensively studied in mammals and yeast but, in Drosophila, very little is known and no mutant has been described so far. Here we report the genetic and molecular characterization of mutations in cyclope (cype) and the cloning of the gene encoding a cytochrome c oxidase subunit VIc homolog. cype is an essential gene whose mutations are lethal and show pleiotropic phenotypes. The 77-amino acid peptide encoded by cype is 46% identical and 59% similar to the human subunit (75 amino acids). The transcripts are expressed maternally and throughout development in localized regions. They are found predominantly in the central nervous system of the embryo; in the central region of imaginal discs; in the germarium, follicular, and nurse cells of the ovary; and in testis. A search in the Genome Annotation Database of Drosophila revealed the absence of subunit VIIb and the presence of 9 putative nuclear cytochrome c oxidase subunits with high identity scores when compared to the 10 human subunits.

A systematic screen for dominant second-site modifiers of Merlin/NF2 phenotypes reveals an interaction with blistered/DSRF and scribbler

Merlin, the Drosophila homologue of the human tumor suppressor gene Neurofibromatosis 2 (NF2), is required for the regulation of cell proliferation and differentiation. To better understand the cellular functions of the NF2 gene product, Merlin, recent work has concentrated on identifying proteins with which it interacts either physically or functionally. In this article, we describe genetic screens designed to isolate second-site modifiers of Merlin phenotypes from which we have identified five multiallelic complementation groups that modify both loss-of-function and dominant-negative Merlin phenotypes. Three of these groups, Group IIa/scribbler (also known as brakeless), Group IIc/blistered, and Group IId/net, are known genes, while two appear to be novel. In addition, two genes, Group IIa/scribbler and Group IIc/blistered, alter Merlin subcellular localization in epithelial and neuronal tissues, suggesting that they regulate Merlin trafficking or function. Furthermore, we show that mutations in scribbler and blistered display second-site noncomplementation with one another. These results suggest that Merlin, blistered, and scribbler function together in a common pathway to regulate Drosophila wing epithelial development.

The role of the Drosophila TAK homologue dTAK during development

The TAK kinases belong to the MAPKKK group and have been implicated in a variety of signaling events. Originally described as a TGF-beta activated kinase (TAK) it has, however, subsequently been demonstrated to signal through p38, Jun N-terminal kinase (JNK) and Nemo types of MAP kinases, and the NFkappaB inducing kinase. Despite these multiple proposed functions, the in vivo role of TAK family kinases remains unclear. Here we report the isolation and genetic characterization of the Drosophila TAK homologue (dTAK). By employing overexpression and double-stranded RNA interference (RNAi) techniques we have analyzed its function during embryogenesis and larval development. Overexpression of dTAK in the embryonic epidermis is sufficient to induce the transcription of the JNK target genes decapentaplegic and puckered. Furthermore, overexpression of dominant negative (DN) or wild-type forms of dTAK in wing and eye imaginal discs, respectively, results in defects in thorax closure and ommatidial planar polarity, two well described phenotypes associated with JNK signaling activity. Surprisingly, RNAi and DN-dTAK expression studies in the embryo argue for a differential requirement of dTAK during developmental processes controlled by JNK signaling, and a redundant or minor role of dTAK in dorsal closure. In addition, dTAK-mediated activation of JNK in the Drosophila eye imaginal disc leads to an eye ablation phenotype due to ectopically induced apoptotic cell death. Genetic analyses in the eye indicate that dTAK can also act through the p38 and Nemo kinases in imaginal discs. Our results suggest that dTAK can act as a JNKKK upstream of JNK in multiple contexts and also other MAPKs in the eye. However, the loss-of-function RNAi studies indicate that it is not strictly required and thus either redundant or playing only a minor role in the context of embryonic dorsal closure.

A new visualization approach for identifying mutations that affect differentiation and organization of the Drosophila ommatidia

The Drosophila eye is widely used as a model system to study neuronal differentiation, survival and axon projection. Photoreceptor differentiation starts with the specification of a founder cell R8, which sequentially recruits other photoreceptor neurons to the ommatidium. The eight photoreceptors that compose each ommatidium exist in two chiral forms organized along two axes of symmetry and this pattern represents a paradigm to study tissue polarity. We have developed a method of fluoroscopy to visualize the different types of photoreceptors and the organization of the ommatidia in living animals. This allowed us to perform an F(1) genetic screen to isolate mutants affecting photoreceptor differentiation, survival or planar polarity. We illustrate the power of this detection system using known genetic backgrounds and new mutations that affect ommatidial differentiation, morphology or chirality.

The tissue polarity gene nemo carries out multiple roles in patterning during Drosophila development

Drosophila nemo was first identified as a gene required for tissue polarity during ommatidial development. We have extended the analysis of nemo and found that it participates in multiple developmental processes. It is required during wing development for wing shape and vein patterning. We observe genetic interactions between nemo and mutations in the Notch, Wingless, Frizzled and Decapentaplegic pathways. Our data support the findings from other organisms that Nemo proteins act as negative regulators of Wingless signaling. nemo mutations cause polarity defects in the adult wing and overexpression of nemo leads to abdominal polarity defects. The expression of nemo during embryogenesis is dynamic and dsRNA inhibition and ectopic expression studies indicate that nemo is essential during embryogenesis.

Lilliputian: an AF4/FMR2-related protein that controls cell identity and cell growth

Members of the AF4/FMR2 family of nuclear proteins are involved in human diseases such as acute lymphoblastic leukemia and mental retardation. Here we report the identification and characterization of the Drosophila lilliputian (lilli) gene, which encodes a nuclear protein related to mammalian AF4 and FMR2. Mutations in lilli suppress excessive neuronal differentiation in response to a constitutively active form of Raf in the eye. In the wild type, Lilli has a partially redundant function in the Ras/MAPK pathway in differentiation but it is essential for normal growth. Loss of Lilli function causes an autonomous reduction in cell size and partially suppresses the increased growth associated with loss of PTEN function. These results suggest that Lilli acts in parallel with the Ras/MAPK and the PI3K/PKB pathways in the control of cell identity and cellular growth.

Transcriptional regulation of cytoskeletal functions and segmentation by a novel maternal pair-rule gene, lilliputian

Transcriptional control during early Drosophila development is governed by maternal and zygotic factors. We have identified a novel maternal transcriptional regulator gene, lilliputian (lilli), which contains an HMG1 (AT-hook) motif and a domain with similarity to the human fragile X mental retardation FMR2 protein and the AF4 proto-oncoprotein. Embryos lacking maternal lilli expression show specific defects in the establishment of a functional cytoskeleton during cellularization, and exhibit a pair-rule segmentation phenotype. These mutant phenotypes correlate with markedly reduced expression of the early zygotic genes serendipity alpha, fushi tarazu and huckebein, which are essential for cellularization and embryonic patterning. In addition, loss of lilli in adult photoreceptor and bristle cells results in a significant decrease in cell size. Our results indicate that lilli represents a novel pair-rule gene that acts in cytoskeleton regulation, segmentation and morphogenesis.

Drosophila homologues of the transcriptional coactivation complex subunits TRAP240 and TRAP230 are required for identical processes in eye-antennal disc development

We have identified mutations in two genes, blind spot and kohtalo, that encode Drosophila homologues of human TRAP240 and TRAP230, components of a large transcriptional coactivation complex homologous to the yeast Mediator complex. Loss of either blind spot or kohtalo has identical effects on the development of the eye-antennal disc. Eye disc cells mutant for either gene can express decapentaplegic and atonal in response to Hedgehog signaling, but they maintain inappropriate expression of these genes and fail to differentiate further. Mutant cells in the antennal disc lose expression of Distal-less and misexpress eyeless, suggesting a partial transformation towards the eye fate. blind spot and kohtalo are not required for cell proliferation or survival, and their absence cannot be rescued by activation of the Hedgehog or Notch signaling pathways. These novel and specific phenotypes suggest that TRAP240 and TRAP230 act in concert to mediate an unknown developmental signal or a combination of signals.

The cell adhesion molecule Echinoid defines a new pathway that antagonizes the Drosophila EGF receptor signaling pathway

Photoreceptor and cone cells in the Drosophila eye are recruited following activation of the epidermal growth factor receptor (EGFR) pathway. We have identified echinoid (ed) as a novel putative cell adhesion molecule that negatively regulates EGFR signaling. The ed mutant phenotype is associated with extra photoreceptor and cone cells. Conversely, ectopic expression of ed in the eye leads to a reduction in the number of photoreceptor cells. ed expression is independent of EGFR signaling and ED is localized to the plasma membrane of every cells throughout the eye disc. We present evidence that ed acts nonautonomously to generate extra R7 cells by a mechanism that is sina-independent but upstream of Tramtrack (TTK88). Together, our results support a model whereby ED defines an independent pathway that antagonizes EGFR signaling by regulating the activity, but not the level, of the TTK88 transcriptional repressor.

Dual-tagging gene trap of novel genes in Drosophila melanogaster

A gene-trap system is established for Drosophila. Unlike the conventional enhancer-trap system, the gene-trap system allows the recovery only of fly lines whose genes are inactivated by a P-element insertion, i.e., mutants. In the gene-trap system, the reporter gene expression reflects precisely the spatial and temporal expression pattern of the trapped gene. Flies in which gene trap occurred are identified by a two-step screening process using two independent markers, mini-w and Gal4, each indicating the integration of the vector downstream of the promoter of a gene (dual tagging). mini-w has its own promoter but lacks a polyadenylation signal. Therefore, mini-w mRNA is transcribed from its own promoter regardless of the vector integration site in the genome. However, the eyes of flies are not orange or red unless the vector is incorporated into a gene enabling mini-w to be spliced to a downstream exon of the host gene and polyadenylated at the 3' end. The promoter-less Gal4 reporter is expressed as a fusion mRNA only when it is integrated downstream of the promoter of a host gene. The exons of trapped genes can be readily cloned by vectorette RT-PCR, followed by RACE and PCR using cDNA libraries. Thus, the dual-tagging gene-trap system provides a means for (i) efficient mutagenesis, (ii) unequivocal identification of genes responsible for mutant phenotypes, (iii) precise detection of expression patterns of trapped genes, and (iv) rapid cloning of trapped genes.

A scanning electron-microscopic study of apical contacts in the eye during postembryonic development of Drosophila melanogaster

The apical surface of the ommatidium plays a major role during development of the compound eye. Cell-cell contacts leading to induction seem to be initiated at this surface. The pupal eye of Drosophila was examined, using scanning electron microscopy, from a few hours after eversion of the imaginal disc (19 h after pupariation, 25 degrees C) until shortly after the onset of the corneal secretion (46 h after pupariation, 25 degrees C). At 19 h, the primary pigment cells are in the process of encircling the cone cells. At this time, tufts formed by the cone cell microvilli are the most prominent feature of the eye's surface. Shortly thereafter, the interommatidial cells become more prominent. Their surfaces are raised to form ridges that enclose primary pigment cells and cone cells. From 21 h onwards and lasting for 5-6 h, the interommatidial cells form slim cytoplasmic extensions that spread over the surfaces of the surrounding cells. These extensions contact neighbouring interommatidial or primary pigment cells, but also non-adjacent cells such as cone cells. The fates of these interommatidial cells presumably are determined during that time. The cell-cell interactions may play a role in determining cell fates, for example by providing positional information.

Neurogenesis in the developing visual system of the branchiopod crustacean Triops longicaudatus (LeConte, 1846): corresponding patterns of compound-eye formation in Crustacea and Insecta?

In the discussion on arthropod phylogeny, the structural evolution of compound eyes and optic ganglia in Crustacea and Insecta is an important topic. On the one hand, many morphological features as well as developmental aspects of the visual system in Insecta and Crustacea correspond in so much detail that eye design in these two groups is likely to have a common euarthropodan ancestor. On the other hand, however, some authors advocate a convergent evolution of the crustacean and insect visual system founding their arguments on differences in the arrangement of the visual neuropils and the fibre connections between Malacostraca and Entomostraca (the "entomostracan enigma"). Therefore, information about cellular aspects of visual system formation in entomostracan Crustacea is likely to enliven this debate, but is not yet available. To fill this gap, we examined the proliferation of neuronal stem cells in the developing visual system of the tadpole shrimp Triops longicaudatus (LeConte, 1846) (Entomostraca, Branchiopoda, Phyllopoda, Calmanostraca, Notostraca) by in vivo incorporation of the proliferation marker bromodeoxyuridine and subsequent immunohistochemical detection. Our results indicate that in the developing visual system of T. longicaudatus, three band-shaped zones containing neuronal stem cells are present corresponding to the proliferation zones found in Malacostraca. We therefore conclude that the ontogenetic mechanisms of visual-system formation are evolutionarily conserved (homologous) in Branchiopoda, Malacostraca, and Insecta.

Roughex mediates G(1) arrest through a physical association with cyclin A

Differentiation in the developing Drosophila eye requires synchronization of cells in the G(1) phase of the cell cycle. The roughex gene product plays a key role in this synchronization by negatively regulating cyclin A protein levels in G(1). We show here that coexpressed Roughex and cyclin A physically interact in vivo. Roughex is a nuclear protein, while cyclin A was previously shown to be exclusively cytoplasmic during interphase in the embryo. In contrast, we demonstrate that in interphase cells in the eye imaginal disk cyclin A is present in both the nucleus and the cytoplasm. In the presence of ectopic Roughex, cyclin A becomes strictly nuclear and is later degraded. Nuclear targeting of both Roughex and cyclin A under these conditions is dependent on a C-terminal nuclear localization signal in Roughex. Disruption of this signal results in cytoplasmic localization of both Roughex and cyclin A, confirming a physical interaction between these molecules. Cyclin A interacts with both Cdc2 and Cdc2c, the Drosophila Cdk2 homolog, and Roughex inhibits the histone H1 kinase activities of both cyclin A-Cdc2 and cyclin A-Cdc2c complexes in whole-cell extracts. Two-hybrid experiments suggested that the inhibition of kinase activity by Roughex results from competition with the cyclin-dependent kinase subunit for binding to cyclin A. These findings suggest that Roughex can influence the intracellular distribution of cyclin A and define Roughex as a distinct and specialized cell cycle inhibitor for cyclin A-dependent kinase activity.

Peripodial cells regulate proliferation and patterning of Drosophila imaginal discs

Cells employ a diverse array of signaling mechanisms to establish spatial patterns during development. Nowhere is this better understood than in Drosophila, where the limbs and eyes arise from discrete epithelial sacs called imaginal discs. Molecular-genetic analyses of pattern formation have generally treated discs as single epithelial sheets. Anatomically, however, discs comprise a columnar cell monolayer covered by a squamous epithelium known as the peripodial membrane. Here we demonstrate that during development, peripodial cells signal to disc columnar cells via microtubule-based apical extensions. Ablation and targeted gene misexpression experiments demonstrate that peripodial cell signaling contributes to growth control and pattern formation in the eye and wing primordia. These findings challenge the traditional view of discs as monolayers and provide foundational evidence for peripodial cell function in Drosophila appendage development.

Novel signaling from the peripodial membrane is essential for eye disc patterning in Drosophila

The Drosophila eye disc is a sac of single layer epithelium with two opposing sides, the peripodial membrane (PM) and the disc proper (DP). Retinal morphogenesis is organized by Notch signaling at the dorsoventral (DV) boundary in the DP. Functions of the PM in coordinating growth and patterning of the DP are unknown. We show that the secreted proteins, Hedgehog, Wingless, and Decapentaplegic, are expressed in the PM, yet they control DP expression of Notch ligands, Delta and Serrate. Peripodial clones expressing Hedgehog induce Serrate in the DP while loss of peripodial Hedgehog disrupts disc growth. Furthermore, PM cells extend cellular processes to the DP. Therefore, peripodial signaling is critical for eye pattern formation and may be mediated by peripodial processes.

Senseless, a Zn finger transcription factor, is necessary and sufficient for sensory organ development in Drosophila

The senseless (sens) gene is required for proper development of most cell types of the embryonic and adult peripheral nervous system (PNS) of Drosophila. Sens is a nuclear protein with four Zn fingers that is expressed and required in the sensory organ precursors (SOP) for proper proneural gene expression. Ectopic expression of Sens in many ectodermal cells causes induction of PNS external sensory organ formation and is able to recreate an ectopic proneural field. Hence, sens is both necessary and sufficient for PNS development. Our data indicate that proneural genes activate sens expression. Sens is then in turn required to further activate and maintain proneural gene expression. This feedback mechanism is essential for selective enhancement and maintenance of proneural gene expression in the SOPs.

homothorax and iroquois-C genes are required for the establishment of territories within the developing eye disc

In Drosophila the eye-antennal disc gives rise to most adult structures of the fly's head. Yet the molecular basis for its regionalization during development is poorly understood. Here we show that homothorax is required early during development for normal eye development and is necessary for the formation of the ventral head capsule. In the ventral region of the disc only, homothorax and wingless are involved in a positive feedback loop necessary to restrict eye formation. homothorax is able to prevent the initiation and progression of the morphogenetic furrow without inducing wingless, which points to homothorax as a key negative regulator of eye development. In addition, we show that the iroquois-complex genes are required for dorsal head development antagonizing the function of homothorax in this region of the disc.

Domina (Dom), a new Drosophila member of the FKH/WH gene family, affects morphogenesis and is a suppressor of position-effect variegation

Domina (Dom) is a novel member of the FKH/WH transcription factor gene family of Drosophila. Two alternatively polyadenylated Dom transcripts of 2.9 and 3.9 kb encode a 719-amino-acid protein with a FKH/WH domain and a putative acidic transactivation domain. Dom is mainly expressed in the central and peripheral nervous system. Homozygous mutants show rough eyes, irregular arrangement of bristles, extended wings, defective posterior wing margins, and a severely diminished vitality and fertility. Heterozygous Dom flies are morphologically wild type but show suppression of position-effect variegation. Consistently with this chromatin effect DOM protein is accumulated in the chromocenter and, as expected from a transcription factor, is found at specific euchromatic loci. Sequence comparison suggests that DOM of Drosophila is homologous to the chordate WHN proteins. The chromatin modifying capability of DOM is probably based on the FKH/WH domain, which shows a remarkable structural similarity to the winged-helix structures of H1 and the central globular domain of H5.

Retinal morphogenesis in Drosophila: hints from an eye-specific decapentaplegic allele

Decapentaplegic (dpp) regulates many aspects of imaginal disc growth and patterning in Drosophila. We have analyzed the phenotype of an eye-specific dpp allele, dppblk, which causes a reduction in the size of the retina due to a loss of ventral ommatidia. Prior to the onset of differentiation, dppblk eye discs are normal regarding size, shape, and ability to express dorsal and ventral markers. However, expression of a dpp-lacZ reporter is reduced at the ventral margin. Additional dorsoventral asymmetry appears during retinal differentiation: the morphogenetic furrow (MF) initiates normally at the posterior tip of the disc, but fails to propagate into the ventral epithelium. This defect can be rescued by increasing dpp expression along the ventral margin by local removal of patched function. We propose that the primary defect in dppblk is an inability to activate dpp expression properly at the ventral margin. This has two consequences: it prevents initiation from the ventral margin, and it renders the ventral epithelium unresponsive to differentiation signals emanating from the MF.

Rasputin, the Drosophila homologue of the RasGAP SH3 binding protein, functions in ras- and Rho-mediated signaling

The small GTPase Ras plays an important role in many cellular signaling processes. Ras activity is negatively regulated by GTPase activating proteins (GAPs). It has been proposed that RasGAP may also function as an effector of Ras activity. We have identified and characterized the Drosophila homologue of the RasGAP-binding protein G3BP encoded by rasputin (rin). rin mutants are viable and display defects in photoreceptor recruitment and ommatidial polarity in the eye. Mutations in rin/G3BP genetically interact with components of the Ras signaling pathway that function at the level of Ras and above, but not with Raf/MAPK pathway components. These interactions suggest that Rin is required as an effector in Ras signaling during eye development, supporting an effector role for RasGAP. The ommatidial polarity phenotypes of rin are similar to those of RhoA and the polarity genes, e.g. fz and dsh. Although rin/G3BP interacts genetically with RhoA, affecting both photoreceptor differentiation and polarity, it does not interact with the gain-of-function genotypes of fz and dsh. These data suggest that Rin is not a general component of polarity generation, but serves a function specific to Ras and RhoA signaling pathways.

The Drosophila gene abstrakt, required for visual system development, encodes a putative RNA helicase of the DEAD box protein family

The molecular mechanisms underlying axonal pathfinding are not well understood. In a genetic screen for mutations affecting the projection of the larval optic nerve we isolated the abstrakt locus. abstrakt is required for pathfinding of the larval optic nerve, and it also affects development in both the adult visual system and the embryonic CNS. Here we report the molecular characterization of abstrakt. It encodes a putative ATP-dependent RNA helicase of the DEAD box protein family, with two rare substitutions in the PTRELA and the RG-D motifs, thought to be involved in oligonucleotide binding: serine for threonine, and lysine for arginine, respectively. Two mutant alleles of abstrakt show amino acid exchanges in highly conserved positions. A glycine to serine exchange in the HRIGR motif, which is involved in RNA binding and ATP hydrolysis, results in a complete loss of protein function; and a proline to leucine exchange located between the highly conserved ATPase A and PTRELA motifs results in temperature-sensitive protein function. Both the broad requirement for abstrakt gene function and its ubiquitous expression are consistent with a molecular function of the abstrakt protein in mRNA splicing or translational control.

Determination of Drosophila photoreceptors: timing is everything

This review covers recent findings concerning the specification of the photoreceptor subtypes in the Drosophila eye. Particular attention is paid to aspects of retinal patterning and differentiation where relative timing of events seems to be tightly controlled and essential for proper assembly of the compound eye. For example, specification of the founding photoreceptors of each cluster requires sequential positive and negative signaling through the Notch pathway, and reiterated signaling through the epidermal growth factor receptor leads to the pairwise recruitment of the distinct types of photoreceptors in discrete zones across the eye. Results suggest that different signaling environments for these two receptors may exist across the disc, and that receiving cells may constantly shift their predisposition to respond to such signals by adopting given fates. In addition, considerable data exist that the rate of expansion of retinal patterning across the disc is restricted to allow the orderly patterning of retinal precursors, and that one mechanism for controlling this rate may be the co-ordinated expression anterior to the furrow of factors which both inhibit and promote the expansion of retinal patterning. Finally, this review considers the possibility that the morphogenetic furrow serves as a moving source of morphogens which supply spatial information to both anterior and posterior tissue, providing temporal cues that regulate the many events involved in orderly assembly of the precise array of retinal cell types in the compound eye.

Decoding vectorial information from a gradient: sequential roles of the receptors Frizzled and Notch in establishing planar polarity in the Drosophila eye

The Drosophila eye is composed of several hundred ommatidia that can exist in either of two chiral forms, depending on position: ommatidia in the dorsal half of the eye adopt one chiral form, whereas ommatidia in the ventral half adopt the other. Chirality appears to be specified by a polarizing signal with a high activity at the interface between the two halves (the ‘equator’), which declines in opposite directions towards the dorsal and ventral poles. Here, using genetic mosaics, we show that this polarizing signal is decoded by the sequential use of two receptor systems. The first depends on the seven-transmembrane receptor Frizzled (Fz) and distinguishes between the two members of the R3/R4 pair of presumptive photoreceptor cells, predisposing the cell that is located closer to the equator and having higher Fz activity towards the R3 photoreceptor fate and the cell further away towards the R4 fate. This bias is then amplified by subsequent interactions between the two cells mediated by the receptor Notch (N) and its ligand Delta (Dl), ensuring that the equatorial cell becomes the R3 photoreceptor while the polar cell becomes the R4 photoreceptor. As a consequence of this reciprocal cell fate decision, the R4 cell moves asymmetrically relative to the R3 cell, initiating the appropriate chiral pattern of the remaining cells of the ommatidium.