Wingless and patched are negative regulators of the morphogenetic furrow and can affect tissue polarity in the developing Drosophila compound eye

Restriction of ectopic eye formation by Drosophila teashirt and tiptop to the developing antenna

In Drosophila, the retinal determination network comprises a set of nuclear factors whose loss-of-function phenotypes often include the complete or near total elimination of the developing eye. These genes also share the ability of being able to induce ectopic eye formation when forcibly expressed in nonretinal tissues such as the antennae, legs, halteres, wings, and genitals. However, it appears that the ability to redirect and transform tissue fates is limited; not all tissues and cell populations can be forced into adopting an eye fate. In this report, we demonstrate that ectopic eye formation by teashirt and its paralog tiptop, a potential new eye specification gene, is restricted to the developing antennae. Of interest, tiptop appears to be a more effective inducer of retinal formation than teashirt. A genetic screen for interacting proteins failed to identify paralog-specific relationships suggesting that the differences between these two genes may be attributed instead to structural differences between the duplicates. We also demonstrate that in addition to being expressed in coincident patterns within the developing eye, both paralogs are transcribed at very similar levels.

Retinal determination genes as targets and possible effectors of extracellular signals

Retinal determination genes are sufficient to specify eyes in ectopic locations, raising the question of how these master regulatory genes define an eye developmental field. Genetic mosaic studies establish that expression of the retinal determination genes eyeless, teashirt, homothorax, eyes absent, sine oculis, and dachshund are each regulated by combinations of Dpp, Hh, N, Wg, and Ras signals in Drosophila. Dpp and Hh control eyeless, teashirt, sine oculis, and dachshund expression, Dpp and Ras control homothorax, and all the signaling pathways affect eyes absent expression. These results suggest that eye-specific development uses retinal determination gene expression to relay positional information to eye target genes, because the distinct, overlapping patterns of retinal determination gene expression reflect the activities of the extracellular signaling pathways.

An antennal-specific role for bowl in repressing supernumerary appendage development in Drosophila

In Drosophila, antennae and legs are serially homologous appendages, and yet they develop into organs of very different structure and function. This implies that different genetic mechanisms operate onto a common developmental ground state to produce antennae and legs. Still few such mechanisms have been uncovered. During leg development, bowl, a member of the odd-skipped gene family, has been shown to participate in the formation of the leg segmental joints. Here we report that, in the antennal disc, bowl has a dramatically different role: bowl is expressed in the ventral antennal disc to prevent inappropriate expression of wg early during development. The removal of bowl function leads to the activation of wg in the dpp-expressing domain. This ectopic expression of wg, together with dpp, results in a new proximo-distal axis that promotes non-autonomous antennal duplications. The role of bowl in suppressing a supernumerary PD axis is maintained even when the antennal disc is homeotically transformed into a leg-like appendage. Therefore, bowl is part of a genetic program that suppresses the formation of supernumerary appendages specifically in the fly's head.

Temporal switching of regulation and function of eye gone (eyg) in Drosophila eye development

The Pax gene eyg is important for Drosophila eye development. eyg expression in the visual system changes dynamically during development. In this study, we found that the transcriptional regulation of eyg can be separated into four distinct temporal phases (E, L1, L2, and L3) and each is regulated by distinct cis-regulatory elements. Utilizing these enhancers for temporal and spatially specific manipulations, we addressed the regulation and function of eyg at different developmental stages. We found that Notch signaling is required and sufficient for eyg expression and this activity is restricted only to the L2 stage. We further showed that the function of eyg in eye development is required only at the second instar larval stage, while its function for head and antenna development can be provided at any time during embryo and larval development. Thus there is a temporal switch of the regulatory mechanism and function of eyg. We propose that eyg expression at L2 is induced and maintained by N signaling, and is turned off at L3 by a negative feedback loop involving the morphogenetic furrow.

Hedgehog and Dpp signaling induce cadherin Cad86C expression in the morphogenetic furrow during Drosophila eye development

During Drosophila eye development, cell differentiation is preceded by the formation of a morphogenetic furrow, which progresses across the epithelium from posterior to anterior. Cells within the morphogenetic furrow are apically constricted and shortened along their apical-basal axis. However, how these cell shape changes and, thus, the progression of the morphogenetic furrow are controlled is not well understood. Here we show that cells simultaneously lacking Hedgehog and Dpp signal transduction fail to shorten and do not enter the morphogenetic furrow. Moreover, we have identified a gene, cadherin Cad86C, which is highly expressed in cells of the leading flank of the morphogenetic furrow. Ectopic activation of either the Hedgehog or Dpp signal transduction pathway results in elevated Cad86C expression. Conversely, simultaneous loss of both Hedgehog and Dpp signal transduction leads to decreased Cad86C expression. Finally, ectopic expression of Cad86C in either eye-antennal imaginal discs or wing imaginal discs results in apical constriction and shortening of cells. We conclude that Hedgehog and Dpp signaling promote the shortening of cells within the morphogenetic furrow. Induction of Cad86C expression might be one mechanism through which Hedgehog and Dpp promote these cell shape changes.

Wingless signaling in Drosophila eye development

The secreted morphogen Wingless (Wg) has a variety of functions throughout Drosophila eye development, controlling tissue specification, growth, and patterning. Wg plays a critical role in subdividing the eye imaginal disc into separate primordia that will give rise to the adult retina and the surrounding head capsule. During larval development, wg is expressed in the anterior lateral margins of the eye disc, regions that will give rise to head cuticle; Wg signaling promotes the head fate and prevents these marginal regions from initiating ectopic photoreceptor differentiation. Expression of wg at the dorsal margin is earlier and stronger than at the ventral margin, allowing Wg to contribute to specifying the dorsal domain of the eye disc. Finally, during the pupal stages, wg expression surrounds the entire eye and a concentric gradient of Wg establishes several distinct peripheral retinal cell fates. This chapter reviews these aspects of Wg function and describes how to generate clones of cells mutant for genes encoding components of the Wg signaling pathway in the eye disc and examine their effects on photoreceptor differentiation by immunohistochemistry.

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

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

Expanded and fat regulate growth and differentiation in the Drosophila eye through multiple signaling pathways

Mutations in the expanded gene act as hyperplastic tumor suppressors, interfere with cell competition and elevate Dpp signaling. Unlike Dpp overexpression, ex causes few patterning defects. Our data suggest that patterning effects are partly masked by antagonistic roles of other signaling pathways that are also activated. ex causes proliferation of cells in the posterior eye disc that are normally postmitotic. ex mutations elevate Wg signaling, but Dpp signaling antagonizes patterning effects of Wg. By contrast, if Dpp signaling is blocked in ex mutant cells, the elevated Wg signaling preserves an immature developmental state and prevents retinal differentiation. An effect of ex mutations on vesicle transport is suggested by evidence for altered sterol distribution. Mutations in ft show effects on proliferation, Wg signaling and sterols very similar to those of ex mutations. During disc growth, ex was largely epistatic to ft, and the Warts pathway mutation hippo largely epistatic to ex. Our data suggest that ft and ex act partially through the Warts pathway.

The Drosophila tumor suppressors Expanded and Merlin differentially regulate cell cycle exit, apoptosis, and Wingless signaling

Mutations that inactivate either merlin (mer) or expanded (ex) result in increased cell growth and proliferation in Drosophila. Both Mer and Ex are members of the Band 4.1 protein superfamily, and, based on analyses of mer ex double mutants, they are proposed to function together in at least a partially redundant manner upstream of the Hippo (Hpo) and Warts (Wts) proteins to regulate cell growth and division. By individually analyzing ex and mer mutant phenotypes, we have found important qualitative and quantitative differences in the ways Mer and Ex function to regulate cell proliferation and cell survival. Though both mer and ex restrict cell and tissue growth, ex clones exhibit delayed cell cycle exit in the developing eye, while mer clones do not. Conversely, loss of mer substantially compromises normal developmental apoptosis in the pupal retina, while loss of ex has only mild effects. Finally, ex has a role in regulating Wingless protein levels in the eye that is not obviously shared by either mer or hpo. Taken together, our data suggest that Mer and Ex differentially regulate multiple downstream pathways.

Ancient mechanisms of visual sense organ development based on comparison of the gene networks controlling larval eye, ocellus, and compound eye specification in Drosophila

Key mechanisms of development are strongly constrained, and hence often shared in the formation of highly diversified homologous organs. This diagnostic is applied to uncovering ancient gene activities in the control of visual sense organ development by comparing the gene networks, which regulate larval eye, ocellus and compound eye specification in Drosophila. The comparison reveals a suite of shared aspects that are likely to predate the diversification of arthropod visual sense organs and, consistent with this, have notable similarities in the developing vertebrate visual system: (I) Pax-6 genes participate in the patterning of primordia of complex visual organs. (II) Primordium determination and differentiation depends on formation of a transcription factor complex that contains the products of the selector genes Eyes absent and Sine oculis. (III) The TGF-beta signaling factor Decapentaplegic exerts transcriptional activation of eyes absent and sine oculis. (IV) Canonical Wnt signaling contributes to primordium patterning by repression of eyes absent and sine oculis. (V) Initiation of determination and differentiation is controlled by hedgehog signaling. (VI) Egfr signaling drives retinal cell fate specification. (VII) The proneural transcription factor atonal regulates photoreceptor specification. (VII) The zinc finger gene glass regulates photoreceptor specification and differentiation.

Continuity versus split and reconstitution: exploring the molecular developmental corollaries of insect eye primordium evolution

Holometabolous insects like Drosophila proceed through two phases of visual system development. The embryonic phase generates simple eyes of the larva. The postembryonic phase produces the adult specific compound eyes during late larval development and pupation. In primitive insects, by contrast, eye development persists seemingly continuously from embryogenesis through the end of postembryogenesis. Comparative literature suggests that the evolutionary transition from continuous to biphasic eye development occurred via transient developmental arrest. This review investigates how the developmental arrest model relates to the gene networks regulating larval and adult eye development in Drosophila, and embryonic compound eye development in primitive insects. Consistent with the developmental arrest model, the available data suggest that the determination of the anlage of the rudimentary Drosophila larval eye is homologous to the embryonic specification of the juvenile compound eye in directly developing insects while the Drosophila compound eye primordium is evolutionarily related to the yet little studied stem cell based postembryonic eye primordium of primitive insects.

The Drosophila ramshackle gene encodes a chromatin-associated protein required for cell morphology in the developing eye

We have identified ramshackle (ram) as a dominant suppressor of hedgehog loss-of-function in the developing Drosophila eye. We have characterized the gene and it encodes a double bromodomain protein with eight WD40 repeats. The Ram protein is localized predominantly to polytene chromosome interbands and is required for the transcription of some genes. ram is an essential gene and null mutants die during larval life. In the developing retina, ram mutant cells have morphological defects including disrupted apical junctions, disorganized actin cytoskeletons and mislocalized nuclei, which are followed by delays in cell-cycle transitions and the expression of differentiation markers. ram is a conserved gene: its vertebrate homolog (WDR9), which lies in Down's Syndrome Critical region 2 (DCR2) is also known to be associated with Brahma-Related-Gene 1 (BRG1).

Decapentaplegic head capsule mutations disrupt novel peripodial expression controlling the morphogenesis of the Drosophila ventral head

Drosophila adult structures derive from imaginal discs, which are sacs with apposed epithelial sheets, the disc proper (DP) and the peripodial epithelium (PE). The Drosophila TGF-beta family member decapentaplegic (dpp) contributes to the development of adult structures through expression in all imaginal discs, driven by enhancers from the 3' cis-regulatory region of the gene. In the eye/antennal disc, there is 3' directed dpp expression in both the DP and PE associated with cell proliferation and eye formation. Here, we analyze a new class of dpp cis-regulatory mutations, which specifically disrupt a previously unknown region of dpp expression, controlled by enhancers in the 5' regulatory region of the gene and limited to the PE of eye/antennal discs. These are the first described Drosophila mutations that act by solely disrupting PE gene expression. The mutants display defects in the ventral adult head and alter peripodial but not DP expression of known dpp targets. However, apoptosis is observed in the underlying DP, suggesting that this peripodial dpp signaling source supports cell survival in the DP.

rap gene encodes Fizzy-related protein (Fzr) and regulates cell proliferation and pattern formation in the developing Drosophila eye-antennal disc

The rap (retina aberrant in pattern) gene encodes the Fizzy-related protein (Fzr), which as an activator of the ubiquitin ligase complex; APC/C (anaphase promoting complex/cyclosome) facilitates the cell cycle stage-specific degradation of cyclins. Loss-of-function mutations in rap cause unscheduled accumulation of cyclin B in the developing eye imaginal disc, resulting in additional mitotic cycles and defective patterning of the developing Drosophila eye. Targeted mis-expression of rap/fzr in the eye primordial cells causes precocious cell cycle exit, and smaller primordial eye fields, which either eliminate or drastically reduce the size of the adult eye. Although mitosis is inhibited in the mis-expression animals, cells with abnormally large nuclei form tumor-like structures from continued endoreplication, cell growth and retinal differentiation. Interestingly, overexpression of Rap/Fzr in the eye primordia also increases the size of the antennal primordium resulting in the induction of ectopic antennae. These results suggest that Rap/Fzr plays an essential role in the timely exit of precursor cells from mitotic cycles and indicate that mechanisms that regulate cell cycle exit are critical during pattern formation and morphogenesis.

A 3' cis-regulatory region controls wingless expression in the Drosophila eye and leg primordia

The precise regulation of wingless (wg) expression in the Drosophila eye disc is key to control the anteroposterior and dorsoventral patterning of this disc. Here, we identify an eye disc-specific wg cis-regulatory element that functions as a regulatory rheostat. Pannier (Pnr), a transcription factor previously proposed to act as an upstream activator of wg, is sufficient to activate the eye disc enhancer but required for wg expression only in the peripodial epithelium of the disc. We propose that this regulation of wg by Pnr appeared associated to the development of the peripodial epithelium in higher dipterans and was added to an existing mechanism regulating the deployment of wingless in the dorsal region of the eye primordium. In addition, our analysis identifies a separate ventral disc enhancer that lies adjacent to the eye-specific one, and thus altogether, they define a 1-kb genomic region where disc-specific enhancers of the wg gene are located.

Smoothened, thickveins and the genetic control of cell cycle and cell fate in the developing Drosophila eye

The Hedgehog and Decapentaplegic pathways have several well-characterized functions in the developing Drosophila compound eye, including initiation and progression of the morphogenetic furrow. Other functions involve control of cell cycle and cell survival as well as cell type specification. Here we have used the mosaic clone analysis of null mutations of the smoothened and thickveins genes (which encode the receptors for these two signals) both alone and in combination, to study cell cycle and cell fate in the developing eye. We conclude that both pathways have several, but differing roles in furrow induction and cell fate and survival, but that neither directly affects cell type specification.

Control of planar cell polarity by interaction of DWnt4 and four-jointed

The Drosophila eye and the wing display specific planar cell polarity. Although Frizzled (Fz) signaling has been implicated in the establishment of ommatidial and wing hair polarity, evidence for the Wnt gene function has been limited. Here we examined the function of a Drosophila homolog of Wnt4 (DWnt4) in the control of planar polarity. We show that DWnt4 mRNA and protein are preferentially expressed in the ventral region of eye disc. DWnt4 mutant eyes show polarity reversals mostly in the ventral domain, consistent with the ventral expression of DWnt4. Ectopic expression of DWnt4 in the dorsoventral (DV) polar margins is insufficient to induce ommatidial polarity but becomes inductive when coexpressed with Four-jointed (Fj). Similarly, DWnt4 and Fj result in synergistic induction of hair polarity toward the source of expression in the wing. Consistent with genetic interaction, we provide evidence for direct interaction of DWnt4 and Fj transmembrane protein. The extracellular domain of Fj is required for direct binding to DWnt4 and for the induction of hair polarity. In contrast to the synergy between DWnt4 and Fj, DWnt4 antagonizes the polarizing effect of Fz. Our results suggest that DWnt4 is involved in ommatidial polarity signaling in the ventral region of the eye and its function is mediated by interacting with Fj.

Genetic interaction of Lobe with its modifiers in dorsoventral patterning and growth of the Drosophila eye

Dorsoventral (DV) patterning is essential for growth of the Drosophila eye. Recent studies suggest that ventral is the default state of the early eye, which depends on Lobe (L) function, and that the dorsal fate is established later by the expression of the dorsal selector gene pannier (pnr). However, the mechanisms of regulatory interactions between L and dorsal genes are not well understood. For studying the mechanisms of DV patterning in the early eye disc, we performed a dominant modifier screen to identify additional genes that interact with L. The criterion of the dominant interaction was either enhancement or suppression of the L ventral eye loss phenotype. We identified 48 modifiers that correspond to 16 genes, which include fringe (fng), a gene involved in ventral eye patterning, and members of both Hedgehog (Hh) and Decapentaplegic (Dpp) signaling pathways, which promote L function in the ventral eye. Interestingly, 29% of the modifiers (6 enhancers and 9 suppressors) identified either are known to interact genetically with pnr or are members of the Wingless (Wg) pathway, which acts downstream from pnr. The detailed analysis of genetic interactions revealed that pnr and L mutually antagonize each other during second instar of larval development to restrict their functional domains in the eye. This time window coincides with the emergence of pnr expression in the eye. Our results suggest that L function is regulated by multiple signaling pathways and that the mutual antagonism between L and dorsal genes is crucial for balanced eye growth.

Organ specification-growth control connection: new in-sights from the Drosophila eye-antennal disc

The eye-antennal disc of Drosophila is serving a guiding role in the studies of how eye identity is specified, as well as how the retina is patterned. However, this system also holds a great potential for studying the coordination between organ growth and specification when various distinct organs form from a common primordium. The eye-antennal disc gives origin not only to the compound eye but also to the head capsule, ocelli, maxillary palp, and antenna, and these organs develop bearing constant size proportions with each other. Here, we review recent results that have shed light on the mechanisms that control the specification and growth of organs of the eye-antennal disc and discuss how these controls are intertwined during the development of neighboring organs to ensure their constant shape and relative sizes.

Drosophila eye disc margin is a center for organizing long-range planar polarity

Planar polarity patterning involves long-range signaling and signal transduction. In Drosophila eye, Dishevelled (Dsh) is not only crucial for cell-autonomous transduction of a polarity signal(s) but is also involved in nonautonomous signaling function. To identify the sites for long-range polarity signaling in eye disc, we examined spatial and temporal conditions for nonautonomous Dsh function. Here we show that Dsh and its downstream factor Armadillo (Arm) are required in the border region of eye disc between the peripodial membrane (PM) and the disc proper (DP) for nonautonomous signaling. Conditional misexpression of Dsh or Arm at the posterior margin of the disc was sufficient to induce nonautonomous polarity reversals. A critical time window for the induction of such changes was approximately coincident with the timing of morphogenetic furrow initiation. Our data suggest that the disc margin is an essential site for organizing planar polarity during the initial stage of retinal morphogenesis.

CREB binding protein functions during successive stages of eye development in Drosophila

During the development of the compound eye of Drosophila several signaling pathways exert both positive and inhibitory influences upon an array of nuclear transcription factors to produce a near-perfect lattice of unit eyes or ommatidia. Individual cells within the eye are exposed to many extracellular signals, express multiple surface receptors, and make use of a large complement of cell-subtype-specific DNA-binding transcription factors. Despite this enormous complexity, each cell will make the correct developmental choice and adopt the appropriate cell fate. How this process is managed remains a poorly understood paradigm. Members of the CREB binding protein (CBP)/p300 family have been shown to influence development by (1) acting as bridging molecules between the basal transcriptional machinery and specific DNA-binding transcription factors, (2) physically interacting with terminal members of signaling cascades, (3) acting as transcriptional coactivators of downstream target genes, and (4) playing a key role in chromatin remodeling. In a screen for new genes involved in eye development we have identified the Drosophila homolog of CBP as a key player in both eye specification and cell fate determination. We have used a variety of approaches to define the role of CBP in eye development on a cell-by-cell basis.

Comparative analysis of Wingless patterning in the embryonic grasshopper eye

The signaling factor Wingless regulates multiple steps during the postembryonic development of the Drosophila eye. To obtain insight into the molecular regulation of embryonic eye development in primitive insects, we studied the expression of wg and genes projected to interact with wg in the grasshopper Schistocerca americana. We find that the dynamic and complex expression of wg in the early grasshopper procephalon results in three paired expression domains with relevance to eye primordium development. By comparison with Drosophila, these domains are compatible with a conserved function of wg during anteroposterior and dorsoventral axis formation by repression of retinal differentiation and stimulation of tissue proliferation. This is further supported by the expression of grasshopper orthologs of the retina determination genes sine oculis and eyes absent, and by inhibition of retina differentiation in grasshopper eye primordia cultured with LiCl. Surprisingly, the expression of wg and the grasshopper orthologs of pannier, fringe, Delta, and Iroquois complex is inconsistent with induction of midline centered Notch signaling activity, which is essential for Drosophila retina development. Similarly substantial evolutionary divergence is found concerning the control of retina versus head epidermis specification. The transcription factor Extradenticle (Exd), which cooperates with wg in specifying the Drosophila head epidermis, is not detected outside the labral and antennal primordia in the embryonic grasshopper head. Our results, which provide the first insight into the molecular control of eye primordium formation in primitive insects, suggest substantial modification of this process during the evolution of the Drosophila mode of postembryonic eye development.

Novel dominant-negative mutation within the six domain of the conserved eye specification gene sine oculis inhibits eye development in Drosophila

The development of the compound eye of Drosophila is controlled, in part, by the concerted actions of several nuclear proteins that form an intricate regulatory system. One member of this network is sine oculis (so), the founding member of the Six gene family. Mutations within so affect the entire visual system, including the compound eye. The vertebrate homologs Six3 and Six6 also appear to play crucial roles in retinal formation. Mutations in Six3 inhibit retinal formation in chickens and fish, whereas those in Six6 are the underlying cause of bilateral anophthalmia in humans. Together, these phenotypes suggest a conserved role for the Six genes in eye development. In this report, we describe the effects of a dominant-negative mutation of sine oculis on the development of the compound eye of Drosophila. The mutation resides within the Six domain and may have implications for eye development and disease.

A mosaic genetic screen reveals distinct roles for trithorax and polycomb group genes in Drosophila eye development

The wave of differentiation that traverses the Drosophila eye disc requires rapid transitions in gene expression that are controlled by a number of signaling molecules also required in other developmental processes. We have used a mosaic genetic screen to systematically identify autosomal genes required for the normal pattern of photoreceptor differentiation, independent of their requirements for viability. In addition to genes known to be important for eye development and to known and novel components of the Hedgehog, Decapentaplegic, Wingless, Epidermal growth factor receptor, and Notch signaling pathways, we identified several members of the Polycomb and trithorax classes of genes encoding general transcriptional regulators. Mutations in these genes disrupt the transitions between zones along the anterior-posterior axis of the eye disc that express different combinations of transcription factors. Different trithorax group genes have very different mutant phenotypes, indicating that target genes differ in their requirements for chromatin remodeling, histone modification, and coactivation factors.

Coming to our senses

Sensory organs are specialized to receive different kinds of input from the outside world. However, common features of their development suggest that they could have a shared evolutionary origin. In a recent paper, Niwa et al. show that three Drosophila adult sensory organs all rely on the spatial signals Decapentaplegic and Wingless to specify their position, and the temporal signal ecdysone to initiate their development. The proneural gene atonal is an important site for integration of these regulatory inputs. These results suggest the existence of a primitive sensory organ precursor, which would differentiate according to the identity of its segment of origin. The authors argue that the eyeless gene controls eye disc identity, indirectly producing an eye from the sensory organ precursor within this disc.

The Tribolium homologue of glass and the evolution of insect larval eyes

While non-arthropod orthologues have been found for many Drosophila eye developmental genes, this has not been the case for the glass (gl) gene, which encodes a zinc finger transcription factor required for photoreceptor cell specification, differentiation, and survival. This study reports sequence and expression analysis of the gl orthologue of the flour beetle Tribolium castaneum. A strongly conserved C-terminal zinc finger binding region and a moderately conserved N-terminal transcriptional activation domain characterize the putative Tribolium gl protein. Tribolium gl transcripts were detected in the developing photoreceptors of the larval and adult visual system, the corpora cardiaca, and subsets of cells in the developing brain. This suggests that the gl function of specifying predominantly neuronal cells is strongly conserved. Using gl as a marker for the onset of larval photoreceptor differentiation, we studied the embryonic development of the Tribolium visual system. We find that the Tribolium larval eyes originate at the posterior margin of the embryonic eye lobes as defined by eye-field-specific wingless expression domains. This is consistent with the hypothesis that the larval visual organs (stemmata) of holometabolous insects were derived from and are therefore homologous to the posterior-most ommatidia of the adult retina in primitive nonholometabolous insects.

Growth and specification: flyPax6 homologseyegone andeyeless have distinct functions

Development requires not only the correct specification of organs and cell types in the right places (pattern), but also the control of their size and shape (growth). Many signaling pathways control both pattern and growth and how these two are distinguished has been something of a mystery. In the fly eye, a Pax6 homolog (eyeless) controls eye specification together with several other genes. Now Dominguez et al.1 show that Notch signaling controls eye growth through a second Pax6 protein (Eyegone). In mice and humans the single Pax6 gene appears to encode both specification and growth controlling proteins through alternative mRNA splicing.

Patterning the peripheral retina of the fly: decoding a gradient

The peripheral regions of the fly eye show a number of specializations. First, immediately interior to the circumscribing head capsule and completely encircling the rest of the eye lies a thick band of pigment cells (pigment rim; PR). Second, in the dorsal periphery of the eye directly interior to the PR lie the dorsal rim (DR) ommatidia that are specialized polarized light detectors. The equivalent position in the ventral eye is occupied by standard ommatidia. Third, ommatidia characteristically project mechanosensory hairs above their lenses, but in the most peripheral rows (including the DR) the ommatidia are bald. Wingless secreted from the head capsule appears to organize all these peripheral specializations. Higher Wg levels induce PR, intermediate levels induce DR, and lower levels induce baldness. The predisposition of dorsal cells to generate DR ommatidia appears to be endowed by the exclusive dorsal expression of Iroquois genes.

Hedgehog Signaling in the Drosophila Eye and Head: An Analysis of the Effects of Differentpatched Trans-heterozygotes

Characterization of different alleles of the Hedgehog receptor patched (ptc) indicates that they can be grouped into several classes. Most mutations result in complete loss of Ptc function. However, missense mutations located within the putative sterol-sensing domain (SSD) or C terminus of ptc encode antimorphic proteins that are unable to repress Smo activity and inhibit wild-type Ptc from doing so, but retain the ability to bind and sequester Hh. Analysis of the eye and head phenotypes of Drosophila melanogaster in various ptc/ptctuf1 heteroallelic combinations shows that these two classes of ptc allele can be easily distinguished by their eye phenotype, but not by their head phenotype. Adult eye size is inversely correlated with head vertex size, suggesting an alteration of cell fate within the eye-antennal disc. A balance between excess cell division and cell death in the mutant eye discs may also contribute to final eye size. In addition, contrary to results reported recently, the role of Hh signaling in the Drosophila head vertex appears to be primarily in patterning rather than in proliferation, with Ptc and Smo having opposing effects on formation of medial structures.

Coordinating proliferation and tissue specification to promote regional identity in the Drosophila head

The Decapentaplegic and Notch signaling pathways are thought to direct regional specification in the Drosophila eye-antennal epithelium by controlling the expression of selector genes for the eye (Eyeless/Pax6, Eyes absent) and/or antenna (Distal-less). Here, we investigate the function of these signaling pathways in this process. We find that organ primordia formation is indeed controlled at the level of Decapentaplegic expression but critical steps in regional specification occur earlier than previously proposed. Contrary to previous findings, Notch does not specify eye field identity by promoting Eyeless expression but it influences eye primordium formation through its control of proliferation. Our analysis of Notch function reveals an important connection between proliferation, field size, and regional specification. We propose that field size modulates the interaction between the Decapentaplegic and Wingless pathways, thereby linking proliferation and patterning in eye primordium development.

The hernandez and fernandez genes of Drosophila specify eye and antenna

The formation of different structures in Drosophila depends on the combined activities of selector genes and signaling pathways. For instance, the antenna requires the selector gene homothorax, which distinguishes between the leg and the antenna and can specify distal antenna if expressed ectopically. Similarly, the eye is formed by a group of "eye-specifying" genes, among them eyeless, which can direct eye development ectopically. We report here the characterization of the hernandez and fernandez genes, expressed in the antennal and eye primordia of the eye-antenna imaginal disc. The predicted proteins encoded by these two genes have 27% common amino acids and include a Pipsqueak domain. Reduced expression of either hernandez or fernandez mildly affects antenna and eye development, while the inactivation of both genes partially transforms distal antenna into leg. Ectopic expression of either of the two genes results in two different phenotypes: it can form distal antenna, activating genes like homothorax, spineless, and spalt, and it can promote eye development and activates eyeless. Reciprocally, eyeless can induce hernandez and fernandez expression, and homothorax and spineless can activate both hernandez and fernandez when ectopically expressed. The formation of eye by these genes seems to require Notch signaling, since the induction of ectopic eyes and the activation of eyeless by the hernandez gene are suppressed when the Notch function is compromised. Our results show that the hernandez and fernandez genes are required for antennal and eye development and are also able to specify eye or antenna ectopically.

Eyeless collaborates with Hedgehog and Decapentaplegic signaling in Drosophila eye induction

eyeless (ey) is a key regulator of the eye development pathway in Drosophila. Ectopic expression of ey can induce the expression of several eye-specification genes (eya, so, and dac) and induce eye formation in multiple locations on the body. However, ey does not induce eye formation everywhere where it is ectopically expressed, suggesting that EY needs to collaborate with additional factors for eye induction. We examined ectopic eye induction by EY in the wing disc and found that eye induction was spatially restricted to the posterior compartment and the anterior-posterior (A/P) compartmental border, suggesting a requirement for both HH and DPP signaling. Although EY in the anterior compartment induced dpp and dac, these were not sufficient for eye induction. Coexpression experiments show that EY needs to collaborate with high level of HH and DPP to induce ectopic eye formation. Ectopic eye formation also requires the activation of an eye-specific enhancer of the endogenous hh gene.

Nonautonomous planar polarity patterning in Drosophila: dishevelled-independent functions of frizzled

The frizzled (fz) gene of Drosophila is required for planar polarity establishment in the adult cuticle, acting both cell autonomously and nonautonomously. We demonstrate that these two activities of fz in planar polarity are temporally separable in both the eye and wing. The nonautonomous function is dishevelled (dsh) independent, and its loss results in polarity phenotypes that resemble those seen for mutations in dachsous (ds). Genetic interactions and epistasis analysis suggest that fz, ds, and fat (ft) act together in the long-range propagation of polarity signals in the eye and wing. We also find evidence that polarity information may be propagated by modulation of the binding affinities of the cadherins encoded by the ds and ft loci.

Potassium-dependent sodium-calcium exchange through the eye of the fly

In this review, we describe the characterization of a Drosophila sodium/calcium-potassium exchanger, Nckx30C. Sodium/calcium (-potassium) exchangers (NCX and NCKX) are required for the rapid removal of calcium in excitable cells. The deduced protein topology for NCKX30C is similar to that of mammalian NCKX, with 5 hydrophobic domains in the amino terminus separated from 6 at the carboxy-terminal end by a large intracellular loop. NCKX30C functions as a potassium-dependent sodium-calcium exchanger and is expressed in adult neurons and during ventral nerve cord development in the embryo. Nckx30C is expressed in a dorsal/ventral pattern in the eye-antennal disc, suggesting that large fluxes of calcium may be occurring during imaginal disc development in the larvae. NCKX30C may play a critical role in modulating calcium during development as well as in the removal of calcium and maintenance of calcium homeostasis in adults.

Combinatorial control of Drosophila eye development by eyeless, homothorax, and teashirt

In Drosophila, the development of the compound eye depends on the movement of a morphogenetic furrow (MF) from the posterior (P) to the anterior (A) of the eye imaginal disc. We define several subdomains along the A-P axis of the eye disc that express distinct combinations of transcription factors. One subdomain, anterior to the MF, expresses two homeobox genes, eyeless (ey) and homothorax (hth), and the zinc-finger gene teashirt (tsh). We provide evidence that this combination of transcription factors may function as a complex and that it plays at least two roles in eye development: it blocks the expression of later-acting transcription factors in the eye development cascade, and it promotes cell proliferation. A key step in the transition from an immature proliferative state to a committed state in eye development is the repression of hth by the BMP-4 homolog Decapentaplegic (Dpp).

The transcriptional repressor Brinker antagonizes Wingless signaling

In the embryonic midgut of Drosophila, Wingless (Wg) signaling elicits threshold-specific transcriptional response, that is, low-signaling levels activate target genes, whereas high-signaling levels repress them. Wg-mediated repression of the HOX gene Ultrabithorax (Ubx) is conferred by a response sequence within the Ubx B midgut enhancer, called WRS-R. It further depends on the Teashirt (Tsh) repressor, which acts through the WRS-R without binding to it. Here, we show that Wg-mediated repression of Ubx B depends on Brinker, which binds to the WRS-R. Furthermore, Brinker blocks transcriptional activation by ubiquitous Wg signaling. Brinker binds to Tsh in vitro, recruits Tsh to the WRS-R, and we find mutual physical interactions between Brinker, Tsh, and the corepressor dCtBP. This suggests that the three proteins may form a ternary repressor complex at the WRS-R to quench the activity of the nearby-bound dTCF/Armadillo transcription complex. Finally, brinker and tsh produce similar mutant phenotypes in the ventral epidermis, and double mutants mimic overactive Wg signaling in this tissue. This suggests that Brinker may have a widespread function in antagonizing Wg signaling.

Planar polarity: photoreceptors on a high fat diet

Differential activity of Frizzled in the R3/R4 photo-receptors of Drosophila regulates the orientation of ommatidia. New evidence suggests that the cadherins Dachsous and Fat act upstream of Frizzled in this process.

Human wild-type tau interacts with wingless pathway components and produces neurofibrillary pathology in Drosophila

Pathologic alterations in the microtubule-associated protein tau have been implicated in a number of neurodegenerative disorders, including Alzheimer's disease (AD), progressive supranuclear palsy (PSP), and frontotemporal dementia (FTD). Here, we show that tau overexpression, in combination with phosphorylation by the Drosophila glycogen synthase kinase-3 (GSK-3) homolog and wingless pathway component (Shaggy), exacerbated neurodegeneration induced by tau overexpression alone, leading to neurofibrillary pathology in the fly. Furthermore, manipulation of other wingless signaling molecules downstream from shaggy demonstrated that components of the Wnt signaling pathway modulate neurodegeneration induced by tau pathology in vivo but suggested that tau phosphorylation by GSK-3beta differs from canonical Wnt effects on beta-catenin stability and TCF activity. The genetic system we have established provides a powerful reagent for identification of novel modifiers of tau-induced neurodegeneration that may serve as future therapeutic targets.

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.

Analyzing the repressive function of ultraspiracle, the Drosophila RXR, in Drosophila eye development

Response to the insect hormone ecdysone is mediated by a nuclear receptor complex containing Ultraspiracle (USP) and the Ecdysone Receptor (EcR). Among other phenotypes, loss of functional USP in Drosophila eye development results in an accelerated morphogenetic furrow, although loss of ecdysone arrests the furrow. We have shown that USP both represses and activates a gene affecting furrow movement, the ecdysone-responsive Z1 isoform of Broad-Complex, and we report additional usp mutant phenotypes. Using targeted replacement of USP to rescue usp mutant clones in the eye, we have mapped various USP functions and tested whether the USP nuclear receptor has an activating as well as a repressive effect on furrow movement. Furrow movement and related phenotypes are rescued by the presence of USP in a limited domain near the furrow while other phenotypes are rescued by USP expression posterior to the furrow. Our data indicate roles for USP activity at multiple developmental stages and help explain why loss of functional USP leads to furrow advancement while loss of ecdysone stops furrow movement.

Dominant Drop mutants are gain-of-function alleles of the muscle segment homeobox gene (msh) whose overexpression leads to the arrest of eye development

Dominant Drop (Dr) mutations are nearly eyeless and have additional recessive phenotypes including lethality and patterning defects in eye and sensory bristles due to cis-regulatory lesions in the cell cycle regulator string (stg). Genetic analysis demonstrates that the dominant small eye phenotype is the result of separate gain-of-function mutations in the closely linked muscle segment homeobox (msh) gene, encoding a homeodomain transcription factor required for patterning of muscle and nervous system. Reversion of the Dr(Mio) allele was coincident with the generation of lethal loss-of-function mutations in msh in cis, suggesting that the dominant eye phenotype is the result of ectopic expression. Molecular genetic analysis revealed that two dominant Dr alleles contain lesions upstream of the msh transcription start site. In the Dr(Mio) mutant, a 3S18 retrotransposon insertion is the target of second-site mutations (P-element insertions or deletions) which suppress the dominant eye phenotype following reversion. The pattern of 3S18 expression and the absence of msh in eye imaginal discs suggest that transcriptional activation of the msh promoter accounts for ectopic expression. Dr dominant mutations arrest eye development by blocking the progression of the morphogenetic furrow leading to photoreceptor cell loss via apoptosis. Gal4-mediated ubiquitous expression of msh in third-instar larvae was sufficient to arrest the morphogenetic furrow in the eye imaginal disc and resulted in lethality prior to eclosion. Dominant mutations in the human msx2 gene, one of the vertebrate homologs of msh, are associated with craniosynostosis, a disease affecting cranial development. The Dr mutations are the first example of gain-of-function mutations in the msh/msx gene family identified in a genetically tractible model organism and may serve as a useful tool to identify additional genes that regulate this class of homeodomain proteins.

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

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

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.

A screen for dominant modifiers of ro(Dom), a mutation that disrupts morphogenetic furrow progression in Drosophila, identifies groucho and hairless as regulators of atonal expression

ro(Dom) is a dominant allele of rough (ro) that results in reduced eye size due to premature arrest in morphogenetic furrow (MF) progression. We found that the ro(Dom) stop-furrow phenotype was sensitive to the dosage of genes known to affect retinal differentiation, in particular members of the hedgehog (hh) signaling cascade. We demonstrate that ro(Dom) interferes with Hh's ability to induce the retina-specific proneural gene atonal (ato) in the MF and that normal eye size can be restored by providing excess Ato protein. We used ro(Dom) as a sensitive genetic background in which to identify mutations that affect hh signal transduction or regulation of ato expression. In addition to mutations in several unknown loci, we recovered multiple alleles of groucho (gro) and Hairless (H). Analysis of their phenotypes in somatic clones suggests that both normally act to restrict neuronal cell fate in the retina, although they control different aspects of ato's complex expression pattern.

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.

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.