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.     

The EGF receptor and notch signaling pathways control the initiation of the morphogenetic furrow during Drosophila eye development

The onset of pattern formation in the developing Drosophila retina begins with the initiation of the morphogenetic furrow, the leading edge of a wave of retinal development that transforms a uniform epithelium, the eye imaginal disc into a near crystalline array of ommatidial elements. The initiation of this wave of morphogenesis is under the control of the secreted morphogens Hedgehog (Hh), Decapentaplegic (Dpp) and Wingless (Wg). We show that the Epidermal Growth Factor Receptor and Notch signaling cascades are crucial components that are also required to initiate retinal development. We also show that the initiation of the morphogenetic furrow is the sum of two genetically separable processes: (1) the 'birth' of pattern formation at the posterior margin of the eye imaginal disc; and (2) the subsequent 'reincarnation' of retinal development across the epithelium.     

Modulation of Drosophila retinal epithelial integrity by the adhesion proteins capricious and tartan

Background

The development of the Drosophila eye imaginal disc requires complex epithelial rearrangements. Cells of the morphogenetic furrow are apically constricted and this leads to a physical indentation in the epithelium. Posterior to the furrow, cells start to rearrange into distinct clusters and eventually form a precisely patterned array of ommatidia. These morphogenetic processes include regulated changes of adhesion between cells.

Methodology/Principal Findings

Here, we show that two transmembrane adhesion proteins, Capricious and Tartan, have dynamic and complementary expression patterns in the eye imaginal disc. We also describe novel null mutations in capricious and double null mutations in capricious and tartan. We report that they have redundant functions in regulating the architecture of the morphogenetic furrow and ommatidial spacing.

Conclusions/Significance

We conclude that Capricious and Tartan contribute to the adhesive properties of the cells in the morphogenetic furrow and that this regulated adhesion participates in the control of spacing ommatidial clusters.     

Myosin II regulates complex cellular arrangement and epithelial architecture in Drosophila

Remodeling epithelia is a primary driver of morphogenesis. Here, we report a central role of myosin II in regulating several aspects of complex epithelial architecture in the Drosophila eye imaginal disc. The epithelial indentation of the morphogenetic furrow is established from a pattern of myosin II activation defined by the developmental signals Hedgehog and Decapentaplegic. More generally, patterned myosin activation can control diverse three-dimensional epithelial sculpting. We have developed a technique to image eye disc development in real time, and we show that myosin II also regulates higher-order organization of cells in the plane of the epithelium. This includes the clustering of cells into ommatidial units and their subsequent coordinated rotation. This later clustering function of myosin II depends on EGF receptor signaling. Our work implies that regulation of the actomyosin cytoskeleton can control morphogenesis by regulating both individual cell shapes and their complex two-dimensional arrangement within epithelia.     

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.     

Localization of DER and the pattern of cell divisions in wild-type and Ellipse eye imaginal discs.

The compound eye of Drosophila develops from a uniform layer of epithelial cells in the eye imaginal disc. One intriguing aspect of eye development is the establishment of the correct number and spacing of the photoreceptor clusters which give rise to the mature ommatidia. Ellipse (Elp) has been implicated as playing a role in this process because the Elp dominant gain of function mutation dramatically reduces the number of photoreceptor clusters in the compound eye without affecting the morphology of individual clusters that are formed (Baker and Rubin, 1989). Since Elp represents an allele of the Drosophila EGF receptor (DER) locus, it encodes a protein which is structurally capable of mediating inductive cell-cell interactions. In an effort to better understand the role of the DER locus in ommatidial patterning, we compared the localization of DER protein in eye imaginal discs of wild-type and Elp larvae. The distribution of this receptor is consistent with the notion of its mediating interactions between cells at the initial stages of photoreceptor precluster positioning and differentiation. However, the basis of the Elp gain of function mutation is not ectopic or increased expression of the DER protein. Rather, expression of the Elp form of the EGF receptor homolog in the normal localization leads to changes in the proliferative pattern of cells dividing posterior to the morphogenetic furrow.     

Cell cycle arrest by a gradient of Dpp signaling during Drosophila eye development

Background  

The secreted morphogen Dpp plays important roles in spatial regulation of gene expression and cell cycle progression in the developing Drosophila eye. Dpp signaling is required for timely cell cycle arrest ahead of the morphogenetic furrow as a prelude to differentiation, and is also important for eye disc growth. The dpp gene is expressed at multiple locations in the eye imaginal disc, including the morphogenetic furrow that sweeps across the eye disc as differentiation initiates.     

Regulation of Drosophila neural development by a putative secreted protein

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

Cell surface binding sites for peanut agglutinin in the differentiating eye disc of Drosophila

The distribution of peanut agglutinin (PNA) binding sites in imaginal discs is described using fluorescence and electron microscopy. PNA binds preferentially to the photoreceptor cell precursors in eye discs resulting in a rectilinear array of fluorescent spots that reflects that lattice-like arrangement of the presumptive ommatidia. The lectin binds to the apical surface of fixed disc cells and is taken up in presumed endocytotic vesicles in living discs. Photoreceptor precursors can be visualized with fluorescein isothyocyanate-PNA from the time they first form preclusters in the morphogenetic furrow and this technique is used to demonstrate a temperature-sensitive defect in precluster formation in the mutant shibire. PNA is localized along the sides of microvilli of disc cells, in general. The preferential binding of PNA to photoreceptor precursors is related in part to the high density of apical microvilli on these cells.     

Broad-complex, but not ecdysone receptor, is required for progression of the morphogenetic furrow in the Drosophila eye

The progression of the morphogenetic furrow in the developing Drosophila eye is an early metamorphic, ecdysteroid-dependent event. Although Ecdysone receptor-encoded nuclear receptor isoforms are the only known ecdysteroid receptors, we show that the Ecdysone receptor gene is not required for furrow function. DHR78, which encodes another candidate ecdysteroid receptor, is also not required. In contrast, zinc finger-containing isoforms encoded by the early ecdysone response gene Broad-complex regulate furrow progression and photoreceptor specification. br-encoded Broad-complex subfunctions are required for furrow progression and proper R8 specification, and are antagonized by other subfunctions of Broad-complex. There is a switch from Broad complex Z2 to Z1 zinc-finger isoform expression at the furrow which requires Z2 expression and responds to Hedgehog signals. These results suggest that a novel hormone transduction hierarchy involving an uncharacterized receptor operates in the eye disc.     

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.