The Drosophila patched gene encodes a putative membrane protein required for segmental patterning

Ommatidial polarity in the Drosophila eye is determined by the direction of furrow progression and local interactions

The adult eye of Drosophila is a highly ordered structure. It is composed of about 800 ommatidia, each displaying precise polarity. The ommatidia are arranged about an axis of mirror image symmetry, the equator, which lies along the dorsoventral midline of the eye. We use hedgehog pathway mutants to induce ectopic morphogenetic furrows and use these as a tool to investigate the establishment of ommatidial polarity. Our results show that ommatidial clusters are self-organising units whose polarity in one axis is determined by the direction of furrow progression, and which can independently define the position of an equator without reference to the global coordinates of the eye disc.

wingless is required for the formation of a subset of muscle founder cells during Drosophila embryogenesis

The final pattern of the Drosophila larval body wall muscles depends critically on the prior segregation of muscle founder cells. We would like to understand the underlying molecular mechanisms which ensure the precise allocation and placement of these muscle founder cells. We have begun our analysis by examining the role of the segment polarity genes, known to be involved in the patterning of the ectoderm. Mutations in only one member of this class, wingless (wg), lead to the complete loss of a subset of muscle founder cells characterised by the expression of S59. Using the GAL4-targetted expression system, we find that Wingless, a secreted glycoprotein and well characterized signalling molecule, acts directly on the mesoderm to ensure the formation of S59-expressing founder cells. Moreover, we present evidence that Wg can signal across germ layers and that, in the wild-type embryo, Wg from the ectoderm could constitute an inductive signal for the initiation of the development of a subset of somatic muscles.

Selection and characterization of sequences with high affinity for the engrailed protein of Drosophila

The engrailed gene helps to direct Drosophila melanogaster development by encoding a homeodomain-containing DNA binding protein. To identify genes whose transcription engrailed regulates, we developed a method to isolate genomic sequences to which engrailed protein binds with high affinity. Fragments of genomic DNA were fractionated on an engrailed protein affinity column, and fragments that were retained in the presence of 0.4-1.0 M KCl were isolated and cloned. The isolated fragments include regions of the engrailed and cubitus interruptus gene promoters, both of which are candidate targets of engrailed, and most fragments contain regions that engrailed protein protects from DNaseI digestion. Chromosomal deletions that remove some of the engrailed binding sites (located either at 64D, 96B or 99D) interact genetically with engrailed. Characterization of a transcript encoded in region 64D revealed its dependence on engrailed protein.

Signalling by hedgehog family proteins in Drosophila and vertebrate development

Members of the hedgehog gene family encode a novel class of secreted proteins and are expressed in embryonic cells known to possess important signalling activities in organisms as diverse as flies and chickens. Proteins of the hedgehog family act in these different developmental contexts as both permissive and instructive signals. How this signalling activity is transduced is (as yet) poorly understood, but recent studies point to the involvement of protein kinase A in both Drosophila and vertebrates.

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

In the developing Drosophila compound eye, a wave of pattern formation and cell-type determination sweeps across the presumptive eye epithelium. This ‘morphogenetic furrow’ coordinates the epithelial cells' division cycle, shape and gene expression to produce evenly spaced neural cell clusters that will eventually form the adult ommatidia. As these clusters develop, they rotate inwards to face the eye's equator and establish tissue polarity. We have found that wingless is strongly expressed in the dorsal margin of the presumptive eye field, ahead of the morphogenetic furrow. We have shown that inactivation of Wingless results in the induction of an ectopic furrow that proceeds ventrally from the dorsal margin. This ectopic furrow is normal in most respects, however the clusters formed by it fail to rotate, and we propose a two-vector model to account for normal rotation and tissue polarity in the retina. A second consequence of this inactivation of Wingless is that the dorsal head is largely deleted. We have also found that patched loss-of-function mosaic clones induce circular ectopic morphogenetic furrows (consistent with the observations of other workers with the hedgehog, and PKA genes). We use such patched induced furrows to test the two-vector model for cluster rotation and tissue polarity.

The Drosophila cubitus interruptus protein and its role in the wingless and hedgehog signal transduction pathways

The segment polarity gene cubitus interruptus (ci) is required to maintain expression of the wingless gene and to specify naked cuticle within each epidermal segment. Antibodies were generated against the ci protein and used to analyze its pattern of expression. By stage 11, post-transcriptional regulation of ci is observed. ci transcript levels are uniform across the anterior compartment, but protein levels are higher next to the compartment boundaries. The distribution of the ci protein is altered in fused, hedgehog and wingless mutants suggesting cell-cell signaling may regulate ci protein levels. The role of ci in cell-cell signaling and pattern formation was examined in double mutants of ci with patched and zeste-white3/shaggy.

Inserting the Ftz homeodomain into engrailed creates a dominant transcriptional repressor that specifically turns off Ftz target genes in vivo

The Engrailed homeodomain protein is an ‘active’ or dominant transcriptional repressor in cultured cells. In contrast, the Fushi Tarazu homeodomain protein is an activator, both in cultured cells and in Drosophila embryos, where it activates several known target genes, including its own gene. This auto-activation has been shown to depend on targeting to a fushi tarazu enhancer by the Fushi Tarazu homeodomain. We combined Fushi Tarazu targeting and Engrailed active repression in a chimeric regulator, EFE. When EFE is ubiquitously expressed, it overrides endogenous Fushi Tarazu and causes a fushi tarazu mutant phenotype. Normal Fushi Tarazu target genes are affected as they are in fushi tarazu mutants. One such target gene is repressed by EFE even where Fushi Tarazu is not expressed, suggesting that the repression is active. This is confirmed by showing that the in vivo activity of EFE depends on a domain that is required for active repression in culture. A derivative that lacks this domain, while it cannot repress the endogenous fushi tarazu gene, can still reduce the activity of the fushi tarazu autoregulatory enhancer, suggesting that it competes with endogenous Fushi Tarazu for binding sites in vivo. However, this passive repression is much less effective than active repression.

Analysis of cubitus interruptus regulation in Drosophila embryos and imaginal disks

The cubitus interruptus (ci) gene of Drosophila is expressed in all anterior compartment cells in both embryos and imaginal disks where it encodes a putative zinc-finger protein related to the vertebrate Gli and C. elegans Tra-1 proteins. Using ci/lacZ fusions, we located regulatory sequences responsible for the normal pattern of ci expression, and obtained evidence that separate elements regulate its expression in embryos and imaginal disks. Mutants that delete a portion of this regulatory region express ci ectopically in the posterior compartments of their wing imaginal disks and have wings with malformed posterior compartments. Similar deletions of ci/lacZ fusion constructs also result in ectopic posterior compartment expression. Evidence that the engrailed protein normally represses ci in posterior compartments includes the expansion of ci expression into posterior compartment cells that lack engrailed function, diminution of ci expression upon overexpression of engrailed protein in anterior compartment cells, and the ability of engrailed protein to bind to the ci regulatory region in vivo and in vitro. We suggest that engrailed protein directly represses ci expression in posterior compartment cells.

Differential requirements for segment polarity genes in wingless signaling

The segment polarity genes wingless and engrailed are required throughout development of Drosophila. During early embryogenesis, these two genes are expressed in adjacent domains, in an inter-dependent way. Later, their expression is regulated by different mechanisms and becomes maintained by auto-regulation. To dissect the genetic requirements for the initial signaling between wingless and engrailed expressing cells, we have previously used a transgenic Drosophila strain that expresses wingless under the control of the heat shock promoter (HS-wg). Focusing on the later phases of wingless and engrailed regulation, we have now extended these studies, using embryos carrying various combinations of segment polarity mutations and the HS-wg transgene. We confirm some of the existing models of regulation of the expression of wingless and engrailed. In addition, we find that HS-wg embryos require engrailed for induction of ectopic endogenous wingless expression. Signaling from engrailed cells to this novel wingless expression domain is dependent on hedgehog but also on porcupine. We further demonstrate a novel requirement for hedgehog in maintenance of expression of engrailed itself.

Control of epithelial morphogenesis by cell signaling and integrin molecules in the Drosophila foregut

Coordinated cell movements are critical for tissue and organ morphogenesis in animal development. We show that the Drosophila genes hedgehog and wingless, which encode signaling molecules, and the gene myospheroid, which encodes a beta subunit of the integrins, are required for epithelial morphogenesis during proventriculus development. In contrast, this morphogenetic process is suppressed by the decapentaplegic gene, which encodes a member of the TGF beta family of growth factors. These results identify a novel cell signaling center in the foregut that directs the formation of a multiply folded organ from a simple epithelial tube.

Cell adhesion molecules and intracellular signalling: from fly to man

Analysis of the wingless pathway in Drosophila and the Wnt pathway in vertebrates has led to a detailed knowledge of the signalling events involved in this morphogenetic system. Molecules involved in cell-cell adhesion and junction formation are implicated at several steps in the pathway, highlighting the high level of interaction between cell adhesive systems and classical signalling pathways.

Protein kinase A and hedgehog signaling in Drosophila limb development

The Drosophila hedgehog (hh) gene encodes a secreted protein involved in organizing growth and patterning in many developmental processes. Hh appears to act by inducing the localized expression of at least two other signaling molecules, decapentaplegic (dpp) and wingless (wg), which then govern cell proliferation and patterning in surrounding tissue. Here, we demonstrate that cyclic AMP (cAMP)-dependent protein kinase A (PKA) is essential during limb development to prevent inappropriate dpp and wg expression. We also show that a constitutively active form of PKA can prevent inappropriate dpp and wg expression, but does not interfere with their normal induction by hh. We propose that the basal activity of PKA imposes a block on the transcription of dpp and wg and that hh exerts its organizing influence by alleviating this block.

cAMP-dependent protein kinase and hedgehog act antagonistically in regulating decapentaplegic transcription in Drosophila imaginal discs

Localized expression of decapentaplegic (dpp) is required for proper development of the Drosophila imaginal discs. Using genetic mosaics, we show that in the anterior compartment of appendage discs and anterior to the morphogenetic furrow in the eye disc, cells that lack cAMP-dependent protein kinase (PKA) activity ectopically express dpp. Pka- cells can influence the fate of neighboring cells to reorganize anterior patterns in appendages and trigger ectopic morphogenetic furrows in the developing retina. This organizing activity of Pka mutant cells depends on dpp activity. Our findings suggest that PKA is a component of a signaling pathway that represses dpp expression and that hh antagonizes this pathway to maintain dpp expression at the anterior-posterior compartment border and in the morphogenetic furrow.

Function of protein kinase A in hedgehog signal transduction and Drosophila imaginal disc development

Reduced protein kinase A (PKA) activity in anterior imaginal disc cells leads to cell-autonomous induction of decapentaplegic (dpp), wingless (wg), and patched (ptc) transcription that is independent of hedgehog (hh) gene activity. The resulting nonautonomous adult wing and leg pattern duplications are largely due to induced dpp and wg expression and resemble phenotypes elicited by ectopic hh expression. Inhibition of PKA in anterior cells close to the posterior compartment can substitute for hh activity to promote growth of imaginal discs, whereas overexpression of PKA can counteract transcriptional induction of ptc by hh in these cells. PKA therefore appears to be an integral component of the mechanism by which hh regulates the expression of key patterning molecules in imaginal discs.

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

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

Signal transduction by cAMP-dependent protein kinase A in Drosophila limb patterning

Interaction between distinctly specified cells in adjacent compartments establishes organizing centres that control growth and specify cell fate in the developing limbs of Drosophila. Localized expression of the secreted Hedgehog protein (Hh) by cells in the posterior compartment induces expression of the secreted signalling molecules decapentaplegic (dpp) or wingless (wg) in nearby anterior cells. wg and dpp in turn organize spatial pattern in the wing and leg imaginal discs. The Hh signal is thought to act by antagonizing the ability of the patched (ptc) gene product to repress wg and dpp expression. Here we present evidence that removing activity of the gene encoding cyclic AMP-dependent protein kinase A (pka) is functionally equivalent to removing ptc activity or to providing cells with the Hh signal. These findings suggest that cyclic AMP-dependent protein kinase A is a component of the signal transduction pathway through which Hh and Ptc direct localized expression of dpp (or wg) and establish the compartment boundary organizer.

Mutations that alter the timing and pattern of cubitus interruptus gene expression in Drosophila melanogaster

The cubitus interruptus (ci) gene is a member of the Drosophila segment polarity gene family and encodes a protein with a zinc finger domain homologous to the vertebrate Gli genes and the nematode tra-1 gene. Three classes of existing mutations in the ci locus alter the regulation of ci expression and can be used to examine ci function during development. The first class of ci mutations causes interruptions in wing veins four and five due to inappropriate expression of the ci product in the posterior compartment of imaginal discs. The second class of mutations eliminates ci protein early in embryogenesis and causes the deletion of structures that are derived from the region including and adjacent to the engrailed expressing cells. The third class of mutations eliminates ci protein later in embryogenesis and blocks the formation of the ventral naked cuticle. The loss of ci expression at these two different stages in embryonic development correlates with the subsequent elimination of wingless expression. Adults heterozygous for the unique ciCe mutation have deletions between wing veins three and four. A similar wing defect is present in animals mutant for the segment polarity gene fused that encodes a putative serine/threonine kinase. In ciCe/+ and fused mutants, the deletions between wing veins three and four correlate with increased ci protein levels in the anterior compartment. Thus, proper regulation of both the ci mRNA and protein appears to be critical for normal Drosophila development.

Positional signaling by hedgehog in Drosophila imaginal disc development

We describe a dominant gain-of-function allele of the segment polarity gene hedgehog. This mutation causes ectopic expression of hedgehog mRNA in the anterior compartment of wing discs, leading to overgrowth of tissue in the anterior of the wing and partial duplication of distal wing structures. The posterior compartment of the wing is unaffected. Other imaginal derivatives are affected, resulting in duplications of legs and antennae and malformations of eyes. In mutant imaginal wing discs, expression of the decapentaplegic gene, which is implicated in the hedgehog signaling pathway, is also perturbed. The results suggest that hedgehog protein acts in the wing as a signal to instruct neighboring cells to adopt fates appropriate to the region of the wing just anterior to the compartmental boundary.

Distinct pathways for autocrine and paracrine Wingless signalling in Drosophila embryos

Two secreted proteins, Wingless and Hedgehog, instruct cell fates within the segmented epidermis of Drosophila embryos (reviewed in ref. 5). Wingless (Wg) is expressed by the most posterior cells in each parasegment; Hedgehog (Hh) is expressed in the most anterior cells of the next parasegment. Immediately after gastrulation, the two cell types are mutually dependent. Local Wg signalling stabilizes Hh expression and local Hh signalling stabilizes Wg expression. Direct Wg autoregulation (autocrine signalling) is masked by its paracrine role in maintaining hh, which in turn maintains wg. I have used zeste-white3 (zw3) and patched (ptc) mutant backgrounds to uncouple genetically this positive-feedback loop and to study autocrine Wg signalling. I report here that direct Wg autoregulation differs from Wg signalling to adjacent cells in the importance of fused (fu), smoothened (smo) and cubitus interruptus (ci) relative to zw3 and armadillo (arm). I also find that Wg autoregulation during this early hh-dependent phase differs from later Wg autoregulation by lack of gooseberry (gsb) participation.

Induction of a mirror-image duplication of anterior wing structures by localized hedgehog expression in the anterior compartment of Drosophila melanogaster wing imaginal discs

The segment polarity gene hedgehog (hh) encodes a secretory protein involved in cell-cell communication in Drosophila melanogaster. The hh gene is expressed in the posterior compartment and is essential for the establishment and maintenance of the anterior/posterior-compartment boundary of each embryonic parasegment [Ingham, P.W., Nature 366 (1993) 560-562]. To clarify possible hh functions in adult appendage formation, we isolated a fly line (h9D) associated with a wing malformation from among fly lines with an hh transgene whose expression is under the control of trapped enhancers. In h9D flies, the ectopic expression of hh occurred in the anterior edge of wing pouch in the wing disc. This abnormal hh expression resulted in not only a mirror-image duplication and ectopic outgrowth in the anterior wing compartment, but also the ectopic expression of patched and decapentaplegic, strongly suggesting that the hh product serves as a morphogen or an inducer essential for wing development, including the proximal/distal axis formation.

Localized expression of sloppy paired protein maintains the polarity of Drosophila parasegments

During germ-band extension in the Drosophila embryo, intercellular communication is required to maintain gene expression patterns initiated at cellular blastoderm. For example, the wingless (wg) single-cell-wide stripe in each parasegment (PS) is dependent on a signal from the adjacent, posterior cells, which express engrailed (eN). This signal is thought to be the hedgehog (hh) gene product, which antagonizes the activity of patched (ptc), a repressor of wg expression. Genetic evidence indicates that the hh signal is bidirectional, but wg transcription is only derepressed on the anterior side of the en/hh stripes. To explain the asymmetric response of the wg promoter to the hh signal, current models predict that each PS is divided into cells that are competent to express either wg or en, but not both. The sloppy paired (slp) locus contains two transcription units, both encoding proteins containing a forkhead domain, a DNA-binding motif. Removal of slp gene function causes embryos to exhibit a severe pair-rule/segment polarity phenotype. We show that the en stripes expand anteriorly in slp mutant embryos and that slp activity is an absolute requirement for maintenance of wg expression at the same time that wg transcription is dependent on hh. The slp proteins are expressed in broad stripes just anterior of the en-positive cells, overlapping the narrow wg stripes. We propose that by virtue of their ability to activate wg and repress en expression, the distribution of the slp proteins define the wg-competent and en-competent groups. Consistent with this hypothesis, ubiquitous expression of slp protein throughout the PS abolishes en expression and, in ptc mutant embryos, results in a near ubiquitous distribution of wg transcripts. In addition to demonstrating the role of slp in maintaining segment polarity, our results suggest that slp works in, or parallel with, the ptc/hh signal transduction pathway to regulate wg transcription.

Subcellular localization of the segment polarity protein patched suggests an interaction with the wingless reception complex in Drosophila embryos

The product of the segment polarity gene patched is a transmembrane protein involved in the cell communication processes that establish polarity within the embryonic segments of Drosophila. Monoclonal antibodies have been raised against the patched protein, and by immunoelectron microscopy part of the patched staining is found associated with discrete regions of the lateral plasma membrane of the embryonic epidermal cells. Using a mutation affecting endocytosis (shibire) we find that patched is a membrane-bound protein, which is internalized by endocytosis, and that the preferential sites of accumulation resemble the described localization of the cell-cell adhesive junctions of the epidermal cells. patched partially co-localizes with the wingless protein in the wingless-expressing and nearby cells, in structures that seem to be endocytic vesicles. These data suggest the interaction of patched protein with elements of the reception complex of wingless, as a way to control the wingless expression.

Genetic factors controlling the expression of the abdominal-A gene of Drosophila within its domain

The homeotic gene abdominal-A (abd-A) is normally expressed in parasegments 7 to 13. We find that the initial distribution of the product is approximately uniform within this domain, but the subsequent elaboration of the expression pattern results in differences between, as well as within, parasegments. We have investigated the possible role of several pair-rule, e.g. fushi tarazu, even-skipped, runt, hairy, paired, and segment polarity e.g. engrailed, wingless, naked, patched and cubitus interruptus genes on the patterning of abd-A expression. We find that the establishment of the original abd-A expression domain is independent of any of these genes, but most of them are required for the subsequent elaboration of abd-A expression within the domain. The genes fushi tarazu, and especially engrailed, appear to act as transcriptional activating factors of abd-A.

Chromophore-assisted laser inactivation of patched protein switches cell fate in the larval visual system of Drosophila

The Drosophila segment-polarity gene patched (ptc) is an integral component of the segmentation gene cascade acting in the early embryo. At later stages of embryogenesis, ptc is expressed in the primordia of epithelial placodes of a specific portion of the brain, the optic lobes. Mutant analysis shows that the lack of ptc activity alters the fate of optic-lobe primordia precursors. In ptc mutants they give rise to supernumerary neurons in the larval light-sensory system, termed Bolwig organ, which is derived from precursor cells next to the optic-lobe anlagen. We specifically eliminated ptc protein by chromophore-assisted laser inactivation (CALI) in late wild-type embryos. Such embryos show a normal segment pattern, but they develop phenocopies equivalent to the phenotype of ptc mutant Bolwig organs. Our results demonstrate that the CALI technique can be applied to separate genetic functions at different developmental stages of a living organism and that the segment-polarity gene ptc is redeployed to functionally discriminate between distinct developmental pathways in adjacent pools of precursor cells.

The Serrate locus of Drosophila and its role in morphogenesis of the wing imaginal discs: control of cell proliferation

The Drosophila gene Serrate encodes a transmembrane protein with 14 EGF-like repeats in its extracellular domain. Here we show that loss-of-function mutations in this gene lead to larval lethality. Homozygous mutant larvae fail to differentiate the anterior spiracles, exhibit poorly developed mouth-hooks and show a severe reduction in the size of the wing and haltere primordia, which is not due to cell death. The few homozygous mutant escapers that pupariate develop into pharate adults that almost completely lack wings and halteres. Clonal analysis in the adult epidermis demonstrates a requirement for Serrate during wing and haltere development. Targeted ectopic expression of Serrate in the imaginal discs using the yeast transcriptional activator Gal4 results in regionally restricted induction of cell proliferation, e.g. the ventral tissues in the case of the wings and halteres. The results suggest that the wild-type function of Serrate is required for the control of position-specific cell proliferation during development of meso- and metathoracic dorsal discs, which in turn exerts a direct effect on morphogenesis.

Drosophila hedgehog acts as a morphogen in cellular patterning

The patterning of cell types in embryogenesis is specified by signals emanating from specialized organizer regions. We demonstrate that engrailed-expressing cells in the Drosophila epidermis have organizer properties. These cells influence the pattern of cell type differentiation across the segment. We show that this function is mediated by the hedgehog (hh) gene. The results of modulating the levels of hh in the embryo suggest that hh acts as a morphogen, specifying distinct cell fates by a concentration-dependent mechanism. We present a model that integrates the role of hh with that of the wingless signal in establishing the segmental array of cell type diversity.

Hedgehog is a signaling protein with a key role in patterning Drosophila imaginal discs

The segment polarity genes hedgehog and engrailed are expressed in identical posterior-compartment-specific patterns in both Drosophila embryos and imaginal discs. We show here that the hedgehog protein is secreted, and it can cross embryo parasegment borders and the anterior-posterior compartment border of imaginal discs to neighboring cells that express neither engrailed nor hedgehog. In these cells, it is localized in discrete punctate structures that are sequestered within the polarized epithelium. Analysis of animals that have expressed hedgehog ectopically, or of a mutant that expresses hedgehog abnormally in the anterior compartment of the wing disc, indicates that hedgehog is involved in regulating patched. In the embryo, hedgehog regulation of patched apparently facilitates patched and wingless expression. In the discs, hedgehog regulation of patched and other genes in the anterior compartment helps to establish the proximodistal axis. We propose that the cell-cell communication mediated by hedgehog links the special properties of compartment borders with specification of the proximodistal axis in imaginal development.

A central role for epidermal segment border cells in the induction of muscle patterning in the Drosophila embryo

The correct patterning of muscles in the Drosophila embryo depends on the migration of developing muscles over the ectoderm and on the attachment of these muscles to specific attachment sites. We investigate the mechanisms that are involved in this process and describe experiments that allow a genetic dissection of the role of the ectoderm in muscle migration and attachment. We show that cells along the segmental border in the ectoderm are used by the developing muscles to reach their attachment sites. These segment border cells are recognized by dissociated myotubes in single suspensions in culture. Thus, developing muscles have properties that allow the specific recognition of the segment border cells and migrate to attach to these cells. The segment border cells are absent in the mutant wingless and naked. In these mutants, the muscles are severely disorganized. We show that this is not a mere consequence of disruption of the epidermis, since, in the mutant patched, where segmental patterning is affected, the segment border cells are present near their normal position; the muscles in this mutant are relatively organized. Similarly, in the mutant lines where ectopic segment border cells are present, the observed muscle derangement correlates well with the ectopic attachment sites that are present. Finally, we have analyzed mutants at the stripe locus and have shown that lethal alleles disrupt muscle organization during embryogenesis. Enhancer-trap alleles of stripe that we have analyzed show reporter gene expression in the segment border cells. Our results indicate a role for the segment border cells in guidance of migrating muscle fibers to their attachment sites.

The Drosophila segment polarity gene patched interacts with decapentaplegic in wing development

The decapentaplegic (dpp) gene of Drosophila melanogaster encodes a polypeptide of the transforming growth factor-beta family of secreted factors. It is required for the proper development of both embryonic and adult structures, and may act as a morphogen in the embryo. In wing imaginal discs, dpp is expressed and required in a stripe of cells near the anterior-posterior compartment boundary. Here we show that viable mutations in the segment polarity genes patched (ptc) and costal-2 (cos2) cause specific alterations in dpp expression within the anterior compartment of the wing imaginal disc. The interaction between ptc and dpp is particularly interesting; both genes are expressed with similar patterns at the anterior-posterior compartment boundary of the disc, and mis-expressed in a similar way in segment polarity mutant backgrounds like ptc and cos2. This mis-expression of dpp could be correlated with some of the features of the adult mutant phenotypes. We propose that ptc controls dpp expression in the imaginal discs, and that the restricted expression of dpp near the anterior-posterior compartment boundary is essential to maintain the wild-type morphology of the wing disc.

Sonic hedgehog, a member of a family of putative signaling molecules, is implicated in the regulation of CNS polarity

We have identified three members of a mouse gene family related to the Drosophila segment polarity gene, hedgehog (hh). Like hh, they encode putative secreted proteins and are thus implicated in cell-cell interactions. One of these, Sonic hh (Shh), is expressed in the notochord, the floor plate, and the zone of polarizing activity, signaling centers that are thought to mediate central nervous system (CNS) and limb polarity. Ectopic expression of Shh in the mouse CNS leads to the activation of floor plate-expressed genes. These results suggest that Shh may play a role in the normal inductive interactions that pattern the ventral CNS.

Localized hedgehog activity controls spatial limits of wingless transcription in the Drosophila embryo

Cell patterning in the body segments of the Drosophila embryo requires activity of the segment polarity genes, a molecularly heterogeneous group defined by a generic mutant phenotype. Two of these genes, wingless (wg) and hedgehog (hh), encode proteins that enter the secretory pathway, implicating them as signals that instruct the fates of neighbouring cells. Genetic analysis has identified wg transcription as one of the targets of hh activity and it has been suggested that the spatial control of wg expression depends on the limited range of the hh signal and the differential competence of responding cells. I have tested this model by driving ubiquitous expression of the hh gene using the Hsp70 promoter. Here I report that, as predicted, this causes the ectopic activation of wg in only a subset of the cells of each parasegment. Using another target of hh activity as a probe, I demonstrate that the competence of cells to express wg is independent of their ability to receive the hh signal. Finally, I show that wg activation requires the function of the segment polarity gene fused, suggesting that the putative hh signal is transduced by the serine/threonine kinase that fused encodes.

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

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

Segment polarity gene interactions modulate epidermal patterning in Drosophila embryos

Each segment of a Drosophila larva shows a precisely organized pattern of cuticular structures, indicating diverse cellular identities in the underlying epidermis. Mutations in the segment polarity genes alter the cuticle pattern secreted by the epidermal cells; these mutant patterns provide clues about the role that each gene product plays in the development of wild-type epidermal pattern. We have analyzed embryos that are multiply mutant for five key patterning genes: wingless, patched, engrailed, naked and hedgehog. Our results indicate that wild-type activity of these five segment polarity genes can account for most of the ventral pattern elements and that their gene products interact extensively to specify the diverse cellular identities within the epidermis. Two pattern elements can be correlated with individual gene action: wingless is required for formation of naked cuticle and engrailed is required for formation of the first row of denticles in each abdominal denticle belt. The remaining cell types can be produced by different combinations of the five gene activities. wingless activity generates the diversity of cell types within the segment, but each specific cell identity depends on the activity of patched, engrailed, naked and hedgehog. These molecules modulate the distribution and interpretation of wingless signalling activity in the ventral epidermal cells and, in addition, each can contribute to pattern through a pathway independent of the wingless signalling pathway.

Segmentation of the Drosophila embryo

Segmentation in Drosophila is a sequential process directed by at least 30 genes that encode various types of proteins, including: many transcription factors; a putative RNA-binding protein; a membrane-associated receptor kinase; several intracellular protein kinases; a number of secreted signaling molecules; and others of unknown function. Although the detailed molecular reactions used to generate the metameric subdivisions of the embryo are not yet understood, a general outline of the processes involved has been described. The manner in which spatial relations in the developing embryo are established can now be described.

Contrasting distributions of patched and hedgehog proteins in the Drosophila embryo

The segment polarity genes patched (ptc) and hedgehog (hh) are thought to encode a receptor and signal molecule respectively, components of a signal transduction pathway that regulates the transcription of the wingless gene in the Drosophila embryo. Here we describe the production of antibodies specific for the products of these two genes and the patterns of protein distribution that they reveal in the developing embryo. The results are consistent with the hh protein being secreted by cells in which it is expressed and support a role for ptc in the reception of the putative hh encoded signal.

The MsK family of alfalfa protein kinase genes encodes homologues of shaggy/glycogen synthase kinase-3 and shows differential expression patterns in plant organs and development

This paper reports on the isolation of a novel class of plant serine/threonine protein kinase genes, MsK-1, MsK-2 and MsK-3. They belong to the superfamily of cdc2-like genes, but show highest identity to the Drosophila shaggy and rat GSK-3 proteins (65-70%). All of these kinases share a highly conserved catalytic protein kinase domain. Different amino-terminal extensions distinguish the different proteins. The different plant kinases do not originate from differential processing of the same gene as is found for shaggy, but are encoded by different members of a gene family. Similarly to the shaggy kinases, the plant kinases show different organ-specific and stage-specific developmental expression patterns. Since the shaggy kinases play an important role in intercellular communication in Drosophila development, the MsK kinases are expected to perform a similar function in plants.

The role of a Drosophila POU homeo domain gene in the specification of neural precursor cell identity in the developing embryonic central nervous system

The Drosophila embryonic central nervous system (CNS) is derived from a stereotypic array of progenitor stem cells called neuroblasts (NBs). Each of the approximately 25 NBs per hemisegment undergoes repeated asymmetric divisions to produce, on average, 5-10 smaller ganglion mother cells (GMCs); each GMC, in turn, divides to produce two neurons. We demonstrate that the protein product encoded by a POU homeo domain gene (dPOU28/pdm-2) is expressed in the cell nuclei of a subset of NBs and GMCs. In the wild-type animal, GMC-1 is the only identified cell in the NB4-2 lineage that expresses dPOU28 protein to a high level, and it divides to produce the RP2 neuron and a second cell of unknown fate. Our results suggest that the presence of ectopically induced dPOU28/pdm-2 protein in the progeny of GMC-1 is sufficient to cause both of these cells to adopt their parental GMC-1 cell fate, leading to duplication of the RP2 neuron (and its sister cell) on the basis of both immunological and morphological criteria. These observations clearly implicate a role for dPOU28/pdm-2 in the specification of GMC-1 cell identity in the NB4-2 lineage and possibly in the specification of cell fate in other NB lineages in the developing embryonic CNS.

Molecular characterization of the her-1 gene suggests a direct role in cell signaling during Caenorhabditis elegans sex determination

We have characterized two transcripts from the male-determining her-1 locus in Caenorhabditis elegans. The larger transcript, which appears more important for male development, is predicted to encode a novel 175-amino-acid, cysteine-rich polypeptide with an apparent amino-terminal signal sequence and potential cleavage and glycosylation sites. Expression of a full-length cDNA construct for the larger transcript driven by a body-wall-myosin promoter causes extensive masculinization of all sexually dimorphic tissues in XX (normally hermaphrodite) animals. This activity is dependent on the presence of the her-1 signal sequence or a substitute synthetic signal sequence in the encoded polypeptide. These results suggest that a secreted product of the her-1 gene dictates male development.

Regulation of wingless transcription in the Drosophila embryo

The segment polarity gene wingless (wg) is expressed in a complex pattern during embryogenesis suggesting that it plays multiple roles in the development of the embryo. The best characterized of these is its role in cell pattening in each parasegment, a process that requires the activity of other segment polarity genes including patched (ptc) and hedgehog (hh). Here we present further evidence that ptc and hh encode components of a signal transduction pathway that regulate the expression of wg transcription following its activation by pair-rule genes. We also show that most other aspects of wg expression are independent of this regulatory network.

wingless signaling acts through zeste-white 3, the Drosophila homolog of glycogen synthase kinase-3, to regulate engrailed and establish cell fate

Intrasegmental patterning in the Drosophila embryo is regulated by cell-cell communication. One of the signaling pathways that operates to specify positional information throughout the segment is mediated by the wingless (wg) protein, which is the homolog of the proto-oncogene Wnt-1. The early role of wg is to stabilize engrailed (en) expression by initiating a phase of en autoregulation in the adjacent more posterior cells. Here, we report that the segment polarity gene zeste-white 3 (zw3; also known as shaggy) acts as a repressor of en autoregulation. Genetic epistasis experiments indicate that wg signaling operates by inactivating the zw3 repression of en autoactivation. In addition, we demonstrate that zw3 encodes the Drosophila homolog of mammalian glycogen synthase kinase-3.

The consequences of ubiquitous expression of the wingless gene in the Drosophila embryo

The segment polarity gene wingless has an essential function in cell-to-cell communication during various stages of Drosophila development. The wingless gene encodes a secreted protein that affects gene expression in surrounding cells but does not spread far from the cells where it is made. In larvae, wingless is necessary to generate naked cuticle in a restricted part of each segment. To test whether the local accumulation of wingless is essential for its function, we made transgenic flies that express wingless under the control of a hsp70 promoter (HS-wg flies). Uniform wingless expression results in a complete naked cuticle, uniform armadillo accumulation and broadening of the engrailed domain. The expression patterns of patched, cubitus interruptus Dominant and Ultrabithorax follow the change in engrailed. The phenotype of heatshocked HS-wg embryos resembles the segment polarity mutant naked, suggesting that embryos that overexpress wingless or lack the naked gene enter similar developmental pathways. The ubiquitous effects of ectopic wingless expression may indicate that most cells in the embryo can receive and interpret the wingless signal. For the development of the wild-type pattern, it is required that wingless is expressed in a subset of these cells.

Secretion and localized transcription suggest a role in positional signaling for products of the segmentation gene hedgehog

The segment polarity genes engrailed and wingless are expressed in neighboring stripes of cells on opposite sides of the Drosophila parasegment boundary. Each gene is mutually required for maintenance of the other's expression; continued expression of both also requires several other segment polarity genes. We show here that one such gene, hedgehog, encodes a protein targeted to the secretory pathway and is expressed coincidently with engrailed in embryos and in imaginal discs; maintenance of the hedgehog expression pattern is itself dependent upon other segment polarity genes including engrailed and wingless. Expression of hedgehog thus functions in, and is sensitive to, positional signaling. These properties are consistent with the non-cell autonomous requirement for hedgehog in cuticular patterning and in maintenance of wingless expression.

Molecular organization and embryonic expression of the hedgehog gene involved in cell-cell communication in segmental patterning of Drosophila

hedgehog is a segment polarity gene necessary to maintain the proper organization of each segment of the Drosophila embryo. We have identified the physical location of a number of rearrangement breakpoints associated with hedgehog mutations. The corresponding hh RNA is expressed in a series of segmental stripes starting at cellular blastoderm in the posterior portion of each segment. This RNA is localized predominantly within nuclei until stage 10, when the localization becomes primarily cytoplasmic. Expression of hh RNA in the posterior compartment is independent of most other segment polarity genes, including en, until the late extended germ-band stage (stage 11). Sequence analysis of the hedgehog locus suggests the protein product is a transmembrane protein, which may, therefore, be directly involved in cell-cell communication.

Molecular genetics of sex determination in C. elegans

Sexual fate in the nematode Caenorhabditis elegans is controlled by a group of genetically well-characterized genes. Several of these sex-determining genes have now been analysed at the molecular level. Transcriptional regulation is likely to control both commitment to a single sexual fate and maintenance of that decision; in addition, intercellular signalling appears to coordinate the sexual fates of cells throughout the animal to adopt a single sexual fate.

tra-2 encodes a membrane protein and may mediate cell communication in the Caenorhabditis elegans sex determination pathway

The Caenorhabditis elegans sex-determining gene, tra-2, promotes female development in XX animals. In this paper we report the cDNA sequence corresponding to a 4.7 kb tra-2 mRNA and show that it is composed of 23 exons, is trans-spliced to SL2, and contains a perfect direct repeat in the 3' untranslated region. This mRNA is predicted to encode a 1475 amino acid protein, named pTra2A, that has a secretory signal and several potential membrane-spanning domains. The molecular analysis of tra-2 loss-of-function mutations supports our open reading frame identification and suggests that the carboxy-terminal domain is important for tra-2 activity. We propose that in XX animals the carboxy-terminal domain of pTra2A negatively regulates the downstream male promoting fem genes. In XO animals, tra-2 is negatively regulated by her-1, which acts cell nonautonomously. Because hydropathy predictions suggest that pTra2A is an integral membrane protein, pTra2A might act as a receptor for the her-1 protein. We propose that in XO animals, the her-1 protein promotes male development by binding and inactivating pTra2A. The role of cell communication in C. elegans sex determination might be to ensure unified sexual development throughout the animal. If so, then regulation of sexual fate by her-1 and tra-2 might provide a general model for the coordination of groups of cells to follow a single cell fate.