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.     

Allocation and specification of the genital disc precursor cells in Drosophila

The adult structures of Drosophila melanogaster are derived from larval imaginal discs, which originate as clusters of cells within the embryonic ectoderm. The genital imaginal disc is composed of three primordia (female genital, male genital, and anal primordia) that originate from the embryonic tail segments A8, A9, and A10, respectively, and produce the sexually dimorphic genitalia and analia. We show that the genital disc precursor cells (GDPCs) are first detectable during mid-embryogenesis as a 22-cell cluster in the ventral epidermis. Analysis of mutant and double mutant phenotypes of embryonic patterning genes in the GDPCs, together with their expression patterns in these cells, revealed the following with respect to the origins and specification of the GDPCs. The allocation of the GDPCs from the ventral epidermis requires the function of ventral patterning genes, including the EGF receptor and the spitz group of genes. The ventral localization of the GDPCs is further restricted by the action of dorsal patterning genes. Along the anterior-posterior axis, several segment polarity genes (wingless, engrailed, hedgehog, and patched) are required for the proper allocation of the GDPCs. These segment polarity genes are expressed in some, but not all of the GDPCs, indicating that anterior and posterior compartments are not fully established in the GDPCs. In addition, we found that the three primordia of the larval genital disc have already been specified in the GDPCs by the coordinated actions of the homeotic (Hox) genes, abdominal-A, Abdominal-B, and caudal. By identifying how these different patterning networks regulate the allocation and primordial organization of the 22 embryonic precursors of the compound genital disc, we demonstrate that at least some of the organization of the larval disc originates as positional information in the embryo, thus providing a context for further studies on the development of the genital disc.     

Analysis of the expression pattern of Mysidium columbiae wingless provides evidence for conserved mesodermal and retinal patterning processes among insects and crustaceans

The Wnt family includes a number of genes, such as wingless ( wg), which encode secreted glycoproteins that function in numerous developmental patterning processes. In order to gain a better understanding of crustacean pattern formation, a wg orthologue was cloned from the malacostracan crustacean Mysidium columbiae(mysid), and the expression pattern of this gene was compared with that of Drosophila wg. Although Drosophila wg is expressed in many developing tissues, such as the ventral neuroectoderm, M. columbiae wg (mcowg)expression is detected within only a subset of these tissues. mcowg is expressed in the dorsal part of each developing segment and within the developing eye, but not within the ventral neuroectoderm. Dorsal wg expression in Drosophila is required for heart and muscle development, and conservation of this dorsal wgexpression pattern suggests that mcowgmay function to pattern these tissues in mysids. Consistent with this, expression of Even-skipped (Eve) protein in heart precursor and muscle cells, which is dependent on Wg signaling in Drosophila, is also conserved in mysids. Within the developing mysid eye, mcowg is expressed in a pattern that is similar to the expression pattern of Drosophila wg in the fly eye disc. In Drosophila,Wg inhibits neural differentiation at the anterior margin of the eye disc and patterns the dorsal/ventral axis of the eye. These data indicate that mcowg may function similarly during mysid eye development. Analysis of mcowgexpression provides molecular evidence suggesting that the processes of heart, muscle, and eye patterning are likely to be conserved among insects and crustaceans.     

Hox-controlled reorganisation of intrasegmental patterning cues underlies Drosophila posterior spiracle organogenesis

Hox proteins provide axial positional information and control segment morphology in development and evolution. Yet how they specify morphological traits that confer segment identity and how axial positional information interferes with intrasegmental patterning cues during organogenesis remain poorly understood. We have investigated the control of Drosophila posterior spiracle morphogenesis, a segment-specific structure that forms under Abdominal-B (AbdB) Hox control in the eighth abdominal segment (A8). We show that the Hedgehog (Hh), Wingless (Wg) and Epidermal Growth Factor Receptor (Egfr) pathways provide specific inputs for posterior spiracle morphogenesis and act in a genetic network made of multiple and rapidly evolving Hox/signalling interplays. A major function of AbdB during posterior spiracle organogenesis is to reset A8 intrasegmental patterning cues, first by reshaping wg and rhomboid expression patterns, then by reallocating the Hh signal and later by initiating de novo expression of the posterior compartment gene engrailed in anterior compartment cells. These changes in expression patterns confer axial specificity to otherwise reiteratively used segmental patterning cues, linking intrasegmental polarity and acquisition of segment identity.     

Role of segment polarity genes in the definition and maintenance of cell states in the Drosophila embryo

Segment polarity genes are expressed and required in restricted domains within each metameric unit of the Drosophila embryo. We have used the expression of two segment polarity genes engrailed (en) and wingless (wg) to monitor the effects of segment polarity mutants on the basic metameric pattern. Absence of patched (ptc) or naked (nkd) functions triggers a novel sequence of en and wg patterns. In addition, although wg and en are not expressed on the same cells absence of either one has effects on the expression of the other. These observations, together with an analysis of mutant phenotypes during development, lead us to suggest that positional information is encoded in cell states defined and maintained by the activity of segment polarity gene products.     

Expression of 'segmentation' genes during larval and juvenile development in the polychaetes Capitella sp. I and H. elegans

Polychaete annelids and arthropods are both segmented protostome invertebrates. To investigate whether the segmented body plan of these two phyla share a common molecular ground pattern, we report the developmental expression of orthologues of the arthropod segment polarity genes engrailed (en), hedgehog (hh), and wingless (wg/Wnt1) in larval and juvenile stages of the polychaete annelid Capitella sp. I and en in a second polychaete, Hydroides elegans. Temporally, neither Wnt1 nor hh are detected in the segmented region of the larval body until after morphological segmentation is apparent. Expression of CapI-Wnt1 is limited to a ring of ectoderm marking the future anus during larval segmentation. CapI-hh is expressed in a ring of the hindgut internal to that of CapI-Wnt1, as well as in a subset of ventral nerve cord neurons, anterior gut tissue, and mesoderm. In both H. elegans and Capitella sp. I, en is expressed in a spatially and temporally dynamic manner in segmentally iterated structures as well as a population of cells that migrate internally from ectoderm to mesoderm, possibly representing a population of ecto-mesodermal precursors. Significantly, the expression patterns we report for wg, en, and hh orthologues in Capitella sp. I and for en in larval development of H. elegans are not comparable to the highly conserved ectodermal segment polarity pattern observed in arthropods at any life history stage, consistent with distinct origins of segmentation between annelids and arthropods.     

Interactions between segment polarity genes and the generation of the segmental pattern in Drosophila.

Although mutations in the segment polarity genes wingless, engrailed, hedgehog, gooseberry and cubitus-interruptusD all affect the region of naked cuticle within each segment of the Drosophila larva, subtle phenotypic differences suggest that these genes play different roles in segmental patterning. In this paper, the regulative interactions between these genes are analysed. They have revealed that the products of most of these genes accomplish more than one function during embryogenesis. Whereas early on a positive feed-back loop involving wg, en and hh maintains the expression of wg and en in the extremes of each parasegment, later on wg and en become independent from each other. en appears to regulate the expression of hh and ptc, while wg depends on gsb and ciD.     

Embryonic and imaginal requirements for wingless, a segment-polarity gene in Drosophila

In Drosophila melanogaster mutant alleles of the segmentation gene wingless fall into two classes: winglessLethal mutations are embryonic lethals with a segment-polarity phenotype; the wingless1 mutation is viable when homozygous and produces a homeotic transformation in adults. This paper further describes the embryonic lethal phenotype, and also pole-cell transplants, experiments with a temperature-sensitive mutation, and clonal analysis with a winglessLethal mutation. It is argued that the wg gene is zygotically required after gastrulation for the normal patterning of each embryonic segment. The gene is still required in the larval stages, and the cell nonautonomy of this function supports the view that the wg gene product may be involved in intercellular signaling during development.     

Engrailed gene expression in the abdominal segment of Oncopeltus: gradients and cell states in the insect segment

A monoclonal antibody that recognizes the product of the segmental gene, engrailed (en), of Drosophila has been used to analyse expression of the homologous gene of Oncopeltus. engrailed expression in the abdominal segment of larval Oncopeltus is confined to a narrow band of epidermal cells localized immediately anterior to the segment border. Expression varies in intensity during postembryonic development: no gene product is detectable in newly moulted larvae, but reappears soon after initiation of intermoult activities. One possible function of en in this system is revealed by a series of operations confronting cells from different anteroposterior levels in the segment. New segment borders are generated only when en-expressing cells confront cells from the anteriormost region of the segment. All other combinations result in intercalation of intermediate intrasegmental levels. It is therefore suggested that the most important function of en is the establishment of new, and presumably the maintenance of existing, segment borders.     

Interactions between fused, a segment-polarity gene in Drosophila, and other segmentation genes.

Fused (fu) is a segment polarity gene whose product is maternally required in the posterior part of each segment. To define further the role of fused and determine how it interacts with other segmentation genes, we examined the phenotypes obtained by combining fused with mutations of pair rule, homeotic and other segment polarity loci. When it was possible, we also looked at the distribution of corresponding proteins in fused mutant embryos. We observed that fused-naked (fu;nkd) double mutant embryos display a phenotypic suppression of simple mutant phenotypes: both naked cuticle and denticle belts, which would normally have been deleted by one of the two mutants alone, were restored. In fused mutant embryos, engrailed (en) and wingless (wg) expression was normal until germ band extension, but partially and completely disappeared respectively during germ band retraction. In the fu;nkd double mutant embryo, en was expressed as in nkd mutant at germ band extension, but later this expression was restricted and became normal at germ band retraction. On the contrary, wg expression disappeared as in fu simple mutant embryos. We conclude that the requirements for fused, naked and wingless activities for normal segmental patterning are not absolute, and propose mechanisms by which these genes interact to specify anterior and posterior cell fates.     

orthodenticle activity is required for the development of medial structures in the larval and adult epidermis of Drosophila.

Lethal alleles of orthodenticle (= otd) cause abnormalities in the embryonic head that reflect an early role in anterior pattern formation. In addition, otd activity is required for the development of the larval and adult epidermis. Clonal analysis of both viable and lethal alleles shows that the adult requirement for otd is restricted to medial regions of certain discs. When otd activity is reduced or removed, some medial precursor cells produce bristles and cuticle characteristic of more lateral structures. Similar medial defects are observed in the larval epidermis of embryos homozygous for lethal otd alleles. Antibodies to otd recognize a nuclear protein found at high levels in the medial region of the eye antennal discs, the leg discs, the genital discs and along the ventral midline of the ventral epidermis of the embryo. These results suggest that the otd gene product is required to specify medial cell fates in both the larval and adult epidermis.     

The Ultrabithorax gene of Drosophila and the specification of abdominal histoblasts.

Separation of the imaginal and larval developmental pathways in Drosophila occurs early in embryogenesis, resulting in the formation of imaginal discs and abdominal histoblast nests along the larval body wall. The dorsal and ventral histoblast nests within the first abdominal (A1) segment are shown not to be segmentally homologous with the metathoracic (T3) haltere and leg discs, respectively, since they occur at distinct dorso-ventral locations during normal development and can be found together within the same segment in mutants of the Bithorax complex (BX-C) where T3 is transformed towards A2-A4 or A1 towards T3. Several patterning abnormalities are also observed in BX-C mutants. A ventral shift in the A1 ventral nest occurs in partially transformed larvae harboring weak bithoraxoid (bxd) mutations; in more fully transformed larvae (Ubx1/Df) both the anterior dorsal and ventral nests are lost and instead a dorsal and ventral disc bud are formed. Dorso-ventral inversions in the pattern of the ventral nest occur in a random fashion throughout A1-A7 in response to an increase or decrease in the gene dosage of the BX-C. In gain-of-function mutants anterior dorsal histoblast cells form in the homologous anterior as well as the nonhomologous posterior portion of T3. Based on these and other findings it appears that the Ultrabithorax (Ubx) locus (and possibly abdominal-A and Abdominal-B) is required to steer ectodermal cells toward an imaginal histoblast rather than a larval cell fate at specific regions within the first abdominal segment.     

CNS midline cells contribute to maintenance of the initial dorsoventral patterning of the Drosophila ventral neuroectoderm

Dorsoventral patterning of the Drosophila ventral neuroectoderm is established by the expression of three evolutionarily conserved homeodomain genes: ventral nervous system defective (vnd), intermediate neuroblasts defective (ind), and muscle segment homeobox (msh) in the medial, intermediate, and lateral columns of the ventral neuroectoderm, respectively. It was not clear whether extrinsic factor(s) from the CNS midline cells influence the initial dorsoventral patterning by controlling the expression of the dorsoventral patterning genes. We show here that the CNS midline cells, specified by single-minded (sim), are essential for maintaining expression of the dorsoventral patterning genes. Ectopic expression of sim in the ventral neuroectoderm during the blastoderm stage repressed expression of the three homeodomain genes in the ventral neuroectoderm. This indicates that the identity of the CNS midline cells is established by a series of repressions of the three homeodomain genes in the ventral neuroectoderm. Ectopic expression of sim in the ventral neuroectoderm during initial neurogenesis induced ectopic ind expression in the medial column in addition to that in the intermediate column via EGFR signaling between the ventral neuroectoderm and midline cells. In contrast, it repressed the expression of vnd and msh in the medial and lateral columns, respectively. Our findings demonstrate that the CNS midline cells provide extrinsic positional information via EGFR signaling that maintains the initial subdivision of the ventral neuroectoderm into three dorsoventral columns during initial neurogenesis.     

Pattern regulation during the development of the dorsal abdomen in the flesh fly,Sarcophaga agryostoma

Summary In dipteran flies the adult abdominal epidermis is formed from small nests of diploid histoblast cells which spread out and replace the larval epidermis during metamorphosis. The pattern of nest outgrowth and fusion in Sarcophaga shows that the large dorsal hemitergite is normally formed by the two dorsal nests, the spiracle nest and part of the ventral nest (which also forms the hemisternite). By rotating the dorsal histoblast nests, we demonstrate that the adult segment border lies between the flexible intersegmental membrane (ISM) and the naked anterior strip of tergite, the acrotergite. Deletion of histoblast nests often results in a corresponding deletion of adult structures, accompanied by enlargement of adjacent structures within the segment and in neighbouring segments. Pattern formation is not strictly coupled to cell division (as in imaginal discs), since the nests remaining after an ablation, in spreading to fill vacant areas, generate more cells and larger structures than normal. Nest deletions can also result in regeneration, with remaining nests forming additional structures in the dorsal-ventral or anterior-posterior axis of the segment. The deletion of strips of anterior and intersegmental larval epidermis without histoblasts results in the formation of double-posterior duplications of the adult hemitergite. Although these operations damage adjacent histoblast nests, several features of the results suggest that the duplications arise from the interaction (after healing) of histoblasts with larval cells which they would not normally encounter, leading to the intercalation of histoblast cells bearing intervening anterior-posterior positional values. A similar process of intercalation may occur in normal development, as the histoblasts spread from their local origins across the larval epidermal sheet, replacing the larval cells to form the entire epidermis of the adult segment. Offprint requests to: V. French     

Sternopleural Is a Regulatory Mutation of Wingless with Both Dominant and Recessive Effects on Larval Development of Drosophila Melanogaster

The Drosophila wingless (wg) gene encodes a secreted signaling protein that is required for many separate patterning events in both embryonic and larval development. wg functions in the development of the adult structures have been studied using the conditional mutant wg(ts) and also using regulatory mutations of wg that reduce larval functions. Here we present evidence that Sternopleural (Sp) is another regulatory allele of wg that affects a subset of larval functions. Sp has both a recessive loss-of-function component and a gain-of-function component. The loss-of-function component reflects a reduction of wg activity in the notum and in the antenna. The gain-of-function component apparently leads to ectopic wg activity in the dorsal first and second leg disc and thereby generates the dominant Sp phenotype. Sp and other wg alleles show a complex pattern of complementation. We present evidence that these genetic properties are due to transvection. These results have implications for the genetic definition of a null allele at loci subject to transvection.     

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.