Early development of leg and wing primordia in the Drosophila embryo

A cell atlas of adult muscle precursors uncovers early events in fibre-type divergence in Drosophila

In Drosophila, the wing disc‐associated muscle precursor cells give rise to the fibrillar indirect flight muscles (IFM) and the tubular direct flight muscles (DFM). To understand early transcriptional events underlying this muscle diversification, we performed single‐cell RNA‐sequencing experiments and built a cell atlas of myoblasts associated with third instar larval wing disc. Our analysis identified distinct transcriptional signatures for IFM and DFM myoblasts that underlie the molecular basis of their divergence. The atlas further revealed various states of differentiation of myoblasts, thus illustrating previously unappreciated spatial and temporal heterogeneity among them. We identified and validated novel markers for both IFM and DFM myoblasts at various states of differentiation by immunofluorescence and genetic cell‐tracing experiments. Finally, we performed a systematic genetic screen using a panel of markers from the reference cell atlas as an entry point and found a novel gene, Amalgam which is functionally important in muscle development. Our work provides a framework for leveraging scRNA‐seq for gene discovery and details a strategy that can be applied to other scRNA‐seq datasets.

Gene expression reveals evidence for EGFR-dependent proximal-distal limb patterning in a myriapod

Evolution of segmented limbs is one of the key innovations of Arthropoda, allowing development of functionally specific specialized head and trunk appendages, a major factor behind their unmatched evolutionary success. Proximodistal limb patterning is controlled by two regulatory networks in the vinegar fly Drosophila melanogaster, and other insects. The first is represented by the function of the morphogens Wingless (Wg) and Decapentaplegic (Dpp); the second by the EGFR-signaling cascade. While the role of Wg and Dpp has been studied in a wide range of arthropods representing all main branches, that is, Pancrustacea (= Hexapoda + Crustacea), Myriapoda and Chelicerata, investigation of the potential role of EGFR-signaling is restricted to insects (Hexapoda). Gene expression analysis of Egfr, its potential ligands, and putative downstream factors in the pill millipede Glomeris marginata (Myriapoda: Diplopoda), reveals that-in at least mandibulate arthropods-EGFR-signaling is likely a conserved regulatory mechanism in proximodistal limb patterning.

Distal-less homeobox genes of insects and spiders: genomic organization, function, regulation and evolution

The Distal-less (Dll) genes are homeodomain transcription factors that are present in most Metazoa and in representatives of all investigated arthropod groups. In Drosophila, the best studied insect, Dll plays an essential role in forming the proximodistal axis of the legs, antennae and analia, and in specifying antennal identity. The initiation of Dll expression in clusters of cells in mid-lateral regions of the Drosophila embryo represents the earliest genetic marker of limbs. Dll genes are involved in the development of the peripheral nervous system and sensitive organs, and they also function as master regulators of black pigmentation in some insect lineages. Here we analyze the complete genomes of six insects, the nematode Caenorhabditis elegans and Homo sapiens, as well as multiple Dll sequences available in databases in order to examine the structure and protein features of these genes. We also review the function, expression, regulation and evolution of arthropod Dll genes with emphasis on insects and spiders.

The legacy of Drosophila imaginal discs

The study of Drosophila imaginal discs has contributed to a number of discoveries in developmental and cellular biology. In addition to the elucidation of the role of tissue compartments and organ-specific master regulator genes during development, imaginal discs have also become well established as models for studying cellular interactions and complex genetic pathways. Here, we review key discoveries resulting from investigations of these epithelial precursor organs, ranging from cell fate determination and transdetermination to tissue patterning. Furthermore, the design of increasingly sophisticated genetic tools over the last decades has added value to the use of imaginal discs as model systems. As a result of tissue-specific genetic screens, several components of developmentally regulated signaling pathways were identified and epistasis revealed the levels at which they function. Discs have been widely used to assess cellular interactions in their natural tissue context, contributing to a better understanding of growth regulation, tissue regeneration, and cancer. With the continuous implementation of novel tools, imaginal discs retain significant potential as model systems to address emerging questions in biology and medicine.

The Genetic Basis of Pigmentation Differences Within and Between Drosophila Species

In Drosophila, as well as in many other plants and animals, pigmentation is highly variable both within and between species. This variability, combined with powerful genetic and transgenic tools as well as knowledge of how pigment patterns are formed biochemically and developmentally, has made Drosophila pigmentation a premier system for investigating the genetic and molecular mechanisms responsible for phenotypic evolution. In this chapter, we review and synthesize findings from a rapidly growing body of case studies examining the genetic basis of pigmentation differences in the abdomen, thorax, wings, and pupal cases within and between Drosophila species. A core set of genes, including genes required for pigment synthesis (eg, yellow, ebony, tan, Dat) as well as developmental regulators of these genes (eg, bab1, bab2, omb, Dll, and wg), emerge as the primary sources of this variation, with most genes having been shown to contribute to pigmentation differences both within and between species. In cases where specific genetic changes contributing to pigmentation divergence were identified in these genes, the changes were always located in noncoding sequences and affected cis-regulatory activity. We conclude this chapter by discussing these and other lessons learned from evolutionary genetic studies of Drosophila pigmentation and identify topics we think should be the focus of future work with this model system.

Divergent role of the Hox gene Antennapedia in spiders is responsible for the convergent evolution of abdominal limb repression

Evolution often results in morphologically similar solutions in different organisms, a phenomenon known as convergence. However, there is little knowledge of the processes that lead to convergence at the genetic level. The genes of the Hox cluster control morphology in animals. They may also be central to the convergence of morphological traits, but whether morphological similarities also require similar changes in Hox gene function is disputed. In arthropods, body subdivision into a region with locomotory appendages ("thorax") and a region with reduced appendages ("abdomen") has evolved convergently in several groups, e.g., spiders and insects. In insects, legs develop in the expression domain of the Hox gene Antennapedia (Antp), whereas the Hox genes Ultrabithorax (Ubx) and abdominal-A mediate leg repression in the abdomen. Here, we show that, unlike Antp in insects, the Antp gene in the spider Achaearanea tepidariorum represses legs in the first segment of the abdomen (opisthosoma), and that Antp and Ubx are redundant in the following segment. The down-regulation of Antp in A. tepidariorum leads to a striking 10-legged phenotype. We present evidence from ectopic expression of the spider Antp gene in Drosophila embryos and imaginal tissue that this unique function of Antp is not due to changes in the Antp protein, but likely due to divergent evolution of cofactors, Hox collaborators or target genes in spiders and flies. Our results illustrate an interesting example of convergent evolution of abdominal leg repression in arthropods by altering the role of distinct Hox genes at different levels of their action.

Imaginal discs: Renaissance of a model for regenerative biology

Many animals display a capacity to regenerate tissues or even a complete body. One of the main goals of regenerative biology is to identify the genes and genetic networks necessary for this process. Drosophila offers an ideal model system for such studies. The wide range of genetic and genomic approaches available for use in flies has helped in initiating the deciphering of the mechanisms underlying regeneration, and the results may be applicable to other organisms, including mammals. Moreover, most models of regeneration require experimental manipulation, whereas in Drosophila discrete domains can be ablated by genetically induced methods. Here, we present a summary of current research into imaginal disc regeneration and discuss the power of this tissue as a tool for understanding the genetics of regeneration.

Mutual regulation of the Drosophila disconnected (disco) and Distal-less (Dll) genes contributes to proximal-distal patterning of antenna and leg

The Drosophila disconnected (disco) gene encodes a C(2)H(2)-type zinc finger transcription factor required for the development of the central and peripheral nervous systems. We report that disco participates in a positive feedback loop with the Dll gene, a master regulator of ventral appendage development. Dll function is not only required for proper disco expression in antenna and leg discs, but is also sufficient for ectopic expression of disco in the developing retina and wing imaginal discs. Conversely, disco gene function is required for the maintenance of Dll expression. We show that Dll phenotypes are partially rescued by the up-regulation of disco expression in the Dll domain. Reduction in disco gene function disrupts antenna and leg development, and the phenotypes closely resemble that produced by Dll alleles. These observations demonstrate that disco plays a fundamental role in the Dll-dependent patterning of antenna and leg, perhaps as a regulator of Dll gene expression.

Spalt major controls the development of the notum and of wing hinge primordia of the Drosophila melanogaster wing imaginal disc

The Drosophila wing and the dorsal thorax develop from primordia within the wing imaginal disc. Here we show that spalt major (salm) is expressed within the presumptive dorsal body wall primordium early in wing disc development to specify notum and wing hinge tissue. Upon ectopic salm expression, dorsally located second leg disc cells develop notum and wing hinge tissue instead of sternopleural tissue. Similarly, by salm over-expression within the wing disc, wing blade formation is suppressed and a mirror-image duplication of the notum and wing hinge is formed. In large dorsal clones, which lack salm and its neighboring paralogue spalt related (salr), the cells of the notum primordium do not grow; these dorsal cells are not specified as notum, hence no notum outgrowth develops. These results suggest that the zinc finger factors encoded by the salm/salr complex play important roles in defining cells of the early wing disc as dorsal body wall cells, which develop into a large dorsal body wall territory and form mesonotum and some wing hinge tissue, and in delimiting the wing primordium. We also find that salm activity is down-regulated by its own product and by that of the Pax gene eyegone.

Reciprocal roles for bowl and lines in specifying the peripodial epithelium and the disc proper of the Drosophila wing primordium

Central to embryonic development is the generation of molecular asymmetries across fields of undifferentiated cells. The Drosophila wing imaginal disc provides a powerful system with which to understand how such asymmetries are generated and how they contribute to formation of a complex structure. Early in development, the wing primordium is subdivided into a thin layer of peripodial epithelium (PE) and an apposing thickened layer of pseudostratified columnar epithelium (CE), known as the disc proper (DP). The DP gives rise to the wing blade, hinge and dorsal mesothorax, whereas the PE makes only a minor contribution to the ventral hinge and pleura. The mechanisms that generate this major asymmetry and its contribution to wing development are poorly understood. The Lines protein destabilizes the nuclear protein Bowl in ectodermal structures. Here, we show that Bowl accumulates in the PE from early stages of wing development and is absent from the DP. Broad inhibition of Bowl in the PE resulted in the replacement of the PE with a mirror image duplication of the DP. The failure to generate the PE severely compromised wing growth and the formation of the notum. Conversely, the activation of bowl in the DP (by removal or inhibition of lines function) resulted in the transformation of the DP into PE. Thus, we provide evidence that bowl and lines act as a binary switch to subdivide the wing primordium into PE and DP, and assign crucial roles for this asymmetry in wing growth and patterning.

Drosophila TRPA channel modulates sugar-stimulated neural excitation, avoidance and social response

Drosophila melanogaster postfeeding larvae show food-averse migration toward food-free habitats before metamorphosis. This developmental switching from food attraction to aversion is regulated by a neuropeptide Y (NPY)-related brain signaling peptide. We used the fly larva model to delineate the neurobiological basis of age-restricted response to environmental stimuli. Here we provide evidence for a fructose-responsive chemosensory pathway that modulates food-averse migratory and social behaviors. We found that fructose potently elicited larval food-averse behaviors, and painless (pain), a transient receptor potential channel that is responsive to noxious stimuli, was required for the fructose response. A subset of pain-expressing sensory neurons have been identified that show pain-dependent excitation by fructose. Although evolutionarily conserved avoidance mechanisms are widely appreciated for their roles in stress coping and survival, their biological importance in animal physiology and development remains unknown. Our findings demonstrate how an avoidance mechanism is recruited to facilitate animal development.

Co-option of signaling mechanisms from neural induction to telencephalic patterning

This article provides an overview of signaling processes during early specification of the anterior neural tube, with special emphasis on the telencephalon. A series of signaling systems based on the action of distinct morphogens acts at different developmental stages, specifying interacting developmental fields that define axes of differentiation in the rostrocaudal and the dorsoventral domains. Interestingly, many of these signaling systems are co-opted for several differentiation processes. This strategy provides a simple and efficient mechanism to generate novel structures in evolution, and may have been especially important in the origin of the telencephalon and the mammalian cerebral cortex. For example, the action of fibroblast growth factor (FGF) secreted in early stages from the anterior neural ridge, but in later stages from the dorsal anterior forebrain, may have been a key factor in the early differentiation of the ventral telencephalon and in the eventual expansion of the mammalian neocortex. Likewise, bone morphogenetic proteins (BMPs) participate at several stages in neural patterning, even if early neural induction consists of the inhibition of the BMP pathway. BMPs, secreted dorsally, interact with FGFs in the frontal aspect of the hemispheres, and with PAX6-dependent signaling sources located laterally, to pattern the dorsal telencephalon. The actions of other morphogens are also described in this context, such as the ventralizing factor SHH, the dorsalizing element GLI3, and other factors related to the dorsomedial telencephalon such as WNTs and EMXs. The main conclusion we draw from this review is the well-known phylogenetic and developmental conservatism of signaling pathways, which in evolution have been applied in different embryological contexts, generating novel interactions between morphogenetic fields and leading to the generation of new morphological structures.

The appendage role of insect disco genes and possible implications on the evolution of the maggot larval form

Though initially identified as necessary for neural migration, Disconnected and its partially redundant paralog, Disco-related, are required for proper head segment identity during Drosophila embryogenesis. Here, we present evidence that these genes are also required for proper ventral appendage development during development of the adult fly, where they specify medial to distal appendage development. Cells lacking the disco genes cannot contribute to the medial and distal portions of ventral appendages. Further, ectopic disco transforms dorsal appendages toward ventral fates; in wing discs, the medial and distal leg development pathways are activated. Interestingly, this appendage role is conserved in the red flour beetle, Tribolium (where legs develop during embryogenesis), yet in the beetle we found no evidence for a head segmentation role. The lack of an embryonic head specification role in Tribolium could be interpreted as a loss of the head segmentation function in Tribolium or gain of this function during evolution of flies. However, we suggest an alternative explanation. We propose that the disco genes always function as appendage factors, but their appendage nature is masked during Drosophila embryogenesis due to the reduction of limb fields in the maggot style Drosophila larva.

Compartmental modulation of abdominal Hox expression by engrailed and sloppy-paired patterns the fly ectoderm

In Drosophila, segmentation genes partition the early embryo into reiterative segments along the anterior-posterior axis, while Hox genes assign segments their identities. Each segment is also subdivided into distinct anterior (A) and posterior (P) compartments based on the expression of the engrailed (en) segmentation gene. Differences in Hox expression often correlate with compartmental boundaries, but the genetic basis for these differences is not well understood. In this study, we extend previous results to describe a genetic circuit that controls the differential expression of two Hox genes, Ultrabithorax (Ubx) and abdominal-A (abd-A), within the A and P compartments of the abdominal ectoderm. Consistent with earlier findings, we show that en is essential for high Abd-A levels and low Ubx levels in the P compartment, whereas sloppy-paired (slp) is required for high Ubx levels in the A compartment. Overall, these results demonstrate that the compartmental expression of Ubx and abd-A is established through a repressive regulatory network between en, slp, Ubx and abd-A. We also show that abd-A expression in the P compartment is important for the formation of abdominal-specific cell types, suggesting that en and slp modulation of Hox expression within the A and P compartments is essential for embryonic patterning.

Epidermal growth factor receptor and transforming growth factor-beta signaling contributes to variation for wing shape in Drosophila melanogaster

Wing development in Drosophila is a common model system for the dissection of genetic networks and their roles during development. In particular, the RTK and TGF-beta regulatory networks appear to be involved with numerous aspects of wing development, including patterning, cell determination, growth, proliferation, and survival in the developing imaginal wing disc. However, little is known as to how subtle changes in the function of these genes may contribute to quantitative variation for wing shape, per se. In this study 50 insertional mutations, representing 43 loci in the RTK, Hedgehog, TGF-beta pathways, and their genetically interacting factors were used to study the role of these networks on wing shape. To concurrently examine how genetic background modulates the effects of the mutation, each insertion was introgressed into two wild-type genetic backgrounds. Using geometric morphometric methods, it is shown that the majority of these mutations have profound effects on shape but not size of the wing when measured as heterozygotes. To examine the relationships between how each mutation affects wing shape hierarchical clustering was used. Unlike previous observations of environmental canalization, these mutations did not generally increase within-line variation relative to their wild-type counterparts. These results provide an entry point into the genetics of wing shape and are discussed within the framework of the dissection of complex phenotypes.

jing is required for wing development and to establish the proximo-distal axis of the leg in Drosophila melanogaster

The establishment of the proximo-distal (PD) axis in the legs of Drosophila melanogaster requires the expression of a nested set of transcription factors that are activated in discreet domains by secreted signaling molecules. The precise regulation of these transcription factor domains is critical for generating the stereotyped morphological characteristics that exist along the PD axis, such as the positioning of specific bristle types and leg joints. Here we provide evidence that the Zn-finger protein encoded by the gene jing is critical for PD axis formation in the Drosophila legs. Our data suggest that jing represses transcription and that it is necessary to keep the proximal gene homothorax (hth) repressed in the medial domain of the PD axis. We further show that jing is also required for alula and vein development in the adult wing. In the wing, Jing is required to repress another proximal gene, teashirt (tsh), in a small domain that will give rise to the alula. Interestingly, we also demonstrate that two other genes affecting alula development, Alula and elbow, also exhibit tsh derepression in the same region of the wing disc as jing- clones. Finally, we show that jing genetically interacts with several members of the Polycomb (Pc) group of genes during development. Together, our data suggest that jing encodes a transcriptional repressor that may participate in a subset of Pc-dependent activities during Drosophila appendage development.

Evolutionarily conserved domains required for activation and repression functions of the Drosophila Hox protein Ultrabithorax

While testing the functions of deletion mutants in the Hox protein Ultrabithorax (Ubx), we found that the embryonic repression function of Ubx on Distal-less transcription in limb primordia is highly concentration dependent. The steep sigmoidal relationship between in vivo Ubx concentration and Distal-less repression is dependent on the Ubx YPWM motif. This suggests that Ubx cooperatively assembles a multi-protein repression complex on Distal-less regulatory DNA with the YPWM motif as a key protein-protein interface in this complex. Our deletion mutants also provide evidence for a transcriptional activation domain in the N-terminal 19 amino acids of Ubx. This proposed activation domain contains a variant of the SSYF motif that is found at the N termini of many Hox proteins, and is conserved in the activation domain of another Hox protein, Sex combs reduced. These results suggest that the N-terminal region containing the SSYF motif has been conserved in many Hox proteins for its role in transcriptional activation.

Dynamic EGFR-Ras signalling in Drosophila leg development

In Drosophila, as in many other animals, EGFR-Ras signalling has multiple developmental roles from oogenesis to differentiation. In leg development, in particular, it has been described to be responsible for the establishment of distal leg fates in a graded manner. Here, we investigate the patterns of expression of activators of EGFR-Ras signalling, as well as some of the effectors, in order to better understand the patterning of the distal leg, and to investigate further roles of this signalling pathway. These patterns, together with genetic data obtained by different mutant conditions for EGFR-Ras members and transgene expression, suggest two rounds of signalling in leg development. Early, the EGFR ligand Vein is the main player in distal leg patterning, possibly supported later by another ligand activated by Rhomboid. Later, in a second wave of signalling when all the proximal-distal leg fates have been specified, domains of EGFR/Ras activation appear inside each leg segment to regulate Notch-mediated joint development, and also some organs such as tendons and sensory organs. This second wave relies on a ligand activated by Rhomboid.

Requirement of Abdominal-A and Abdominal-B in the developing genitalia of Drosophila breaks the posterior downregulation rule

The genitalia of Drosophila derive from the genital disc and require the activity of the Abdominal-B (Abd-B) Hox gene. This gene encodes two different proteins, Abd-B M and Abd-B R. We show here that the embryonic genital disc, like the larval genital disc, is formed by cells from the eighth (A8), ninth (A9) and tenth (A10) abdominal segments, which most likely express the Abd-B M, Abd-B R and Caudal products, respectively. Abd-B m is needed for the development of A8 derivatives such as the external and internal female genitalia, the latter also requiring abdominal-A (abd-A), whereas Abd-B r shapes male genitalia (A9 in males). Although Abd-B r represses Abd-B m in the embryo, in at least part of the male A9 such regulation does not occur. In the male A9, some Abd-B m(-)r(-) or Abd-B r(-) clones activate Distal-less and transform part of the genitalia into leg or antenna. In the female A8, many Abd-B m(-)r(-) mutant clones produce similar effects, and also downregulate or eliminate abdominal-A expression. By contrast, although Abd-B m is the main or only Abd-B transcript present in the female A8, Abd-B m(-) clones induced in this primordium do not alter Distal-less or abd-A expression, and transform the A8 segment into the A4. The relationship between Abd-B and abd-A in the female genital disc is opposite to that of the embryonic epidermis, and contravenes the rule that posteriorly expressed Hox genes downregulate more anterior ones.

Hierarchical recruitment of polycomb group silencing complexes

Polycomb group (PcG) proteins maintain the transcriptional silence of target genes through many cycles of cell division. Here, we provide evidence for the sequential binding of PcG proteins at a Polycomb response element (PRE) in proliferating cells in which the sequence-specific DNA binding Pho and Phol proteins directly recruit E(z)-containing complexes, which in turn methylate histone H3 at lysine 27 (H3mK27). This provides a tag that facilitates binding by a Pc-containing complex. In wing imaginal discs, these PcG proteins also are present at discrete locations at or downstream of the promoter of a silenced target gene, Ubx. E(z)-dependent H3mK27 is also present near the Ubx promoter and is needed for Pc binding. The location of E(z)- and Pc-containing complexes downstream of the Ubx transcription start site suggests that they may inhibit transcription by interfering with assembly of the preinitiation complex or by blocking transcription initiation or elongation.

The expression of the proximodistal axis patterning genes Distal-less and dachshund in the appendages of Glomeris marginata (Myriapoda: Diplopoda) suggests a special role of these genes in patterning the head appendages

The genes Distal-less, dachshund, extradenticle, and homothorax have been shown in Drosophila to be among the earliest genes that define positional values along the proximal-distal (PD) axis of the developing legs. In order to study PD axis formation in the appendages of the pill millipede Glomeris marginata, we have isolated homologues of these four genes and have studied their expression patterns. In the trunk legs, there are several differences to Drosophila, but the patterns are nevertheless compatible with a conserved role in defining positional values along the PD axis. However, their role in the head appendages is apparently more complex. Distal-less in the mandible and maxilla is expressed in the forming sensory organs and, thus, does not seem to be involved in PD axis patterning. We could not identify in the mouthparts components that are homologous to the distal parts of the trunk legs and antennnae. Interestingly, there is also a transient premorphogenetic expression of Distal-less in the second antennal and second maxillary segment, although no appendages are eventually formed in these segments. The dachshund gene is apparently involved both in PD patterning as well as in sensory organ development in the antenna, maxilla, and mandible. Strong dachshund expression is specifically correlated with the tooth-like part of the mandible, a feature that is shared with other mandibulate arthropods. homothorax is expressed in the proximal and medial parts of the legs, while extradenticle RNA is only seen in the proximal region. This overlap of expression corresponds to the functional overlap between extradenticle and homothorax in Drosophila.

Identification of a human homologue of the DREF transcription factor with a potential role in regulation of the histone H1 gene

A human homologue (hDREF/KIAA0785) of Drosophila DREF, a transcriptional regulatory factor required for expression of genes involved in DNA replication and cell proliferation, was identified by BLAST search. Amino acid sequences corresponding to three regions highly conserved between two Drosophila species also proved to be very similar in the hDREF/KIAA0785 polypeptide. A consensus binding sequence (5'-TGTCG(C/T)GA(C/T)A) for hDREF/KIAA0785, determined by the CASTing method, overlapped with that for the Drosophila DREF (5'-TGTCGATA). We found hDREF/KIAA0785 binding sequences in the promoter regions of human genes related to cell proliferation. Analyses using a specific antibody revealed that an hDREF/KIAA0785 binds to the promoter region of the histone H1 gene. Co-transfection experiments with an hDREF/KIAA0785-expressing plasmid and a histone H1 promoter-directed luciferase reporter plasmid in HeLa cells revealed possible activation of the histone H1 promoter. Immunohistochemical analysis demonstrated that hDREF/KIAA0785 is localized in the nuclei. Although the expression level of the factor was found to be low in serum-deprived human normal fibroblasts, the amount was increased by adding serum to cultures and reached a maximum during S phase. RNA interference experiments targeting hDREF/KIAA0785 resulted in inhibition of S phase entry and reduction of histone H1 mRNA in HeLa cells. These results suggest that expression of hDREF/KIAA0785 may have a role in regulation of human genes related to cell proliferation.

Distinct functions of homothorax in leg development in Drosophila

The Drosophila leg is subdivided into two mutually antagonistic proximal and distal domains. The proximal domain is defined by the activity of the homeobox genes homothorax and extradenticle and the distal one by the Dpp/Wg targets Distal-less (Dll) and dachshund (dac). It is known that hth/exd function prevents the activity of Dpp and Wg response genes and that cells deficient for exd activity in the proximal domain differentiate pattern elements corresponding to more distal leg regions. We report new results on the role of hth/exd antagonising the Dpp pathway. In cells expressing hth in the distal leg, there is a debilitation of the Dpp pathway which is reflected in lower levels of Mad phosphorylation and in increased levels of the receptor thick veins. Ectopic hth expression in the distal leg results in JNK-mediated apoptosis, decreased growth and pattern abnormalities. It also causes a general proximalisation of the appendage, which can be explained by interference with the Dpp and Wg pathways. We also report that the repression by hth/exd of the Dpp and Wg target Distal-less is not achieved at the level of transcription but preventing the activation of Dll target genes. We propose that hth/exd function contributes to the normal identity of proximal cells both by limiting the influence of the Dpp and Wg pathways and by activating proximal genes like teashirt (tsh) and aristaless (al).

Developmental functions of theDistal-less/Dlx homeobox genes

Distal-less is the earliest known gene specifically expressed in developing insect limbs; its expression is maintained throughout limb development. The homeodomain transcription factor encoded by Distal-less is required for the elaboration of proximodistal pattern elements in Drosophila limbs and can initiate proximodistal axis formation when expressed ectopically. Distal-less homologs, the Dlx genes, are expressed in developing appendages in at least six phyla, including chordates, consistent with requirements for Dlx function in normal appendage development across the animal kingdom. Recent work implicates the Dlx genes of vertebrates in a variety of other developmental processes ranging from neurogenesis to hematopoiesis. We review what is known about the invertebrate and vertebrate Dll/Dlx genes and their varied roles during development. We propose revising the vertebrate nomenclature to reflect phylogenetic relationships among the Dlx genes.

Drumstick is a zinc finger protein that antagonizes Lines to control patterning and morphogenesis of theDrosophilahindgut

Elongation of the Drosophila embryonic hindgut epithelium occurs by a process of oriented cell rearrangement requiring the genes drumstick (drm) and lines (lin). The elongating hindgut becomes subdivided into domains – small intestine, large intestine and rectum – each characterized by a specific pattern of gene expression dependent upon normal drm and lin function. We show that drm encodes an 81 amino acid (10 kDa) zinc finger protein that is a member of the Odd-skipped family. drm expression is localized to the developing midgut-hindgut junction and is required to establish the small intestine, while lin is broadly expressed throughout the gut primordium and represses small intestine fate. lin is epistatic to drm, suggesting a model in which localized expression of drm blocks lin activity, thereby allowing small intestine fate to be established. Further supporting this model, ectopic expression of Drm throughout the hindgut produces a lin phenotype. Biochemical and genetic data indicate that the first conserved zinc finger of Drm is essential for its function. We have thus defined a pathway in which a spatially localized zinc finger protein antagonizes a globally expressed protein, thereby leading to specification of a domain (the small intestine) necessary for oriented cell rearrangement.

The Hox gene Abdominal-B antagonizes appendage development in the genital disc of Drosophila

In Drosophila, the Hox gene Abdominal-B is required to specify the posterior abdomen and the genitalia. Homologues of Abdominal-B in other species are also needed to determine the posterior part of the body. We have studied the function of Abdominal-B in the formation of Drosophila genitalia, and show here that absence of Abdominal-B in the genital disc of Drosophila transforms male and female genitalia into leg or, less frequently, into antenna. These transformations are accompanied by the ectopic expression of genes such as Distal-less or dachshund, which are normally required in these appendages. The extent of wild-type and ectopic Distal-less expression depends on the antagonistic activities of the Abdominal-B gene, as a repressor, and of the decapentaplegic and wingless genes as activators. Absence of Abdominal-B also changes the expression of Homothorax, a Hox gene co-factor. Our results suggest that Abdominal-B forms genitalia by modifying an underlying positional information and repressing appendage development. We propose that the genital primordia should be subdivided into two regions, one of them competent to be transformed into an appendage in the absence of Abdominal-B.

The Drosophila genes disconnected and disco-related are redundant with respect to larval head development and accumulation of mRNAs from deformed target genes

HOM-C/hox genes specify body pattern by encoding regionally expressed transcription factors that activate the appropriate target genes necessary for differentiation of each body region. The current model of target gene activation suggests that interactions with cofactors influence DNA-binding ability and target gene activation by the HOM-C/hox proteins. Currently, little is known about the specifics of this process because few target genes and fewer cofactors have been identified. We undertook a deficiency screen in Drosophila melanogaster in an attempt to identify loci potentially encoding cofactors for the protein encoded by the HOM-C gene Deformed (Dfd). We identified a region of the X chromosome that, when absent, leads to loss of specific larval mouthpart structures producing a phenotype similar to that observed in Dfd mutants. The phenotype is correlated with reduced accumulation of mRNAs from Dfd target genes, though there appears to be no effect on Dfd protein accumulation. We show that these defects are due to the loss of two functionally redundant, neighboring genes encoding zinc finger transcription factors, disconnected and a gene we call disco-related. We discuss the role of these genes during differentiation of the gnathal segments and, in light of other recent findings, propose that regionally expressed zinc finger proteins may play a central role with the HOM-C proteins in establishing body pattern.

Dopamine and sensory tissue development inDrosophila melanogaster

Dopamine is an important signaling molecule in the nervous system; it also plays a vital role in the development of diverse non-neuronal tissues in the fruit fly Drosophila melanogaster. The current study demonstrates that males depleted of dopamine as third instar larvae (via inhibition of the biosynthetic enzyme tyrosine hydroxylase) demonstrated abnormalities in courtship behavior as adults. These defects were suggestive of abnormalities in sensory perception and/or processing. Electroretinograms (ERGs) of eyes from adults depleted of dopamine for 1 day as third instar larvae revealed diminished or absent on- and off-transients. These sensory defects were rescued by the addition of L-DOPA in conjunction with tyrosine hydroxylase inhibition during the larval stage. Depletion of dopamine in the first or second larval instar was lethal, but this was not due to a general inhibition of proliferative cells. To establish that dopamine was synthesized in tissues destined to become part of the adult sensory apparatus, transgenic lines were generated containing 1 or 4 kb of 5' upstream sequences from the Drosophila tyrosine hydroxylase gene (DTH) fused to the E. coli beta-galactosidase reporter. The DTH promoters directed expression of the reporter gene in discrete and consistent patterns within the imaginal discs, in addition to the expected expression in gonadal, brain, and cuticular tissues. The beta-galactosidase expression colocalized with tyrosine hydroxylase protein. These results are consistent with a developmental requirement for dopamine in the normal physiology of adult sensory tissues.

Distal-less function during Drosophila appendage and sense organ development

The Drosophila Distal-less (Dll) gene was identified in the early 1980s by means of dominant and recessive mutations that caused both striking antenna-to-leg transformations and leg truncations. The gene initially was named "Bristle on arista" or "Brista" because one aspect of the phenotype is the formation of leg bristles on the antenna (Sato [1984] Drosophila Information Service 60:180-182; Sunkel and Whittle [1987] Wilhelm Roux's. Arch. Dev. Biol. 196:124-132). Subsequent studies have revealed that Dll encodes a homeodomain transcription factor (Cohen et al. [1989] Nature 338:432-434) that is expressed throughout limb development from embryogenesis on (Cohen [1990] Nature 343:173-177; Weigmann and Cohen [1999] Development 126:3823-3830). Dll is required for the elaboration of distal pattern elements in the antenna, the legs, the limb-derived gnathal structures (Cohen and Jurgens [1989] Nature 482-485), and the anal plate (Gorfinkiel et al. [1999] Mech. Dev. 868:113-123) and can initiate proximodistal axis formation when expressed ectopically (Gorfinkiel et al. [1997] Genes Dev. 11:2259-2271). Dll homologs are expressed in developing appendages in at least six coelomate phyla, including chordates (Akimenko et al. [1994] J. Neurosci. 14:3475-3486; Beauchemin and Savard [1992] Dev. Biol. 154:55-65; Bulfone et al. [1993] Mech. Dev. 40:129-140; Dolle et al. [1992] Differentiation 49:93-99; Ferrari et al. [1995] Mech. Dev. 52:257-264; Panganiban et al. [1997] Proc. Natl. Acad. Sci. USA 94:5162-5166; Simeone et al. [1994] Proc. Natl. Acad. Sci. USA 91:2250-2254), consistent with requirements for Dlx function in normal limb development across the animal kingdom. Distal-less also has been implicated in various aspects of vertebrate neurogenesis (see reviews by Kraus and Lufkin [1999] J. Cell. Biochem. 32-33:133-140 and the accompanying review by Beanan and Sargent [2000] Dev. Dyn. 218:000-000). Here, I outline what is known about Dll function and regulation in Drosophila.

Drosophila dumbfounded: a myoblast attractant essential for fusion

Aggregation and fusion of myoblasts to form myotubes is essential for myogenesis in many organisms. In Drosophila the formation of syncytial myotubes is seeded by founder myoblasts. Founders fuse with clusters of fusion-competent myoblasts. Here we identify the gene dumbfounded (duf) and show that it is required for myoblast aggregation and fusion. duf encodes a member of the immunoglobulin superfamily of proteins that is an attractant for fusion-competent myoblasts. It is expressed by founder cells and serves to attract clusters of myoblasts from which myotubes form by fusion.

Autoregulation of the Drosophila disconnected gene in the developing visual system

The Drosophila disconnected (disco) gene is required for the formation of appropriate connections between the larval optic nerve and its target cells in the brain. The disco gene encodes a nuclear protein with two zinc fingers, which suggests that the gene product is a transcription factor. Here, we present data supporting this notion. We find that disco expression in the optic lobe primordium, a group of cells contacted by the developing optic nerve, depends on an autoregulatory feedback loop. We show that wild-type disco function is required for maintenance of disco mRNA and protein expression in the developing optic lobe. In addition, we demonstrate that ubiquitous Disco activity supplied by a heat-inducible gene construct activates expression from the endogenous disco gene specifically in the optic lobe primordium. Consistent with a role of Disco as a transcriptional regulatory protein, we show that portions of the Disco protein are capable of activating the transcription of reporter constructs in a heterologous system. Moreover, we find that the zinc finger portion of Disco binds in vitro to sequences located near the disco transcription unit, suggesting that Disco autoregulates its transcription in the optic lobe primordium by direct binding to a regulatory element in its own promoter.

Dlx-5 in limb initiation in the chick embryo

Dlx-5 is a vertebrate homolog of the Drosophila Distal-less gene, one of the first genetic signals for limb formation in the fly. In the present study we have explored the possible role of Dlx-5 in limb initiation in the chick embryo. At stage 14 which is well before the initial formation of limb buds Dlx-5 is highly and specifically expressed in the ectoderm of the presumptive wing and leg forming regions of the lateral plate, but not in the intervening non-limb forming prospective flank. Thus, Dlx-5 expression distinguishes the limb-forming territories prior to limb budding, and is one of the first molecular markers of vertebrate limb initiation. Furthermore, Dlx-5 expression is induced in the non-limb-forming flank within 12 hours after implantation of an FGF2-soaked bead, a procedure that results in the induction of an ectopic limb. The rapid induction of Dlx-5 expression in response to a signal which ultimately leads to supernumerary limb formation is consistent with a role for Dlx-5 in limb initiation. We have also examined the expression of Dlx-5 in the limb buds of amelic limbless mutant chick embryos, which undergo normal limb formation but do not form an AER and thus fail to undergo further outgrowth. Dlx-5 is transiently expressed by the ectoderm of emergent limbless limb buds, consistent with a role for Dlx-5 in limb initiation. Together, our results suggest that Dlx-5 may be involved in the specification of the limb territories of the lateral plate, and in the initial formation of the limb bud from these regions. Dev Dyn 1999;216:10-15.

Regulation of the Sex combs reduced gene in Drosophila

The Sex combs reduced gene of the Antennapedia complex specifics the identities of the anterior thoracic and posterior head segments, including the primordium of the larval salivary gland. The Sex combs reduced transcription unit spans over 30 kb of genomic DNA, with another 40 kb of upstream cis-regulatory sequences. The pattern of Sex combs reduced transcription is set in the early embryo by the segmentation genes and is then maintained by two competing sets of proteins, the Polycomb group and the trithorax group. One of the trithorax group genes required for activation, the brahma gene, encodes an evolutionarily conserved DNA-stimulated ATPase that is part of a large protein complex. This complex facilitates the action of sequence-specific, DNA-binding proteins in regulating target genes, possibly by altering chromatin structure.

Wing surface interactions in venation patterning in Drosophila

The adult wing of Drosophila consists of two wing surfaces apposed by their basal membranes which first came into contact following disc eversion at metamorphosis. Veins appear in these surfaces in a dorsal-ventral symmetric pattern, but are 'corrugated' (vein cells are more compacted and more pigmented) in a dorsal-ventral asymmetric pattern. We prevented dorsal-ventral contact apposition during wing imaginal disc morphogenesis by implanting fragments of discs into metamorphosing hosts. In these implants, longitudinal veins differentiate but with wider corrugation and in both surfaces. These results and those of genetic mosaics of mutants removing veins or causing ectopic veins reveal mutual dorso-ventral induction/inhibition at work to modulate the final vein differentiation pattern and corrugation.

Nerve dependency of regeneration: the role of Distal-less and FGF signaling in amphibian limb regeneration

Dlx-3, a homolog of Drosophila Dll, has been isolated from an axolotl blastema cDNA library, and its expression in developing and regenerating limbs characterized. The normal expression pattern, and the changes that occur during experimental treatments, indicate a correlation between Dlx-3 expression and the establishment of the outgrowth-permitting epidermis. Dlx-3 is expressed at high levels in a distal-to-proximal gradient in the epidermis of developing limb buds, and is upregulated in the apical ectodermal cap (AEC) during limb regeneration. Expression is maximal at the late bud stage of regeneration, coincident with the transition from the early phase of nerve dependency to the later phase of nerve independence. Dlx-3 expression in the epidermis is rapidly downregulated by denervation during the nerve-dependent phase and is unaffected by denervation during the nerve-independent phase. We investigated this relationship between nerves and Dlx-3 expression by implanting FGF-2 beads into regenerates that had been denervated at a nerve-dependent stage. Dlx-3 expression was maintained by FGF-2 after denervation, and regeneration progressed to completion. In addition, we detected FGF-2 protein in the AEC and in nerves, and observed that the level of expression in both tissues decreases dramatically in response to denervation. We conclude that both limb development and regeneration require a permissive epidermis, characterized by Dlx-3 and FGF expression, both of which are maintained by FGF through an autocrine loop. The transformation of the limb epidermis into a functional AEC that produces and responds to FGF autocatalytically, is presumed to be induced by FGF. Since nerves appear to be a source of this priming FGF, it is possible that a member of the FGF family of growth factors is the elusive neurotrophic factor of limb regeneration.

Sequence and developmental expression of AmphiDll, an amphioxus Distal-less gene transcribed in the ectoderm, epidermis and nervous system: insights into evolution of craniate forebrain and neural crest

The dynamic expression patterns of the single amphioxus Distal-less homolog (AmphiDll) during development are consistent with successive roles of this gene in global regionalization of the ectoderm, establishment of the dorsoventral axis, specification of migratory epidermal cells early in neurulation and the specification of forebrain. Such a multiplicity of Distal-less functions probably represents an ancestral chordate condition and, during craniate evolution, when this gene diversified into a family of six or so members, the original functions evidently tended to be parcelled out among the descendant genes. In the amphioxus gastrula, AmphiDll is expressed throughout the animal hemisphere (presumptive ectoderm), but is soon downregulated dorsally (in the presumptive neural plate). During early neurulation, AmphiDll-expressing epidermal cells flanking the neural plate extend lamellipodia, appear to migrate over it and meet mid-dorsally. Midway in neurulation, cells near the anterior end of the neural plate begin expressing AmphiDll and, as neurulation terminates, these cells are incorporated into the dorsal part of the neural tube, which forms by a curling of the neural plate. This group of AmphiDll-expressing neural cells and a second group expressing the gene a little later and even more anteriorly in the neural tube demarcate a region that comprises the anterior three/fourths of the cerebral vesicle; this region of the amphioxus neural tube, as judged by neural expression domains of craniate Distal-less-related genes, is evidently homologous to the craniate forebrain. Our results suggest that craniates evolved from an amphioxus-like creature that had the beginnings of a forebrain and possibly a precursor of neural crest - namely, the cell population leading the epidermal overgrowth of the neural plate during early neurulation.

Establishment of neuronal connectivity during development of the Drosophila larval visual system

We used confocal microscopy in conjunction with specific antibodies and enhancer trap strains to investigate the development of specific neuronal connections in a simple model system, the larval visual system of Drosophila. We find that the establishment of axonal projections from the larval photoreceptor neurons to their central nervous system targets involves a series of discrete steps. During embryogenesis, the larval optic nerve contacts several different cell types, including optic lobe pioneer (OLP) neurons and a number of glial cells. We demonstrate that OLP neurons are present and project normally in glass (gl) mutant embryos in which the larval optic nerve fails to develop, suggesting that they do not depend on interactions with the larval optic nerve for differentiation and proper axonal projection. The OLPs fail to differentiate properly is disconnected (disco) mutant embryos, where appropriate connections between the larval optic nerve and its targets in the brain are not formed. The disco gene is expressed in the OLPs and may therefore act autonomously to direct the differentiation of these cells. Taken together, our results suggest that the OLPs act as an intermediate target required for the establishment of normal optic nerve projection and connectivity.

Dachsous encodes a member of the cadherin superfamily that controls imaginal disc morphogenesis in Drosophila

Mutations in the dachsous gene of Drosophila lead to striking defects in the morphogenesis of the thorax, legs, and wings. The dachsous gene has been cloned and shown to encode a huge transmembrane protein that is a member of the cadherin superfamily, similar to the fat gene reported previously. Both the Dachsous and Fat proteins contain large tandem arrays of cadherin domains--27 and 34, respectively--as compared with 4 cadherin domains in classic vertebrate cadherins. In addition, Dachsous and Fat each has a cytoplasmic domain with sequence similarity to the cytoplasmic beta-catenin-binding domain of classic vertebrate cadherins. The dachsous gene is expressed in the ectoderm of embryos, whereas its expression in larvae is restricted to imaginal discs and specific regions of the brain. The phenotypes of, and genetic interactions between dachsous and fat are consistent with a model in which cell proliferation and morphogenesis of imaginal structures depends on the coupled equilibria between homo- and heterophilic interactions of the Dachsous and Fat cadherin proteins.

Homeotic genes and the regulation and evolution of insect wing number

The evolution of wings catalysed the radiation of insects which make up some 75 per cent of known animals. Fossil evidence suggests that wings evolved from a segment of the leg and that early pterygotes bore wings on all thoracic and abdominal segments. The pterygote body plan subsequently diverged producing orders bearing three, two or just one pair of thoracic wings. We have investigated the role of homeotic genes in pterygote evolution by examining their function in Drosophila wing development and their expression in a primitive apterygote. Wing formation is not promoted by any homeotic gene, but is repressed in different segments by different homeotic genes. We suggest here that wings first arose without any homeotic gene involvement in an ancestor with a homeotic 'groundplan' similar to modern winged insects and that wing formation subsequently fell under the negative control of individual homeotic genes at different stages of pterygote evolution.

Compartments and appendage development in Drosophila

The appendages of Drosophila develop from the imaginal discs. During the extensive growth of these discs cell lineages are for the most part unfixed, suggesting a strong role for cell-cell interactions in controlling the final pattern of differentiation. However, during early and middle stages of development, discs are subdivided by strict lineage restrictions into a small number of spatially distinct compartments. These compartments appear to be maintained by stably inheriting states of gene expression; the compartment-specific expression of two such 'selector'-like genes, engrailed and apterous, are critical for anterior-posterior and dorso-ventral compartmentalization, respectively. Recent work suggests that one purpose of compartmentalization is to establish regions of specialized cells near compartment boundaries via intercompartmental induction, using molecules like the hedgehog protein. Thus, compartments can act as organizing centers for patterning within compartments. Evidence for non-compartmental patterning mechanisms will also be discussed.

Engrailed-mediated repression of Ultrabithorax is necessary for the parasegment 6 identity in Drosophila

The homeotic genes of Drosophila are expressed in overlapping domains along the anterior-to-posterior axis and specify the distinct morphological patterns of each parasegment. Within single parasegments, the levels of homeotic gene expression are often modulated, in part because of cross-regulation by other homeotic gene products. However, the functional significance of different levels of homeotic gene expression is unclear. Here modulations in Ultrabithorax (Ubx) expression within parasegment 6 are examined. Specifically, Ubx is shown to be down-regulated in the posterior compartment of this parasegment by engrailed (en). The significance of Ubx repression by en was demonstrated by characterizing the expression of the Ubx target gene, Distal-less (Dll). In the posterior compartment of parasegment 6, Dll is normally expressed in a small cluster of cells. If Ubx is expressed uniformly via a heat-shock promoter, Dll is inappropriately repressed in these posterior compartment cells. In the anterior compartment of parasegment 6, Dll is normally repressed by high levels of Ubx. However, if en is expressed uniformly via a heat-shock promoter, Ubx is repressed and Dll is derepressed. Because Dll is required for the development of larval sensory structures, these results demonstrate that en-mediated repression of Ubx in the posterior compartment is necessary for the morphology of parasegment 6. Thus, different levels of homeotic gene expression can be important for their segmental patterning functions.

Diploidy of Drosophila imaginal cells is maintained by a transcriptional repressor encoded by escargot

The Drosophila escargot (esg) gene encodes a C2-H2-type zinc finger protein that is expressed in the imaginal discs and histoblasts. In some esg mutants, the abdominal histoblasts become polyploid. It has therefore been suggested that the role of esg is to maintain diploidy of the imaginal cells. We show that esg encodes a DNA-binding protein with high affinity for G/ACAGGTG, the consensus-binding sequence for the basic helix-loop-helix (bHLH) family of transcription factors (E2 box). This DNA-binding activity is essential for esg function in vivo as the strong embryonic lethal allele esgVS8 is caused by an amino acid change within the zinc finger region, leading to reduced affinity for DNA. In cultured cells, a heterodimer of the bHLH proteins Scute and Daughterless activates transcription from promoters containing E2 boxes. The esg protein strongly inhibits this activation, suggesting that esg may regulate developmental processes dependent on bHLH proteins. In larvae, esg protein expressed by the heat shock promoter can rescue the polyploid phenotype of abdominal histoblasts, demonstrating that the phenotype is attributable to a loss of esg function. esg must be expressed continuously during the larval period for efficient rescue. Ectopic expression of esg in the salivary glands inhibits endoreplication of DNA. These results suggest that esg is involved in transcriptional inhibition of genes required for endoreplication.

The role of apterous in the control of dorsoventral compartmentalization and PS integrin gene expression in the developing wing of Drosophila

During the development of Drosophila appendages from imaginal discs lineage restrictions appear that prevent dividing cells from crossing between regionally distinct compartments. These compartments correspond not only to regions of cell lineage restrictions but also to regions of specific gene expression. When compartments were first discovered, it was proposed that their formation relied on compartment-specific ‘selector’ gene activity; engrailed is thought to play such a role for the early-arising anterior-posterior restriction. Recent results suggest that the dorsally expressed transcription factor encoded by apterous may control dorsoventral identity in the wing. In this study we use mosaic analysis to show that apterous maintains the late-arising dorsoventral lineage restriction in a manner that strongly supports the selector gene hypothesis: loss of apterous function from dorsal cells after the formation of the boundary causes them to cross into the ventral compartment. Moreover, we show that apterous plays a role controlling patterns of gene expression in the developing wing disc. The PS1 and PS2 integrins are normally expressed in primarily dorsal-specific and ventral-specific patterns, respectively. We show that ectopic expression of apterous induces ectopic ventral expression of PS1 integrin and alpha PS1 mRNA, while loss of apterous can induce the ectopic dorsal expression of PS2 integrin. Thus, apterous plays a selector-like role both in terms of the control of lineage restrictions and the regulation of downstream gene expression.

wingless acts through the shaggy/zeste-white 3 kinase to direct dorsal-ventral axis formation in the Drosophila leg

The secreted glycoproteins encoded by Wnt genes are thought to function as intercellular signaling molecules which convey positional information. Localized expression of Wingless protein is required to specify the fate of ventral cells in the developing Drosophila leg. We report here that Wingless acts through inactivation of the shaggy/zeste white 3 protein kinase to specify ventral cell fate in the leg. Ectopic expression of Wingless outside its normal ventral domain has been shown reorganize the dorsal-ventral axis of the leg in a non-autonomous manner. Using genetic mosaics, we show that cells that lack shaggy/zeste white 3 activity can influence the fate of neighboring cells to reorganize dorsal-ventral pattern in the leg, in the same manner as Wingless-expressing cells. Therefore, clones of cells that lack shaggy/zeste white 3 activity exhibit all of the organizer activity previously attributed to Wingless-expressing cells, but do so without expressing wingless. We also show that the organizing activity of ventral cells depends upon the location of the clone along the dorsal-ventral axis. These findings suggest that Wingless protein does not function as a morphogen in the dorsal-ventral axis of the leg.

Interactions of decapentaplegic, wingless, and Distal-less in the Drosophila leg

The genes decapentaplegic, wingless, and Distalless appear to be instrumental in constructing the anatomy of the adult Drosophila leg. In order to investigate how these genes function and whether they act coordinately, we analyzed the leg phenotypes of the single mutants and their inter se double mutant compounds. In decapentaplegic the tarsi frequently exhibit dorsal deficiencies which suggest that the focus of gene action may reside dorsally rather than distally. In wingless the tarsal hinges are typically duplicated along with other dorsal structures, confirming that the hinges arise dorsally. The plane of symmetry in double-ventral duplications caused by decapentaplegic is virtually the same as the plane in double-dorsal duplications caused by wingless. It divides the fate map into two parts, each bisected by the dorsoventral axis. In the double mutant decapentaplegic wingless the most ventral and dorsal tarsal structures are missing, consistent with the notion that both gene products function as morphogens. In wingless Distal-less compounds the legs are severely truncated, indicating an important interaction between these genes. Distal-less and decapentaplegic manifest a relatively mild synergism when combined.

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.

Pattern formation in the limbs of Drosophila: bric a brac is expressed in both a gradient and a wave-like pattern and is required for specification and proper segmentation of the tarsus

We have identified the gene bric a brac and show that it is required for pattern formation along the proximal-distal axis of the leg and antenna of Drosophila. In bric a brac mutant legs, the bristle pattern of the three central tarsal segments is transformed towards the pattern of the most proximal tarsal segment. In addition, bric a brac mutant legs and antennae have segmentation defects. bric a brac encodes a nuclear protein that shares a highly conserved domain with two transcription factors from Drosophila. bric a brac function is dosage dependent and is required in a graded manner for the specification of tarsal segments. The graded requirement for bric a brac correlates with its graded expression pattern, suggesting that the concentration of BRIC A BRAC protein specifies segment identity in the tarsus.