extradenticle determines segmental identities throughout Drosophila development

Control of vertebrate limb outgrowth by the proximal factor Meis2 and distal antagonism of BMPs by Gremlin

The mechanisms controlling growth and patterning along the proximal-distal axis of the vertebrate limb are yet to be understood. We show that restriction of expression of the homeobox gene Meis2 to proximal regions of the limb bud is essential for limb development, since ectopic Meis2 severely disrupts limb outgrowth. We also uncover an antagonistic relationship between the secreted factors Gremlin and BMPs required to maintain the Shh/FGF loop that regulates distal outgrowth. These proximal and distal factors have coordinated activities: Meis2 can repress distal genes, and Bmps and Hoxd genes restrict Meis2 expression to the proximal limb bud. Moreover, combinations of BMPs and AER factors are sufficient to distalize proximal limb cells. Our results unveil a novel set of proximal-distal regulatory interactions that establish and maintain outgrowth of the vertebrate limb.

The Berkeley Drosophila Genome Project gene disruption project: Single P-element insertions mutating 25% of vital Drosophila genes

A fundamental goal of genetics and functional genomics is to identify and mutate every gene in model organisms such as Drosophila melanogaster. The Berkeley Drosophila Genome Project (BDGP) gene disruption project generates single P-element insertion strains that each mutate unique genomic open reading frames. Such strains strongly facilitate further genetic and molecular studies of the disrupted loci, but it has remained unclear if P elements can be used to mutate all Drosophila genes. We now report that the primary collection has grown to contain 1045 strains that disrupt more than 25% of the estimated 3600 Drosophila genes that are essential for adult viability. Of these P insertions, 67% have been verified by genetic tests to cause the associated recessive mutant phenotypes, and the validity of most of the remaining lines is predicted on statistical grounds. Sequences flanking >920 insertions have been determined to exactly position them in the genome and to identify 376 potentially affected transcripts from collections of EST sequences. Strains in the BDGP collection are available from the Bloomington Stock Center and have already assisted the research community in characterizing >250 Drosophila genes. The likely identity of 131 additional genes in the collection is reported here. Our results show that Drosophila genes have a wide range of sensitivity to inactivation by P elements, and provide a rationale for greatly expanding the BDGP primary collection based entirely on insertion site sequencing. We predict that this approach can bring >85% of all Drosophila open reading frames under experimental control.

Regeneration in insects

@9cIntroduction@21T issues exhibit an impressive ability to respond to a myriad of insults by repairing and regenerating complex structures. The elegant and orderly process of regeneration provides clues to the mechanisms of pattern formation but also offers the hope that the process might one day be manipulated to replace damaged body parts. To manipulate the process, it will be necessary to understand the genetic basis of the process. In the case of the insect leg, we are coming close to such a level of understanding and many of the lessons learned are relevant to vertebrate systems. A dynamic web of gene regulatory networks appears to create a robust self-organizing system that is at once extremely intricate but also perhaps simple in its reliance on a few key signaling pathways and a few simple processes, e.g. autoactivation and lateral inhibition. Here we will summarize what has been learned about the networks of gene regulation present in the Drosophila leg discs and then we will explore how the regenerative responses to different insults can be understood as predictable responses to these networks. Each of the regulatory networks could themselves serve as the subject of a detailed review and that is beyond the scope of this discussion. Here we will focus on the interplay between the regulatory networks in patterning the tissue.

Proximal to distal cell communication in the Drosophila leg provides a basis for an intercalary mechanism of limb patterning

Proximodistal patterning in the Drosophila leg is elaborated from the circular arrangement of the proximal domain expressing escargot and homothorax, and the distal domain expressing Distal-less that are allocated during embryogenesis. The distal domain differentiates multiply segmented distal appendages by activating additional genes such as dachshund. Secreted signaling molecules Wingless and Decapentaplegic, expressed along the anterior-posterior compartment boundary, are required for activation of Distal-less and dachshund and repression of homothorax in the distal domain. However, whether Wingless and Decapentaplegic are sufficient for the circular pattern of gene expression is not known. Here we show that a proximal gene escargot and its activator homothorax regulate proximodistal patterning in the distal domain. Clones of cells expressing escargot or homothorax placed in the distal domain induce intercalary expression of dachshund in surrounding cells and reorient planar cell polarity of those cells. Escargot and homothorax-expressing cells also sort out from other cells in the distal domain. We suggest that inductive cell communication between the proximodistal domains, which is maintained in part by a cell-sorting mechanism, is the cellular basis for an intercalary mechanism of the proximodistal axis patterning of the limb.

A common mechanism for antenna-to-Leg transformation in Drosophila: suppression of homothorax transcription by four HOM-C genes

The Drosophila HOM-C genes encode transcription factors containing the DNA-binding homeodomain. Mutations in the HOM-C genes can cause specific homeotic transformation, suggesting that the HOM-C genes determine segmental identities by acting on different target genes. However, misexpression of several HOM-C genes in the antenna disc causes similar antenna-to-leg transformations. Here we show that the Scr, Antp, Ubx, and abd-A HOM-C genes all exert their effects through a common mechanism: suppressing the transcription of the homothorax (hth) homeobox gene and thereby preventing the nuclear localization of the Extradenticle homeodomain protein. We also show that ectopic hth expression can cause duplication of the proximodistal axis of the antenna, suggesting that it is involved in proximodistal development of the antenna.

Patterning mechanisms in the body trunk and the appendages of Drosophila

During evolution, many animal groups have developed specialised outgrowths of the body wall, limbs or appendages. The type of appendage depends on the identity of the segment where they appear, indicating that the Hox genes contribute to appendage specification. Moreover, work carried out principally in Drosophila has identified the gene products and the mechanisms involved in pattern formation in the appendages. In this essay, we compare the morphogenetic processes in the appendages and the body wall; the function of the Hox genes and the response to the signalling molecules involved in local patterning. We speculate that, although the basic mechanisms are similar, there are significant differences in the manner the body trunk and appendages respond to them.

Notch-mediated segmentation and growth control of the Drosophila leg

The possession of segmented appendages is a defining characteristic of the arthropods. By analyzing both loss-of-function and ectopic expression experiments, we show that the Notch signaling pathway plays a fundamental role in the segmentation and growth of the Drosophila leg. Local activation of Notch is necessary and sufficient to promote the formation of joints between segments. This segmentation process requires the participation of the Notch ligands, Serrate and Delta, as well as Fringe. These three proteins are each expressed in the developing leg and antennal imaginal discs in a segmentally repeated pattern that is regulated downstream of the action of Wingless and Decapentaplegic. Our studies further show that Notch activation is both necessary and sufficient to promote leg growth. We also identify target genes regulated both positively and negatively downstream of Notch signaling that are required for normal leg development. Together, these observations outline a regulatory hierarchy for the segmentation and growth of the leg. The Notch pathway is also deployed for segmentation during vertebrate somitogenesis, which raises the possibility of a common origin for the segmentation of these distinct tissues.

HOXA9 forms triple complexes with PBX2 and MEIS1 in myeloid cells

Aberrant activation of the HOX, MEIS, and PBX homeodomain protein families is associated with leukemias, and retrovirally driven coexpression of HOXA9 and MEIS1 is sufficient to induce myeloid leukemia in mice. Previous studies have demonstrated that HOX-9 and HOX-10 paralog proteins are unique among HOX homeodomain proteins in their capacity to form in vitro cooperative DNA binding complexes with either the PBX or MEIS protein. Furthermore, PBX and MEIS proteins have been shown to form in vivo heterodimeric DNA binding complexes with each other. We now show that in vitro DNA site selection for MEIS1 in the presence of HOXA9 and PBX yields a consensus PBX-HOXA9 site. MEIS1 enhances in vitro HOXA9-PBX protein complex formation in the absence of DNA and forms a trimeric electrophoretic mobility shift assay (EMSA) complex with these proteins on an oligonucleotide containing a PBX-HOXA9 site. Myeloid cell nuclear extracts produce EMSA complexes which appear to contain HOXA9, PBX2, and MEIS1, while immunoprecipitation of HOXA9 from these extracts results in coprecipitation of PBX2 and MEIS1. In myeloid cells, HOXA9, MEIS1, and PBX2 are all strongly expressed in the nucleus, where a portion of their signals are colocalized within nuclear speckles. However, cotransfection of HOXA9 and PBX2 with or without MEIS1 minimally influences transcription of a reporter gene containing multiple PBX-HOXA9 binding sites. Taken together, these data suggest that in myeloid leukemia cells MEIS1 forms trimeric complexes with PBX and HOXA9, which in turn can bind to consensus PBX-HOXA9 DNA targets.

Structure of a DNA-bound Ultrabithorax-Extradenticle homeodomain complex

During the development of multicellular organisms, gene expression must be tightly regulated, both spatially and temporally. One set of transcription factors that are important in animal development is encoded by the homeotic (Hox) genes, which govern the choice between alternative developmental pathways along the anterior-posterior axis. Hox proteins, such as Drosophila Ultrabithorax, have low DNA-binding specificity by themselves but gain affinity and specificity when they bind together with the homeoprotein Extradenticle (or Pbxl in mammals). To understand the structural basis of Hox-Extradenticle pairing, we determine here the crystal structure of an Ultrabithorax-Extradenticle-DNA complex at 2.4 A resolution, using the minimal polypeptides that form a cooperative heterodimer. The Ultrabithorax and Extradenticle homeodomains bind opposite faces of the DNA, with their DNA-recognition helices almost touching each other. However, most of the cooperative interactions arise from the YPWM amino-acid motif of Ultrabithorax-located amino-terminally to its homeodomain-which forms a reverse turn and inserts into a hydrophobic pocket on the Extradenticle homeodomain surface. Together, these protein-DNA and protein-protein interactions define the general principles by which homeotic proteins interact with Extradenticle (or Pbx1) to affect development along the anterior-posterior axis of animals.

Functional and cooperative interactions between the homeodomain PDX1, Pbx, and Prep1 factors on the somatostatin promoter

Expression of the somatostatin gene in endocrine pancreatic cells is controlled by several regulatory cis-elements located in the promoter region. Among these, the adjacent UE-A and TSEI elements, located from -113 to -85 relative to the transcription initiation site, function in combination and act as a pancreas-specific mini-enhancer. The TSEI element is recognized by the pancreatic homeodomain factor PDX1. In the present study, we show that the UE-A element binds a heterodimeric complex composed of a Pbx factor and the Prep1 protein, both belonging to the atypical three-amino acid loop extension homeodomain family. Recombinant Pbx1 and Prep1 proteins bind cooperatively to the UE-A site, whereas neither protein can bind this site alone. Transient transfection experiments reveal that both Pbx1 and Prep1 are required to generate a strong transcriptional activation from the UE-A element when this element is inserted close to the TATA box. In contrast, in the context of the intact somatostatin promoter or mini-enhancer, Pbx1 and Prep1 alone have no effect, but they produce a drastic activation when the pancreatic homeodomain factor PDX1 is also coexpressed. Thus, the activity of the somatostatin mini-enhancer is mediated by a cooperative interaction between the Pbx-Prep1 heterodimeric complex and the pancreatic factor PDX1.

Proximodistal axis formation in the Drosophila leg: subdivision into proximal and distal domains by Homothorax and Distal-less

The developing legs of Drosophila are subdivided into proximal and distal domains by the activity of the homeodomain proteins Homothorax (Hth) and Distal-less (Dll). The expression domains of Dll and Hth are initially reciprocal. Wingless and Dpp define both domains by activating Dll and by repressing Hth in the distal region of the disc. Wg and Dpp do not act through Dll to repress Hth. Hth functions to reduce the sensitivity of proximal cells to Wg and Dpp. This serves to limit the effective range of these signals in regulating later-acting genes such as Dac. We present evidence that proximal and distal cells tend to sort-out from one another. Cells forced to express Hth are unable to mix with distal cells. Likewise, cells forced to express Dll are unable to mix with proximal cells. Clones of cells unable to express Dll in the distal region sort-out from the disc. Clones of cells unable to express Hth lose the specialized population of cells at the interface between proximal and distal territories and cause fusion between body wall and leg segments. These observations suggest that sorting-out behavior of Hth- and Dll-expressing cells contributes to subdivision of the leg into proximal and distal domains.

A genetic screen of the Drosophila X chromosome for mutations that modify Deformed function

We have screened the Drosophila X chromosome for genes whose dosage affects the function of the homeotic gene Deformed. One of these genes, extradenticle, encodes a homeodomain transcription factor that heterodimerizes with Deformed and other homeotic Hox proteins. Mutations in the nejire gene, which encodes a transcriptional adaptor protein belonging to the CBP/p300 family, also interact with Deformed. The other previously characterized gene identified as a Deformed interactor is Notch, which encodes a transmembrane receptor. These three genes underscore the importance of transcriptional regulation and cell-cell signaling in Hox function. Four novel genes were also identified in the screen. One of these, rancor, is required for appropriate embryonic expression of Deformed and another homeotic gene, labial. Both Notch and nejire affect the function of another Hox gene, Ultrabithorax, indicating they may be required for homeotic activity in general.

Generation of multiple antagonistic domains along the proximodistal axis during Drosophila leg development

homothorax (hth) is a Drosophila member of the Meis family of homeobox genes. hth function is required for the nuclear localization of the Hox cofactor Extradenticle (EXD). We show here that there is also a post-transcriptional control of HTH by exd: exd activity is required for the apparent stability of the HTH protein. In leg imaginal discs, hth expression is limited to the domain of exd function and this domain is complementary to the domain in which the Wingless (WG) and Decapentaplegic (DPP) signals are active. We demonstrate that WG and DPP act together through their targets Distal-less (Dll) and dachshund (dac) to restrict hth expression, and therefore EXD's nuclear localization, to the most proximal regions of the leg disc. Furthermore, there is a reciprocal repression exerted by HTH on these and other DPP and WG downstream targets that restricts their expression to non-hth-expressing cells. Thus, there exists in the leg disc a set of mutually antagonistic interactions between proximal cells, which we define as those that express hth, and distal cells, or those that do not express hth. In addition, we show that dac negatively regulates Dll. We suggest that these antagonistic relationships help to convert the WG and DPP activity gradients into discreet domains of gene expression along the proximodistal axis.

Analysis in Drosophila melanogaster of the interaction between sex combs reduced and extradenticle activity in the determination of tarsus and arista identity

Sex Combs Reduced (SCR) activity is proposed to be required cell nonautonomously for determination of tarsus identity, and Extradenticle (EXD) activity is required cell autonomously for determination of arista identity. Using the ability of Proboscipedia to inhibit the SCR activity required for determination of tarsus identity, we found that loss-of-EXD activity is epistatic to loss-of-SCR activity in tarsus vs. arista determination. This suggests that in the sequence leading to arista determination SCR activity is OFF while EXD activity is ON, and in the sequence leading to tarsus determination SCR activity is ON, which turns EXD activity OFF. Immunolocalization of EXD in early third-instar larval imaginal discs reveals that EXD is localized in the nuclei of antennal imaginal disc cells and localized in the cytoplasm of distal imaginal leg disc cells. We propose that EXD localized to the nucleus suppresses tarsus determination and activates arista determination. We further propose that in the mesodermal adepithelial cells of the leg imaginal discs, SCR is required for the synthesis of a tarsus-inducer that when secreted acts on the ectoderm cells inhibiting nuclear accumulation of EXD, such that tarsus determination is no longer suppressed and arista determination is no longer activated.

Antagonism between extradenticle function and Hedgehog signalling in the developing limb

The Drosophila homeobox gene extradenticle (exd) encodes a highly conserved cofactor of Hox proteins. exd activity is regulated post-translationally by a mechanism involving nuclear translocation; only nuclear Exd protein is functional. The exd gene is required for patterning of the proximal region of the leg, whereas patterning of the distal region requires signalling by the Wingless (Wg) and Decapentaplegic (Dpp) proteins, which are in turn activated by Hedgehog (Hh). Here we show that exd function and Dpp/Wg signalling are antagonistic and divide the leg into two mutually exclusive domains. In the proximal domain, exd activity prevents cells from responding to Dpp and Wg. Conversely, in the distal domain, exd function is suppressed by the Dpp/Wg response gene Distal-less (Dll), which prevents the nuclear transport of Exd. We also found that the product of a murine homologue of exd (Pbx1) is regulated at the subcellular level, and that its pattern of nuclear localization in the mouse limb resembles that of Exd in the Drosophila leg. These findings suggest that the division of the limb into two antagonistic domains, as defined by exd (Pbx1) function and Hh signalling, may be a general feature of limb development.

Experimental analysis of character coupling across a complex life cycle: pigment pattern metamorphosis in the tiger salamander, Ambystoma tigrinum tigrinum

Developmental relationships among characters are expected to bias patterns of morphological variation at the population level. Studies of character development thus can provide insights into processes of adaptation and the evolutionary diversification of morphologies. Here I use experimental manipulations to test whether larval and adult pigment patterns are coupled across metamorphosis in the tiger salamander, Ambystoma tigrinum tigrinum (Ambystomatidae). Previous investigations showed that the early larval pigment pattern depends on interactions between pigment cells and the lateral line sensory system. In contrast, the results of this study demonstrate that the major features of the adult pigment pattern develop largely independently of both the early larval pattern and the lateral lines. These results suggest that ontogenetic changes that occur across metamorphosis decouple larval and adult pigment patterns and could thereby facilitate independent evolutionary modifications to the patterns during different stages of the life cycle.

Control of antennal versus leg development in Drosophila

During the evolution of insects from a millipede-like ancestor, the Hox genes are thought to have promoted the diversification of originally identical body structures. In Drosophila melanogaster, antennae and legs are homologous structures that differ from each other as a result of the Hox gene Antennapedia (Antp), which promotes leg identities by repressing unknown antennal-determining genes. Here we present four lines of evidence that identify extradenticle (exd) and homothorax (hth) as antennal-determining genes. First, removing the function of exd or hth, which is required for the nuclear localization of Exd protein, transforms the antenna into leg; such transformations occur without activation of Antp. Second, hth is expressed and Exd is nuclear in most antennal cells, whereas both are restricted to proximal cells of the leg. Third, Antp is a repressor of hth. Fourth, ectopic expression of Meis1, a murine hth homologue, can trigger antennal development elsewhere in the fly. Taken together, these data indicate that hth is an antennal selector gene, and that Antp promotes leg development by repressing hth and consequently nuclear Exd.

Functional and regulatory interactions between Hox and extradenticle genes

The homeobox gene extradenticle (exd) acts as a cofactor of Hox function both in Drosophila and vertebrates. It has been shown that the distribution of the Exd protein is developmentally regulated at the post-translational level; in the regions where exd is not functional Exd is present only in the cell cytoplasm, whereas it accumulates in the nuclei of cells requiring exd function. We show that the subcellular localization of Exd is regulated by the BX-C genes and that each BX-C gene can prevent or reduce nuclear translocation of Exd to different extents. In spite of this negative regulation, two BX-C genes, Ultrabithorax and abdominal-A, require exd activity for their maintenance and function. We propose that mutual interactions between Exd and BX-C proteins ensure the correct amounts of interacting molecules. As the Hoxd10 gene has the same properties as Drosophila BX-C genes, we suggest that the control mechanism of subcellular distribution of Exd found in Drosophila probably operates in other organisms as well.

Nuclear translocation of extradenticle requires homothorax, which encodes an extradenticle-related homeodomain protein

We show that homothorax (hth) is required for the Hox genes to pattern the body of the fruit fly, Drosophila melanogaster. hth is necessary for the nuclear localization of an essential HOX cofactor, Extradenticle (EXD), and encodes a homeodomain protein that shares extensive identity with the product of Meis1, a murine proto-oncogene. MEIS1 is able to rescue hth mutant phenotypes and can induce the cytoplasmic-to-nuclear translocation of EXD in cell culture and Drosophila embryos. Thus, Meis1 is a murine homolog of hth. MEIS1/HTH also specifically binds to EXD with high affinity in vitro. These data suggest a novel and evolutionarily conserved mechanism for regulating HOX activity in which a direct protein-protein interaction between EXD and HTH results in EXD's nuclear translocation.

A novel fibroblast growth factor gene expressed in the developing nervous system is a downstream target of the chimeric homeodomain oncoprotein E2A-Pbx1

Pbx1 is a homeodomain transcription factor that has the ability to form heterodimers with homeodomain proteins encoded by the homeotic selector (Hox) gene complexes and increase their DNA-binding affinity and specificity. A current hypothesis proposes that interactions with Pbx1 are necessary for Hox proteins to regulate downstream target genes that in turn control growth, differentiation and morphogenesis during development. In pre B cell leukemias containing the t(1;19) chromosome translocation, Pbx1 is converted into a strong transactivator by fusion to the activation domain of the bHLH transcription factor E2A. The E2A-Pbx1 fusion protein should therefore activate transcription of genes normally regulated by Pbx1. We have used the subtractive process of representational difference analysis to identify targets of E2A-Pbx1. We show that E2A-Pbx1 can directly activate transcription of a novel member of the fibroblast growth factor family of intercellular signalling molecules, FGF-15. The FGF-15 gene is expressed in a regionally restricted pattern in the developing nervous system, suggesting that FGF-15 may play an important role in regulating cell division and patterning within specific regions of the embryonic brain, spinal cord and sensory organs.

Switching the in vivo specificity of a minimal Hox-responsive element

The homeodomain proteins encoded by the Hox complex genes do not bind DNA with high specificity. In vitro, Hox specificity can be increased by binding to DNA cooperatively with the homeodomain protein extradenticle or its vertebrate homologs, the pbx proteins (together, the PBC family). Here we show that a two basepair change in a Hox-PBC binding site switches the Hox-dependent expression pattern generated in vivo, from labial to Deformed. The change in vivo correlates with an altered Hox binding specificity in vitro. Further, we identify similar Deformed-PBC binding sites in the Deformed and Hoxb-4 genes and show that they generate Deformed or Hoxb-4 expression patterns in Drosophila and mouse embryos, respectively. These results suggest a model in which Hox-PBC binding sites play an instructive role in Hox specificity by promoting the formation of different Hox-PBC heterodimers in vivo. Thus, the choice of Hox partner, and therefore Hox target genes, depends on subtle differences between Hox-PBC binding sites.

The Pbx family of proteins is strongly upregulated by a post-transcriptional mechanism during retinoic acid-induced differentiation of P19 embryonal carcinoma cells

Retinoic acid (RA) induces expression of genes encoding the Hox family of transcription factors, whose differential expression orchestrates developmental programs specifying anterior-posterior structures during embryogenesis. Hox proteins bind DNA as monomers and heterodimers with Pbx proteins. Here we show that RA upregulates Pbx protein abundance coincident with transcriptional activation of Hox genes in P19 embryonal carcinoma cells undergoing neuronal differentiation. However, in contrast to Hox induction, Pbx upregulation is predominantly a result of post-transcriptional mechanisms. Interestingly, Pbx1, Pbx2, and Pbx3 exhibit different profiles of upregulation, suggesting possible functional divergence. The parallel upregulation of Pbx and Hox proteins in this model suggests an important role for transcriptional control by Pbx-Hox heterodimers during neurogenesis, and argues for precise control by RA.

Nucleocytoplasmic localisation of extradenticle protein is spatially regulated throughout development in Drosophila

The extradenticle protein is a homeodomain transcription factor which has an important role regulating the DNA-binding specificity of homeotic selector proteins. We have made a monoclonal antibody against extradenticle and have studied the expression of the protein in the embryo and in imaginal discs. We find that extradenticle is initially uniformly distributed as expected but strikingly is excluded from nuclei until gastrulation. During the extended germ band stage the protein remains predominantly cytoplasmic and does not accumulate in nuclei until germ band retraction. Nuclear accumulation occurs in a highly spatially regulated pattern. In the imaginal discs the nuclear accumulation of extradenticle is also spatially regulated and, in the wing and leg discs, distal regions exhibit cytoplasmic extradenticle whereas proximally the protein is nuclear. We suggest that this regulation of the sub-cellular localisation of extradenticle is important for the interactions between extradenticle and the homeotic selector proteins and that extradenticle is not simply a ubiquitously available cofactor.

Genetic evidence for the subdivision of the arthropod limb into coxopodite and telopodite

Arthropod appendages are thought to have evolved as outgrowths from the body wall of a limbless ancestor. Snodgrass, in his Principles of Insect Morphology (1935), proposed that, during evolution, expansion of the body wall would originate the base of the appendages, or coxopodite, upon which the most distal elements that represent the true outer limb, or telopodite, would develop. The homeobox gene Distal-less (Dll), which is required in the Drosophila appendages for development of distal regions, has been proposed to promote formation of telopodite structures above the evolutionary ground-state of non-limb or body wall. Here, we present evidence that another homeobox gene, extradenticle (exd), which is required for appropriate development of the trunk and the proximal parts of the appendages, represents a coxopodite gene. We show that exd function is eliminated from the distal precursors in the developing limb and remains restricted to proximal precursors throughout development. This elimination is important because, when ectopically expressed, exd prevents distal development and gives rise to truncated appendages lacking distal elements. Moreover, the maintenance of exd expression during larval stages, contrary to Dll, does not require the hedgehog (hh) signaling pathway, suggesting that the proximal regions of the appendages develop independently of hh function. Finally, we show that in the crustacean Artemia, exd and Dll are expressed in comparable patterns as in Drosophila, suggesting a conserved genetic mechanism subdividing the arthropod limb.

Nuclear import of the homeodomain protein extradenticle in response to Wg and Dpp signalling

In Drosophila, Decapentaplegic (Dpp) and Wingless (Wg) are two secreted signalling proteins of the transforming growth factor (TGF)-beta and Wnt families, respectively. Although both are often required during development, only a few downstream components of these signalling pathways have been described. Here we present evidence that in the embryonic midgut both signalling pathways control the subcellular localization of the homeodomain protein encoded by the extradenticle (exd) gene. Exd protein is predominantly nuclear in endoderm cells close to Dpp-and Wg-secreting cells of the visceral mesoderm, but is in the cytoplasm in more distant endoderm cells. Both dpp and wg are required for the nuclear localization of Exd in the endoderm, whereas ectopic expression of dpp and wg expands the domain of nuclear Exd. Furthermore, the nuclear import of Exd correlates with the transcription of an exd-dependent reporter gene in the endoderm. Thus one mechanism by which extracellular signals might control pattern is by directing the graded nuclear localization of homeodomain proteins such as Exd that directly control the expression of target genes.