Homeotic genes of the bithorax complex repress limb development in the abdomen of the Drosophila embryo through the target gene Distal-less

Hox proteins meet more partners

The Hox genes are clustered sets of homeobox-containing genes that play a central role in animal development. Recent genetic and molecular data suggest that Hox proteins interact with pre-existing homeodomain protein complexes. These complexes may help to regulate Hox activity and Hox specificity, and help cells to interpret signaling cascades during development.

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.

Ectopic orthodenticle expression alters segment polarity gene expression but not head segment identity in the Drosophila embryo

The cephalic gap genes specify anterior head development in the Drosophila embryo. However, the mechanisms of action of these genes remain poorly understood. Here, we focused on the cephalic gap gene orthodenticle (otd), which establishes a specific region of the anterior head. It has been proposed that otd acts in a combinatorial fashion with the cephalic gap genes empty spiracles (ems) and buttonhead (btd) to assign segmental identities in this region. To test this model, we used a heat-inducible transgene to generate pulses of ubiquitous otd expression during embryonic development. Ectopic otd expression caused significant defects in head formation, including the duplication of sensory structures derived from otd-dependent segments. However, these defects do not appear to result from the transformation of head segment identities predicted by the combinatorial model. Instead, they correlate with specific regulatory effects of otd on the expression of the segment polarity genes engrailed (en) and wingless (wg). Ectopic otd expression also caused the loss of head structures derived from the maxillary segment, which lies posterior to the otd domain. We show that this effect is associated with otd repression of the homeotic selector gene Deformed (Dfd).

Evolution of the interaction between Hox genes and a downstream target

Segmental identities along the insect body depend on the activities of Hox genes [1,2]. In Drosophila melanogaster, one well-studied Hox regulatory target is Distal-less (Dll), which is required for the development of distal limb structures [3]. In abdominal segments, Dll transcription is prevented when Hox proteins of the Bithorax Complex (BX-C) bind to cis-regulatory elements upstream of the Dll transcription start site [4,5]. Previous evolutionary comparisons of gene expression patterns suggest that this direct repression is conserved between Diptera and Lepidoptera, but is absent in the Crustacea [6,7]. We examined gene expression patterns in three orders of hexapods, all of which develop abdominal appendages, in order to determine when the strong repressive interaction between BX-C proteins and Dll appeared during evolution. In each of the species examined, Dll expression was initiated in abdominal cells despite the presence of high levels of BX-C proteins. It appears that the strong repressive effects of BX-C proteins on Dll expression arose relatively late in insect evolution. We suggest that the regulatory interaction between the BX-C genes and Dll has evolved within the hexapods in a complex, segment-specific manner.

Regulation and formation of the Drosophila salivary glands

The homeotic gene, Sex combs reduced (Scr), is a master regulator of Drosophila salivary gland formation. Embryos in which Scr function is missing do not form salivary glands, and embryos in which SCR protein is expressed everywhere form extra salivary glands. However, other known proteins, including the homeotic protein Abdominal-B, the unusual zinc finger protein Teashirt, and the secreted signaling molecule Decapentaplegic (a TGF-beta family member), limit the recruitment of SCR-expressing cells to salivary glands. To learn the molecular details of how salivary gland gene expression is controlled and as a first step toward understanding how the SCR transcription factor controls salivary gland morphogenesis, we screened for genes expressed in the developing salivary gland. Among our best candidates for potential direct downstream targets of SCR in the salivary gland are the genes trachealess (trh), dCREB-A, jalapeño, and Semaphorin II (SemaII). Our genetic studies suggest distinct and important roles for each of these genes in salivary gland morphogenesis. Current work includes studying the molecular interactions between SCR and these downstream target genes and asking how target genes coordinate their activities to effect the cell biological changes required to build functional salivary glands.

The lines gene of Drosophila is required for specific functions of the Abdominal-B HOX protein

The Hox genes encode homeobox transcription factors that control the formation of segment specific structures in the anterior-posterior axis. HOX proteins regulate the transcription of downstream targets acting both as repressors and as activators. Due to the similarity of their homeoboxes it is likely that much of the specificity of HOX proteins is determined by interaction with transcriptional cofactors, but few HOX cofactor proteins have yet been described. Here I present genetic evidence showing that lines, a segment polarity gene of Drosophila, is required for the function of the Abdominal-B protein. In lines mutant embryos Abdominal-B protein expression is normal but incapable of promoting its normal functions: formation of the posterior spiracles and specification of an eighth abdominal denticle belt. These defects arise because in lines mutant embryos the Abdominal-B protein cannot activate its direct target empty spiracles or other downstream genes while it can function as a repressor of Ultrabithorax and abdominal-A. The lines gene seems to be required exclusively for Abdominal-B but not for the function of other Hox genes.

A novel Drosophila nuclear protein serine/threonine kinase expressed in the germline during its establishment

Nuclear protein kinases are believed to play important roles in regulating gene expression. We report here the identification and developmental expression of Dmnk (Drosophila maternal nuclear kinase), a Drosophila gene encoding a putative nuclear protein serine/threonine kinase with no apparent homology to previously identified protein kinases and located at 38B on the second chromosome. Dmnk mRNAs are transcribed in nurse cells and are subsequently localized in the anterior of oocytes during oogenesis, in a manner similar to several maternal transcripts regulating oogenesis and early embryogenesis. At early cleavage-stages Dmnk transcripts are transiently present throughout the embryo, but become restricted to the posterior pole and then to the newly-formed primordial germ cells (pole cells) by the blastoderm stage. The transcripts are sustained in the pole cells during gastrulation until they pass through the midgut pocket wall into the body cavity. Immunostaining with specific antibodies revealed that Dmnk proteins are localized to the nuclei in a speckled pattern. Dmnk proteins become detectable in both somatic and germ line cell nuclei upon their arrival at the periplasm of the syncytial embryo, but then disappear from the somatic cell nuclei. Consistent with mRNA expression, Dmnk proteins in pole cell nuclei are sustained during gastrulation. Taken together, Dmnk represents a novel class of nuclear protein kinases and the dynamic expression of Dmnk suggests a role in germ line establishment. The results are discussed in the light of recent findings concerning germ line establishment in Caenorhabditis and Drosophila.

Junk DNA and sectorial gene repression

Transcriptional repression in eukaryotes often involves tens or hundreds of kilobase pairs, two to three orders of magnitude more than the bacterial operator/repressor model does. Classical repression, represented by this model, was maintained over the whole span of evolution under different guises, and consists of repressor factors interacting primarily with promoters and, in later evolution, also with enhancers. The use of much larger amounts of DNA in the other mode of repression, here called the sectorial mode ('superrepression'), results in the conceptual transfer of so-called junk DNA to the domain of functional DNA. This contribution to the solution of the c-value paradox involves perhaps 15% of genomic 'junk,' and encompasses the bulk of the introns, thought to fill a stabilizing role in sectorially repressed chromatin structures. In the case of developmental genes, such structures appear to be heterochromatoid in character. However, solid clues regarding general structural features of superrepressed terminal differentiation genes remain elusive. The competition among superrepressible DNA sectors for sectorially binding factors offers, in principle, a molecular mechanism for developmental switches. Position effect variegation may be considered an abnormal manifestation of normal processes that underly development and involve heterochromatoid sectorial repression, which is apparently required for local elimination or modulation of morphological features (morpholysis). Sectorial repression of genes participating either in development or in terminal differentiation is considered instrumental in establishing stable cell types, and provides a basis for the distinction between determination and cell type specification. The gamut of possible stable cell types may have been broadened by the appearance in evolution of heavy isochores. Additional types of relatively frequent GC-rich cis-acting DNA motifs may offer reiterated binding sites to factors endowed with a selective (though not individually strong) affinity for these motifs. The majority of sequence motifs thought to be used in superrepression need not be individually maintained by natural selection. It is re-emphasized that the dispensability of sequences is not an indicator of their nonfunctionality and that in many cases, along noncoding sequences, nucleotides tend to fill functions collectively, rather than individually.

Dlx genes encode DNA-binding proteins that are expressed in an overlapping and sequential pattern during basal ganglia differentiation

The Dlx gene family encodes homeodomain proteins that are required for forebrain and craniofacial development. Towards elucidating the roles for each of these genes, we have isolated cDNA clones encoding the full-coding sequence for murine Dlx-5 and partial coding sequence for murine Dlx-6. Three different classes of sense Dlx-5 cDNA clones were characterized, two of which lack the homeobox. We also identified an antisense Dlx-6 transcript. Genomic analysis shows that the Dlx-5 and -6 genes are linked. Biochemical analysis using gel shift assays demonstrate that DLX-1, -2 and -5 have very similar DNA-binding properties. The expression of Dlx-1, -2, -5, -6 and antisense Dlx-1 and -6 was studied in the midgestation mouse brain. We found that the Dlx genes are expressed in overlapping patterns at different stages of differentiation within the primordia of the basal ganglia. Dlx-1 and -2 are expressed in the least mature cells (in the ventricular and subventricular zones). Dlx-5 appears to be co-expressed with Dlx-1 and -2 in the SVZ, but is also expressed in the postmitotic cells of the mantle. Dlx-6 expression is strongest in the mantle. Antisense Dlx-1 and -6 have their highest expression in the SVZ. These results suggest that each of these Dlx genes may have a distinct role in different steps of differentiation in the basal ganglia.

Relationship between the genomic organization and the overlapping embryonic expression patterns of the zebrafish dlx genes

To understand the relationship between the expression and the genomic organization of the zebrafish dlx genes, we have determined the genomic structure of the dlx2 and dlx4 loci. This led to the identification of the zebrafish dlx1 and dlx6 genes, which are closely linked to dlx2 and dlx4, respectively. Therefore, the inverted convergent configuration of Dlx genes is conserved among vertebrates. Analysis of the expression patterns of dlx1 and dlx6 showed striking similarities to those of dlx2 and dlx4, respectively, the genes to which they are linked. Furthermore, the expression patterns of dlx3 and dlx7, which likely constitute a third pair of convergently transcribed genes, are indistinguishable. Thus, the overlapping expression patterns of linked Dlx genes during embryonic development suggest that they share cis-acting sequences that control their spatiotemporal expression. The evolutionary conservation of the genomic organization and combinatorial expression of Dlx genes in distantly related vertebrates suggest tight control mechanisms that are essential for their function during development.

Cross-interactions between two members of the Dlx family of homeobox-containing genes during zebrafish development

The Dlx homeobox genes of vertebrates are transcribed in multiple cells of the embryo with overlapping patterns but often with different onsets of expression. Here we describe the interaction between two dlx genes, dlx3 and dlx4, during zebrafish development. The observation that dlx3 expression precedes that of dlx4 in the otic vesicle led us to investigate whether dlx3 had the ability to control expression of dlx4. Truncated versions of dlx3 were overexpressed in zebrafish embryos and the expression patterns of dlx4 were examined later in development. Overexpression of truncated forms of Dlx3 or of a Dlx3-Dlx2 chimera was found to result in perturbations in dlx4 expression. In addition, cotransfection experiments indicated the ability of Dlx3 to activate transcription through a 1.7-kb fragment of the 5 prime flanking region of dlx4. These results suggest that dlx4 is one of the target genes of dlx3 in embryos and that cross-regulatory interactions between Dlx genes may be one of the mechanisms responsible for their overlapping expression.

The del(7q) subgroup in uterine leiomyomata: genetic and biologic characteristics. Further evidence for the secondary nature of cytogenetic abnormalities in the pathobiology of uterine leiomyomata

Rearrangement of 7q represents one of the major cytogenetic subgroups in uterine leiomyomata, the most common tumors arising in females. Herein we report cytogenetic analysis on a series of 22 cases of uterine leiomyomata with 7q abnormalities, and describe observations regarding tumor size and maintenance of the tumor cells in culture. We discuss the frequent finding of mosaic tumors containing both cells with the 7q rearrangement and karyotypically normal cells. Clonality studies of three mosaic cases reveal nonrandom X chromosome inactivation substantiating prior findings suggesting the secondary nature of chromosome aberrations in these tumors. Genes mapped in 7q22 and those identified in the pathobiology of other uterine leiomyomata subgroups are discussed in addition to a perspective on the role of the 7q22 leiomyomata gene.

The homeobox gene Distal-less induces ventral appendage development in Drosophila

This study investigates the role of the homeobox gene Distal-less (Dll) in the development of the legs, antennae, and wings of Drosophila. Lack of Dll function causes a change in the identity of ventral appendage cells (legs and antennae) that often results in the loss of the appendage. Ectopic Dll expression in the proximal region of ventral appendages induces nonautonomous duplication of legs and antennae by the activation of wingless and decapentaplegic. Ectopic Dll expression in dorsal appendages produces transformation into corresponding ventral appendages; wings and halteres develop ectopic legs and the head-eye region develops ectopic antennae. In the wing, the exogenous Dll product induces this transformation by activating the endogenous Dll gene and repressing the wing determinant gene vestigial. It is proposed that Dll induces the development of ventral appendages and also participates in a genetic address that specifies the identity of ventral appendages and discriminates the dorsal versus the ventral appendages in the adult. However, unlike other homeotic genes, Dll expression and function is not defined by a cell lineage border. Dll also performs a secondary and late function required for the normal patterning of the wing.

Fossils, genes and the evolution of animal limbs

The morphological and functional evolution of appendages has played a crucial role in the adaptive radiation of tetrapods, arthropods and winged insects. The origin and diversification of fins, wings and other structures, long a focus of palaeontology, can now be approached through developmental genetics. Modifications of appendage number and architecture in each phylum are correlated with regulatory changes in specific patterning genes. Although their respective evolutionary histories are unique, vertebrate, insect and other animal appendages are organized by a similar genetic regulatory system that may have been established in a common ancestor.

Evolution of the entire arthropod Hox gene set predated the origin and radiation of the onychophoran/arthropod clade

BACKGROUND

Dramatic changes in body size and pattern occurred during the radiation of many taxa in the Cambrian, and these changes are best documented for the arthropods. The sudden appearance of such diverse body plans raises the fundamental question of when the genes and the developmental control systems that regulate these designs evolved. As Hox genes regulate arthropod body patterns, the evolution of these genes may have played a role in the origin and diversification of the arthropod body plan from a homonomous ancestor. To trace the origin of arthropod Hox genes, we examined their distribution in a myriapod and in the Onychophora, a sister group to the arthropods.

RESULTS

Despite the limited segmental diversity within myriapods and Onychophora, all insect Hox genes are present in both taxa, including the trunk Hox genes Ultrabithorax and abdominal-A as well as an ortholog of the fushi tarazu gene. Comparative analysis of Hox gene deployment revealed that the anterior boundary of expression of trunk Hox genes has shifted dramatically along the anteroposterior axis between Onychophora and different arthropod classes. Furthermore, we found that repression of expression of the Hox target gene Distal-less is unique to the insect lineage.

CONCLUSIONS

A complete arthropod Hox gene family existed in the ancestor of the onychophoran/arthropod clade. No new Hox genes were therefore required to catalyze the arthropod radiation; instead, arthropod body-plan diversity arose through changes in the regulation of Hox genes and their downstream targets.

Why are Hox genes clustered?

The evolutionarily conserved genomic organization of the Hox genes has been a puzzle ever since it was discovered that their order along the chromosome is similar to the order of their functional domains along the antero-posterior axis. Why has this colinearity been maintained throughout evolution? A close look at regulatory sequences from the mouse Hox clusters suggests that enhancer sharing between adjacent Hox genes may be one reason. Moreover, characterizing the activity of one of these mouse enhancers in Drosophila illustrates that despite many similarities, not all Hox clusters are built in the same way.

Proximal-distal axis formation in the Drosophila leg

Limb development requires the formation of a proximal-distal axis perpendicular to the main anterior-posterior and dorsal-ventral body axes. The secreted signalling proteins Decapentaplegic and Wingless act in a concentration-dependent manner to organize the proximal-distal axis. Discrete domains of proximal-distal gene expression are defined by different thresholds of Decapentaplegic and Wingless activities. Subsequent modulation of the relative sizes of these domains by growth of the leg is required to form the mature pattern.

The Drosophila homeotic gene moira regulates expression of engrailed and HOM genes in imaginal tissues

moira is a member of the trithorax group of homeotic gene regulators in Drosophila melanogaster. We show that moira is required for the function of multiple homeotic genes of the Antennapedia and bithorax complexes (HOM genes) in most imaginal tissues and that the requirement for moira function is at the level of transcription. moira is also required for transcription of the engrailed segmentation gene in the imaginal wing disc. The abnormalities caused by the loss of moira function in germ cells suggests that at least one other target gene requires moira for normal oogenesis.

A role for phosphorylation by casein kinase II in modulating Antennapedia activity in Drosophila

We present evidence that the in vivo activity of the HOX protein Antennapedia (ANTP) is modified because of phosphorylation by the serine/threonine kinase casein kinase II (CKII). Using an in vivo assay a form of ANTP that has alanine substitutions at its CKII target sites has, in addition to wild-type ANTP functions, the ability to alter severely thoracic and abdominal development. The novel functions of this protein suggest that this form of ANTP is not suppressed phenotypically by the more posterior homeotic proteins. In contrast, the in vivo activity of a form of ANTP that contains acidic amino acid substitutions at its CKII target sites, thereby mimicking a constitutively phosphorylated ANTP protein, is greatly reduced. This hypoactive form of ANTP, but not the alanine-substituted form, is also reduced in its ability to bind to DNA cooperatively with the homeodomain protein Extradenticle. Our results suggest that phosphorylation of ANTP by CKII is important for preventing inappropriate activities of this homeotic protein during embryogenesis.

The origin and evolution of animal appendages

Animals have evolved diverse appendages adapted for locomotion, feeding and other functions. The genetics underlying appendage formation are best understood in insects and vertebrates. The expression of the Distal-less (Dll) homeoprotein during arthropod limb outgrowth and of Dll orthologs (Dlx) in fish fin and tetrapod limb buds led us to examine whether expression of this regulatory gene may be a general feature of appendage formation in protostomes and deuterostomes. We find that Dll is expressed along the proximodistal axis of developing polychaete annelid parapodia, onychophoran lobopodia, ascidian ampullae, and even echinoderm tube feet. Dll/Dlx expression in such diverse appendages in these six coelomate phyla could be convergent, but this would have required the independent co-option of Dll/Dlx several times in evolution. It appears more likely that ectodermal Dll/Dlx expression along proximodistal axes originated once in a common ancestor and has been used subsequently to pattern body wall outgrowths in a variety of organisms. We suggest that this pre-Cambrian ancestor of most protostomes and the deuterostomes possessed elements of the genetic machinery for and may have even borne appendages.

Drosophila Hox complex downstream targets and the function of homeotic genes

Hox complex genes are key developmental regulators highly conserved throughout evolution. The encoded proteins share a 60-amino-acid DNA-binding motif, the homeodomain, and function as transcription factors to control axial patterning. An important question concerns the nature and function of genes acting downstream of Hox proteins. This review focuses on Drosophila, as little is known about this question in other organisms. The noticeable progress gained in the field during the past few years has significantly improved our current understanding of how Hox genes control diversified morphogenesis. Here we summarise the strategies deployed to identify Hox target genes and discuss how their function contributes to pattern formation and morphogenesis. The regulation of target genes is also considered with special emphasis on the mechanisms underlying the specificity of action of Hox proteins in the whole animal.

Homeotic transformation of legs to mouthparts by proboscipedia expression in Drosophila imaginal discs

The Drosophila homeotic gene proboscipedia (pb) specifies labial identify and directs formation of the adult distiproboscis from the labial imaginal discs. pb null alleles result in the homeotic transformation of the distiproboscis into prothoracic (T1) legs [Kaufman (1978) Genetics 90, 579-596; Pultz et al. (1988) Genes Dev. 2, 901-920]. Homology with other transcription factors, localization to the nucleus, and restricted embryonic and imaginal expression implicate the pb protein (PB) as a transcription factor. In order to examine the possible roles that PB may play in the specification of adult mouthparts, we have expressed PB in cells of wing, leg and eye-antennal imaginal discs and assayed for effects on the development of adult structures. We report here that the ectopic expression of PB in the imaginal discs under the control of the inducible GAL4 system [Brand and Perrimon (1993) Development 118, 401-415] alters the developmental program of adult legs into maxillary or labial palps. These homeotic transformations have an equal effect on all three sets of legs, indicating an activity that is not solely dependent upon the unique combinations of other homeotic genes present in each of the leg discs. Segment polarity genes required for establishing the AP compartment boundary were found to be undisturbed by ectopic PB. Furthermore, normal patterns of apoptosis are observed in animals expressing ectopic PB, indicating that PB does not alter or affect cell death. These results suggest that molecular events occurring downstream of the establishment of the compartment boundary are affected by ectopic PB expression in imaginal discs and point to a general role in 'palp' formation in addition to the specification of labial identity.

Specification of the embryonic limb primordium by graded activity of Decapentaplegic

Two thoracic limbs of Drosophila, the leg and the wing, originate from a common cluster of cells that include the source of two secreted signaling molecules, Decapentaplegic and Wingless. We show that Wingless, but not Decapentaplegic, is responsible for initial specification of the limb primordia with a distal identity. Limb formation is restricted to the lateral position of the embryo by negative control of the early function of Decapentaplegic and the EGF receptor homolog that determine the global dorsoventral pattern. Late function of Decapentaplegic locally determines two additional cell identities in a dosage dependent manner. Loss of Decapentaplegic activity results in a deletion of the proximal structures of the limb, which is in contrast to the consequence of decapentaplegic mutations in the imaginal disc, which cause a deletion of distal structures. The results indicate that the limb pattern elements are added in a distal to proximal direction in the embryo, which is opposite to what is happening in the growing imaginal disc. We propose that Wingless and Decapentaplegic act sequentially to initiate the proximodistal axis.

Structure of the insect head in ontogeny and phylogeny: a view from Drosophila

Evolutionary, developmental and insect biologists are currently using a three-pronged approach to study the evolution and development of the insect head. First, genetic manipulation of the fruit fly Drosophila melanogaster has led to the identification of many genes, including the segmentation and homeotic genes, that are important for embryonic pattern formation and development. Second, a comparison of orthologous gene expression patterns in other insects reveals that these regulatory genes are deployed in similar, yet distinct, patterns in different insects. Third, comparisons of embryonic morphology with gene expression patterns suggest that in general these genes promote a common insect body plan, but that variations in gene expression can often be correlated to variations in morphology. Here, we present a detailed review of the development of the cephalic ectoderm of Drosophila and extrapolate to development of a generalized insect head. Our analysis of the variations among insect species, in both morphology and gene expression patterns, conducted within an evolutionary framework supported by traditional phylogenies and paleontology provides the basis for hypotheses about the genetic factors governing morphologic and developmental evolution.

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.

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.

The Drosophila teashirt homeotic protein is a DNA-binding protein and modulo, a HOM-C regulated modifier of variegation, is a likely candidate for being a direct target gene

The Drosophila teashirt (tsh) gene has an homeotic function which, in combination with HOM-C genes, determines thoracic and abdominal (trunk) identities. Analysis of TSH protein distribution during embryogenesis using a specific polyclonal antibody shows that it is nuclear. The protein is present with regional modulation in several tissues within the trunk, suggesting additional tsh functions to those already studied. We identified a candidate tsh target shared with some HOM-C genes, the modifier of variegation gene modulo (mod). The TSH zinc-finger protein recognizes in vitro two specific sites within a 5' control element of the mod gene which responds in vivo to tsh activity. TSH is therefore a DNA binding protein and might directly control mod expression.

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.

Neo-Darwinian developmental evolution: can we bridge the gap between pattern and process?

In the past decade, there has been a surge of renewed interest in the study of developmental evolution. One approach that has been taken is to examine the expression patterns of a candidate gene in divergent taxa and to use these results to infer which aspects of a particular genetic pathway are either conserved or altered. Here we consider this approach from the perspective of the neo-Darwinian paradigm for evolutionary change. If adaptations are typically composed of large numbers of gene substitutions that are of small effect individually, then the candidate gene approach is unlikely to bridge the gap between developmental pattern and evolutionary process: changes in gene expression patterns may identify the steps in developmental pathways that have been altered during evolution but fail to identify the actual genetic changes that have occurred. On the other hand, there is growing support for the view that adaptations often involve large-effect genes; fortunately, the candidate gene approach is well suited to this type of genetic architecture.

The homeotic target gene centrosomin encodes an essential centrosomal component

The centrosomin (cnn) gene encodes a protein associated with mitotic centrosomes in Drosophila melanogaster and is a target of homeotic gene regulation. Here we report that CNN is an essential component of the centrosome. Loss of zygotic cnn expression disrupts the development of the second midgut constriction, the gastric caeca, and the nervous system. Embryos that lack maternal as well as zygotic cnn expression display defects in nuclear division, chromosome alignment, and microtubule organization, while adult flies that are mosaic for cnn-cells exhibit defects indicative of a block in cell proliferation. We propose that cnn provides an example of homeotic genes directly regulating the accumulation of essential cellular proteins to carry out segment-specific morphogenetic functions.

Chromosomal binding sites of Ultrabithorax homeotic proteins

Drosophila homeotic genes and their vertebrate cognates, the Hox genes, encode homeodomain proteins that are thought to control segment-specific morphogenesis by regulating subordinate target genes. Although expression of many genes is thought to be influenced by homeotic/Hox function, little is known about the genes they directly regulate in the developing embryo. One of the Drosophila homeotic genes is Ultrabithorax (Ubx) that specifies the identity of specific thoracic and abdominal metameres. Towards identifying genes directly regulated by Ubx we have mapped the binding sites of Ubx proteins (UBX) in polytene chromosomes. We found that the UBX isoforms Ia and IVa accumulate in about 100 discrete chromosomal sites. Most, if not all, the sites are the same for the two UBX isoforms. These sites are all euchromatic, include both bands and interbands and are reproducible from chromosome to chromosome. Some of these sites correspond to the locations of known genes that are good candidates, or are known to be, under direct Ubx control.

Functional domains in the Deformed protein

A chimeric protein consisting of Deformed with a substituted Abdominal-B homeodomain (Dfd/Abd-B) is used to identify protein domains outside the homeodomain that are required for regulatory activity in vivo. A series of deletion proteins were generated based on regions showing amino acid composition similar to known regulatory domains. Each mutant protein can influence regulation of homeotic genes in a manner distinct from the intact protein. Activity was also tested using promoter elements from empty spiracles and Distal-less, two genes known to be directly regulated by Abdominal-B. Removal of the acidic region and the C-tail region convert the chimera from a strong activator to a repressor of the Distal-less element, but had comparatively little effect on the activation of the empty spiracles element. Constructs without a third domain, the N domain, fail to show any regulatory activity. The N domain is the only domain of the Dfd/Abd-B protein which exhibits significant activation activity when fused to a heterologous DNA binding domain. Our results suggest transcriptional activity of the N domain can be modulated by the acidic and C-tail domains.

Determination of wing cell fate by the escargot and snail genes in Drosophila

Inset appendages such as the wing and the leg are formed in response to inductive signals in the embryonic field. In Drosophila, cells receiving such signals initiate developmental programs which allow them to become imaginal discs. Subsequently, these discs autonomously organize patterns specific for each appendage. We here report that two related transcription factors, Escargot and Snail that are expressed in the embryonic wing disc, function as intrinsic determinants of the wing cell fate. In escargot or snail mutant embryos, wing-specific expression of Snail, Vestigial and beta-galactosidase regulated by escargot enhancer were found as well as in wild-type embryos. However, in escargot snail double mutant embryos, wing development proceeded until stage 13, but the marker expression was not maintained in later stages, and the invagination of the primordium was absent. From such analyses, it was concluded that Escargot and Snail expression in the wing disc are maintained by their auto- and crossactivation. Ubiquitous escargot or snail expression induced from the hsp70 promoter rescued the escargot snail double mutant phenotype with the effects confined to the prospective wing cells. Similar DNA binding specificities of Escargot and Snail suggest that they control the same set of genes required for wing development. We thus propose the following scenario for early wing disc development. Prospective wing cells respond to the induction by turning on escargot and snail transcription, and become competent for regulation by Escargot and Snail. Such cells initiate auto- and crossregulatory circuits of escargot and snail. The sustained Escargot and Snail expression then activates vestigial and other target genes that are essential for wing development. This maintains the commitment to the wing cell fate and induces wing-specific cell shape change.

How do single homeotic genes control multiple segment identities?

Each of the homeotic genes of the bithorax complex of Drosophila defines the identities of more than one body segment. The mechanisms by which this occurs have been elusive. In a recent report, Castelli-Gair and Akam analyze in detail the control of parasegment 5 and parasegment 6 identities by the bithorax complex gene Ubx. Their results indicate that differences in the spatial and temporal expression patterns of Ubx are critical in determining differences between these parasegments. However, dose effects observed by others indicate that parasegment-specific differences in the level of Ubx expression are also important. For the other BX-C genes, parasegment-specific expression of protein isoforms, or combinatorial control dependent on the expression patterns of other spatially restricted regulators, may also play a role.

Serrate signals through Notch to establish a Wingless-dependent organizer at the dorsal/ventral compartment boundary of the Drosophila wing

Growth and patterning of the Drosophila wing is controlled by organizing centers located at the anterior-posterior and dorsal-ventral compartment boundaries. Interaction between cells in adjacent compartments establish the organizer. We report here that Serrate and Notch mediate the interaction between dorsal and ventral cells to direct localized expression of Wingless at the D/V boundary. Serrate serves as a spatially localized ligand which directs Wg expression through activation of Notch. Ligand independent activation of Notch is sufficient to direct Wg expression, which in turn mediates the organizing activity of the D/V boundary.

The development of crustacean limbs and the evolution of arthropods

Arthropods exhibit great diversity in the position, number, morphology, and function of their limbs. The evolutionary relations among limb types and among the arthropod groups that bear them (insects, crustaceans, myriapods, and chelicerates) are controversial. Here, the use of molecular probes, including an antibody to proteins encoded by arthropod and vertebrate Distal-less (Dll and Dlx) genes, provided evidence that common genetic mechanisms underlie the development of all arthropod limbs and their branches and that all arthropods derive from a common ancestor. However, differences between crustacean and insect body plans were found to correlate with differences in the deployment of particular homeotic genes and in the ways that these genes regulate limb development.

The Drosophila homeotic target gene centrosomin (cnn) encodes a novel centrosomal protein with leucine zippers and maps to a genomic region required for midgut morphogenesis

The products of the homeotic genes in Drosophila are transcription factors that are necessary to impose regional identity along the anterior-posterior axis of the developing embryo. However, the target genes under homeotic regulation that control this developmental process are largely unknown. We have utilized an immunopurification method to clone target genes of the Antennapedia protein (ANTP). We present here the characterization of centrosomin (cnn), one of the target genes isolated using this approach. The spatial and temporal expression of the cnn gene in the developing visceral mesoderm (VM) of the midgut and the central nervous system (CNS) of wild-type and homeotic mutant embryos is consistent with the idea that cnn is a homeotic target. In the VM, Antp and abdominal-A (abd-A) negatively regulate cnn, while Ultrabithorax (Ubx) shows positive regulation. In the CNS, cnn is regulated positively by Antp and negatively by Ubx and abd-A. Characterization of a cDNA encoding CNN predicts a novel structural protein with three leucine zipper motifs and several coiled-coil domains exhibiting limited homology to the rod portion of myosin. Immunocytochemical results demonstrate that the cnn encoded protein is localized to the centrosome and the accumulation pattern is coupled to the nuclear and centrosome duplication cycles of cleavage. In addition, evidence suggests that the expression of the cnn gene in the VM correlates with the morphogenetic function of Ubx in that tissue, i.e., the formation of the second midgut construction. The centrosomal localization of CNN and the involvement of microtubules in midgut morphogenesis suggest that this protein may participate in mitotic spindle assembly and the mechanics of morphogenesis through an interaction with microtubules, either directly or indirectly.

How the Hox gene Ultrabithorax specifies two different segments: the significance of spatial and temporal regulation within metameres

In Drosophila, the Hox gene Ultrabithorax (Ubx) specifies the development of two different metameres--parasegment 5, which is entirely thoracic, and parasegment 6, which includes most of the first abdominal segment. Here we investigate how a single Hox gene can specify two such different morphologies. We show that, in the early embryo, cells respond similarly to UBX protein in both parasegments. The differences between parasegments 5 and 6 can be explained by the different spatial and temporal pattern of UBX protein expression in these two metameres. We find no evidence for multiple threshold responses to different levels of UBX protein. We examine in particular the role of Ubx in limb development. We show that UBX protein will repress limb primordia before 7 hours, when Ubx is expressed in the abdomen, but not later, when UBX is first expressed in the T3 limb primordium. The regulation of one downstream target of UBX, the Distalless gene, provides a model for this transition at the molecular level.

Homeotic genes and the evolution of arthropods and chordates

Clusters of homeotic genes sculpt the morphology of animal body plans and body parts. Different body patterns may evolve through changes in homeotic gene number, regulation or function. Recent evidence suggests that homeotic gene clusters were duplicated early in vertebrate evolution, but the generation of arthropod and tetrapod diversity has largely involved regulatory changes in the expression of conserved arrays of homeotic genes and the evolution of interactions between homeotic proteins and the genes they regulate.

Hox genes and the diversification of insect and crustacean body plans

Crustaceans and insects share a common origin of segmentation, but the specialization of trunk segments appears to have arisen independently in insects and various crustacean subgroups. Such macroevolutionary changes in body architecture may be investigated by comparative studies of conserved genetic markers. The Hox genes are well suited for this purpose, as they determine positional identity along the body axis in a wide range of animals. Here we examine the expression of four Hox genes in the branchiopod crustacean Artemia franciscana, and compare this with Hox expression patterns from insects. In Artemia the three 'trunk' genes Antp, Ubx and abdA are expressed in largely overlapping domains in the uniform thoracic region, whereas in insects they specify distinct segment types within the thorax and abdomen. Our comparisons suggest a multistep process for the diversification of these Hox gene functions, involving early differences in tissue specificity and the later acquisition of a role in defining segmental differences within the trunk. We propose that the branchiopod thorax may be homologous to the entire pregenital (thoracic and abdominal) region of the insect trunk.

Homeotic genes and diversification of the insect body plan

The homeotic genes regulate segmental identity by repressing the formation of or influencing the morphology of segmental structures during development. The specific roles these genes have played in the generation of insect diversity are discussed relative to abdominal limb formation and to the evolution of wings as novel repeated structures. Other recent findings discussed suggest possible 'homeotic' origins for the non-homeotic genes found within the homeotic complexes of higher insects.

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.

Requirement of MADS domain transcription factor D-MEF2 for muscle formation in Drosophila

Members of the myocyte enhancer binding factor-2 (MEF2) family of MADS (MCM1, agamous, deficiens, and serum response factor) box transcription factors are expressed in the skeletal, cardiac, and smooth muscle lineages of vertebrate and Drosophila embryos. These factors bind an adenine-thymidine-rich DNA sequence associated with muscle-specific genes. The function of MEF2 was determined by generating a loss-of-function of the single mef2 gene in Drosophila (D-mef2). In loss-of-function embryos, somatic, cardiac, and visceral muscle cells did not differentiate, but myoblasts were normally specified and positioned. These results demonstrate that different muscle cell types share a common myogenic differentiation program controlled by MEF2.

DWnt-4, a novel Drosophila Wnt gene acts downstream of homeotic complex genes in the visceral mesoderm

Wnt genes encode putative cell signalling proteins which play crucial roles during development. From a library of DNA fragments associated, in vivo, with Ultrabithorax proteins, we isolated a novel Drosophila Wnt gene, DWnt-4. Neither a paralog nor an ortholog of the gene exist in the current repertoire of full-length Wnt sequences. DWnt-4 maps close (30 kb) to wingless, suggesting that the two Wnt genes derive from a duplication that occurred early in evolution, since they are significantly diverged in sequence and structure. Developmental expression of DWnt-4 partially overlaps that of wingless. The gene is transcribed following a segment polarity-like pattern in the posterior-most cells of each parasegment of the ectoderm, and at two locations that correspond to parasegments 4 and 8 of the visceral mesoderm. The control of DWnt-4 expression in the visceral mesoderm involves a network of regulatory molecules that includes Ultrabithorax and other proteins from the homeotic complex (HOM-C), as well as the TGF-beta decapentaplegic gene product.

Evolution of homeotic gene regulation and function in flies and butterflies

It has been proposed that the evolution of homeotic genes parallels, and to some degree directs, the evolution of segment diversity in the myriapod-insect lineage. But the discovery of discrete Antennapedia complex (ANT-C) and bithorax complex (BX-C) gene members in crustacea, chelicerates, annelids and various insects, as well as in vertebrates, indicates that the expansion and diversification of homeotic genes preceded the diversification of arthropods and insects. How, then, have these genes influenced the evolution of body plans? To address this question, we now examine homeotic gene expression and regulation in butterflies (Lepidoptera), which, unlike flies, possess larval abdominal limbs and two pairs of wings. We show that the difference in larval limb number between these insects results from striking changes in BX-C gene regulation in the butterfly abdomen, and we deduce that the wing-patterning genes regulated by Ultrabithorax have diverged in the course of butterfly and fly evolution. These findings have general implications for the role of homeotic genes in animal evolution.

Regulation of a decapentaplegic midgut enhancer by homeotic proteins

The clustered homeotic genes encode transcription factors that regulate pattern formation in all animals, conferring cell fates by coordinating the activities of downstream ‘target’ genes. In the Drosophila midgut, the Ultrabithorax (Ubx) protein activates and the abdominalA (abd-A) protein represses transcription of the decapentaplegic (dpp) gene, which encodes a secreted signalling protein of the TGF beta class. We have identified an 813 bp dpp enhancer which is capable of driving expression of a lacZ gene in a correct pattern in the embryonic midgut. The enhancer is activated ectopically in the visceral mesoderm by ubiquitous expression of Ubx or Antennapedia but not by Sex combs reduced protein. Ectopic expression of abd-A represses the enhancer. Deletion analysis reveals regions required for repression and activation. A 419 bp subfragment of the 813 bp fragment also drives reporter gene expression in an appropriate pattern, albeit more weakly. Evolutionary sequence conservation suggests other factors work with homeotic proteins to regulate dpp. A candidate cofactor, the extradenticle protein, binds to the dpp enhancer in close proximity to homeotic protein binding sites. Mutation of either this site or another conserved motif compromises enhancer function. A 45 bp fragment of DNA from within the enhancer correctly responds to both UBX and ABD-A in a largely tissue-specific manner, thus representing the smallest in vivo homeotic response element (HOMRE) identified to date.

Differential and overlapping expression domains of Dlx-2 and Dlx-3 suggest distinct roles for Distal-less homeobox genes in craniofacial development

During the development of the vertebrate head, cranial neural crest cells migrate into the branchial arches to form many of the structures of the facial skeleton. These cells follow defined developmental pathways and their fates are determined early. We have isolated and characterized the murine Distal-less homeobox gene Dlx-3 and have performed a comparative analysis of Dlx-3 and Dlx-2 expression during craniofacial development. In contrast to Dlx-2 and other vertebrate Distal-less genes, Dlx-3 is not expressed in the central nervous system and is expressed in a highly restricted region of the branchial arches. Dlx-2 and -3 display temporal and spatial differences in expression in the arches and their derivatives. In later development, these two genes are expressed in both complementary and partially overlapping domains in regions whose development is dependent on epithelial-mesenchymal interactions, such as the developing middle and inner ear, teeth and whisker follicles. The differential expression of Dlx genes in the branchial region suggests that they play key roles in craniofacial patterning and morphogenesis.

Cell interaction between compartments establishes the proximal-distal axis of Drosophila legs

The appendage primordia of Drosophila are subdivided into compartments by the localized expression of transcription factors. Interaction between cells in adjacent compartments establishes organizing centres responsible for generating spatial pattern and promoting cell proliferation in the developing appendages. Localized expression of hedgehog (hh) in the posterior compartment of the leg imaginal disc directs expression of wingless (wg) in ventral-anterior cells and decapentaplegic (dpp) in dorsal-anterior cells near the anterior-posterior compartment boundary; wg then acts to specify ventral cell fate and to organize the dorsal-ventral axis of the leg. Interaction between wg-expressing ventral cells and dorsal cells near the anterior-posterior compartment boundary promotes axis formation in the leg. Here we show that the combined action of wg-expressing cells in the ventral-anterior compartment and dpp-expressing cells in the dorsal-anterior compartment activates expression of Distal-less, a gene required for proximal-distal axis formation in the limbs. These results demonstrate that sequential interaction between anterior-posterior and dorsal-ventral compartments establishes the proximal-distal axis of the limbs.

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.