The homeotic gene fork head encodes a nuclear protein and is expressed in the terminal regions of the Drosophila embryo

The zebrafish forkhead transcription factor Foxi1 specifies epibranchial placode-derived sensory neurons

Vertebrate epibranchial placodes give rise to visceral sensory neurons that transmit vital information such as heart rate, blood pressure and visceral distension. Despite the pivotal roles they play, the molecular program underlying their development is not well understood. Here we report that the zebrafish mutation no soul, in which epibranchial placodes are defective, disrupts the fork headrelated, winged helix domain-containing protein Foxi1. Foxi1 is expressed in lateral placodal progenitor cells. In the absence of foxi1 activity, progenitor cells fail to express the basic helix-loop-helix gene neurogenin that is essential for the formation of neuronal precursors, and the paired homeodomain containing gene phox2a that is essential for neuronal differentiation and maintenance. Consequently, increased cell death is detected indicating that the placodal progenitor cells take on an apoptotic pathway. Furthermore, ectopic expression of foxi1 is sufficient to induce phox2a-positive and neurogenin-positive cells. Taken together, these findings suggest that Foxi1 is an important determination factor for epibranchial placodal progenitor cells to acquire both neuronal fate and subtype visceral sensory identity.

FOXP2 in focus: what can genes tell us about speech and language?

The human capacity for acquiring speech and language must derive, at least in part, from the genome. In 2001, a study described the first case of a gene, FOXP2, which is thought to be implicated in our ability to acquire spoken language. In the present article, we discuss how this gene was discovered, what it might do, how it relates to other genes, and what it could tell us about the nature of speech and language development. We explain how FOXP2 could, without being specific to the brain or to our own species, still provide an invaluable entry-point into understanding the genetic cascades and neural pathways that contribute to our capacity for speech and language.

Fox's in development and disease

Since the first forkhead (Fox) gene was identified, the importance of this family of transcription factors has increased steadily with the discoveries of the diverse range of developmental processes that they regulate in eukaryotes. Among other processes, the Fox factors are important in the establishment of the body axis and the development of tissues from all three germ layers. In this article, we present some of the recent data on this gene family with reference to selected phenotypes observed in patients and model organisms, and the sensitivity of developmental processes to alterations in forkhead gene dosage.

Posterior migration of the salivary gland requires an intact visceral mesoderm and integrin function

The final overall shape of an organ and its position within the developing embryo arise as a consequence of both its intrinsic properties and its interactions with surrounding tissues. Here, we focus on the role of directed cell migration in shaping and positioning the Drosophila salivary gland. We demonstrate that the salivary gland turns and migrates along the visceral mesoderm to become properly oriented with respect to the overall embryo. We show that salivary gland posterior migration requires the activities of genes that position the visceral mesoderm precursors, such as heartless, thickveins, and tinman, but does not require a differentiated visceral mesoderm. We also demonstrate a role for integrin function in salivary gland migration. Although the mutations affecting salivary gland motility and directional migration cause defects in the final positioning of the salivary gland, most do not affect the length or diameter of the salivary gland tube. These findings suggest that salivary tube dimensions may be an intrinsic property of salivary gland cells.

Constructing an organ: the Drosophila salivary gland as a model for tube formation

Tubes are required in metazoans to transport the liquids and gases that sustain life. The conservation of molecules and mechanisms involved in tube formation suggests that what we learn by studying simple systems will apply to related processes in higher animals. Studies over the past 10 years have revealed the molecules that specify cell fate in Drosophila salivary gland and the cellular events that mediate tube morphogenesis. Here, we discuss how anterior-posterior and dorsal-ventral patterning information specifies both the position of salivary-gland primordia and how many cells they contain. We examine the transformation of a polarized epithelial sheet into an elongated, unbranched tube, and the intrinsic and extrinsic factors that influence the final position of the salivary gland.

Early patterning of the spider embryo: a cluster of mesenchymal cells at the cumulus produces Dpp signals received by germ disc epithelial cells

In early embryogenesis of spiders, the cumulus is characteristically observed as a cellular thickening that arises from the center of the germ disc and moves centrifugally. This cumulus movement breaks the radial symmetry of the germ disc morphology, correlating with the development of the dorsal region of the embryo. Classical experiments on spider embryos have shown that a cumulus has the capacity to induce a secondary axis when transplanted ectopically. In this study, we have examined the house spider, Achaearanea tepidariorum, on the basis of knowledge from Drosophila to characterize the cumulus at the cellular and molecular level. In the cumulus, a cluster of about 10 mesenchymal cells, designated the cumulus mesenchymal (CM) cells, is situated beneath the epithelium, where the CM cells migrate to the rim of the germ disc. Germ disc epithelial cells near the migrating CM cells extend cytoneme-like projections from their basal side onto the surface of the CM cells. Molecular cloning and whole-mount in situ hybridization showed that the CM cells expressed a spider homolog of Drosophila decapentaplegic (dpp), which encodes a secreted protein that functions as a dorsal morphogen in the Drosophila embryo. Furthermore, the spider Dpp signal appeared to induce graded levels of the phosphorylated Mothers against dpp (Mad) protein in the nuclei of germ disc epithelial cells. Adding data from spider homologs of fork head, orthodenticle and caudal, we suggest that, in contrast to the Drosophila embryo, the progressive mesenchymal-epithelial cell interactions involving the Dpp-Mad signaling cascade generate dorsoventral polarity in accordance with the anteroposterior axis formation in the spider embryo. Our findings support the idea that the cumulus plays a central role in the axial pattern formation of the spider embryo.

A function for Egf receptor signaling in expanding the developing brain in Drosophila

BACKGROUND

In invertebrates and vertebrates, neural midline cells secrete signals that pattern the central nervous system (CNS). However, an important part of the developing insect brain, involved in functions such as olfaction and feeding behavior, is positioned lateral to the foregut and lacks neural cells at the midline. Could the foregut substitute for neural midline cells and secrete signals that pattern this part of the brain?

RESULTS

In Drosophila embryos, the neural midline marker Single-minded is expressed in foregut cells adjacent to the brain, as are members of the Egf receptor signaling pathway. Removing the function of these molecules results in aberrant proliferation and reduced size in the brain lateral to the foregut.

CONCLUSIONS

Cells of the brain lateral to the foregut receive an Egf signal from the midline and proliferate in response. A likely source of this signal is the foregut. These findings raise the possibility that the brain lateral to the foregut is an evolutionarily recent addition to the arthropod brain, and that the anterior boundary of the brain neural midline is a conserved feature in bilaterally symmetric animals.

The many forks in FOXO's road

The FOXO family of transcription factors constitute an evolutionarily conserved subgroup within the larger family known as winged helix or Forkhead transcriptional regulators. Building upon work in the nematode, researchers have uncovered a role for these proteins in a diverse set of cellular responses that include glucose metabolism, stress response, cell cycle regulation, and apoptosis. At the organismal level, FOXO transcription factors are believed to function in various pathological processes ranging from cancer and diabetes to organismal aging. A number of studies have also shed light on the signaling pathways that regulate FOXO activity in response to external stimuli and have identified multiple FOXO target genes that mediate this varied set of biological responses.

Zebrafish foxi1 mediates otic placode formation and jaw development

The otic placode is a transient embryonic structure that gives rise to the inner ear. Although inductive signals for otic placode formation have been characterized, less is known about the molecules that respond to these signals within otic primordia. Here, we identify a mutation in zebrafish, hearsay, which disrupts the initiation of placode formation. We show that hearsay disrupts foxi1, a forkhead domain-containing gene, which is expressed in otic precursor cells before placodes become visible; foxi1 appears to be the earliest marker known for the otic anlage. We provide evidence that foxi1 regulates expression of pax8, indicating a very early role for this gene in placode formation. In addition, foxi1 is expressed in the developing branchial arches, and jaw formation is disrupted in hearsay mutant embryos.

The expression of planarian brain factor homologs, DjFoxG and DjFoxD

Recent accumulating evidence revealed that planarian central nervous system (CNS) has numerous functional domains distinguished by a large number of neural markers, suggesting that primitive animals which developed CNS already had the framework of the brain development. It is of interest to investigate genes which have been acquired at an early stage of evolution for brain pattern formation. One such candidate is FoxG1 (BF-1), specifically expressed in the telencephalon and implicated in brain development. We identified a FoxG1 (BF-1) homolog gene in planarians (DjFoxG). We also identified a FoxD class gene, DjFoxD. DjFoxG is expressed in the body and brain, with strong expression in the mesenchyme surrounding the gut. During regeneration, an intense anterior signal is detected, but this is not restricted to the head. DjFoxD is expressed in the mid-apex of the head, between the two lobes of the brain. Strong expression was detected in the mid-anterior blastema. Thus, FoxG and FoxD homologs do exist in planarians, but are regulated differently than those in vertebrates.

PAX3-FKHR transformation increases 26 S proteasome-dependent degradation of p27Kip1, a potential role for elevated Skp2 expression

PAX3-FKHR is an oncogenic form of the developmental regulator Pax3 transcription factor. PAX3-FKHR results from a t(2,13) chromosomal translocation, a unique genetic marker of alveolar rhabdomyosarcoma. In this study, we showed that ectopic expression of PAX3-FKHR, but not Pax3, in fibroblasts altered cell cycle control and accelerated G(0)/G(1) to S cell cycle transition. PAX3-FKHR-expressing cells had reduced expression of p27(Kip1) protein, a key cell cycle regulator. The reduction in p27(Kip1) levels by PAX3-FKHR resulted from destabilization of p27(Kip1) as shown by cycloheximide treatment and in vivo pulse-chase labeling experiments. The reduced p27(Kip1) protein level in PAX3-FKHR-expressing cells was restored to the level of control cells by treatment with chemical inhibitors that specifically blocked 26 S proteasome activity. Along with the reduction in p27(Kip1) protein, PAX3-FKHR-expressing cells exhibited elevated expression of F-box Skp2 protein, a substrate-specific component of SCF (Skp1-Cullin-F box protein) ligase involved in the cell cycle-dependent control of p27(Kip1) ubiquitination and 26 S proteasome dependent degradation. Finally, we showed that ectopic expression of p27(Kip1) in PAX3-FKHR-expressing cells significantly reduced the proliferation and colony-forming potential of these cells, implicating that down-regulation of p27(Kip1) protein played an active role in the PAX3-FKHR-directed cell transformation.

Research perspectives in inherited lymphatic disease

The hereditary lymphedemas provide an opportunity to identify genes involved in normal and deranged lymphatic development. Genetic analysis of families with Milroy's disease identified mutations in VEGFR3 as a cause of congenital lymphedema, confirming the importance of VEGFC/VEGFR3 signaling in lymphatic development. These observations led to the identification of a mouse model for primary lymphedema, and subsequent analysis of this mouse model, using transgenic and gene transfer techniques, has provided initial clues to the development of a biologically based therapy for primary lymphedema. Of more importance from a public health perspective is the fact that manipulation of this pathway may lead to effective therapies for the more prevalent forms of secondary lymphedema. Identification of FOXC2 as the gene mutated in the lymphedema-distichiasis syndrome has revealed new molecular insight into lymphatic development. Molecular analysis of the FOXC2 pathway may provide clues to developmental pathways shared by the lymphatic system and the other developmental abnormalities associated with this complex syndrome. With improving knowledge of the human genome, genetic analysis of families with lymphedema continues to offer one of the most promising approaches to identifying genes influencing lymphatic development.

Anterior repression of a Drosophila stripe enhancer requires three position-specific mechanisms

The striped expression pattern of the pair-rule gene even skipped(eve) is established by five stripe-specific enhancers, each of which responds in a unique way to gradients of positional information in the earlyDrosophila embryo. The enhancer for eve stripe 2(eve 2) is directly activated by the morphogens Bicoid (Bcd) and Hunchback (Hb). As these proteins are distributed throughout the anterior half of the embryo, formation of a single stripe requires that enhancer activation is prevented in all nuclei anterior to the stripe 2 position. The gap genegiant (gt) is involved in a repression mechanism that sets the anterior stripe border, but genetic removal of gt (or deletion of Gt-binding sites) causes stripe expansion only in the anterior subregion that lies adjacent to the stripe border. We identify a well-conserved sequence repeat, (GTTT)4, which is required for repression in a more anterior subregion. This site is bound specifically by Sloppy-paired 1 (Slp1),which is expressed in a gap gene-like anterior domain. Ectopic Slp1 activity is sufficient for repression of stripe 2 of the endogenous eve gene,but is not required, suggesting that it is redundant with other anterior factors. Further genetic analysis suggests that the(GTTT)4-mediated mechanism is independent of the Gt-mediated mechanism that sets the anterior stripe border, and suggests that a third mechanism, downregulation of Bcd activity by Torso, prevents activation near the anterior tip. Thus, three distinct mechanisms are required for anterior repression of a single eve enhancer, each in a specific position. Ectopic Slp1 also represses eve stripes 1 and 3 to varying degrees,and the eve 1 and eve 3+7 enhancers each contain GTTT repeats similar to the site in the eve 2 enhancer. These results suggest a common mechanism for preventing anterior activation of three different eve enhancers.

Forkhead-associated domains of the tobacco NtFHA1 transcription activator and the yeast Fhl1 forkhead transcription factor are functionally conserved

NtFHA1 encodes a novel protein containing the forkhead-associated (FHA) domain and the acidic domain in Nicotiana tabacum. NtFHA1 functions as a transactivator and is targeted to the nucleus. The sequence of the FHA domain of NtFHA1 is significantly homologous to that of the Fhl1 forkhead transcription factor of yeast. FHL1 was previously identified as a suppressor of RNA polymerase III mutations, and the fhl1 deletion mutant exhibited severe growth defects and impaired rRNA processing. Ectopic expression of the FHA domain of NtFHA1 (but not its mutant form) resulted in severe growth retardation in yeast. Similarly, expression of Fhl1, its FHA domain, or chimeric Fhl1 containing the NtFHA1 FHA domain also inhibited yeast growth. Yeast cells overexpressing the FHA domains of NtFHA1 and Fhl1 contained lower levels of mature rRNAs and exhibited rRNA-processing defects, similar to the fhl1 null mutant. Chimeric Fhl1 (but not the mutant form with a small deletion in its FHA domain) fully complemented the growth and rRNA-processing defects of the fhl1 null mutant, demonstrating that the FHA domain of NtFHA1 can functionally substitute for the FHA domain of Fhl1. These results demonstrate that the FHA domains of NtFHA1 and Fhl1 are conserved in their structure and function and that the FHA domain of Fhl1 is critically involved in regulation of rRNA processing in yeast. NtFHA1 function in plants may be analogous to Fhl1 function in yeast.

Lymphedema-distichiasis syndrome and FOXC2 gene mutation

PURPOSE

To describe the clinical characteristics of a family with autosomal dominant lymphedema-distichiasis syndrome and to report the results of analysis of the FOXC2 gene

DESIGN

Observational and experimental study.

METHODS

The setting was a clinical practice. The study population was 17 members of a family with lymphedema-distichiasis. Observation procedures were complete ophthalmologic examinations and collection of blood samples. DNA was extracted. Mutation analysis of the coding region of the FOXC2 gene was performed using direct sequencing of polymerase chain reaction (PCR) product and a restriction enzyme assay. The main outcome measure was inheritance of mutation in FOXC2 gene.

RESULTS

Nine patients had distichiasis or lymphedema or both and eight did not. Sequencing of the coding region of the only translated exon of the FOXC2 gene revealed a C to A transversion at position 939 resulting in a Tyr313Stop codon with premature termination of translation and a truncated protein product. The mutation was present in all nine affected individuals and in an asymptomatic 9-year-old boy.

CONCLUSIONS

Distichiasis-lymphedema syndrome results from mutations in FOXC2, a member of the forkhead/winged family of transcription factors. There is intrafamilial variation in the clinical expression of the mutation.

The murine fork head gene Foxn2 is expressed in craniofacial, limb, CNS and somitic tissues during embryogenesis

The fork head domain-containing gene family (Fox) comprises over 20 members in mammals and is defined by a conserved 110 amino-acid motif containing a winged helix structure DNA-binding domain. The members of this gene family have been implicated as key regulators of embryogenesis, cell cycling, cell lineage restriction and cancer. The Foxn2 gene (Ches1) is expressed in postgastrulation embryos in multiple tissues that serve as important signaling centers as well as end-stage-differentiated cell types that arise from different germ layers of the developing embryo. The dynamic and specific expression of Foxn2 during embryonic development suggest multiple independent roles for Foxn2 function during gestation.

FOXD4a and FOXD4b, two new winged helix transcription factors, are expressed in human leukemia cell lines

Winged helix factors are important regulators of embryonal development and tissue differentiation. They are also involved in translocations found in acute leukemias and solid tumors. We have detected transcripts from five known and four novel winged helix genes in leukemia cell lines and CD34(+) blood progenitor cells by reverse trancription-polymerase chain reaction with degenerate primers on the highly conserved DNA binding domain. The genomic clones coding for two new winged helix proteins, FOXD4a and FOXD4b were isolated by high-stringency hybridization of a human phage library. FOXD4a and FOXD4b are encoded by a 1319 and 1250 bp single exon coding for a winged helix DNA binding domain, an amino-terminal acidic region and a carboxy-terminal proline- and alanine-rich region which correspond to putative transcriptional regulatory motifs. TATA box, CCAAT box, and transcription factor binding motifs have been identified in the 5' region of the genes. In addition, foxD4a and foxD4b cDNA has been isolated from NB-4 mRNA. The fox genes are transcribed in a tissue-restricted pattern in adult and fetal human tissues. FoxD4a and foxD4b mRNA was expressed in the leukemia cell lines KG-1, Kasumi, NB-4, HL-60, U937, THP-1, HEL, U266, Jurkat, and Raji. It has already been shown that winged helix factors are also involved in carcinogenesis. Based upon these studies, our results suggest that FOXD4a and FOXD4b may play a role in leukemogenesis.

Regulatory phases of early liver development: paradigms of organogenesis

Genetic analysis, embryonic tissue explantation and in vivo chromatin studies have together identified the distinct regulatory steps that are necessary for the development of endoderm into a bud of liver tissue and, subsequently, into an organ. In this review, I discuss the acquisition of competence to express liver-specific genes by the endoderm, the control of early hepatic growth, the coordination of hepatic and vascular development and the cell differentiation that is necessary to generate a functioning liver. The regulatory mechanisms that underlie these phases are common to the development of many organ systems and might be recapitulated or disrupted during stem-cell differentiation and adult tissue pathogenesis.

FOXC1 transcriptional regulation is mediated by N- and C-terminal activation domains and contains a phosphorylated transcriptional inhibitory domain

Mutations in the FOXC1 gene result in Axenfeld-Rieger malformations of the anterior segment of the eye and lead to an increased susceptibility of glaucoma. To understand how the FOXC1 protein may function in contributing to these malformations, we identified functional regions in FOXC1 required for nuclear localization and transcriptional regulation. Two regions in the FOXC1 forkhead domain, one rich in basic amino acid residues, and a second, highly conserved among all FOX proteins, were necessary for nuclear localization of the FOXC1 protein. However, only the basic region was sufficient for nuclear localization. Two transcriptional activation domains were identified in the extreme N- and C-terminal regions of FOXC1. A transcription inhibitory domain was located at the central region of the protein. This region was able to reduce the trans-activation potential of the C-terminal activation domain, as well as the GAL4 activation domain. Lastly, we demonstrate that FOXC1 is a phosphoprotein, and a number of residues predicted to be phosphorylated were localized to the FOXC1 inhibitory domain. Removal of residues 215-366 resulted in a transcriptionally hyperactive FOXC1 protein, which displayed a reduced level of phosphorylation. These results indicate that FOXC1 is under complex regulatory control with multiple functional domains modulating FOXC1 transcriptional regulation.

It takes guts: the Drosophila hindgut as a model system for organogenesis

The Drosophila hindgut is fruitful territory for investigation of events common to many types of organogenesis. The development of the Drosophila hindgut provides, in microcosm, a genetic model system for studying processes such as establishment (patterning) of an epithelial primordium, its internalization by gastrulation, development of left--right asymmetric looping, patterning in both the anteroposterior and dorsoventral axes, innervation, investment of an epithelium with mesoderm, reciprocal epitheliomesenchymal interactions, cell shape change, and cell rearrangement. We review the genetic control of these processes during development of the Drosophila hindgut, and compare these to related processes in other bilaterians, particularly vertebrates. We propose that caudal/Cdx, brachyenteron/Brachyury, fork head/HNF-3, and wingless/Wnt constitute a conserved "cassette" of genes expressed in the blastopore and later in the gut, involved in posterior patterning, cell rearrangement, and gut maintenance. Elongation of the internalized Drosophila hindgut primordium is similar to elongation of the archenteron and also of the entire embryonic axis (both during and after gastrulation), as well as of various tubules (e.g., nephric ducts, Malpighian tubules), as it is driven by cell rearrangement. The genes drumstick, bowl, and lines (which encode putative transcriptional regulators) are required for this cell rearrangement, as well as for spatially localized gene expression required to establish the three morphologically distinct subregions of the hindgut. Expression of signaling molecules regulated by drumstick, bowl, and lines, in particular of the JAK/STAT activator Unpaired at the hindgut anterior, may play a role in controlling hindgut cell rearrangement. Other cell signaling molecules expressed in the hindgut epithelium are required to establish its normal size (Dpp and Hh), and to establish and maintain the hindgut visceral mesoderm (Wg and Hh). Both maternal gene activity and zygotic gene activity are required for asymmetric left--right looping of the hindgut. Some of the same genes (caudal and brachyenteron) required for embryonic hindgut development also act during pupation to construct a new hindgut from imaginal cells. Application of the plethora of genetic techniques available in Drosophila, including forward genetic screens, should identify additional genes controlling hindgut development and thus shed light on a variety of common morphogenetic processes.

AmphiFoxE4, an amphioxus winged helix/forkhead gene encoding a protein closely related to vertebrate thyroid transcription factor-2: expression during pharyngeal development

The full-length sequence and developmental expression of amphioxus AmphiFoxE4 are described. Transcripts of the gene are first detected in the pharyngeal endoderm, where the club-shaped gland is forming and subsequently in the definitive gland itself. AmphiFoxE4 is closely related to vertebrate genes encoding the thyroid-specific transcription factor-2 (TTF2), which plays an early developmental role in the morphogenesis of the thyroid gland and a later role in hormone-mediated control of thyroid function. In amphioxus, AmphiFoxE4 expression is not thyroid specific because the club-shaped gland, the only structure expressing the gene, is not homologous to the vertebrate thyroid; instead, the thyroid homologue of amphioxus is a specialized region of the pharyngeal endoderm called the endostyle. We propose that (a) the pharynx of an amphioxus-like ancestor of the vertebrates included a club-shaped gland that expressed FoxE4 as well as an endostyle that did not, and (b) the club-shaped gland soon disappeared in the vertebrate line of descent but (c) not before there was a homeogenetic transfer of FoxE4 expression from the club-shaped gland to the nearby endostyle. Such a transfer could have provided part of the genetic program enabling the endostyle to separate from the pharyngeal endoderm and migrate away as the rudiment of the thyroid gland.

drumstick, bowl, and lines are required for patterning and cell rearrangement in the Drosophila embryonic hindgut

The Drosophila embryonic hindgut is a robust system for the study of patterning and morphogenesis of epithelial organs. We show that, in a period of about 10 h, and in the absence of significant cell division or apoptosis, the hindgut epithelium undergoes morphogenesis by changes in cell shape and size and by cell rearrangement. The epithelium concomitantly becomes surrounded by visceral mesoderm and is characterized by distinct gene expression patterns that forecast the development of three morphological subdomains: small intestine, large intestine, and rectum. At least three genes encoding putative transcriptional regulators, drumstick (drm), bowl, and lines (lin), are required to establish normal hindgut morphology. We show that the defect in hindgut elongation in drm, bowl, and lin mutants is due, in large part, to the requirement of these genes in the process of cell rearrangement. Further, we show that drm, bowl, and lin are required for patterning of the hindgut, i.e., for correct expression in the prospective small intestine, large intestine, and rectum of genes encoding cell signals (wingless, hedgehog, unpaired, Serrate, dpp) and transcription factors (engrailed, dead ringer). The close association of both cell rearrangement and patterning defects in all three mutants suggest that proper patterning of the hindgut into small intestine and large intestine is likely required for its correct morphogenesis.

Distinct and overlapping functions of insulin and IGF-I receptors

Targeted gene mutations have established distinct, yet overlapping, developmental roles for receptors of the insulin/IGF family. IGF-I receptor mediates IGF-I and IGF-II action on prenatal growth and IGF-I action on postnatal growth. Insulin receptor mediates prenatal growth in response to IGF-II and postnatal metabolism in response to insulin. In rodents, unlike humans, insulin does not participate in embryonic growth until late gestation. The ability of the insulin receptor to act as a bona fide IGF-II-dependent growth promoter is underscored by its rescue of double knockout Igf1r/Igf2r mice. Thus, IGF-II is a true bifunctional ligand that is able to stimulate both insulin and IGF-I receptor signaling, although with different potencies. In contrast, the IGF-II/cation-independent mannose-6-phosphate receptor regulates IGF-II clearance. The growth retardation of mice lacking IGF-I and/or insulin receptors is due to reduced cell number, resulting from decreased proliferation. Evidence from genetically engineered mice does not support the view that insulin and IGF receptors promote cellular differentiation in vivo or that they are required for early embryonic development. The phenotypes of insulin receptor gene mutations in humans and in mice indicate important differences between the developmental roles of insulin and its receptor in the two species.

Organ formation in Drosophila: specification and morphogenesis of the salivary gland

The Drosophila salivary gland has emerged as an outstanding model system for the process of organ formation. Many of the component steps, from initial regional specification through cell specialization and morphogenesis, are known and many of the genes required for these different processes have been identified. The salivary gland is a relatively simple organ; the entire gland comprises of only two major cell types, which derive from a single contiguous primordium. Salivary cells cease dividing once they are specified, and organ growth is achieved simply by an increase in size of individual cells, thus eliminating concerns about the potential unequal distribution of determinants during mitosis. Drosophila salivary glands form by the same cellular mechanisms as organs in higher organisms, including regulated cell shape changes, cell intercalation and directed cell migration. Thus, learning how these events are coordinated for tissue morphogenesis in an organism for which the genetic and molecular tools are unsurpassed should provide excellent paradigms for dissecting related processes in the more intricate organs of more complicated species.

Structural characterization of the mouse Foxf1a gene

Members in the superfamily of the forkhead/winged-helix transcription factors are known to play a critical role in the control of cell differentiation and tissue development. To understand the regulation and function of these genes, we have initially isolated and characterized the mouse Foxf1a gene, a novel forkhead gene predominantly expressed in the lung. The mouse gene consists of two exons with the forkhead domain contained in exon 1, and is located at band E1 on chromosome 8. Amino acid sequence of the mouse protein shares a high degree of homology to that of the corresponding human protein. The tissue specificity of expression of the mouse gene also resembles that found in the human gene. This gene is primarily expressed in the lung, and to a lesser extent in placenta and tissues in gastrointestinal tract. The transcription start site was mapped to 113 nucleotides upstream from the putative translation initiation site. The promoter of the mouse gene is highly GC rich and contains neither a CAAT nor a TATA box. A series of luciferase report constructs driven by the promoter and various deletions in the 5' flanking region of the gene were constructed and employed in transient transfection studies using a line of SV40 transformed mouse lymph node endothelial cells (SVEC4-10), which express the endogenous Foxf1a gene, and a line of mouse hepatoma cells (Hepa 1-6), in which Foxf1a is not expressed. To our surprise, these reporter genes are equally active in both cell lines. Further studies have shown that the proximal 5' flanking sequence and exon 1 of the endogenous gene are highly methylated in Hepa 1-6 cells but not in SVEC4-10 cells, suggesting that DNA methylation but not cell-specific transcription factor(s) regulates cell specificity of gene expression in these cultured cells.

foxD5a, a Xenopus winged helix gene, maintains an immature neural ectoderm via transcriptional repression that is dependent on the C-terminal domain

Xenopus foxD5a, the full-length fork head gene previously described as a PCR fragment (XFLIP), is first detectable at stage II of oogenesis. Low-abundance maternal transcripts are localized to the animal hemisphere of the cleavage embryo, and protein can be translocated to the nucleus prior to the onset of zygotic transcription. Zygotic expression is strongest in the presumptive neural ectoderm at gastrula and neural plate stages, but there is minor paraxial mesodermal expression during primary gastrulation that becomes significant in the tail bud during secondary gastrulation. Expression of foxD5a in animal cap explants induces elongation and expression of mesodermal, neural-inducing, and early neural-specifying genes, indicating a role in dorsal axis formation. Zygotic foxD5a expression is induced strongly by siamois, moderately by cerberus, weakly by Wnt8 and noggin, and not by chordin in animal cap explants. Expression of foxD5a in whole embryos has differential dorsal and ventral effects. Ventral mRNA injection induces partial secondary axes composed of expanded mesodermal and epidermal tissues, but does not induce ectopic neural tissues. Dorsal mRNA injection causes hypertrophy of the neural plate and expansion of early neural genes (sox3 and otx2), but this is not the result of increased proliferation or expanded neural-inducing mesoderm. The neural plate appears to be maintained in an immature state because otx2 expression is expanded and expression of en2, Krox20, proneural genes (Xnrgn1, neuroD) and a neural differentiation gene (n-tubulin) is repressed in foxD5a-expressing cells. These results indicate that foxD5a maintains an undifferentiated neural ectoderm after neural induction. Expression of foxD5a constructs fused with the engrailed repressor domain or with the VP16 activation domain demonstrates that FoxD5a acts as a transcriptional repressor in axis formation and neural plate expansion. Deletion constructs indicate that this activity requires the C-terminal domain of the protein.

Hindgut visceral mesoderm requires an ectodermal template for normal development in Drosophila

During Drosophila embryogenesis, the development of the midgut endoderm depends on interactions with the overlying visceral mesoderm. Here we show that the development of the hindgut also depends on cellular interactions, in this case between the inner ectoderm and outer visceral mesoderm. In this section of the gut, the ectoderm is essential for the proper specification and differentiation of the mesoderm, whereas the mesoderm is not required for the normal development of the ectoderm. Wingless and the fibroblast growth factor receptor Heartless act over sequential but interdependent phases of hindgut visceral mesoderm development. Wingless is required to establish the primordium and to enhance Heartless expression. Later, Heartless is required to promote the proper differentiation of the hindgut visceral mesoderm itself.

Planaria FoxA (HNF3) homologue is specifically expressed in the pharynx-forming cells

We have isolated a planarian Forkhead box A (FoxA, a new name for a gene group containing HNF3 alpha,beta,gamma)-related gene, DjFoxA, and examined its spatial and temporal distribution in both intact and regenerating planarians by in situ hybridization. In intact worms, DjFoxA is specifically expressed in the cells participating in pharynx development in the region surrounding the pharynx, which is located in the central portion of the body. During regeneration, DjFoxA-positive cells appear in the pharynx-forming region and migrate to the midline to form a pharynx rudiment. These results suggest that DjFoxA is specifically expressed in the cells participating in pharynx formation and has an evolutionarily conserved function in digestive tract formation.

Early genes required for salivary gland fate determination and morphogenesis in Drosophila melanogaster

Studies of Drosophila salivary gland formation have elucidated the regulatory pathway by which the salivary gland fate is determined and the morphogenetic processes by which the primordial cells are internalized to form the tubular glands. Both the position of the salivary primordia and the number of cells recruited to a salivary gland fate are established through a combination of the localized expression of the transcription factors SEX COMBS REDUCED (SCR), TEASHIRT (TSH) and ABDOMINAL-B (ABD-B), and localized DPP-signaling. Similarly, the distinction between the two major cell types, duct and secretory, is determined by spatially limited EGF-signaling. Salivary gland formation also requires the function of two transcription factors expressed in nearly all cells of the developing embryo, EXTRADENTICLE (EXD) and HOMOTHORAX (HTH). Once the salivary gland fate is determined, cells of the secretory primordia are internalized by an apical constriction mode of invagination. We have characterized three genes encoding transcription factors, trachealess (trh), hückebein (hkb), and fork head (fkh), that are downstream targets of the salivary gland regulators. Mutations in these transcription factors profoundly affect salivary gland morphogenesis. trh is required for the formation of the salivary duct tubes. hkb determines the order of secretory cell invagination, a regulated process critical for determining the final shape of the salivary gland. fkh has two early roles in salivary gland formation. fkh both promotes secretory cell survival and facilitates secretory cell internalization. trh, hkb, and fkh are involved in the formation of not only the salivary duct and secretory tubes, but also of other tubular structures, such as the trachea and the gut endoderm. We propose that trh, hkb, and fkh may serve as "morphogenetic cassettes" responsible for forming tubular structures in a variety of tissues.

Mutations in FOXC2 (MFH-1), a forkhead family transcription factor, are responsible for the hereditary lymphedema-distichiasis syndrome

Lymphedema-distichiasis (LD) is an autosomal dominant disorder that classically presents as lymphedema of the limbs, with variable age at onset, and double rows of eyelashes (distichiasis). Other complications may include cardiac defects, cleft palate, extradural cysts, and photophobia, suggesting a defect in a gene with pleiotrophic effects acting during development. We previously reported neonatal lymphedema, similar to that in Turner syndrome, associated with a t(Y;16)(q12;q24.3) translocation. A candidate gene was not found on the Y chromosome, and we directed our efforts toward the chromosome 16 breakpoint. Subsequently, a gene for LD was mapped, by linkage studies, to a 16-cM region at 16q24.3. By FISH, we determined that the translocation breakpoint was within this critical region and further narrowed the breakpoint to a 20-kb interval. Because the translocation did not appear to interrupt a gene, we considered candidate genes in the immediate region that might be inactivated by position effect. In two additional unrelated families with LD, we identified inactivating mutations-a nonsense mutation and a frameshift mutation-in the FOXC2 (MFH-1) gene. FOXC2 is a member of the forkhead/winged-helix family of transcription factors, whose members are involved in diverse developmental pathways. FOXC2 knockout mice display cardiovascular, craniofacial, and vertebral abnormalities similar to those seen in LD syndrome. Our findings show that FOXC2 haploinsufficiency results in LD. FOXC2 represents the second known gene to result in hereditary lymphedema, and LD is only the second hereditary disorder known to be caused by a mutation in a forkhead-family gene.

Null mutations in the lin-31 gene indicate two functions during Caenorhabditis elegans vulval development

The lin-31 gene is required for the proper specification of vulval cell fates in the nematode Caenorhabditis elegans and encodes a member of the winged-helix family of transcription factors. Members of this important family have been identified in many organisms and are known to bind specific DNA targets involved in a variety of developmental processes. DNA sequencing of 13 lin-31 alleles revealed six nonsense mutations and two missense mutations within the DNA-binding domain, plus three deletions, one transposon insertion, and one frameshift mutation that all cause large-scale disruptions in the gene. The missense mutations are amino acid substitutions in the DNA-binding domain and probably disrupt interactions of the LIN-31 transcription factor with its DNA target. In addition, detailed phenotypic analysis of all 19 alleles showed similar penetrance for several characteristics examined. From our analysis we conclude: (1) the null phenotype of lin-31 is the phenotype displayed by almost all of the existing alleles, (2) the DNA-binding domain plays a critical role in LIN-31 function, and (3) direct screens for multivulva and vulvaless mutants will probably yield only null (or strong) alleles of lin-31.

Repressive and restrictive mesodermal interactions with gut endoderm: possible relation to Meckel's Diverticulum

The midgut and hindgut endoderm of the mouse embryo give rise to the intestinal epithelium, yet it is not known how the intestinal program is chosen in contrast to other endoderm-derived cell types. Previous tissue explant studies with embryos at 8.5 to 11.5 days gestation (d) showed that when the gut mesoderm is removed from the prospective intestinal endoderm, the endoderm activates the expression of liver-specific genes such as serum albumin, demonstrating the endoderm's pluripotence. This reversible repression of liver genes does not affect the expression of the endodermal transcription factors HNF3 and GATA4, nor these factors' ability to engage target sites in chromatin. We have now found that at 13.5 d, the mesoderm gains a second inhibitory activity, resulting in the irreversible loss of expression of HNF3 (Foxa2) and GATA factors in the endoderm and the absence of factors binding to their target sites in chromatin. The second inhibitory activity causes the endoderm to lose the potential to activate a liver gene, and this restriction precedes the normal cytodifferentiation of the intestinal epithelium. In summary, two inhibitory interactions with mesoderm successively restrict the developmental potential of the gut endoderm, leading to intestinal differentiation. We also observed rare gut bud structures in midgestation embryos that appear to represent murine examples of Meckel's Diverticulum, a congenital abnormality in human development. The absence of restrictive mesodermal interactions could explain how Meckel's diverticula express diverse non-intestinal, endoderm-derived cell types.

Fork head prevents apoptosis and promotes cell shape change during formation of the Drosophila salivary glands

The secretory tubes of the Drosophila salivary glands are formed by the regulated, sequential internalization of the primordia. Secretory cell invagination occurs by a change in cell shape that includes basal nuclear migration and apical membrane constriction. In embryos mutant for fork head (fkh), which encodes a transcription factor homologous to mammalian hepatocyte nuclear factor 3beta (HNF-3beta), the secretory primordia are not internalized and secretory tubes do not form. Here, we show that secretory cells of fkh mutant embryos undergo extensive apoptotic cell death following the elevated expression of the apoptotic activator genes, reaper and head involution defective. We rescue the secretory cell death in the fkh mutants and show that the rescued cells still do not invaginate. The rescued fkh secretory cells undergo basal nuclear migration in the same spatial and temporal pattern as in wild-type secretory cells, but do not constrict their apical surface membranes. Our findings suggest at least two roles for fkh in formation of the embryonic salivary glands: an early role in promoting survival of the secretory cells, and a later role in secretory cell invagination, specifically in the constriction of the apical surface membrane.

Homologous tails? Or tales of homology?

Classical mutations at the mouse Brachyury (T) locus were discovered because they lead to shortened tails in heterozygous newborns. no tail (ntl) mutants in the zebrafish, as their name suggests, show a similar phenotype. In Drosophila, mutants in the brachyenteron (byn) gene disrupt hindgut formation. These genes all encode T-box proteins, a class of sequence-specific DNA binding proteins and transcription factors. Mutations in the C. elegans mab-9 gene cause massive defects in the male tail because of failed fate decisions in two tail progenitor cells. In a recent paper, Woollard and Hodgkin have cloned the mab-9 gene and found that it too encodes a T-box protein, similar to Brachyury in vertebrates and brachyenteron in Drosophila. The authors suggest that their results support models for an evolutionarily ancient role for these genes in hindgut formation. We will discuss this proposal and try to decide whether the gene sequences, gene interactions and gene expression patterns allow any conclusions to be made about the rear end of the ancestral metazoan.

The hepatocyte nuclear factor 3 (HNF3 or FOXA) family in metabolism

The genes encoding hepatocyte nuclear factor 3 (HNF3) proteins play a pivotal role in the regulation of metabolism and in the differentiation of metabolic tissues such as the pancreas and liver. HNF3 transcription factors bind to cis-regulatory elements in hundreds of genes encoding gluconeogenic and glycolytic enzymes, serum proteins and hormones. Genetic analysis in mice has shown that HNF3 beta is necessary for the development of the foregut endoderm, from which the liver and pancreas arise. HNF3 alpha is required for the full activation of glucagon in the pancreas, whereas HNF3 gamma induces the activation of gluconeogenic enzymes to prevent hypoglycemia during fasting.

FHX.L and FHX.S, two isoforms of the human fork-head factor FHX (FOXJ2) with differential activity

Many biological phenomena are dependent on mechanisms that fine-tune the expression levels of particular genes. This can be achieved by altering the relative activity of a single transcription factor, by post-translational modifications or by interaction with regulatory molecules. An alternative mechanism is based on competition between two or more differently active isoforms of the same transcription factor. We found that FHX, a recently characterized human fork-head transcriptional activator, may show such a mechanism for balancing its activity by expressing two differently sized isoforms, FHX.S and FHX.L, encoded by a single gene located on human chromosome 12. FHX. L and FHX.S showed different transcriptional capacities, the larger form, FHX.L, behaving as the more potent transactivator. A transactivation domain of the acidic type present only in FHX.L would account for this functional difference. The relative concentrations of these two FHX isoforms appear to vary in a number of cell types, a circumstance that may regulate the final activity of this transcription factor.

A member of Forkhead family transcription factor, FKHRL1, is one of the downstream molecules of phosphatidylinositol 3-kinase-Akt activation pathway in erythropoietin signal transduction

The phosphatidylinositol 3-kinase (PI3K) signaling pathway is important for the regulation of a number of cellular responses. Serine/threonine kinase Akt (protein kinase B; PKB) is downstream of PI3K and activated by growth factors. This study found that erythropoietin (EPO) induced tyrosine phosphorylation of Akt in a time- and dose-dependent manner in EPO-dependent human leukemia cell line UT-7/EPO. In vitro kinase assay using histone H2B and glucose synthase kinase as substrates demonstrated that Akt was actually activated by EPO. EPO-induced phosphorylation of Akt was completely blocked by a PI3K-specific inhibitor, LY294002, at 10 μmol/L, indicating that activation of Akt by EPO is dependent on PI3K activity. In addition, overexpression of the constitutively active form of Akt on UT-7/EPO cells partially blocked apoptosis induced by withdrawal of EPO from the culture medium. This finding suggested that the PI3K-Akt activation pathway plays some role in the antiapoptotic effect of EPO. EPO induced phosphorylation of a member of the trancription factor Forkhead family, FKHRL1, at threonine 32 and serine 253 in a dose- and time-dependent manner in UT-7/EPO cells. Moreover, results showed that Akt kinase activated by EPO directly phosphorylated FKHRL1 protein and that FKHRL1 phosphorylation was completely dependent on PI3K activity as is the case for Akt. In conjunction with the evidence that FKHRL1 is expressed in normal human erythroid progenitor cells and erythroblasts, the results suggest that FKHRL1 plays an important role in erythropoiesis as one of the downstream target molecules of PI3K-Akt.

A member of Forkhead family transcription factor, FKHRL1, is one of the downstream molecules of phosphatidylinositol 3-kinase-Akt activation pathway in erythropoietin signal transduction

The phosphatidylinositol 3-kinase (PI3K) signaling pathway is important for the regulation of a number of cellular responses. Serine/threonine kinase Akt (protein kinase B; PKB) is downstream of PI3K and activated by growth factors. This study found that erythropoietin (EPO) induced tyrosine phosphorylation of Akt in a time- and dose-dependent manner in EPO-dependent human leukemia cell line UT-7/EPO. In vitro kinase assay using histone H2B and glucose synthase kinase as substrates demonstrated that Akt was actually activated by EPO. EPO-induced phosphorylation of Akt was completely blocked by a PI3K-specific inhibitor, LY294002, at 10 μmol/L, indicating that activation of Akt by EPO is dependent on PI3K activity. In addition, overexpression of the constitutively active form of Akt on UT-7/EPO cells partially blocked apoptosis induced by withdrawal of EPO from the culture medium. This finding suggested that the PI3K-Akt activation pathway plays some role in the antiapoptotic effect of EPO. EPO induced phosphorylation of a member of the trancription factor Forkhead family, FKHRL1, at threonine 32 and serine 253 in a dose- and time-dependent manner in UT-7/EPO cells. Moreover, results showed that Akt kinase activated by EPO directly phosphorylated FKHRL1 protein and that FKHRL1 phosphorylation was completely dependent on PI3K activity as is the case for Akt. In conjunction with the evidence that FKHRL1 is expressed in normal human erythroid progenitor cells and erythroblasts, the results suggest that FKHRL1 plays an important role in erythropoiesis as one of the downstream target molecules of PI3K-Akt.

Domina (Dom), a new Drosophila member of the FKH/WH gene family, affects morphogenesis and is a suppressor of position-effect variegation

Domina (Dom) is a novel member of the FKH/WH transcription factor gene family of Drosophila. Two alternatively polyadenylated Dom transcripts of 2.9 and 3.9 kb encode a 719-amino-acid protein with a FKH/WH domain and a putative acidic transactivation domain. Dom is mainly expressed in the central and peripheral nervous system. Homozygous mutants show rough eyes, irregular arrangement of bristles, extended wings, defective posterior wing margins, and a severely diminished vitality and fertility. Heterozygous Dom flies are morphologically wild type but show suppression of position-effect variegation. Consistently with this chromatin effect DOM protein is accumulated in the chromocenter and, as expected from a transcription factor, is found at specific euchromatic loci. Sequence comparison suggests that DOM of Drosophila is homologous to the chordate WHN proteins. The chromatin modifying capability of DOM is probably based on the FKH/WH domain, which shows a remarkable structural similarity to the winged-helix structures of H1 and the central globular domain of H5.

Jumeaux, a novel Drosophila winged-helix family protein, is required for generating asymmetric sibling neuronal cell fates

The great majority of neurons in the Drosophila embryonic CNS are generated through two successive asymmetric cell divisions; neuroblasts (NBs) divide to produce another NB and a smaller ganglion mother cell (GMC); GMCs divide to generate two sibling neurons which can adopt distinct identities. During the division of the first born GMC from the NB4-2 lineage, GMC4-2a, Inscuteable (Insc) is localised to the apical cortex, Pon/Numb is localised to the basal cortex and two daughters with distinct identities, the RP2 motoneuron and its sibling RP2sib, are born. Resolution of distinct sibling neuronal fates requires correct apical localisation of Insc to facilitate the asymmetric localisation and preferential segregation of Pon/Numb to the basal daughter destined to become RP2. Here we report that jumeaux (jumu), which encodes a new member of the winged-helix family of transcription factors, is required to mediate the asymmetric localisation and segregation of Pon/Numb but is dispensable for Insc apical localisation during the GMC4-2a cell division. In jumu mutants GMC4-2a Pon/Numb asymmetric localisation is defective and both daughter neurons can adopt the RP2 identity. Jumu protein shows nuclear localisation and within the NB4-2 lineage is first detected only after the first neuroblast cell division, in GMC4-2a. Our results suggest that in addition to the correct formation of an apical complex, transcription mediated by Jumu is also necessary to facilitate the correct asymmetric localisation and segregation of Pon/Numb.

Two yeast forkhead genes regulate the cell cycle and pseudohyphal growth

There are about 800 genes in Saccharomyces cerevisiae whose transcription is cell-cycle regulated. Some of these form clusters of co-regulated genes. The 'CLB2' cluster contains 33 genes whose transcription peaks early in mitosis, including CLB1, CLB2, SWI5, ACE2, CDC5, CDC20 and other genes important for mitosis. Here we find that the genes in this cluster lose their cell cycle regulation in a mutant that lacks two forkhead transcription factors, Fkh1 and Fkh2. Fkh2 protein is associated with the promoters of CLB2, SWI5 and other genes of the cluster. These results indicate that Fkh proteins are transcription factors for the CLB2 cluster. The fkh1 fkh2 mutant also displays aberrant regulation of the 'SIC1' cluster, whose member genes are expressed in the M-G1 interval and are involved in mitotic exit. This aberrant regulation may be due to aberrant expression of the transcription factors Swi5 and Ace2, which are members of the CLB2 cluster and controllers of the SIC1 cluster. Thus, a cascade of transcription factors operates late in the cell cycle. Finally, the fkh1 fkh2 mutant displays a constitutive pseudohyphal morphology, indicating that Fkh1 and Fkh2 may help control the switch to this mode of growth.

Hepatocyte nuclear factor 3beta (Foxa2) is dispensable for maintaining the differentiated state of the adult hepatocyte

Liver-specific gene expression is controlled by a heterogeneous group of hepatocyte-enriched transcription factors. One of these, the winged helix transcription factor hepatocyte nuclear factor 3beta (HNF3beta or Foxa2) is essential for multiple stages of embryonic development. Recently, HNF3beta has been shown to be an important regulator of other hepatocyte-enriched transcription factors as well as the expression of liver-specific structural genes. We have addressed the role of HNF3beta in maintenance of the hepatocyte phenotype by inactivation of HNF3beta in the liver. Remarkably, adult mice lacking HNF3beta expression specifically in hepatocytes are viable, with histologically normal livers and normal liver function. Moreover, analysis of >8,000 mRNAs by array hybridization revealed that lack of HNF3beta affects the expression of only very few genes. Based on earlier work it appears that HNF3beta plays a critical role in early liver development; however, our studies demonstrate that HNF3beta is not required for maintenance of the adult hepatocyte or for normal liver function. This is the first example of such functional dichotomy for a tissue specification transcription factor.

The evolution of the serotonergic nervous system

The pattern of development of the serotonergic nervous system is described from the larvae of ctenophores, platyhelminths, nemerteans, entoprocts, ectoprocts (bryozoans), molluscs, polychaetes, brachiopods, phoronids, echinoderms, enteropneusts and lampreys. The larval brain (apical ganglion) of spiralian protostomes (except nermerteans) generally has three serotonergic neurons and the lateral pair always innervates the ciliary band of the prototroch. In contrast, brachiopods, phoronids, echinoderms and enteropneusts have numerous serotonergic neurons in the apical ganglion from which the ciliary band is innervated. This pattern of development is much like the pattern seen in lamprey embryos and larvae, which leads the author to conclude that the serotonergic raphe system found in vertebrates originated in the larval brain of deuterostome invertebrates. Further, the neural tube of chordates appears to be derived, at least in part, from the ciliary band of deuterostome invertebrate larvae. The evidence shows no sign of a shift in the dorsal ventral orientation within the line leading to the chordates.

Conserved and divergent aspects of terminal patterning in the beetle Tribolium castaneum

To infer similarities and differences in terminal pattern formation in insects, we analyzed several of the key genes of this process in the beetle Tribolium castaneum. We cloned two genes of the terminal pattern cascade, namely tailless (tll) and forkhead (fkh), from Tribolium and studied their expression patterns. In addition, we analyzed the pattern of MAP kinase activation at blastoderm stage as a possible signature for torso-dependent signaling. Further, we analyzed the late expression of the previously cloned Tribolium caudal (Tc-cad) gene. Finally, we used the upstream region of Tc-tll to drive a reporter gene construct in Drosophila. We find that this construct is activated at the terminal regions in Drosophila, suggesting that the torso-dependent pathway is conserved between the species. We show that most of the expression patterns of the genes studied here are similar in Drosophila and Tribolium, suggesting conserved functions. There is, however, one exception, namely the early function of Tc-tll at the posterior pole. In Drosophila, the posterior tll expression is involved in the direct regulation of the target genes of the terminal pathway. In Tribolium, posterior Tc-tll expression occurs only for a short time and ceases before the target genes known from Drosophila are activated. Thus, we infer that Tc-tll does not function as a direct regulator of segmentation genes at the posterior end. It is more likely to be involved in the early specification of a group of "terminal" cells, which begin to differentiate only at a later stage of embryogenesis, when much of the abdominal segmentation process is complete. Thus, there appears to have been a major shift in tll function during the evolutionary transition from short germ to long germ embryogenesis.