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.

Drosophila arc encodes a novel adherens junction-associated PDZ domain protein required for wing and eye development.

Loss of arc function results in a downwardly curved wing and smaller eyes with a reduced number of ommatidia. Consistent with this phenotype, molecular analysis shows that arc mRNA and protein are expressed in the wing imaginal disc and in clusters of cells in the morphogenetic furrow of the eye imaginal disc. The 36-kb arc transcription unit contains 10 exons that are spliced to form a 5. 5-kb mRNA. The encoded Arc protein is 143,000 Da and contains two PDZ (PSD-95, Discs large, ZO-1) domains; there is no close structural similarity to other PDZ proteins. In addition to its expression in imaginal discs, arc is expressed during embryogenesis in epithelia undergoing morphogenesis, including the invaginating posterior midgut, evaginating Malpighian tubule buds, elongating hindgut, invaginating salivary glands, intersegmental grooves, and developing tracheae. Arc protein colocalizes with Armadillo (beta-catenin) to the apical (luminal) surface of these developing epithelia, indicating that it is associated with adherens junctions. Genes that are required for patterning of embryonic epithelia (e.g., tailless, Krüppel, fork head, and brachyenteron) or for progression of the morphogenetic furrow (i. e., hedgehog) are required to establish or maintain the regional expression of arc. Misexpression of arc in the eye imaginal discs results in rough and larger eyes with fused ommatidia. We propose that arc affects eye development by modulating adherens junctions of the developing ommatidium.

The control of cell fate in the embryonic visual system by atonal, tailless and EGFR signaling.

We describe here the role of the transcription factors encoding genes tailless (tll), atonal (ato), sine oculis (so), eyeless (ey) and eyes absent (eya), and EGFR signaling in establishing the Drosophila embryonic visual system. The embryonic visual system consists of the optic lobe primordium, which, during later larval life, develops into the prominent optic lobe neuropiles, and the larval photoreceptor (Bolwig's organ). Both structures derive from a neurectodermal placode in the embryonic head. Expression of tll is normally confined to the optic lobe primordium, whereas ato appears in a subset of Bolwig's organ cells that we call Bolwig's organ founders. Phenotypic analysis, using specific markers for Bolwig's organ and the optic lobe, of tll loss- and gain-of-function mutant embryos reveals that tll functions to drive cells to optic lobe as opposed to Bolwig's organ fate. Similar experiments indicate that ato has the opposite effect, namely driving cells to a Bolwig's organ fate. Since we can show that tll and ato do not regulate each other, we propose a model wherein tll expression restricts the ability of cells to respond to signaling arising from ato-expressing Bolwig's organ pioneers. Our data further suggest that the Bolwig's organ founder cells produce Spitz (the Drosophila TGFalpha homolog) signal, which is passed to the neighboring secondary Bolwig's organ cells where it activates the EGFR signaling cascade and maintains the fate of these secondary cells. The regulators of tll expression in the embryonic visual system remain elusive, as we were unable to find evidence for regulation by the 'early eye genes' so, eya and ey, or by EGFR signaling.

Identification of genes controlling malpighian tubule and other epithelial morphogenesis in Drosophila melanogaster.

The Drosophila Malpighian tubule is a model system for studying genetic mechanisms that control epithelial morphogenesis. From a screen of 1800 second chromosome lethal lines, by observing uric acid deposits in unfixed inviable embryos, we identified five previously described genes (barr, fas, flb, raw, and thr) and one novel gene, walrus (wal), that affect Malpighian tubule morphogenesis. Phenotypic analysis of these mutant embryos allows us to place these genes, along with other previously described genes, into a genetic pathway that controls Malpighian tubule development. Specifically, wal affects evagination of the Malpighian tubule buds, fas and thr affect bud extension, and barr, flb, raw, and thr affect tubule elongation. In addition, these genes were found to have different effects on development of other epithelial structures, such as foregut and hindgut morphogenesis. Finally, from the same screen, we identified a second novel gene, drumstick, that affects only foregut and hindgut morphogenesis.

Role of caudal in hindgut specification and gastrulation suggests homology between Drosophila amnioproctodeal invagination and vertebrate blastopore.

During early embryogenesis in Drosophila, caudal mRNA is distributed as a gradient with its highest level at the posterior of the embryo. This suggests that the Caudal homeodomain transcription factor might play a role in establishing the posterior domains of the embryo that undergo gastrulation and give rise to the posterior gut. By generating embryos lacking both the maternal and zygotic mRNA contribution, we show that caudal is essential for invagination of the hindgut primordium and for further specification and development of the hindgut. These effects are achieved by the function of caudal in activating different target genes, namely folded gastrulation, which is required for invagination of the posterior gut primordium, and fork head and wingless, which are required to promote development of the internalized hindgut primordium. caudal is not sufficient for hindgut gastrulation and development, however, as it does not play a significant role in activating expression of the genes tailless, huckebein, brachyenteron and bowel. We argue that caudal and other genes expressed at the posterior of the Drosophila embryo (fork head, brachyenteron and wingless) constitute a conserved constellation of genes that plays a required role in gastrulation and gut development.

The role of morphogenetic cell death during Drosophila embryonic head development.

This article addresses the role of programmed cell death (apoptosis) during embryonic head development of Drosophila. Previous studies showed that reaper (rpr) is expressed in and required by cells undergoing apoptosis. We have analyzed the correlation between the pattern of expression of rpr and morphogenetic movements affecting head development. Furthermore, we have investigated the defects in head development resulting from the absence of apoptosis in embryos deficient for rpr. Our results show that, in the head, domains of high incidence of cell death as marked by expression of rpr correlate with regions where most morphogenetic movements occur; these regions are involved in formation of mouth structures, the internalization of neural progenitors, and head involution. Cellular events driving these movements are delamination, invagination, and intercalation as well as disruption and reformation of contacts among epithelial cells. The analysis of rpr-deficient embryos demonstrates that, despite of the widespread occurrence of apoptosis during normal head morphogenesis, many aspects of this process proceed in an apparently unperturbed manner even when cell death is blocked. In particular, movements that happen early during embryonic development and that are evolutionarily more ancient (e.g., formation of the dorsal ridge and the pharynx) take place almost normally in rpr-deficient embryos. Later events which are mostly associated with head involution (e.g., retraction of the clypeolabrum, formation of the dorsal pouch, fusion of lateral gnathal lobes) are evolutionarily more recent and fail to occur normally in rpr-deficient embryos.

Posterior gut development in Drosophila: a model system for identifying genes controlling epithelial morphogenesis.

The posterior gut of the Drosophila embryo, consisting of hindgut and Malpighian tubules, provides a simple, well-defined system where it is possible to use a genetic approach to define components essential for epithelial morphogenesis. We review here the advantages of Drosophila as a model genetic organism, the morphogenesis of the epithelial structures of the posterior gut, and what is known about the genetic requirements to form these structures. In overview, primordia are patterned by expression of hierarchies of transcription factors; this leads to localized expression of cell signaling molecules, and finally, to the least understood step: modulation of cell adhesion and cell shape. We describe approaches to identify additional genes that are required for morphogenesis of these simple epithelia, particularly those that might play a structural role by affecting cell adhesion and cell shape.

Conversion of dorsal from an activator to a repressor by the global corepressor Groucho.

The Dorsal morphogen acts as both an activator and a repressor of transcription in the Drosophila embryo to regulate the expression of dorsal/ventral patterning genes. Circumstantial evidence has suggested that Dorsal is an intrinsic activator and that additional factors (corepressors) convert it into a repressor. These corepressors, however, have previously eluded definitive identification. We show here, via the analysis of embryos lacking the maternally encoded Groucho corepressor and via protein-binding assays, that recruitment of Groucho to the template by protein:protein interactions is required for the conversion of Dorsal from an activator to a repressor. Groucho is therefore a critical component of the dorsal/ventral patterning system.

Complex regulatory region mediating tailless expression in early embryonic patterning and brain development.

tailless encodes a transcription factor expressed in multiple domains in the developing embryo. Early and transient expression at the posterior pole is required to establish a domain from which the eighth abdominal segment, telson and posterior gut arise. Just a few nuclear cycles later, a brain-specific domain is initiated at the anterior; expression in this domain is maintained with complex modulations throughout embryogenesis. Expression of tailless in this domain is required to establish the most anterior region of the brain. To understand the function and regulation of these different domains of expression, we provide a detailed description of tailless expression in brain neuroblasts and show that this expression is not detectably regulated by the head gap genes buttonhead or orthodenticle, by the proneural gene lethal of scute or by tailless itself. We show that approximately 6 kb of sequenced upstream regulatory DNA can drive lacZ expression in a pattern that mimics the full tailless embryonic expression pattern. Within this sequence we identify multiple modules responsible for different aspects of the tailless pattern. In addition to identifying additional torso response elements that mediate early blastoderm polar expression, we show that the complex brain expression pattern is driven by a combination of modules; thus expression at a low level throughout the brain and at a high level in the dorsal medial portion of the brain and in the optic lobe, as well as neuroblast-specific repression are mediated by different DNA regions.

Expression and sequence analysis of the Drosophila blastoderm-specific gene bsg25A.

By differential screening of a genomic library, we have cloned a gene expressed specifically during the blastoderm stage of Drosophila embryogenesis. Northern blot analysis and in situ hybridization to embryos reveal that the transcript is maximally expressed during the late syncytial blastoderm stage, disappears rapidly during the cellular blastoderm stage and is not detected at any other point in the Drosophila life cycle. On the basis of its temporally restricted expression and its polytene chromosomal map position at 25A1,2, we have designated this gene blastoderm-specific gene 25A (bsg25A). bsg25A encodes a novel protein of 23 kDa.

A Drosophila neurexin is required for septate junction and blood-nerve barrier formation and function.

Septate and tight junctions are thought to seal neighboring cells together and to function as barriers between epithelial cells. We have characterized a novel member of the neurexin family, Neurexin IV (NRX), which is localized to septate junctions (SJs) of epithelial and glial cells. NRX is a transmembrane protein with a cytoplasmic domain homologous to glycophorin C, a protein required for anchoring protein 4.1 in the red blood cell. Absence of NRX results in mislocalization of Coracle, a Drosophila protein 4.1 homolog, at SJs and causes dorsal closure defects similar to those observed in coracle mutants. nrx mutant embryos are paralyzed, and electrophysiological studies indicate that the lack of NRX in glial-glial SJs causes a breakdown of the blood-brain barrier. Electron microscopy demonstrates that nrx mutants lack the ladder-like intercellular septa characteristic of pleated SJs (pSJs). These studies identify NRX as the first transmembrane protein of SJ and demonstrate a requirement for NRX in the formation of septate-junction septa and intercellular barriers.

Drosophila brachyenteron regulates gene activity and morphogenesis in the gut.

Chromosomal region 68D/E is required for various aspects of Drosophila gut development; within this region maps the Brachyury homolog T-related gene (Trg), DNA of which rescues the hindgut defects of deficiency 68D/E. From a screen of 13,000 mutagenized chromosomes we identified six non-complementing alleles that are lethal over deficiencies of 68D/E and show a hindgut phenotype. These mutations constitute an allelic series and are all rescued to viability by a Trg transgene. We have named the mutant alleles and the genetic locus they define brachyenteron (byn); phenotypic characterization of the strongest alleles allows determination of the role of byn in embryogenesis. byn expression is activated by tailless, but byn does not regulate itself. byn expression in the hindgut and anal pad primordia is required for the regulation of genes encoding transcription factors (even-skipped, engrailed, caudal, AbdominalB and orthopedia) and cell signaling molecules (wingless and decapentaplegic). In byn mutant embryos, the defective program of gene activity in these primordia is followed by apoptosis (initiated by reaper expression and completed by macrophage engulfment), resulting in severely reduced hindgut and anal pads. Although byn is not expressed in the midgut or the Malpighian tubules, it is required for the formation of midgut constrictions and for the elongation of the Malpighian tubules.

Graded effect of tailless on posterior gut development: molecular basis of an allelic series of a nuclear receptor gene.

By marking cells of early gastrula stage embryos, we showed that in embryos mutant for a strong tll allele the fate map is shifted posteriorly and the hindgut anlage is deleted. We therefore used aspects of hindgut development to characterize the phenotype of new and previously described tll alleles. In embryos mutant for the various alleles, relative levels of blastoderm expression of Trg (T-related gene, required to establish the hindgut) and of mature hindgut size were determined; the results of these assays correlated with each other. Of the alleles that map to the sequence encoding the Tailless nuclear receptor protein, all (four) affect the zinc fingers of the DNA binding domain; surprisingly, substitutions of highly conserved residues allow a range of activities as detected by our bioassays.

The torso response element binds GAGA and NTF-1/Elf-1, and regulates tailless by relief of repression.

Modulation of transcription factor activity leading to changes in cell behavior (e.g., differentiation versus proliferation) is one of the critical outcomes of receptor tyrosine kinase (RTK) stimulation. In the early Drosophila embryo, activation of the torso (tor) RTK at the poles of the embryo activates a phosphorylation cascade that leads to the spatially specific transcription of the tailless (tll) gene. Our analysis of the tor response element (tor-RE) in the tll promoter indicates that the key activity modulated by the tor RTK pathway is a repressor present throughout the embryo. We have mapped the tor-RE to an 11-bp sequence; using this sequence as the basis for protein purification, we have determined that the proteins GAGA and NTF-1 (also known as Elf-1, product of the grainyhead gene) bind to the tor-RE. We demonstrate that NTF-1 can be phosphorylated by MAPK (mitogen-activated protein kinase), and that tll expression is expanded in embryos lacking maternal NTF-1 activity; these results make NTF-1 a likely target for modulation by the tor RTK pathway in vivo. The data presented here support a model in which activation of the tor RTK at the poles of the embryos leads to inactivation of the repressor and therefore, to transcriptional activation (by activators present throughout the embryo) of the tll gene at the poles of the embryo.

Binding sites for transcription factor NTF-1/Elf-1 contribute to the ventral repression of decapentaplegic.

The Dorsal morphogen is a transcription factor that activates some genes and represses others to establish multiple domains of gene expression along the dorsal/ventral axis of the early Drosophila embryo. Repression by Dorsal appears to require accessory proteins that bind to corepression elements in Dorsal-dependent regulatory modules called ventral repression regions (VRRs). We have identified a corepression element in decapentaplegic (dpp), a zygotically active gene that is repressed by the Dorsal morphogen. This dpp repression element (DRE) is located within a previously identified VRR and close to essential Dorsal-binding sites. We have purified a factor from Drosophila embryo extracts that binds to the DRE but not to mutant forms of the DRE that fail to support efficient repression. This protein also binds to an apparently essential region in a VRR associated with the zerknüllt (zen) gene. One of the DREs in the dpp VRR overlaps the binding site for a potential activator protein suggesting that one mechanism of ventral repression may be the mutually exclusive binding of repressor and activator proteins. We have found the DRE-binding protein to be identical to NTF-1 (equivalent to Elf-1, the product of the grainyhead gene), a factor originally identified as an activator of the Ultrabithorax and Dopa decarboxylase promoters. NTF-1 mRNA is synthesized during oogenesis and deposited in the developing oocyte where it is available to contribute to ventral repression during early embryogenesis. Previous studies have shown that overexpression of NTF-1 in the postblastoderm embryo results in a phenotype that is consistent with a role for this factor in the repression of dpp later in embryogenesis.

Posterior stripe expression of hunchback is driven from two promoters by a common enhancer element.

The gap gene hunchback (hb) is required for the formation and segmentation of two regions of the Drosophila embryo, a broad anterior domain and a narrow posterior domain. Accumulation of hb transcript in the posterior of the embryo occurs in two phases, an initial cap covering the terminal 15% of the embryo followed by a stripe at the anterior edge of this region. By in situ hybridization with transcript-specific probes, we show that the cap is composed only of mRNA from the distal transcription initiation site (P1), while the later posterior stripe is composed of mRNA from both the distal and proximal (P2) transcription initiation sites. Using a series of genomic rescue constructs and promoter-lacZ fusion genes, we define a 1.4 kb fragment of the hb upstream region that is both necessary and sufficient for posterior expression. Sequences within this fragment mediate regulation by the terminal gap genes tailless (tll) and a huckebein, which direct the formation of the posterior hb stripe. We show that the tll protein binds in vitro to specific sites within the 1.4 kb posterior enhancer region, providing the first direct evidence for activation of gene expression by tll. We propose a model in which the anterior border of the posterior hb stripe is determined by tll concentration in a manner analogous to the activation of anterior hb expression by bicoid.

The thick veins gene of Drosophila is required for dorsoventral polarity of the embryo.

We have discovered a new member of the class of genes controlling embryonic dorsoventral patterning. Mutants of the thick veins (tkv) gene have been described previously (as slater alleles) as embryonic lethal, lacking dorsal epidermis, but not as showing a recognizable dorsoventral phenotype. We show here that maternal alteration of function coupled with zygotic reduction of function of tkv is strongly ventralizing. In addition, in double heterozygous combinations in the mother, tkv mutations increase the ventralizing effect of dominant, weakly ventralizing alleles of the maternal effect, dorsoventral genes easter and cactus. An interaction is also seen with zygotic dorsoventral genes: tkv interacts maternally and zygotically in double heterozygotes with decapentaplegic and zygotically with screw in double homozygotes. We conclude that both maternally and zygotically supplied wild-type tkv product can play a role in dorsoventral patterning of the early embryo. On the basis of the phenotype of trans-heterozygous adult escapers, we propose that tkv might act by potentiating the activity of the zygotically acting decapentaplegic gene.

Characterization of downstream elements in a Raf-1 pathway.

At the poles of the Drosophila embryo, cell fate is established by a pathway that begins with the activation of a membrane-associated tyrosine kinase (the torso gene product); this then leads to activation of a serine/threonine kinase (Drosophila Raf-1). Activated Raf-1 then leads, by an undefined mechanism, to the transcriptional activation of the tailless (tll) gene; the tll gene product, itself a transcription factor, subsequently regulates the expression of an array of target genes. To further define this pathway, we have utilized sequence comparison between Drosophila melanogaster and Drosophila virilis to identify conserved elements in the tll promoter region. As assessed by DNase I footprinting and promoter dissection experiments, two of these elements are potential regulatory targets of Raf-1-activated transcription factors. Sequence comparison also reveals that the unique residues in the DNA-binding domain of the tll protein, the next component in the pathway, are conserved. One of these residues, the alanine after the last cysteine in the first zinc finger, may be responsible for part of the difference between the tll protein DNA binding site and the closely related half-site of the retinoid/estrogen receptors. Consistent with the rapid turnover of the tll protein, it contains a PEST sequence (rich in proline, glutamate and aspartate, serine, and threonine) that is also conserved.

Control of tailless expression by bicoid, dorsal and synergistically interacting terminal system regulatory elements.

Three different maternal morphogen gradients regulate expression of the gap gene tailless (tll), which is required to establish the acron and telson of the Drosophila embryo. To identify elements in the tll promoter that respond to these different maternal systems, we have generated promoter-lacZ fusions and transformed them into the germline. Expression of these constructs in both wild type and mutant embryos revealed the presence of at least two separate but synergistically interacting regions that mediate tll expression by the terminal system. This functional synergism between regulatory elements may play a role in the translation of the torso (tor) morphogen gradient into the sharp boundary of tll gene activity. In addition to regions mediating activation by the terminal system, regions mediating both activation and repression by bicoid (bcd), and repression by dorsal (dl) were identified. Binding sites of bcd protein in a 0.5 kb region, revealed by DNaseI footprinting, could be crucial for the bcd-dependent activation of tll expression in the anterior stripe.

Single amino acid exchanges in separate domains of the Drosophila serendipity delta zinc finger protein cause embryonic and sex biased lethality.

The Drosophila serendipity (sry) delta (delta) zinc finger protein is a sequence-specific DNA binding protein, maternally inherited by the embryo and present in nuclei of transcriptionally active cells throughout fly development. We report here the isolation and characterization of four ethyl methanesulfate-induced zygotic lethal mutations of different strengths in the sry delta gene. For the stronger allele, all of the lethality occurs during late embryogenesis or the first larval instar. In the cases of the three weaker alleles, most of the lethality occurs during pupation; moreover, those adult escapers that emerge are sterile males lacking partially or completely in spermatozoa bundles. Genetic analysis of sry delta thus indicates that it is an essential gene, whose continued expression throughout the life cycle, notably during embryogenesis and pupal stage, is required for viability. Phenotypic analysis of sry delta hemizygote escaper males further suggests that sry delta may be involved in regulation of two different sets of genes: genes required for viability and genes involved in gonadal development. All four sry delta alleles are fully rescued by a wild-type copy of sry delta, but not by an additional copy of the sry beta gene, reinforcing the view that, although structurally related, these two genes exert distinct functions. Molecular characterization of the four sry delta mutations revealed that these mutations correspond to single amino acid replacements in the sry delta protein. Three of these replacements map to the same (third out of seven) zinc finger in the carboxy-terminal DNA binding domain; interestingly, none affects the zinc finger consensus residues. The fourth mutation is located in the NH2-proximal part of the protein, in a domain proposed to be involved in specific protein-protein interactions.