Regulation of Krüppel expression in the anlage of the Malpighian tubules in the Drosophila embryo

hkb is required for DIP-α expression and target recognition in the Drosophila neuromuscular circuit

Our nervous system contains billions of neurons that form precise connections with each other through interactions between cell surface proteins. In Drosophila, the Dpr and DIP immunoglobulin protein subfamilies form homophilic or heterophilic interactions to instruct synaptic connectivity, synaptic growth, and cell survival. However, the upstream regulatory mechanisms of Dprs and DIPs are not clear. On the other hand, while transcription factors have been implicated in target recognition, their downstream cell surface proteins remain mostly unknown. We conduct an F1 dominant modifier genetic screen to identify regulators of Dprs and DIPs. We identify huckebein (hkb), a transcription factor previously implicated in target recognition of the dorsal Is motor neuron. We show that hkb genetically interacts with DIP-α and loss of hkb leads to complete removal of DIP-α expression specifically in dorsal Is motor neurons. We then confirm that this specificity is through the dorsal Is motor neuron specific transcription factor, even-skipped (eve), which acts downstream of hkb. Analysis of the genetic interaction between hkb and eve reveals that they act in the same pathway to regulate dorsal Is motor neuron connectivity. Our study provides insight into the transcriptional regulation of DIP-α and suggests that distinct regulatory mechanisms exist for the same CSP in different neurons.

Coordination of cell cycle and morphogenesis during organ formation

Organ formation requires precise regulation of cell cycle and morphogenetic events. Using the Drosophila embryonic salivary gland (SG) as a model, we uncover the role of the SP1/KLF transcription factor Huckebein (Hkb) in coordinating cell cycle regulation and morphogenesis. The hkb mutant SG exhibits defects in invagination positioning and organ size due to the abnormal death of SG cells. Normal SG development involves distal-to-proximal progression of endoreplication (endocycle), whereas hkb mutant SG cells undergo abnormal cell division, leading to cell death. Hkb represses the expression of key cell cycle and pro-apoptotic genes in the SG. Knockdown of cyclin E or cyclin-dependent kinase 1, or overexpression of fizzy-related rescues most of the morphogenetic defects observed in the hkb mutant SG. These results indicate that Hkb plays a critical role in controlling endoreplication by regulating the transcription of key cell cycle effectors to ensure proper organ formation.

hkb is required for DIP-α expression and target recognition in the Drosophila neuromuscular circuit

Our nervous system contains billions of neurons that form precise connections with each other through interactions between cell surface proteins (CSPs). In Drosophila, the Dpr and DIP immunoglobulin protein subfamilies form homophilic or heterophilic interactions to instruct synaptic connectivity, synaptic growth and cell survival. However, the upstream regulation and downstream signaling mechanisms of Dprs and DIPs are not clear. In the Drosophila larval neuromuscular system, DIP-α is expressed in the dorsal and ventral type-Is motor neurons (MNs). We conducted an F1 dominant modifier genetic screen to identify regulators of Dprs and DIPs. We found that the transcription factor, huckebein (hkb), genetically interacts with DIP-α and is important for target recognition specifically in the dorsal Is MN, but not the ventral Is MN. Loss of hkb led to complete removal of DIP-α expression. We then confirmed that this specificity is through the dorsal Is MN specific transcription factor, even-skipped (eve), which acts downstream of hkb. Genetic interaction between hkb and eve revealed that they act in the same pathway to regulate dorsal Is MN connectivity. Our study provides insight into the transcriptional regulation of DIP-α and suggests that distinct regulatory mechanisms exist for the same CSP in different neurons.

A timer gene network is spatially regulated by the terminal system in the Drosophila embryo

In insect embryos, anteroposterior patterning is coordinated by the sequential expression of the 'timer' genes caudal, Dichaete, and odd-paired, whose expression dynamics correlate with the mode of segmentation. In Drosophila, the timer genes are expressed broadly across much of the blastoderm, which segments simultaneously, but their expression is delayed in a small 'tail' region, just anterior to the hindgut, which segments during germband extension. Specification of the tail and the hindgut depends on the terminal gap gene tailless, but beyond this the regulation of the timer genes is poorly understood. We used a combination of multiplexed imaging, mutant analysis, and gene network modelling to resolve the regulation of the timer genes, identifying 11 new regulatory interactions and clarifying the mechanism of posterior terminal patterning. We propose that a dynamic Tailless expression gradient modulates the intrinsic dynamics of a timer gene cross-regulatory module, delineating the tail region and delaying its developmental maturation.

A comprehensive study of arthropod and onychophoran Fox gene expression patterns

Fox genes represent an evolutionary old class of transcription factor encoding genes that evolved in the last common ancestor of fungi and animals. They represent key-components of multiple gene regulatory networks (GRNs) that are essential for embryonic development. Most of our knowledge about the function of Fox genes comes from vertebrate research, and for arthropods the only comprehensive gene expression analysis is that of the fly Drosophila melanogaster. For other arthropods, only selected Fox genes have been investigated. In this study, we provide the first comprehensive gene expression analysis of arthropod Fox genes including representative species of all main groups of arthropods, Pancrustacea, Myriapoda and Chelicerata. We also provide the first comprehensive analysis of Fox gene expression in an onychophoran species. Our data show that many of the Fox genes likely retained their function during panarthropod evolution highlighting their importance in development. Comparison with published data from other groups of animals shows that this high degree of evolutionary conservation often dates back beyond the last common ancestor of Panarthropoda.

Physiology, Development, and Disease Modeling in the Drosophila Excretory System

The insect excretory system contains two organ systems acting in concert: the Malpighian tubules and the hindgut perform essential roles in excretion and ionic and osmotic homeostasis. For over 350 years, these two organs have fascinated biologists as a model of organ structure and function. As part of a recent surge in interest, research on the Malpighian tubules and hindgut of Drosophila have uncovered important paradigms of organ physiology and development. Further, many human disease processes can be modeled in these organs. Here, focusing on discoveries in the past 10 years, we provide an overview of the anatomy and physiology of the Drosophila excretory system. We describe the major developmental events that build these organs during embryogenesis, remodel them during metamorphosis, and repair them following injury. Finally, we highlight the use of the Malpighian tubules and hindgut as accessible models of human disease biology. The Malpighian tubule is a particularly excellent model to study rapid fluid transport, neuroendocrine control of renal function, and modeling of numerous human renal conditions such as kidney stones, while the hindgut provides an outstanding model for processes such as the role of cell chirality in development, nonstem cell-based injury repair, cancer-promoting processes, and communication between the intestine and nervous system.

Drosophila Malpighian Tubules: A Model for Understanding Kidney Development, Function, and Disease

The Malpighian tubules of insects are structurally simple but functionally important organs, and their integrity is important for the normal excretory process. They are functional analogs of human kidneys which are important physiological organs as they maintain water and electrolyte balance in the blood and simultaneously help the body to get rid of waste and toxic products after various metabolic activities. In addition, it receives early indications of insults to the body such as immune challenge and other toxic components and is essential for sustaining life. According to National Vital Statistics Reports 2016, renal dysfunction has been ranked as the ninth most abundant cause of death in the USA. This chapter provides detailed descriptions of Drosophila Malpighian tubule development, physiology, immune function and also presents evidences that Malpighian tubules can be used as a model organ system to address the fundamental questions in developmental and functional disorders of the kidney.

Ecdysone regulates morphogenesis and function of Malpighian tubules in Drosophila melanogaster through EcR-B2 isoform

Malpighian tubules are the osmoregulatory and detoxifying organs of Drosophila and its proper development is critical for the survival of the organism. They are made up of two major cell types, the ectodermal principal cells and mesodermal stellate cells. The principal and stellate cells are structurally and physiologically distinct from each other, but coordinate together for production of isotonic fluid. Proper integration of these cells during the course of development is an important pre-requisite for the proper functioning of the tubules. We have conclusively determined an essential role of ecdysone hormone in the development and function of Malpighian tubules. Disruption of ecdysone signaling interferes with the organization of principal and stellate cells resulting in malformed tubules and early larval lethality. Abnormalities include reduction in the number of cells and the clustering of cells rather than their arrangement in characteristic wild type pattern. Organization of F-actin and β-tubulin also show aberrant distribution pattern. Malformed tubules show reduced uric acid deposition and altered expression of Na(+)/K(+)-ATPase pump. B2 isoform of ecdysone receptor is critical for the development of Malpighian tubules and is expressed from early stages of its development.

Morphogenesis at criticality

Spatial patterns in the early fruit fly embryo emerge from a network of interactions among transcription factors, the gap genes, driven by maternal inputs. Such networks can exhibit many qualitatively different behaviors, separated by critical surfaces. At criticality, we should observe strong correlations in the fluctuations of different genes around their mean expression levels, a slowing of the dynamics along some but not all directions in the space of possible expression levels, correlations of expression fluctuations over long distances in the embryo, and departures from a Gaussian distribution of these fluctuations. Analysis of recent experiments on the gap gene network shows that all these signatures are observed, and that the different signatures are related in ways predicted by theory. Although there might be other explanations for these individual phenomena, the confluence of evidence suggests that this genetic network is tuned to criticality.

Efficient reverse-engineering of a developmental gene regulatory network

Understanding the complex regulatory networks underlying development and evolution of multi-cellular organisms is a major problem in biology. Computational models can be used as tools to extract the regulatory structure and dynamics of such networks from gene expression data. This approach is called reverse engineering. It has been successfully applied to many gene networks in various biological systems. However, to reconstitute the structure and non-linear dynamics of a developmental gene network in its spatial context remains a considerable challenge. Here, we address this challenge using a case study: the gap gene network involved in segment determination during early development of Drosophila melanogaster. A major problem for reverse-engineering pattern-forming networks is the significant amount of time and effort required to acquire and quantify spatial gene expression data. We have developed a simplified data processing pipeline that considerably increases the throughput of the method, but results in data of reduced accuracy compared to those previously used for gap gene network inference. We demonstrate that we can infer the correct network structure using our reduced data set, and investigate minimal data requirements for successful reverse engineering. Our results show that timing and position of expression domain boundaries are the crucial features for determining regulatory network structure from data, while it is less important to precisely measure expression levels. Based on this, we define minimal data requirements for gap gene network inference. Our results demonstrate the feasibility of reverse-engineering with much reduced experimental effort. This enables more widespread use of the method in different developmental contexts and organisms. Such systematic application of data-driven models to real-world networks has enormous potential. Only the quantitative investigation of a large number of developmental gene regulatory networks will allow us to discover whether there are rules or regularities governing development and evolution of complex multi-cellular organisms.

Spatiotemporal network motif reveals the biological traits of developmental gene regulatory networks in Drosophila melanogaster

Background

Network motifs provided a “conceptual tool” for understanding the functional principles of biological networks, but such motifs have primarily been used to consider static network structures. Static networks, however, cannot be used to reveal time- and region-specific traits of biological systems. To overcome this limitation, we proposed the concept of a “spatiotemporal network motif,” a spatiotemporal sequence of network motifs of sub-networks which are active only at specific time points and body parts.

Results

On the basis of this concept, we analyzed the developmental gene regulatory network of the Drosophila melanogaster embryo. We identified spatiotemporal network motifs and investigated their distribution pattern in time and space. As a result, we found how key developmental processes are temporally and spatially regulated by the gene network. In particular, we found that nested feedback loops appeared frequently throughout the entire developmental process. From mathematical simulations, we found that mutual inhibition in the nested feedback loops contributes to the formation of spatial expression patterns.

Conclusions

Taken together, the proposed concept and the simulations can be used to unravel the design principle of developmental gene regulatory networks.

The gap gene network

Gap genes are involved in segment determination during the early development of the fruit fly Drosophila melanogaster as well as in other insects. This review attempts to synthesize the current knowledge of the gap gene network through a comprehensive survey of the experimental literature. I focus on genetic and molecular evidence, which provides us with an almost-complete picture of the regulatory interactions responsible for trunk gap gene expression. I discuss the regulatory mechanisms involved, and highlight the remaining ambiguities and gaps in the evidence. This is followed by a brief discussion of molecular regulatory mechanisms for transcriptional regulation, as well as precision and size-regulation provided by the system. Finally, I discuss evidence on the evolution of gap gene expression from species other than Drosophila. My survey concludes that studies of the gap gene system continue to reveal interesting and important new insights into the role of gene regulatory networks in development and evolution.

Multipotent stem cells in the Malpighian tubules of adult Drosophila melanogaster

Excretion is an essential process of an organism's removal of the waste products of metabolism to maintain a constant chemical composition of the body fluids despite changes in the external environment. Excretion is performed by the kidneys in vertebrates and by Malpighian tubules (MTs) in Drosophila. The kidney serves as an excellent model organ to investigate the cellular and molecular mechanisms underlying organogenesis. Mammals and Drosophila share common principles of renal development. Tissue homeostasis, which is accomplished through self-renewal or differentiation of stem cells, is critical for the maintenance of adult tissues throughout the lifetime of an animal. Growing evidence suggests that stem cell self-renewal and differentiation is controlled by both intrinsic and extrinsic factors. Deregulation of stem cell behavior results in cancer formation, tissue degeneration, and premature aging. The mammalian kidney has a low rate of cellular turnover but has a great capacity for tissue regeneration following an ischemic injury. However, there is an ongoing controversy about the source of regenerating cells in the adult kidney that repopulate injured renal tissues. Recently, we identified multipotent stem cells in the MTs of adult Drosophila and found that these stem cells are able to proliferate and differentiate in several types of cells in MTs. Furthermore, we demonstrated that an autocrine JAK-STAT (Janus kinase-signal transducers and activators of transcription) signaling regulates stem cell self-renewal or differentiation of renal stem cells. The Drosophila MTs provide an excellent in vivo system for studying the renal stem cells at cellular and molecular levels. Understanding the molecular mechanisms governing stem cell self-renewal or differentiation in vivo is not only crucial to using stem cells for future regenerative medicine and gene therapy, but it also will increase our understanding of the mechanisms underlying cancer formation, aging and degenerative diseases. Identifying and understanding the cellular processes underlying the development and repair of the mammalian kidney may enable more effective, targeted therapies for acute and chronic kidney diseases in humans.

Drosophila female sterile (1) homeotic is a multifunctional transcriptional regulator that is modulated by Ras signaling

The Drosophila (fs(1)h) gene encodes small (Fs(1)hS) and large (Fs(1)hL) chromatin-binding BET protein transcription factor isoforms. Zygotic mutations cause either lethality or female sterility, whereas maternal mutations cause segmental deletions and thoracic homeotic transformations. Here, we describe novel fs(1)h embryonic phenotypes: homeosis of the head in zygotic mutants and deletion of head and tail regions in maternal mutants, similar to those caused by dominant torso (tor(D)) alleles. tor activates transcription of tailless (tll) and hückebein (hkb) by means of a canonical Ras pathway, through inactivation of Groucho (Gro), Capicua (Cic) and, possibly, Grainy-head (Grh) repressors. Expression of both tailless and hückebein are de-repressed in fs(1)h maternal mutants, as in tor(D), gro, grh, and cic mutant animals, indicating fs(1)h is also necessary for tll and hkb repression. These data link Ras signaling with modulation of a chromatin-binding transcription factor, Fs(1)h, suggesting a novel mechanism by which Ras can modulate gene expression.

Genetic regulation of patterned tubular branching in Drosophila

A common theme in organogenesis is the branching of epithelial tubes, for example in the lung, liver, or kidney. The later morphogenesis of these branched epithelia dictates the final form and function of the mature tissue. Epithelial branching requires the specification of branch cells, the eversion process itself, and, frequently, patterned morphogenesis to produce branches of specific shape and orientation. Using the branching of renal tubule primordia from the hindgut in Drosophila, we show that these aspects are coordinately regulated. Cell specification depends on Wnt signaling along the tubular gut and results in the spatially restricted coexpression of two transcription factors, Krüppel and Cut, in the hindgut, whose activity drives cells toward renal tubule fate. Significantly, these transcription factors also confer the competence to respond to a second signal; TGF-beta induces branching to form the four renal tubule buds. Differential activation of the TGF-beta pathway also patterns the tubules, resulting in the asymmetry in size and positioning that is characteristic of the two tubule pairs. High levels of TGF-beta promote the expression of Dorsocross1-3 and anterior tubule growth, whereas low levels allow the expression of the transcriptional repressor, Brinker, and thus promote posterior tubule identity. We show that patterning of the tubule primordium into two distinct pairs is critical for the eversion of tubule branches, as well as for their asymmetric morphogenesis.

EVOPRINTER, a multigenomic comparative tool for rapid identification of functionally important DNA

Here, we describe a multigenomic DNA sequence-analysis tool, evoprinter, that facilitates the rapid identification of evolutionary conserved sequences within the context of a single species. The evoprinter output identifies multispecies-conserved DNA sequences as they exist in a reference DNA. This identification is accomplished by superimposing multiple reference DNA vs. test-genome pairwise blat (blast-like alignment tool) readouts of the reference DNA to identify conserved nucleotides that are shared by all orthologous DNAs. evoprinter analysis of well characterized genes reveals that most, if not all, of the conserved sequences are essential for gene function. For example, analysis of orthologous genes that are shared by many vertebrates identifies conserved DNA in both protein-encoding sequences and noncoding cis-regulatory regions, including enhancers and mRNA microRNA binding sites. In Drosophila, the combined mutational histories of five or more species affords near-base pair resolution of conserved transcription factor DNA-binding sites, and essential amino acids are revealed by the nucleotide flexibility of their codon-wobble position(s). Conserved small peptide-encoding genes, which had been undetected by conventional gene-prediction algorithms, are identified by the codon-wobble signatures of invariant amino acids. Also, evoprinter allows one to assess the degree of evolutionary divergence between orthologous DNAs by highlighting differences between a selected species and the other test species.

Making tubes in the Drosophila embryo

Epithelial and endothelial tubes come in various shapes and sizes and form the basic units of many tubular organs. During embryonic development, single unbranched tubes as well as highly branched networks of tubes form from simple sheets of cells by several morphogenic movements. Studies of tube formation in the Drosophila embryo have greatly advanced our understanding of the cellular and molecular mechanisms by which tubes are formed. This review highlights recent progress on formation of the hindgut, Malpighian tubules, proventriculus, salivary gland, and trachea of the Drosophila embryo, focusing on the cellular events that form each tube and their genetic requirements.

Amnioserosa is required for dorsal closure in Drosophila

Dorsal closure in the fruit fly Drosophila melanogaster is a complex morphogenetic process, driven by sequential signaling cascades and involving multiple forces, which contribute to cell movements and rearrangements as well as to changes in cell shape. During closure, lateral epidermal cells elongate along the dorsoventral axis and subsequently spread dorsally to cover the embryonic dorsal surface. Amnioserosal cells, which are the original occupants of the most dorsal position in the developing embryo, constrict during closure; thus, the increase in epidermal surface area is accommodated by a reduction in the amnioserosal surface area. Several of the epidermal requirements for closure have been established in functional assays. In contrast, amnioserosal requirements for closure have remained elusive, in part because laser ablation and clonal approaches are limited to only subsets of amnioserosal cells. Here, we report our use of the UAS-GAL4 system to target expression of the cell autonomous toxin Ricin-A to all cells of the amnioserosa. We show that ablation of the amnioserosa leads to clear defects in dorsal closure and, thus, directly demonstrate a role for the amnioserosa in dorsal closure. We also show that DJNK (Drosophila Jun N-terminal kinase) signaling, an epidermal trigger of closure, is unaffected by amnioserosal ablation. These data, together with our demonstration that amnioserosal ablated and Dpp signaling mutant embryos exhibit shared loss-of-function phenotypes, point to a requirement for the amnioserosa in dorsal closure that is downstream of Dpp, perhaps as part of a paracrine response to this signaling cascade.

The signals that drive kidney development: a view from the fly eye

PURPOSE OF REVIEW

Development of the mammalian kidney is a complex process involving numerous signals and signaling pathways. Other complex tissues have benefited enormously from studies in lower, simpler organisms. The present review provides an update on what we have learned from the fruitfly Drosophila melanogaster, and argues that Drosophila is an important but under-utilized organism for study of renal development.

RECENT FINDINGS

The Malpighian tubules provide renal function to the fly. These require a number of signaling pathways for their development that are also seen in vertebrate kidney development, including the Notch, Ras, and Wnt signaling pathways, as well as nuclear factors such as Krüppel and Cut/Cux-1. Many of these factors are shared between early Malpighian tubule development and ureteric bud formation. The Ret signaling receptor, which is central to mammalian renal development, is poorly understood in flies, although its expression pattern is intriguing. Surprisingly, other signaling factors such as Neph-1, Pax2, and Wilms' tumor suppressor-1 appear to work within later fly retinal development, providing a surprising link between these two disparate tissues.

SUMMARY

Drosophila offers a powerful palate of tools for dissecting developmental processes. Importantly, these tools can often be examined at the level of single cells, permitting us to address issues of differentiation with high resolution. If we are to take full advantage of Drosophila, however, then we must target specific issues and gain a better understanding of the details of Malpighian tubule development.

Axon guidance of Drosophila SNb motoneurons depends on the cooperative action of muscular Krüppel and neuronal capricious activities

The body wall musculature of the Drosophila larva consists of a stereotyped pattern of 30 muscles per abdominal hemisegment which are innervated by about 40 distinct motoneurons. Proper innervation by motoneurons is established during late embryogenesis. Guidance of motor axons to specific muscles requires appropriate pathfinding decisions as they follow their pathways within the central nervous system and on the surface of muscles. Once the appropriate targets are reached, stable synaptic contacts between motoneurons and muscles are formed. Recent studies revealed a number of molecular components required for proper motor axon pathfinding and demonstrated specific roles in fasciculation/defasciculation events, a key process in the formation of discrete motoneuron pathways. The gene capricious (caps), which encodes a cell-surface protein, functions as a recognition molecule in motor axon guidance, regulating the formation of the selective connections between the SNb-derived motoneuron RP5 and muscle 12. Here we show that Krüppel (Kr), best known as a segmentation gene of the gap class, functionally interacts with caps in establishing the proper axonal pathway of SNb including the RP5 axons. The results suggest that the transcription factor Krüppel participates in proper control of cell-surface molecules which are necessary for the SNb neurons to navigate in a caps-dependent manner within the array of the ventral longitudinal target muscles.

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.

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.

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.

Krüppel acts as a developmental switch gene that mediates Notch signalling-dependent tip cell differentiation in the excretory organs of Drosophila

Cell proliferation in the excretory organs of Drosophila, the Malpighian tubules (MT), is under the control of a neural tip cell. This unique cell is singled out from equivalent MT primordial cells in response to Notch signalling. We show that the gene Krüppel (Kr), best known for its segmentation function in the early embryo, is under the control of the Notch-dependent signalling process. Lack-of-function and gain-of-function experiments demonstrate that Kr activity determines the neural fate of tip cells by acting as a direct downstream target of proneural basic helix-loop-helix (bHLH) proteins that are restricted in response to Notch signalling. We have identified a unique cis-acting element that mediates all spatial and temporal aspects of Kr gene expression during MT development. This element contains functional binding sites for the restricted proneural bHLH factors and Fork head protein which is expressed in all MT cells. Our results suggest a mechanism in which these transcription factors cooperate to set up a unique cell fate within an equivalence group of cells by restricting the activity of the developmental switch gene Kr in response to Notch signalling.

A modifier screen in the eye reveals control genes for Krüppel activity in the Drosophila embryo

Irregular facets (If) is a dominant mutation of Drosophila that results in small eyes with fused ommatidia. Previous results showed that the gene Krüppel (Kr), which is best known for its early segmentation function, is expressed ectopically in If mutant eye discs. However, it was not known whether ectopic Kr activity is either the cause or the result of the If mutation. Here, we show that If is a gain-of-function allele of Kr. We then used the If mutation in a genetic screen to identify dominant enhancers and suppressors of Kr activity on the third chromosome. Of 30 identified Kr-interacting loci, two were cloned, and we examined whether they also represent components of a natural Kr-dependent developmental pathway of the embryo. We show that the two genes, eyelid (eld) and extramacrochaetae (emc), which encode a Bright family-type DNA binding protein and a helix-loop-helix factor, respectively, are necessary to achieve the singling-out of a unique Kr-expressing cell during the development of the Malpighian tubules, the excretory organs of the fly. The results indicate that the Kr gain-of-function mutation If provides a tool to identify genes that are active during eye development and that a number of them function also in the control of Kr-dependent developmental processes.

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.

Five years on the wings of fork head

Since its discovery five years ago the conserved family of fork head/HNF-3-related transcription factors has gained increasing importance for the analysis of gene regulatory mechanisms during embryonic development and in differentiated cells. Different members of this family, which is defined by a conserved 110 amino acid residues encompassing DNA binding domain of winged helix structure, serve as regulatory keys in embryogenesis, in tumorigenesis or in the maintenance of differentiated cell states. The purpose of this review is to summarize the accumulating amount of data on structure, expression and function of fork head/HNF-3-related transcription factors.

Invagination centers within the Drosophila stomatogastric nervous system anlage are positioned by Notch-mediated signaling which is spatially controlled through wingless

The gut-innervating stomatogastric nervous system of Drosophila, unlike the central and the peripheral nervous system, derives from a compact, single layered epithelial anlage. Here we report how this anlage is initially defined during embryogenesis by the expression of proneural genes of the achaete-scute complex in response to the maternal terminal pattern forming system. Within the stomatogastric nervous system anlage, the wingless-dependent intercellular communication system adjusts the cellular range of Notch-dependent lateral inhibition to single-out three achaete-expressing cells. Those cells define distinct invagination centers which orchestrate the behavior of neighboring cells to form epithelial infoldings, each headed by an achaete-expressing tip cell. Our results suggest that the wingless pathway acts not as an instructive signal, but as a permissive factor which coordinates the spatial activity of morphoregulatory signals within the stomatogastric nervous system anlage.

The interaction of DNA with the DNA-binding domain encoded by the Drosophila gene fork head

The Drosophila gene fork head (fkh) encodes a nuclear protein which shares sequence similarity with the rat hepatocyte-enriched transcription factor family HNF3 alpha-gamma. The sequence similarity is restricted to the region that has been defined as the DNA-binding domain of these proteins, termed the fork head domain. In this study, we investigate the structural properties of the fork head domain of the prototype of this protein family encoded by fkh and its interaction with DNA. The core sequence required for DNA binding of the fork head domain consists of 114 amino acids and represents a stable and highly compact monomer of globular structure with an alpha-helix content of 37%. The fork head domain binds specifically to the DNA target sequence of HNF3 alpha-gamma and to an enhancer element that is derived from a regulatory sequence of an in vivo Drosophila target gene. The specific interaction between the DNA-binding domain of the fkh-encoded protein and its target DNA is mediated by two contact regions which are separated from each other by one turn of the DNA. Our data are consistent with a structural model which derived rom X-ray diffraction analysis of the DNA-binding domain of HNF3 gamma. Differences concerning the DNA contact sites between the DNA-binding domain of the fkh-encoded protein and the HNF3 protein family are discussed.

The gene serpent has homeotic properties and specifies endoderm versus ectoderm within the Drosophila gut

The gut of Drosophila consists of ectodermally derived foregut and hindgut and endodermally derived midgut. Here I show that the gene serpent plays a key role in the development of the endoderm. serpent embryos lack the entire midgut and do not show endodermal differentiation. They gastrulate normally and form proper amnioproctodeal and anterior midgut invaginations. However, the prospective anterior midgut cells acquire properties that are usually found in ectodermal foregut cells. In the posterior region of the embryo, the prospective posterior midgut forms an additional hindgut which is contiguous with the normal hindgut and which appears to be a serial duplication, not a mere enlargement of the hindgut. The fate shifts in both the anterior and the posterior part of the srp embryo can be described in terms of homeotic transformations of anterior midgut to foregut and of posterior midgut to hindgut. serpent appears to act as a homeotic gene downstream of the terminal gap gene huckebein and to promote morphogenesis and differentiation of anterior and posterior midgut.

Development of sibling inbred sea urchins: normal embryogenesis, but frequent postembryonic malformation, arrest and lethality

Inbred lines of Strongylocentrotus purpuratus descended from a single pair of wild animals were constructed by sibling mating. We describe results from a systematic series of crosses in which eggs from F2 and from F3 females were fertilized respectively with sperm from their sibling males. Observations were also made on self-fertilized cultures derived from several naturally occurring hermaphrodites. Morphological development, survival efficiency, and expression of three territorial embryonic markers were assayed in the embryos developing from these crosses. Unexpectedly, out of > 90 controlled crosses, we observed no developmental failures whatsoever, up to the end of embryogenesis (i.e., onset of feeding) that could be attributed to homozygous, zygotically acting recessive genes. However, during postembryonic larval development, lethality, morphological malformation, and arrest are observed in inbred cultures at a high frequency. The incidence of these zygotic developmental failures is such that it appears that there is at least one recessive genetic defect affecting larval development per haploid parental genome. The relative imperviousness of the basic embryonic process to defects arising from homozygosity is consistent with other evidence implying that territorial specification in sea urchin embryogenesis is controlled by maternally rather than zygotically expressed gene products.

orthodenticle activity is required for the development of medial structures in the larval and adult epidermis of Drosophila

Lethal alleles of orthodenticle (= otd) cause abnormalities in the embryonic head that reflect an early role in anterior pattern formation. In addition, otd activity is required for the development of the larval and adult epidermis. Clonal analysis of both viable and lethal alleles shows that the adult requirement for otd is restricted to medial regions of certain discs. When otd activity is reduced or removed, some medial precursor cells produce bristles and cuticle characteristic of more lateral structures. Similar medial defects are observed in the larval epidermis of embryos homozygous for lethal otd alleles. Antibodies to otd recognize a nuclear protein found at high levels in the medial region of the eye antennal discs, the leg discs, the genital discs and along the ventral midline of the ventral epidermis of the embryo. These results suggest that the otd gene product is required to specify medial cell fates in both the larval and adult epidermis.