Seven Wnt homologues in Drosophila: a case study of the developing tracheae

The Wnt Co-Receptor PTK7/Otk and Its Homolog Otk-2 in Neurogenesis and Patterning

Wnt signaling is a highly conserved metazoan pathway that plays a crucial role in cell fate determination and morphogenesis during development. Wnt ligands can induce disparate cellular responses. The exact mechanism behind these different outcomes is not fully understood but may be due to interactions with different receptors on the cell membrane. PTK7/Otk is a transmembrane receptor that is implicated in various developmental and physiological processes including cell polarity, cell migration, and invasion. Here, we examine two roles of Otk-1 and Otk-2 in patterning and neurogenesis. We find that Otk-1 is a positive regulator of signaling and Otk-2 functions as its inhibitor. We propose that PTK7/Otk functions in signaling, cell migration, and polarity contributing to the diversity of cellular responses seen in Wnt-mediated processes.

Transcriptome analysis and identification of genes related to environmental adaptation of Grylloprimevala jilina Zhou & Ren 2023

Grylloprimevala jilina is a true cave insect living in the dark areas of caves. It has the characteristics of sparse skin pigmentation, degeneration of the compound eyes and monocular eyes, and obvious preference for high-humidity and low-temperature environments. Given the highly specialized, rare, and limited distribution, G. jilina is considered an endangered species and also a first-level national protected insect in China. Cave creatures often undergo dramatic morphological changes in their sensory systems to adapt to the cave environment. Most previous studies mainly focused on morphological adaptive changes in cave insects, and only a few studied the changes at the gene level. In this study, we performed transcriptome analysis of G. jilina and constructed phylogenetic trees of genes that are related to environmental adaptation, including chemosensory, visual-related, reproduction-related, temperature adaptation-related, and winged morph differentiation-related genes. Besides, the expression levels of environmental adaption-related genes in different tissues, including antennae, heads, thoraxes, abdomens, legs, and tails, were analyzed. The results showed the loss of chemosensory genes and vision-related genes, the conservation of reproduction-related genes and temperature adaptation-related genes, and the conservation of wing-related genes despite the loss of wings, and the results were consistent with other cave insects. The identification and expression study of genes possibly related to the environmental adaptability in G. jilina provided basic data for the protection of this endangered species and increased knowledge about insect evolution in general.

Molecular mechanisms of tubulogenesis revealed in the sea star hydro-vascular organ

A fundamental goal in the organogenesis field is to understand how cells organize into tubular shapes. Toward this aim, we have established the hydro-vascular organ in the sea star Patiria miniata as a model for tubulogenesis. In this animal, bilateral tubes grow out from the tip of the developing gut, and precisely extend to specific sites in the larva. This growth involves cell migration coupled with mitosis in distinct zones. Cell proliferation requires FGF signaling, whereas the three-dimensional orientation of the organ depends on Wnt signaling. Specification and maintenance of tube cell fate requires Delta/Notch signaling. Moreover, we identify target genes of the FGF pathway that contribute to tube morphology, revealing molecular mechanisms for tube outgrowth. Finally, we report that FGF activates the Six1/2 transcription factor, which serves as an evolutionarily ancient regulator of branching morphogenesis. This study uncovers distinct mechanisms of tubulogenesis in vivo and we propose that cellular dynamics in the sea star hydro-vascular organ represents a key comparison for understanding the evolution of vertebrate organs.

Molecular versatility during pluripotency progression

A challenge during development is to ensure lineage segregation while preserving plasticity. Using pluripotency progression as a paradigm, we review how developmental transitions are coordinated by redeployments, rather than global resettings, of cellular components. We highlight how changes in response to extrinsic cues (FGF, WNT, Activin/Nodal, Netrin-1), context- and stoichiometry-dependent action of transcription factors (Oct4, Nanog) and reconfigurations of epigenetic regulators (enhancers, promoters, TrxG, PRC) may confer robustness to naïve to primed pluripotency transition. We propose the notion of Molecular Versatility to regroup mechanisms by which molecules are repurposed to exert different, sometimes opposite, functions in close stem cell configurations.

The Wnt gene family in Tenebrio molitor and other coleopterans

The Wnt gene family is involved in a wide range of developmental processes. Despite its significance, the evolution and function of Wnt genes remain largely unclear. Here, an exhaustive survey of Wnt genes was conducted in Tenebrio molitor and 17 other beetle genomes. A total of 146 Wnt genes were identified, creating a comprehensive coleopteran Wnt gene catalog. Comparative genomics indicates that dynamic evolutionary patterns of Wnt gene loss and duplication occurred in Coleoptera, leading to the diverse Wnt gene repertoire in various beetles. A striking loss of particular Wnt gene subfamilies occurs in Coleoptera. Remarkably, Wnt gene duplication was discovered for the first time in insects. Further analysis of Wnt gene expression in T. molitor indicates that each Wnt gene, including the duplicated ones, has a unique spatial or temporal expression pattern. The current study provides valuable insight into the evolution and functional validation of Wnt genes in Coleoptera.

Ligand-receptor promiscuity enables cellular addressing

In multicellular organisms, secreted ligands selectively activate, or "address," specific target cell populations to control cell fate decision-making and other processes. Key cell-cell communication pathways use multiple promiscuously interacting ligands and receptors, provoking the question of how addressing specificity can emerge from molecular promiscuity. To investigate this issue, we developed a general mathematical modeling framework based on the bone morphogenetic protein (BMP) pathway architecture. We find that promiscuously interacting ligand-receptor systems allow a small number of ligands, acting in combinations, to address a larger number of individual cell types, defined by their receptor expression profiles. Promiscuous systems outperform seemingly more specific one-to-one signaling architectures in addressing capability. Combinatorial addressing extends to groups of cell types, is robust to receptor expression noise, grows more powerful with increases in the number of receptor variants, and is maximized by specific biochemical parameter relationships. Together, these results identify design principles governing cellular addressing by ligand combinations.

Diversity and robustness of bone morphogenetic protein pattern formation

Pattern formation by bone morphogenetic proteins (BMPs) demonstrates remarkable plasticity and utility in several contexts, such as early embryonic development, tissue patterning and the maintenance of stem cell niches. BMPs pattern tissues over many temporal and spatial scales: BMP gradients as short as 1-2 cell diameters maintain the stem cell niche of the Drosophila germarium over a 24-h cycle, and BMP gradients of several hundred microns establish dorsal-ventral tissue specification in Drosophila, zebrafish and Xenopus embryos in timescales between 30 min and several hours. The mechanisms that shape BMP signaling gradients are also incredibly diverse. Although ligand diffusion plays a dominant role in forming the gradient, a cast of diffusible and non-diffusible regulators modulate gradient formation and confer robustness, including scale invariance and adaptability to perturbations in gene expression and growth. In this Review, we document the diverse ways that BMP gradients are formed and refined, and we identify the core principles that they share to achieve reliable performance.

Receptor binding competition: A paradigm for regulating TGF-β family action

The transforming growth factor (TGF)-β family is a group of structurally related, multifunctional growth factors, or ligands that are crucially involved in the development, regulation, and maintenance of animal tissues. In humans, the family counts over 33 members. These secreted ligands typically form multimeric complexes with two type I and two type II receptors to activate one of two distinct signal transduction branches. A striking feature of the family is its promiscuity, i.e., many ligands bind the same receptors and compete with each other for binding to these receptors. Although several explanations for this feature have been considered, its functional significance has remained puzzling. However, several recent reports have promoted the idea that ligand-receptor binding promiscuity and competition are critical features of the TGF-β family that provide an essential regulating function. Namely, they allow a cell to read and process multi-ligand inputs. This capability may be necessary for producing subtle, distinctive, or adaptive responses and, possibly, for facilitating developmental plasticity. Here, we review the molecular basis for ligand competition, with emphasis on molecular structures and binding affinities. We give an overview of methods that were used to establish experimentally ligand competition. Finally, we discuss how the concept of ligand competition may be fundamentally tied to human physiology, disease, and therapy.

Genome-wide identification and transcriptome-based expression profiling of Wnt gene family in Ruditapes philippinarum

The Wnt genes encode a set of conserved glycoproteins regulating early development, cell proliferation and differentiation, and tissue regeneration in metazoans. In some mollusks, the knowledge of Wnt gene family has been limited because of the short of the genomic and transcriptomic resources. Ruditapes philippinarum is an economically important bivalve with a variety of shell coloration patterns and ability to regenerate its siphon. To gain a greater understanding of the evolutionary dynamics of Wnt gene family, we carried out a genome-wide identification and phylogenetic analysis of Wnt gene family in R. philippinarum and other four mollusks. A total of 12 Wnt genes were identified in the genome of R. philippinarum, and the dynamic patterns of gene conservation, loss and duplication of Wnt genes were analyzed in mollusks and model organisms. Furthermore, the transcriptome analyses demonstrated the expression profiles of the Wnt genes at different developmental stage, in adult tissues, during siphon regeneration, in four different shell color strains, and at uncolored and colored developmental stages in two different shell color strains. These findings suggest that the expansion of Wnt genes may play vital roles in the larval development, the formation of shell color pattern and siphon regeneration in R. philippinarum. This study provides a valuable insight into Wnt function and evolution in mollusks.

Wnt gene regulation and function during maxillary palp development in Drosophila melanogaster

Wnt genes encode secreted ligands that play many important roles in the development of metazoans. There are thirteen known Wnt gene subfamilies and seven of these are represented in Drosophila melanogaster. While wingless (wg) is the best understood and most widely studied Wnt gene in Drosophila, the functions of many of the other Drosophila Wnt genes are less well understood. For example, relatively little is known about Wnt6, which is an ancient paralog of wg and they form a conserved Wnt cluster together with Wnt9 (Dwnt4) and Wnt10. Wg and Wnt6 encode similar proteins and exhibit overlapping expression in several tissues during development. Both wg and Wnt6 were previously shown to regulate the development of maxillary palps, important olfactory organs in flies, but it remained unclear how these two ligands may combine to carry out specific functions and how this is regulated. Here, we have further analysed Wnt6 function in the context of maxillary palp development. Surprisingly, we found that Wnt6 does not appear to be necessary for development of maxillary palps. While a deletion of the 5' region of Wnt6 results in very small maxillary palps, we show that this effect is more likely to be a consequence of removing cis-regulatory elements that may regulate wg expression in this tissue rather than through the loss of Wnt6 function. Although, we cannot completely exclude the possibility that Wnt6 may subtly regulate maxillary palp development in combination with wg, our analysis of Wnt6 loss of function mutants suggests this ligand plays a more general role in regulating growth during development. Taken together our results provide new insights into maxillary palp formation and Wnt6 functions in Drosophila, and further evidence for a complex cis-regulatory landscape in the Wnt9-wg-Wnt6-Wnt10 cluster, which may help explain its evolutionary conservation.

Formation, interpretation, and regulation of the Drosophila Dorsal/NF-κB gradient

The morphogen gradient of the transcription factor Dorsal in the early Drosophila embryo has become one of the most widely studied tissue patterning systems. Dorsal is a Drosophila homolog of mammalian NF-κB and patterns the dorsal-ventral axis of the blastoderm embryo into several tissue types by spatially regulating upwards of 100 zygotic genes. Recent studies using fluorescence microscopy and live imaging have quantified the Dorsal gradient and its target genes, which has paved the way for mechanistic modeling of the gradient. In this review, we describe the mechanisms behind the initiation of the Dorsal gradient and its regulation of target genes. The main focus of the review is a discussion of quantitative and computational studies of the Dl gradient system, including regulation of the Dl gradient. We conclude with a discussion of potential future directions.

Genome-Wide Identification and Expression Profiling of Wnt Family Genes in the Silkworm, Bombyx mori

Wnt is a family of conserved glycoproteins that participate in a variety of important biological processes including embryo development, cell proliferation and differentiation, and tissue regeneration. The Wnt family is a metazoan novelty found in all animal phyla. Studies have revealed that the number of Wnt genes varies among species, presumably due to reproduction and loss of genes during evolution. However, a comprehensive inventory of Wnt genes in Lepidoptera is lacking. In this study, we identified the repertoire of Wnt genes in the silkworm and seven other species of Lepidoptera and obtained eight Wnt genes (Wnt1, Wnt5–Wnt7, Wnt9–Wnt11, and WntA) in each species. Four of these Wnt genes are clustered in two orientations (5′-Wnt9-Wnt1-Wnt6-Wnt10-3′ and 5′-Wnt10-Wnt6-Wnt1-Wnt9-3′) in both moths and butterflies. Transcript analysis of Wnt in silkworm embryonic stages showed that each BmWnt gene had a unique expression pattern during embryological development. Analysis of a larval stage revealed differential expression of Wnt family members in diverse tissues. Our study provides an overview of the Wnt family in Lepidoptera and will inspire further functional study of the Wnt genes in the silkworm.

Combinatorial Signal Perception in the BMP Pathway

The bone morphogenetic protein (BMP) signaling pathway comprises multiple ligands and receptors that interact promiscuously with one another and typically appear in combinations. This feature is often explained in terms of redundancy and regulatory flexibility, but it has remained unclear what signal-processing capabilities it provides. Here, we show that the BMP pathway processes multi-ligand inputs using a specific repertoire of computations, including ratiometric sensing, balance detection, and imbalance detection. These computations operate on the relative levels of different ligands and can arise directly from competitive receptor-ligand interactions. Furthermore, cells can select different computations to perform on the same ligand combination through expression of alternative sets of receptor variants. These results provide a direct signal-processing role for promiscuous receptor-ligand interactions and establish operational principles for quantitatively controlling cells with BMP ligands. Similar principles could apply to other promiscuous signaling pathways.

An embryonic system to assess direct and indirect Wnt transcriptional targets

During animal development, complex signals determine and organize a vast number of tissues using a very small number of signal transduction pathways. These developmental signaling pathways determine cell fates through a coordinated transcriptional response that remains poorly understood. The Wnt pathway is involved in a variety of these cellular functions, and its signals are transmitted in part through a β-catenin/TCF transcriptional complex. Here we report an in vivo Drosophila assay that can be used to distinguish between activation, de-repression and repression of transcriptional responses, separating upstream and downstream pathway activation and canonical/non-canonical Wnt signals in embryos. We find specific sets of genes downstream of both β-catenin and TCF with an additional group of genes regulated by Wnt, while the non-canonical Wnt4 regulates a separate cohort of genes. We correlate transcriptional changes with phenotypic outcomes of cell differentiation and embryo size, showing our model can be used to characterize developmental signaling compartmentalization in vivo.

DNMT3L enables accumulation and inheritance of epimutations in transgenic Drosophila

DNMT3L is an important epigenetic regulator in mammals, integrating DNA methylation and histone modification based epigenetic circuits. Here we show DNMT3L to be a part of the machinery that enables inheritance of epigenetic modifications from one generation to the next. Ectopic expression of DNMT3L in Drosophila, which lacks DNMT3L and its normal interacting partners DNMT3A and DNMT3B, lead to nuclear reprogramming that was gradual and progressive, resulting in melanotic tumors that were observed only when these flies were maintained for five generations. This global gene expression misregulation was accompanied by aberrations in the levels of H3K4me₃ and H3K36me₃, globally as well as at specific gene promoters. The levels of these epigenetic aberrations (epimutations) also increased progressively across successive generations. The accumulation and inheritance of epimutations across multiple generations recapitulates the important role of DNMT3L in intergenerational epigenetic inheritance in mammals.

Multipotent versus differentiated cell fate selection in the developing Drosophila airways

Developmental potentials of cells are tightly controlled at multiple levels. The embryonic Drosophila airway tree is roughly subdivided into two types of cells with distinct developmental potentials: a proximally located group of multipotent adult precursor cells (P-fate) and a distally located population of more differentiated cells (D-fate). We show that the GATA-family transcription factor (TF) Grain promotes the P-fate and the POU-homeobox TF Ventral veinless (Vvl/Drifter/U-turned) stimulates the D-fate. Hedgehog and receptor tyrosine kinase (RTK) signaling cooperate with Vvl to drive the D-fate at the expense of the P-fate while negative regulators of either of these signaling pathways ensure P-fate specification. Local concentrations of Decapentaplegic/BMP, Wingless/Wnt, and Hedgehog signals differentially regulate the expression of D-factors and P-factors to transform an equipotent primordial field into a concentric pattern of radially different morphogenetic potentials, which gradually gives rise to the distal-proximal organization of distinct cell types in the mature airway.

DOI:http://dx.doi.org/10.7554/eLife.09646.001

Many organs are composed of tubes of different sizes, shapes and patterns that transport vital substances from one site to another. In the fruit fly species Drosophila melanogaster, oxygen is transported by a tubular network, which divides into finer tubes that allow the oxygen to reach every part of the body.

Different parts of the fruit fly’s airways develop from different groups of tracheal precursor cells. P-fate cells form the most 'proximal' tubes (which are found next to the outer layer of the fly). These cells are 'multipotent' stem cells, and have the ability to specialize into many different types of cells during metamorphosis. The more 'distal' branches that emerge from the proximal tubes develop from D-fate cells. These are cells that generally acquire a narrower range of cell identities.

By performing a genetic analysis of fruit fly embryos, Matsuda et al. have now identified several proteins and signaling molecules that control whether tracheal precursor cells become D-fate or P-fate cells. For example, several signaling pathways work with a protein called Ventral veinless to cause D-fate cells to develop instead of P-fate cells. However, molecules that prevent signaling occurring via these pathways help P-fate cells to form. Different amounts of the molecules that either promote or hinder these signaling processes are present in different parts of the fly embryo; this helps the airways of the fly to develop in the correct pattern.

This work provides a comprehensive view of how cell types with different developmental potentials are positioned in a complex tubular network. This sets a basis for future studies addressing how the respiratory organs – and indeed the entire organism – are sustained.

DOI:http://dx.doi.org/10.7554/eLife.09646.002

Weevil endosymbiont dynamics is associated with a clamping of immunity

BACKGROUND

Insects subsisting on nutritionally unbalanced diets have evolved long-term mutualistic relationships with intracellular symbiotic bacteria (endosymbionts). The endosymbiont population load undergoes changes along with insect development. In the cereal weevil Sitophilus oryzae, the midgut endosymbionts Sodalis pierantonius drastically multiply following adult metamorphosis and rapidly decline until total elimination when the insect achieves its cuticle synthesis. Whilst symbiont load was shown to timely meet insect metabolic needs, little is known about the host molecular and immune processes underlying this dynamics.

METHODS

We performed RNA sequencing analysis on weevil midguts at three representative phases of the endosymbiont dynamics (i.e. increase, climax and decrease). To screen genes which transcriptional changes are specifically related to symbiont dynamics and not to the intrinsic development of the midgut, we further have monitored by RT-qPCR sixteen gene transcript levels in symbiotic and artificially non-symbiotic (aposymbiotic) weevils. We also localized the endosymbionts during the elimination process by fluorescence microscopy.

RESULTS

Functional analysis of the host differentially expressed genes by RNA sequencing showed that the main transcriptional changes occur during endosymbiont growth phase and affect cell proliferation, apoptosis, autophagy, phagocytosis, and metabolism of fatty acids and nucleic acids. We also showed that symbiont dynamics alters the expression of several genes involved in insect development. Our results strengthened the implication of apoptosis and autophagy processes in symbiont elimination and recycling. Remarkably, apart from the coleoptericin A that is known to target endosymbionts and controls their division and location, no gene coding antimicrobial peptide was upregulated during the symbiont growth and elimination phases.

CONCLUSION

We show that endosymbiont dynamics parallels numerous transcriptional changes in weevil developing adults and affects several biological processes, including metabolism and development. It also triggers cell apoptosis, autophagy and gut epithelial cell swelling and delamination. Strikingly, immunity is repressed during the whole process, presumably avoiding tissue inflammation and allowing insects to optimize nutrient recovery from recycled endosymbiont.

The intersection of the extrinsic hedgehog and WNT/wingless signals with the intrinsic Hox code underpins branching pattern and tube shape diversity in the drosophila airways

The tubular networks of the Drosophila respiratory system and our vasculature show distinct branching patterns and tube shapes in different body regions. These local variations are crucial for organ function and organismal fitness. Organotypic patterns and tube geometries in branched networks are typically controlled by variations of extrinsic signaling but the impact of intrinsic factors on branch patterns and shapes is not well explored. Here, we show that the intersection of extrinsic hedgehog(hh) and WNT/wingless (wg) signaling with the tube-intrinsic Hox code of distinct segments specifies the tube pattern and shape of the Drosophila airways. In the cephalic part of the airways, hh signaling induces expression of the transcription factor (TF) knirps (kni) in the anterior dorsal trunk (DTa1). kni represses the expression of another TF spalt major (salm), making DTa1 a narrow and long tube. In DTa branches of more posterior metameres, Bithorax Complex (BX-C) Hox genes autonomously divert hh signaling from inducing kni, thereby allowing DTa branches to develop as salm-dependent thick and short tubes. Moreover, the differential expression of BX-C genes is partly responsible for the anterior-to-posterior gradual increase of the DT tube diameter through regulating the expression level of Salm, a transcriptional target of WNT/wg signaling. Thus, our results highlight how tube intrinsic differential competence can diversify tube morphology without changing availabilities of extrinsic factors.

Tubes are common structural elements of many internal organs, facilitating fluid flow and material exchange. To meet the local needs of diverse tissues, the branching patterns and tube shapes vary regionally. Diametric tapering and specialized branch targeting to the brain represent two common examples of variations with organismal benefits in the Drosophila airways and our vascular system. Several extrinsic signals instruct tube diversifications but the impact of intrinsic factors remains underexplored. Here, we show that the local, tube-intrinsic Hox code instructs the pattern and shape of the dorsal trunk (DT), the main Drosophila airway. In the cephalic part (DT1), where Bithorax Complex (BX-C) Hox genes are not expressed, the extrinsic Hedgehog signal is epistatic to WNT/Wingless signals. Hedgehog instructs anterior DT1 cells to take a long and narrow tube fate targeting the brain. In more posterior metameres, BX-C genes make the extrinsic WNT/Wingless signals epistatic over Hedgehog. There, WNT/Wingless instruct all DT cells to take the thick and short tube fate. Moreover, BX-C genes modulate the outputs of WNT/wingless signaling, making the DT tubes thicker in more posterior metameres. We provide a model for how intrinsic factors modify extrinsic signaling to control regional tube morphologies in a network.

The PTK7-related transmembrane proteins off-track and off-track 2 are co-receptors for Drosophila Wnt2 required for male fertility

Wnt proteins regulate many developmental processes and are required for tissue homeostasis in adult animals. The cellular responses to Wnts are manifold and are determined by the respective Wnt ligand and its specific receptor complex in the plasma membrane. Wnt receptor complexes contain a member of the Frizzled family of serpentine receptors and a co-receptor, which commonly is a single-pass transmembrane protein. Vertebrate protein tyrosine kinase 7 (PTK7) was identified as a Wnt co-receptor required for control of planar cell polarity (PCP) in frogs and mice. We found that flies homozygous for a complete knock-out of the Drosophila PTK7 homolog off track (otk) are viable and fertile and do not show PCP phenotypes. We discovered an otk paralog (otk2, CG8964), which is co-expressed with otk throughout embryonic and larval development. Otk and Otk2 bind to each other and form complexes with Frizzled, Frizzled2 and Wnt2, pointing to a function as Wnt co-receptors. Flies lacking both otk and otk2 are viable but male sterile due to defective morphogenesis of the ejaculatory duct. Overexpression of Otk causes female sterility due to malformation of the oviduct, indicating that Otk and Otk2 are specifically involved in the sexually dimorphic development of the genital tract.

Wnts are secreted, growth factor-like proteins that are important for the development of many tissues and organs in animals. They are also required in adult animals and humans for controlling the balance between growth and differentiation. Wnts are bound at the cell surface by Wnt receptors, which are dimers composed of a Frizzled protein and a co-receptor. Here we have analyzed the Drosophila Wnt co-receptors Off-track (Otk) and Off-track 2 (Otk2), which are closely related to vertebrate Protein tyrosine kinase 7 (PTK7). We found that in contrast to PTK7 in mice and frogs, which controls planar cell polarity (PCP), Otk and Otk2 together are needed in males for development of the ejaculatory duct, a tube-like organ that transports the mature sperm. Our data furthermore indicate that Otk and Otk2 are co-receptors for Wnt2. The sterile phenotype of Wnt2 mutant males is not identical to that of otk, otk2 double mutants, so additional Wnts may be involved in this process. Interestingly, the function of Wnt2 in male fertility appears to be evolutionarily conserved, because male mice mutant for Wnt7A, the vertebrate homolog of Drosophila Wnt2, are sterile due to abnormal development of the vas deferens, which corresponds to the fly ejaculatory duct.

Analysis of the Wnt gene repertoire in an onychophoran provides new insights into the evolution of segmentation

BACKGROUND

The Onychophora are a probable sister group to Arthropoda, one of the most intensively studied animal phyla from a developmental perspective. Pioneering work on the fruit fly Drosophila melanogaster and subsequent investigation of other arthropods has revealed important roles for Wnt genes during many developmental processes in these animals.

RESULTS

We screened the embryonic transcriptome of the onychophoran Euperipatoides kanangrensis and found that at least 11 Wnt genes are expressed during embryogenesis. These genes represent 11 of the 13 known subfamilies of Wnt genes.

CONCLUSIONS

Many onychophoran Wnt genes are expressed in segment polarity gene-like patterns, suggesting a general role for these ligands during segment regionalization, as has been described in arthropods. During early stages of development, Wnt2, Wnt4, and Wnt5 are expressed in broad multiple segment-wide domains that are reminiscent of arthropod gap and Hox gene expression patterns, which suggests an early instructive role for Wnt genes during E. kanangrensis segmentation.

Comparative molecular developmental aspects of the mammalian- and the avian lungs, and the insectan tracheal system by branching morphogenesis: recent advances and future directions

Gas exchangers fundamentally form by branching morphogenesis (BM), a mechanistically profoundly complex process which derives from coherent expression and regulation of multiple genes that direct cell-to-cell interactions, differentiation, and movements by signaling of various molecular morphogenetic cues at specific times and particular places in the developing organ. Coordinated expression of growth-instructing factors determines sizes and sites where bifurcation occurs, by how much a part elongates before it divides, and the angle at which branching occurs. BM is essentially induced by dualities of factors where through feedback- or feed forward loops agonists/antagonists are activated or repressed. The intricate transactions between the development orchestrating molecular factors determine the ultimate phenotype. From the primeval time when the transformation of unicellular organisms to multicellular ones occurred by systematic accretion of cells, BM has been perpetually conserved. Canonical signalling, transcriptional pathways, and other instructive molecular factors are commonly employed within and across species, tissues, and stages of development. While much still remain to be elucidated and some of what has been reported corroborated and reconciled with rest of existing data, notable progress has in recent times been made in understanding the mechanism of BM. By identifying and characterizing the morphogenetic drivers, and markers and their regulatory dynamics, the elemental underpinnings of BM have been more precisely explained. Broadening these insights will allow more effective diagnostic and therapeutic interventions of developmental abnormalities and pathologies in pre- and postnatal lungs. Conservation of the molecular factors which are involved in the development of the lung (and other branched organs) is a classic example of nature's astuteness in economically utilizing finite resources. Once purposefully formed, well-tested and tried ways and means are adopted, preserved, and widely used to engineer the most optimal phenotypes. The material and time costs of developing utterly new instruments and routines with every drastic biological change (e.g. adaptation and speciation) are circumvented. This should assure the best possible structures and therefore functions, ensuring survival and evolutionary success.

Canonical Wnt signaling in the visceral muscle is required for left-right asymmetric development of the Drosophila midgut

Many animals develop left-right (LR) asymmetry in their internal organs. The mechanisms of LR asymmetric development are evolutionarily divergent, and are poorly understood in invertebrates. Therefore, we studied the genetic pathway of LR asymmetric development in Drosophila. Drosophila has several organs that show directional and stereotypic LR asymmetry, including the embryonic gut, which is the first organ to develop LR asymmetry during Drosophila development. In this study, we found that genes encoding components of the Wnt-signaling pathway are required for LR asymmetric development of the anterior part of the embryonic midgut (AMG). frizzled 2 (fz2) and Wnt4, which encode a receptor and ligand of Wnt signaling, respectively, were required for the LR asymmetric development of the AMG. arrow (arr), an ortholog of the mammalian gene encoding low-density lipoprotein receptor-related protein 5/6, which is a co-receptor of the Wnt-signaling pathway, was also essential for LR asymmetric development of the AMG. These results are the first demonstration that Wnt signaling contributes to LR asymmetric development in invertebrates, as it does in vertebrates. The AMG consists of visceral muscle and an epithelial tube. Our genetic analyses revealed that Wnt signaling in the visceral muscle but not the epithelium of the midgut is required for the AMG to develop its normal laterality. Furthermore, fz2 and Wnt4 were expressed in the visceral muscles of the midgut. Consistent with these results, we observed that the LR asymmetric rearrangement of the visceral muscle cells, the first visible asymmetry of the developing AMG, did not occur in embryos lacking Wnt4 expression. Our results also suggest that canonical Wnt/β-catenin signaling, but not non-canonical Wnt signaling, is responsible for the LR asymmetric development of the AMG. Canonical Wnt/β-catenin signaling is reported to have important roles in LR asymmetric development in zebrafish. Thus, the contribution of canonical Wnt/β-catenin signaling to LR asymmetric development may be an evolutionarily conserved feature between vertebrates and invertebrates.

Conservation, loss, and redeployment of Wnt ligands in protostomes: implications for understanding the evolution of segment formation

Background

The Wnt genes encode secreted glycoprotein ligands that regulate a wide range of developmental processes, including axis elongation and segmentation. There are thirteen subfamilies of Wnt genes in metazoans and this gene diversity appeared early in animal evolution. The loss of Wnt subfamilies appears to be common in insects, but little is known about the Wnt repertoire in other arthropods, and moreover the expression and function of these genes have only been investigated in a few protostomes outside the relatively Wnt-poor model species Drosophila melanogaster and Caenorhabditis elegans. To investigate the evolution of this important gene family more broadly in protostomes, we surveyed the Wnt gene diversity in the crustacean Daphnia pulex, the chelicerates Ixodes scapularis and Achaearanea tepidariorum, the myriapod Glomeris marginata and the annelid Platynereis dumerilii. We also characterised Wnt gene expression in the latter three species, and further investigated expression of these genes in the beetle Tribolium castaneum.

Results

We found that Daphnia and Platynereis both contain twelve Wnt subfamilies demonstrating that the common ancestors of arthropods, ecdysozoans and protostomes possessed all members of all Wnt subfamilies except Wnt3. Furthermore, although there is striking loss of Wnt genes in insects, other arthropods have maintained greater Wnt gene diversity. The expression of many Wnt genes overlap in segmentally reiterated patterns and in the segment addition zone, and while these patterns can be relatively conserved among arthropods and the annelid, there have also been changes in the expression of some Wnt genes in the course of protostome evolution. Nevertheless, our results strongly support the parasegment as the primary segmental unit in arthropods, and suggest further similarities between segmental and parasegmental regulation by Wnt genes in annelids and arthropods respectively.

Conclusions

Despite frequent losses of Wnt gene subfamilies in lineages such as insects, nematodes and leeches, most protostomes have probably maintained much of their ancestral repertoire of twelve Wnt genes. The maintenance of a large set of these ligands could be in part due to their combinatorial activity in various tissues rather than functional redundancy. The activity of such Wnt 'landscapes' as opposed to the function of individual ligands could explain the patterns of conservation and redeployment of these genes in important developmental processes across metazoans. This requires further analysis of the expression and function of these genes in a wider range of taxa.

The function and evolution of Wnt genes in arthropods

Wnt signalling is required for a wide range of developmental processes, from cleavage to patterning and cell migration. There are 13 subfamilies of Wnt ligand genes and this diverse repertoire appeared very early in metazoan evolution. In this review, we first summarise the known Wnt gene repertoire in various arthropods. Insects appear to have lost several Wnt subfamilies, either generally, such as Wnt3, or in lineage specific patterns, for example, the loss of Wnt7 in Anopheles. In Drosophila and Acyrthosiphon, only seven and six Wnt subfamilies are represented, respectively; however, the finding of nine Wnt genes in Tribolium suggests that arthropods had a larger repertoire ancestrally. We then discuss what is currently known about the expression and developmental function of Wnt ligands in Drosophila and other insects in comparison to other arthropods, such as the spiders Achaearanea and Cupiennius. We conclude that studies of Wnt genes have given us much insight into the developmental roles of some of these ligands. However, given the frequent loss of Wnt genes in insects and the derived development of Drosophila, further studies of these important genes are required in a broader range of arthropods to fully understand their developmental function and evolution.

Wnt to build a tube: contributions of Wnt signaling to epithelial tubulogenesis

Epithelial tubes are crucial to the function of organ systems including the cardiovascular system, pulmonary system, gastrointestinal tract, reproductive organ systems, excretory system, and auditory system. Using a variety of animal model systems, recent studies have substantiated the role of Wnt signaling via the canonical/beta-catenin-mediated trajectory, the non-canonical Wnt trajectories, or both, in forming epithelial tubular tissues. This review focuses on the involvement of the Wnt pathways in the induction, specification, proliferation, and morphogenesis involved in tubulogenesis within tissues including the lungs, kidneys, ears, mammary glands, gut, and heart. The ultimate goal is to describe the developmental processes forming the various tubulogenic organ systems to determine the relationships between these processes.

Towards an integrated view of Wnt signaling in development

Wnt signaling is crucial for embryonic development in all animal species studied to date. The interaction between Wnt proteins and cell surface receptors can result in a variety of intracellular responses. A key remaining question is how these specific responses take shape in the context of a complex, multicellular organism. Recent studies suggest that we have to revise some of our most basic ideas about Wnt signal transduction. Rather than thinking about Wnt signaling in terms of distinct, linear, cellular signaling pathways, we propose a novel view that considers the integration of multiple, often simultaneous, inputs at the level of both Wnt-receptor binding and the downstream, intracellular response.

Wingless secretion promotes and requires retromer-dependent cycling of Wntless

Wnt ligands are lipid-modified, secreted glycoproteins that control multiple steps during embryogenesis and adult-tissue homeostasis. Little is known about the mechanisms underlying Wnt secretion. Recently, Wntless (Wls/Evi/Srt) was identified as a conserved multi-pass transmembrane protein whose function seems to be dedicated to promoting the release of Wnts. Here, we describe Wls accumulation in the Golgi apparatus of Wnt/Wingless (Wg)-producing cells in Drosophila, and show that this localization is essential for Wg secretion. Moreover, Wls localization and levels critically depend on retromer, a conserved protein complex that mediates endosome-to-Golgi protein trafficking in yeast. In the absence of the retromer components Dvps35 or Dvps26, but in presence of Wg, Wls is degraded and Wg secretion impaired. Our results indicate that Wg, clathrin-mediated endocytosis and retromer sustain a Wls traffic loop from the Golgi to the plasma membrane and back to the Golgi, thereby enabling Wls to direct Wnt secretion.

Tramtrack regulates different morphogenetic events duringDrosophilatracheal development

Tramtrack (Ttk) is a widely expressed transcription factor, the function of which has been analysed in different adult and embryonic tissues in Drosophila. So far, the described roles of Ttk have been mainly related to cell fate specification, cell proliferation and cell cycle regulation. Using the tracheal system of Drosophila as a morphogenetic model, we have undertaken a detailed analysis of Ttk function. Ttk is autonomously and non-autonomously required during embryonic tracheal formation. Remarkably, besides a role in the specification of different tracheal cell identities, we have found that Ttk is directly involved and required for different cellular responses and morphogenetic events. In particular, Ttk appears to be a new positive regulator of tracheal cell intercalation. Analysis of this process in ttk mutants has unveiled cell shape changes as a key requirement for intercalation and has identified Ttk as a novel regulator of its progression. Moreover, we define Ttk as the first identified regulator of intracellular lumen formation and show that it is autonomously involved in the control of tracheal tube size by regulating septate junction activity and cuticle formation. In summary, the involvement of Ttk in different steps of tube morphogenesis identifies it as a key player in tracheal development.

Evolution of the vertebrate twist family and synfunctionalization: a mechanism for differential gene loss through merging of expression domains

Twist genes are essential for embryonic development and are conserved from jellyfish to human. To study the vertebrate twist family and its evolution, the entire complement of twist genes was obtained for 9 representative species. Phylogenetic analysis showed that a single protochordate twist gene was duplicated at least twice before the teleost-tetrapod split to give rise to 3 ancestral genes, which were further duplicated or deleted, resulting in fluctuating number of twist paralogs in different vertebrate lineages. To find whether changes in gene copy number were associated with changes in gene function, embryonic expression patterns of twist orthologs were evaluated against the number of twist paralogs in different species. The results showed evidence for both neo- and subfunctionalization, and, in addition, for loss of an ancestral regulatory gene. For example, in Xenopus, twist2 was lost, but the twist1 paralog acquired, and therefore preserved, twist2 function. A general model is proposed to explain the data. In this process, termed synfunctionalization, one paralog acquires the expression domain(s) of another. The merging may lead to function shuffle. Alternatively, it may leave one paralog redundant and thus subject to deletion--while its function is retained by the surviving paralog(s). Synfunctionalization is a mechanism that, together with neo- and subfunctionalization, may work to establish equilibrium in the number of genes that regulate developmental processes; it may regulate the complexity of regulatory regions as well as gene copy number and therefore may play a role in evolution of gene function and the structure of genome.

Proteome analysis of silk gland proteins from the silkworm,Bombyx mori

The silk gland of Bombyx mori is an organ specialized for the synthesis and secretion of silk proteins. We report here the resolution of silk gland proteins by 2-DE and the identification of many of those proteins. This was accomplished by dissecting the glands into several sections, with each exhibiting more than 400 protein spots by 2-DE, of which 100 spots were excised and characterized by in-gel digestion followed by PMF. Ninety-three proteins were tentatively identified. These were then categorized into groups involved in silk protein secretion, transport, lipid metabolism, defense, etc. Western blotting of a 2-DE gel using an antibody of the carotenoid binding protein confirmed the presence of this protein in the silk gland. Proteins including fibroin L-chain and P25 were found as multiple isoforms, some of which contained differential amounts of phosphate residues as analyzed by on-probe dephosphorylation. The current analysis contributes to our understanding of proteins expressed by the silk gland not only of the model lepidopteran B. mori, but also to proteins from other silk-producing insects such as Philosamia cynthia ricini.

Wingless signaling in a large insect, the blowflyLucilia sericata: A beautiful example of evolutionary developmental biology

Blowflies are the primary facultative agent in causing myiasis of domestic sheep in the whole world and, at the same time, it is an important tool for forensic medicine. Surprisingly, and in contrast to its importance, almost no data regarding the embryology and molecular markers are known for this insect. In this report, we present a detailed description of the blowfly Lucilia sericata embryogenesis and of imaginal disc development. The embryogenesis of Lucilia strongly resembles that of Drosophila, despite their apparent size difference. Moreover, imaginal disc development appears to be equally well conserved. Through cloning, expression, and functional studies, we show that the Lucilia Wingless (Wg) protein is highly conserved between the two species. We further show that parasegments are established in Lucilia, however, engrailed expression shows a more dynamic expression pattern than expected in comparison to Drosophila. Over-expression of Lucilia Wingless in Drosophila shows wingless-like wing phenotypes, suggesting that Lucilia Wingless blocks the signalling activity of Drosophila Wingless. Upon injection of wg dsRNA, we observe a "lawn of denticle" phenotype, closely resembling that of Drosophila. Due to the large size of the insect, the distance over which Wingless exerts signalling activity is up to three times larger than in Drosophila, yet the consequences are very similar. Our data demonstrate long-range wingless signaling mechanisms adapted for patterning large domains of naked cuticle and suggest signaling properties of Lucilia Wingless that are distinct from those of Drosophila Wingless.

Expression of 'segmentation' genes during larval and juvenile development in the polychaetes Capitella sp. I and H. elegans

Polychaete annelids and arthropods are both segmented protostome invertebrates. To investigate whether the segmented body plan of these two phyla share a common molecular ground pattern, we report the developmental expression of orthologues of the arthropod segment polarity genes engrailed (en), hedgehog (hh), and wingless (wg/Wnt1) in larval and juvenile stages of the polychaete annelid Capitella sp. I and en in a second polychaete, Hydroides elegans. Temporally, neither Wnt1 nor hh are detected in the segmented region of the larval body until after morphological segmentation is apparent. Expression of CapI-Wnt1 is limited to a ring of ectoderm marking the future anus during larval segmentation. CapI-hh is expressed in a ring of the hindgut internal to that of CapI-Wnt1, as well as in a subset of ventral nerve cord neurons, anterior gut tissue, and mesoderm. In both H. elegans and Capitella sp. I, en is expressed in a spatially and temporally dynamic manner in segmentally iterated structures as well as a population of cells that migrate internally from ectoderm to mesoderm, possibly representing a population of ecto-mesodermal precursors. Significantly, the expression patterns we report for wg, en, and hh orthologues in Capitella sp. I and for en in larval development of H. elegans are not comparable to the highly conserved ectodermal segment polarity pattern observed in arthropods at any life history stage, consistent with distinct origins of segmentation between annelids and arthropods.

WntD is a feedback inhibitor of Dorsal/NF-kappaB in Drosophila development and immunity

Regulating the nuclear factor-kappaB (NF-kappaB) family of transcription factors is of critical importance to animals, with consequences of misregulation that include cancer, chronic inflammatory diseases and developmental defects. Studies in Drosophila melanogaster have proved fruitful in determining the signals used to control NF-kappaB proteins, beginning with the discovery that the Toll/NF-kappaB pathway, in addition to patterning the dorsal-ventral axis of the fly embryo, defines a major component of the innate immune response in both Drosophila and mammals. Here, we characterize the Drosophila wntD (Wnt inhibitor of Dorsal) gene. We show that WntD acts as a feedback inhibitor of the NF-kappaB homologue Dorsal during both embryonic patterning and the innate immune response to infection. wntD expression is under the control of Toll/Dorsal signalling, and increased levels of WntD block Dorsal nuclear accumulation, even in the absence of the IkappaB homologue Cactus. The WntD signal is independent of the common Wnt signalling component Armadillo (beta-catenin). By engineering a gene knockout, we show that wntD loss-of-function mutants have immune defects and exhibit increased levels of Toll/Dorsal signalling. Furthermore, the wntD mutant phenotype is suppressed by loss of zygotic dorsal. These results describe the first secreted feedback antagonist of Toll signalling, and demonstrate a novel Wnt activity in the fly.

Shifted, the Drosophila Ortholog of Wnt Inhibitory Factor-1, Controls the Distribution and Movement of Hedgehog

We here identify and characterize an extracellular modulator of Hedgehog signaling in Drosophila, Shifted. Shifted is required for high levels of long-range signaling in the developing wing imaginal disc. Surprisingly, shifted encodes the only Drosophila ortholog of the secreted vertebrate protein Wnt Inhibitory Factor-1 (WIF-1), whose known role is to bind to extracellular Wnts and inhibit their activity. However, Shifted does not regulate Hedgehog signaling by affecting Wingless or Wnt signaling. We show instead that Shifted is a secreted protein that acts over a long distance and is required for the normal accumulation of Hh protein and its movement in the wing. Our data further indicate that Shf interacts with Hh and the heparan sulfate proteoglycans. Therefore, we propose that Shf stabilizes the interaction between Hh and the proteoglycans, an unexpected role for a member of the WIF-1 family.

Involvement of Wingless/Armadillo signaling in the posterior sequential segmentation in the cricket, Gryllus bimaculatus (Orthoptera), as revealed by RNAi analysis

In insects, there are two different modes of segmentation. In the higher dipteran insects (like Drosophila), their segmentation takes place almost simultaneously in the syncytial blastoderm. By contrast, in the orthopteran insects (like Schistocerca (grasshopper)), the anterior segments form almost simultaneously in the cellular blastoderm and then the remaining posterior part elongates to form segments sequentially from the posterior proliferative zone. Although most of their orthologues of the Drosophila segmentation genes may be involved in their segmentation, little is known about their roles. We have investigated segmentation processes of Gryllus bimaculatus, focusing on its orthologues of the Drosophila segment-polarity genes, G. bimaculatus wingless (Gbwg), armadillo (Gbarm) and hedgehog (Gbhh). Gbhh and Gbwg were observed to be expressed in the each anterior segment and the posterior proliferative zone. In order to know their roles, we used RNA interference (RNAi). We could not observed any significant effects of RNAi for Gbwg and Gbhh on segmentation, probably due to functional replacement by another member of the corresponding gene families. Embryos obtained by RNAi for Gbarm exhibited abnormal anterior segments and lack of the abdomen. Our results suggest that GbWg/GbArm signaling is involved in the posterior sequential segmentation in the G. bimaculatus embryos, while Gbwg, Gbarm and Gbhh are likely to act as the segment-polarity genes in the anterior segmentation similarly as in Drosophila.

Branching Morphogenesis of theDrosophilaTracheal System

Many organs including the mammalian lung and vascular system consist of branched tubular networks that transport essential gases or fluids, but the genetic programs that control the development of these complex three-dimensional structures are not well understood. The Drosophila melanogaster tracheal (respiratory) system is a network of interconnected epithelial tubes that transports oxygen and other gases in the body and provides a paradigm of branching morphogenesis. It develops by sequential sprouting of primary, secondary, and terminal branches from an epithelial sac of approximately 80 cells in each body segment of the embryo. Mapping of the cell movements and shape changes during the sprouting process has revealed that distinct mechanisms of epithelial migration and tube formation are used at each stage of branching. Genetic dissection of the process has identified a general program in which a fibroblast growth factor (FGF) and fibroblast growth factor receptor (FGFR) are used repeatedly to control branch budding and outgrowth. At each stage of branching, the mechanisms controlling FGF expression and the downstream signal transduction pathway change, altering the pattern and structure of the branches that form. During terminal branching, FGF expression is regulated by hypoxia, ensuring that tracheal structure matches cellular oxygen need. A branch diversification program operates in parallel to the general budding program: Regional signals locally modify the general program, conferring specific structural features and other properties on individual branches, such as their substrate outgrowth preferences, differences in tube size and shape, and the ability to fuse to other branches to interconnect the network.

Environmental oxidant pollutant effects on biologic systems: a focus on micronutrient antioxidant-oxidant interactions

Oxidative atmospheric pollutants represent a significant source of stress to both terrestrial plants and animals. The biosurfaces of plants and surface-living organisms are directly exposed to these pollutant stresses. These surfaces, including respiratory tract surfaces, contain integrated antioxidant systems that would be expected to provide a primary defense against environmental threats caused by atmospheric reactive oxygen species. When the biosurface antioxidant defenses are overwhelmed, oxidative stress to the cellular components of the exposed biosurfaces can be expected, inducing inflammatory, adaptive, injurious, and reparative processes. Studies of mutants and/or transformed plants and insects, with specific alterations in key components of antioxidant defense systems, offer opportunities to dissect the complex systems that maintain surface defenses against environmental oxidants. In this article, we use a comparative approach to consider interactions of atmospheric oxidant pollutants with selected biosystems, with focus on O3 as the pollutant; plants, flies, skin, and lungs as the exposed biosystems; and nonenzymatic micronutrient antioxidants as significant contributors to overall antioxidant defense strategies of these varied biosystems. Parallelisms among several living organisms, with regard to their protective strategies against environmental atmospheric oxidants, are presented.

Elbow and Noc define a family of zinc finger proteins controlling morphogenesis of specific tracheal branches

The elbow (elB) gene encodes a conserved nuclear protein with a single zinc finger. Expression of ElB is restricted to a specific subset of tracheal cells, namely the dorsal branch and the lateral trunks. Stalled or aberrant migration of these branches is observed in elB mutant embryos. Conversely, ElB misexpression in the trachea gave rise to absence of the visceral branch and an increase in the number of cells forming the dorsal branch. These results imply that the restricted expression of ElB contributes to the specification of distinct branch fates, as reflected in their stereotypic pattern of migration. As elB loss-of-function tracheal phenotypes are reminiscent of defects in Dpp signaling, the relationship between ElB and the Dpp pathway was examined. By using pMad antibodies that detect the activation pattern of the Dpp pathway, we show that Dpp signaling in the trachea is not impaired in elB mutants. In addition, expression of the Dpp target gene kni was unaltered. The opposite is true as well, because expression of elB is independent of Dpp signaling. ElB thus defines a parallel input, which determines the identity of the lateral trunk and dorsal branch cells. No ocelli (Noc) is the Drosophila protein most similar to ElB. Mutations in noc give rise to a similar tracheal phenotype. Noc is capable of associating with ElB, suggesting that they can function as a heterodimer. ElB also associates with the Groucho protein, indicating that the complex has the capacity to repress transcription of target genes. Indeed, in elB or noc mutants, expanded expression of tracheal branch-specific genes was observed.

Towards a model of the organisation of planar polarity and pattern in theDrosophilaabdomen

The abdomen of adult Drosophila consists of a chain of alternating anterior (A) and posterior (P) compartments which are themselves subdivided into stripes of different types of cuticle. Most of the cuticle is decorated with hairs and bristles that point posteriorly, indicating the planar polarity of the cells. Here we research the link between pattern and polarity.Previously we showed that the pattern of the A compartment depends on the local concentration (the scalar) of a Hedgehog morphogen produced by cells in the P compartment. Here we present evidence that the P compartment is patterned by another morphogen, Wingless, which is induced by Hedgehog in A compartment cells and then spreads back into the P compartment. We also find that both Hedgehog and Wingless appear to specify pattern by activating the optomotor blind gene, which encodes a transcription factor.We re-examine our working model that planar polarity is determined by the cells reading the gradient in concentration (the vector) of a morphogen ‘X’ which is produced on receipt of Hedgehog. We present evidence that Hedgehog induces X production by driving optomotor blind expression. We tried but failed to identify X and present data that X is not likely to operate through the conventional Notch, Decapentaplegic, EGF or FGF transduction pathways, or to encode a Wnt. However, we argue that Wingless may act to enhance the production or organise the distribution of X. A simple model that accommodates our results is that X forms a monotonic gradient extending from the back of the A compartment to the front of the P compartment in the next segment, a unit constituting a parasegment.

DWnt4 regulates cell movement and focal adhesion kinase during Drosophila ovarian morphogenesis

Cell motility is regulated by extracellular cues and by intracellular factors that accumulate at sites of contact between cells and the extracellular matrix. One of these factors, focal adhesion kinase (FAK), regulates the cycle of focal adhesion formation and disassembly that is required for cell movement to occur. Recently, Wnt signaling has also been implicated in the control of cell movement in vertebrates, but the mechanism through which Wnt proteins influence motility is unclear. We demonstrate that Drosphila Wnt4 is required for cell movement and FAK regulation during ovarian morphogenesis. Dfrizzled2, Disheveled, and protein kinase C are also required. The DWnt4 cell motility pathway is distinct from both the canonical Wnt pathway and the planar polarity pathway. Our data suggest that DWnt4 facilitates motility through regulation of focal adhesions.