Regulation of BMP and Activin Signaling in Drosophila

Physiological and molecular mechanisms of insect appendage regeneration

Regeneration, as a fascinating scientific field, refers to the ability of animals replacing lost tissue or body parts. Many metazoan organisms have been reported with the regeneration phenomena, but showing evolutionarily variable abilities. As the most diverse metazoan taxon, hundreds of insects show strong appendage regeneration ability. The regeneration process and ability are dependent on many factors, including macroscopic physiological conditions and microscopic molecular mechanisms. This article reviews research progress on the physiological conditions and internal underlying mechanisms controlling appendage regeneration in insects.

Gbb Regulates Blood Cell Proliferation and Differentiation through JNK and EGFR Signaling Pathways in the Drosophila Lymph Gland

The Drosophila lymph gland is an ideal model for studying hematopoiesis, and unraveling the mechanisms of Drosophila hematopoiesis can improve our understanding of the pathogenesis of human hematopoietic malignancies. Bone morphogenetic protein (BMP) signaling is involved in a variety of biological processes and is highly conserved between Drosophila and mammals. Decapentaplegic (Dpp)/BMP signaling is known to limit posterior signaling center (PSC) cell proliferation by repressing the protooncogene dmyc. However, the role of two other TGF-β family ligands, Glass bottom boat (Gbb) and Screw (Scw), in Drosophila hematopoiesis is currently largely unknown. Here, we showed that the loss of Gbb in the cortical zone (CZ) induced lamellocyte differentiation by overactivation of the EGFR and JNK pathways and caused excessive differentiation of plasmatocytes, mainly by the hyperactivation of EGFR. Furthermore, we found that Gbb was also required for preventing the hyperproliferation of the lymph glands by inhibiting the overactivation of the Epidermal Growth Factor Receptor (EGFR) and c-Jun N-terminal Kinase (JNK) pathways. These results further advance our understanding of the roles of Gbb protein and the BMP signaling in Drosophila hematopoiesis and the regulatory relationship between the BMP, EGFR, and JNK pathways in the proliferation and differentiation of lymph gland hemocytes.

The NDNF-like factor Nord is a Hedgehog-induced extracellular BMP modulator that regulates Drosophila wing patterning and growth

Hedgehog (Hh) and Bone Morphogenetic Proteins (BMPs) pattern the developing Drosophila wing by functioning as short- and long-range morphogens, respectively. Here, we show that a previously unknown Hh-dependent mechanism fine-tunes the activity of BMPs. Through genome-wide expression profiling of the Drosophila wing imaginal discs, we identify nord as a novel target gene of the Hh signaling pathway. Nord is related to the vertebrate Neuron-Derived Neurotrophic Factor (NDNF) involved in congenital hypogonadotropic hypogonadism and several types of cancer. Loss- and gain-of-function analyses implicate Nord in the regulation of wing growth and proper crossvein patterning. At the molecular level, we present biochemical evidence that Nord is a secreted BMP-binding protein and localizes to the extracellular matrix. Nord binds to Decapentaplegic (Dpp) or the heterodimer Dpp-Glass-bottom boat (Gbb) to modulate their release and activity. Furthermore, we demonstrate that Nord is a dosage-dependent BMP modulator, where low levels of Nord promote and high levels inhibit BMP signaling. Taken together, we propose that Hh-induced Nord expression fine-tunes both the range and strength of BMP signaling in the developing Drosophila wing.

Striking parallels between dorsoventral patterning in Drosophila and Gryllus reveal a complex evolutionary history behind a model gene regulatory network

Dorsoventral pattering relies on Toll and BMP signalling in all insects studied so far, with variations in the relative contributions of both pathways. Drosophila and the beetle Tribolium share extensive dependence on Toll, while representatives of more distantly related lineages like the wasp Nasonia and bug Oncopeltus rely more strongly on BMP signalling. Here, we show that in the cricket Gryllus bimaculatus, an evolutionarily distant outgroup, Toll has, like in Drosophila, a direct patterning role for the ventral half of the embryo. In addition, Toll polarises BMP signalling, although this does not involve the conserved BMP inhibitor Sog/Chordin. Finally, Toll activation relies on ovarian patterning mechanisms with striking similarity to Drosophila. Our data suggest two surprising hypotheses: (1) that Toll's patterning function in Gryllus and Drosophila is the result of convergent evolution or (2) a Drosophila-like system arose early in insect evolution and was extensively altered in multiple independent lineages.

Amyloid-like Assembly Activates a Phosphatase in the Developing Drosophila Embryo

Prion-like proteins can assume distinct conformational and physical states in the same cell. Sequence analysis suggests that prion-like proteins are prevalent in various species; however, it remains unclear what functional space they occupy in multicellular organisms. Here, we report the identification of a prion-like protein, Herzog (CG5830), through a multimodal screen in Drosophila melanogaster. Herzog functions as a membrane-associated phosphatase and controls embryonic patterning, likely being involved in TGF-β/BMP and FGF/EGF signaling pathways. Remarkably, monomeric Herzog is enzymatically inactive and becomes active upon amyloid-like assembly. The prion-like domain of Herzog is necessary for both its assembly and membrane targeting. Removal of the prion-like domain impairs activity, while restoring assembly on the membrane using a heterologous prion-like domain and membrane-targeting motif can restore phosphatase activity. This study provides an example of a prion-like domain that allows an enzyme to gain essential functionality via amyloid-like assembly to control animal development.

Does the bone morphogenetic protein 7 inhibit oocyte maturation by autocrine/paracrine in mud crab?

A bone morphogenetic protein ligand (BMP7) and its two receptors (BMPRIB and BMPRII) were recently cloned and characterized in the mud crab, Scylla paramamosain. However specific functions of BMP7 and the mechanistic pathways regulating its function are largely unidentified. In the present study, we separated oocytes and follicle cells from the ovarian explants of S. paramamosain. Subsequent analysis using semi-quantitative PCR demonstrated that the mRNA of Sp-BMP7 was exclusively expressed in follicle cells while Sp-BMPRs were expressed in both oocytes and follicle cells. In vitro experiments further showed that the mRNA and protein levels of Cyclin B increased but Sp-BMP7 declined in 17α, 20β-Dihydroxyprogesterone (DHP)-induced oocytes. Furthermore, the inhibitory effects of Sp-BMP7 were not affected by the elimination of the contact/gap junction-mediated communication between oocytes and follicle cells. Our data indicate that BMP7 may play a role in the suppression of DHP-induced oocyte maturation by affecting autocrine/paracrine pathways in S. paramamosain.

bantam microRNA is a negative regulator of the Drosophila decapentaplegic pathway

Decapentaplegic (Dpp), the Drosophila homolog of the vertebrate bone morphogenetic protein (BMP2/4), is crucial for patterning and growth in many developmental contexts. The Dpp pathway is regulated at many different levels to exquisitely control its activity. We show that bantam (ban), a microRNA, modulates Dpp signaling activity. Over expression of ban decreases phosphorylated Mothers against decapentaplegic (Mad) levels and negatively affects Dpp pathway transcriptional target genes, while null mutant clones of ban upregulate the pathway. We provide evidence that dpp upregulates ban in the wing imaginal disc, and attenuation of Dpp signaling results in a reduction of ban expression, showing that they function in a feedback loop. Furthermore, we show that this feedback loop is important for maintaining anterior-posterior compartment boundary stability in the wing disc through regulation of optomotor blind (omb), a known target of the pathway. Our results support a model that ban functions with dpp in a negative feedback loop.

In locus analysis of patterning evolution of the BMP type II receptor Wishful thinking

Proper tissue patterning is an essential step during organ formation. During this process, genes are expressed in distinct patterns, defining boundaries for future functional domains. The bone morphogenetic protein (BMP) signaling pathway sets the anterior domain during eggshell patterning. Previously, the Drosophila melanogaster homolog of BMPR2, Wishful thinking (WIT), was shown to be required for BMP signaling and patterning during eggshell formation. Expressed in a conserved anterior pattern, the width of wit patterning in the follicular epithelium is evolutionarily divergent between Drosophila species. We used genome editing to demonstrate how the gene pattern divergence is controlled in cis within the wit locus of D. virilis Furthermore, unlike direct targets of BMP signaling, we demonstrate how one transcription factor binding site shapes the pattern of WIT in D. melanogaster by negative regulation. However, changes in this site are not sufficient to explain the evolution of wit patterning, suggesting that a positive regulatory element that controls pattern divergence remains to be discovered.

Drosophila as a Model to Study the Link between Metabolism and Cancer

Cellular metabolism has recently been recognized as a hallmark of cancer. Investigating the origin and effects of the reprogrammed metabolism of tumor cells, and identifying its genetic mediators, will improve our understanding of how these changes contribute to disease progression and may suggest new approaches to therapy. Drosophila melanogaster is emerging as a valuable model to study multiple aspects of tumor formation and malignant transformation. In this review, we discuss the use of Drosophila as model to study how changes in cellular metabolism, as well as metabolic disease, contribute to cancer.

TGFβ/Activin signalling is required for ribosome biogenesis and cell growth in Drosophila salivary glands

Signalling by TGFβ superfamily factors plays an important role in tissue growth and cell proliferation. In Drosophila, the activity of the TGFβ/Activin signalling branch has been linked to the regulation of cell growth and proliferation, but the cellular and molecular basis for these functions are not fully understood. In this study, we show that both the RII receptor Punt (Put) and the R-Smad Smad2 are strongly required for cell and tissue growth. Knocking down the expression of Put or Smad2 in salivary glands causes alterations in nucleolar structure and functions. Cells with decreased TGFβ/Activin signalling accumulate intermediate pre-rRNA transcripts containing internal transcribed spacer 1 regions accompanied by the nucleolar retention of ribosomal proteins. Thus, our results show that TGFβ/Activin signalling is required for ribosomal biogenesis, a key aspect of cellular growth control. Importantly, overexpression of Put enhanced cell growth induced by Drosophila Myc, a well-characterized inducer of nucleolar hypertrophy and ribosome biogenesis.

Non-canonical dorsoventral patterning in the moth midge Clogmia albipunctata

Background

Bone morphogenetic proteins (BMPs) are of central importance for dorsal–ventral (DV) axis specification. They are core components of a signalling cascade that includes the BMP ligand decapentaplegic (DPP) and its antagonist short gastrulation (SOG) in Drosophila melanogaster. These components are very ancient, with orthologs involved in DV patterning in both protostomes and deuterostomes. Despite such strong conservation, recent comparative work in insects has revealed interesting differences in the way the patterning function of the DV system is achieved in different species.

Results

In this paper, we characterise the expression patterns of the principal components of the BMP DV patterning system, as well as its signalling outputs and downstream targets, in the non-cyclorrhaphan moth midge Clogmia albipunctata (Diptera: Psychodidae). We previously reported ventral expression patterns of dpp in the pole regions of C. albipunctata blastoderm embryos. Strikingly, we also find ventral sog and posteriorly restricted tkv expression, as well as expanded polar activity of pMad. We use our results from gene knock-down by embryonic RNA interference to propose a mechanism of polar morphogen shuttling in C. albipunctata. We compare these results to available data from other species and discuss scenarios for the evolution of DV signalling in the holometabolan insects.

Conclusions

A comparison of gene expression patterns across hemipteran and holometabolan insects reveals that expression of upstream signalling factors in the DV system is very variable, while signalling output is highly conserved. This has two major implications: first, as long as ligand shuttling and other upstream regulatory mechanisms lead to an appropriately localised activation of BMP signalling at the dorsal midline, it is of less importance exactly where the upstream components of the DV system are expressed. This, in turn, explains why the early-acting components of the DV patterning system in insects exhibit extensive amounts of developmental systems drift constrained by highly conserved downstream signalling output.

Recent advances in understanding contextual TGFβ signaling

The appearance of the first animal species on earth coincides with the emergence of transforming growth factor β (TGFβ) pathways. The evolution of these animals into more complex organisms coincides with a progressively increased TGFβ repertoire through gene duplications and divergence, making secreted TGFβ molecules the largest family of morphogenetic proteins in humans. It is therefore not surprising that TGFβ pathways govern numerous aspects of human biology from early embryonic development to regeneration, hematopoiesis, neurogenesis, and immunity. Such heavy reliance on these pathways is reflected in the susceptibility to minor perturbations in pathway components that can lead to dysregulated signaling and a diverse range of human pathologies such as cancer, fibrosis, and developmental disorders. Attempts to comprehensively resolve these signaling cascades are complicated by the long-recognized paradoxical role the pathway plays in cell biology. Recently, several groups have probed examples of the disparate aspects of TGFβ biology in a variety of animal models and uncovered novel context-dependent regulatory mechanisms. Here, we briefly review recent advancements and discuss their overall impact in directing future TGFβ research.

The protein kinase LKB1 negatively regulates bone morphogenetic protein receptor signaling

The protein kinase LKB1 regulates cell metabolism and growth and is implicated in intestinal and lung cancer. Bone morphogenetic protein (BMP) signaling regulates cell differentiation during development and tissue homeostasis. We demonstrate that LKB1 physically interacts with BMP type I receptors and requires Smad7 to promote downregulation of the receptor. Accordingly, LKB1 suppresses BMP-induced osteoblast differentiation and affects BMP signaling in Drosophila wing longitudinal vein morphogenesis. LKB1 protein expression and Smad1 phosphorylation analysis in a cohort of non-small cell lung cancer patients demonstrated a negative correlation predominantly in a subset enriched in adenocarcinomas. Lung cancer patient data analysis indicated strong correlation between LKB1 loss-of-function mutations and high BMP2 expression, and these two events further correlated with expression of a gene subset functionally linked to apoptosis and migration. This new mechanism of BMP receptor regulation by LKB1 has ramifications in physiological organogenesis and disease.

High fat diet-induced TGF-β/Gbb signaling provokes insulin resistance through the tribbles expression

Hyperglycemia, hyperlipidemia, and insulin resistance are hallmarks of obesity-induced type 2 diabetes, which is often caused by a high-fat diet (HFD). However, the molecular mechanisms underlying HFD-induced insulin resistance have not been elucidated in detail. In this study, we established a Drosophila model to investigate the molecular mechanisms of HFD-induced diabetes. HFD model flies recapitulate mammalian diabetic phenotypes including elevated triglyceride and circulating glucose levels, as well as insulin resistance. Expression of glass bottom boat (gbb), a Drosophila homolog of mammalian transforming growth factor-β (TGF-β), is elevated under HFD conditions. Furthermore, overexpression of gbb in the fat body produced obese and insulin-resistant phenotypes similar to those of HFD-fed flies, whereas inhibition of Gbb signaling significantly ameliorated HFD-induced metabolic phenotypes. We also discovered that tribbles, a negative regulator of AKT, is a target gene of Gbb signaling in the fat body. Overexpression of tribbles in flies in the fat body phenocopied the metabolic defects associated with HFD conditions or Gbb overexpression, whereas tribbles knockdown rescued these metabolic phenotypes. These results indicate that HFD-induced TGF-β/Gbb signaling provokes insulin resistance by increasing tribbles expression.

Activins and Inhibins: Roles in Development, Physiology, and Disease

Since their original discovery as regulators of follicle-stimulating hormone (FSH) secretion and erythropoiesis, the TGF-β family members activin and inhibin have been shown to participate in a variety of biological processes, from the earliest stages of embryonic development to highly specialized functions in terminally differentiated cells and tissues. Herein, we present the history, structures, signaling mechanisms, regulation, and biological processes in which activins and inhibins participate, including several recently discovered biological activities and functional antagonists. The potential therapeutic relevance of these advances is also discussed.

Ancient and diverged TGF-β signaling components in Nasonia vitripennis

The transforming growth factor beta (TGF)-β signaling pathway and its modulators are involved in many aspects of cellular growth and differentiation in all metazoa. Although most of the core components of the pathway are highly conserved, many lineage-specific adaptations have been observed including changes regarding paralog number, presence and absence of modulators, and functional relevance for particular processes. In the parasitic jewel wasp Nasonia vitripennis, the bone morphogenetic proteins (BMPs), one of the major subgroups of the TGF-β superfamily, play a more fundamental role in dorsoventral (DV) patterning than in all other insects studied so far. However, Nasonia lacks the BMP antagonist Short gastrulation (Sog)/chordin, which is essential for polarizing the BMP gradient along the DV axis in most bilaterian animals. Here, we present a broad survey of TGF-β signaling in Nasonia with the aim to detect other lineage-specific peculiarities and to identify potential mechanisms, which explain how BMP-dependent DV pattering occurs in the early Nasonia embryo in the absence of Sog.

Dpp/BMP signaling in flies: from molecules to biology

Decapentaplegic (Dpp), the fly homolog of the secreted mammalian BMP2/4 signaling molecules, is involved in almost all aspects of fly development. Dpp has critical functions at all developmental stages, from patterning of the eggshell to the determination of adult intestinal stem cell identity. Here, we focus on recent findings regarding the transcriptional regulatory logic of the pathway, on a new feedback regulator, Pentagone, and on Dpp's roles in scaling and growth of the Drosophila wing.

Systemic Activin signaling independently regulates sugar homeostasis, cellular metabolism, and pH balance in Drosophila melanogaster

The ability to maintain cellular and physiological metabolic homeostasis is key for the survival of multicellular organisms in changing environmental conditions. However, our understanding of extracellular signaling pathways that modulate metabolic processes remains limited. In this study we show that the Activin-like ligand Dawdle (Daw) is a major regulator of systemic metabolic homeostasis and cellular metabolism in Drosophila. We find that loss of canonical Smad signaling downstream of Daw leads to defects in sugar and systemic pH homeostasis. Although Daw regulates sugar homeostasis by positively influencing insulin release, we find that the effect of Daw on pH balance is independent of its role in insulin signaling and is caused by accumulation of organic acids that are primarily tricarboxylic acid (TCA) cycle intermediates. RNA sequencing reveals that a number of TCA cycle enzymes and nuclear-encoded mitochondrial genes including genes involved in oxidative phosphorylation and β-oxidation are up-regulated in the daw mutants, indicating either a direct or indirect role of Daw in regulating these genes. These findings establish Activin signaling as a major metabolic regulator and uncover a functional link between TGF-β signaling, insulin signaling, and metabolism in Drosophila.

Comparative gene expression analysis of Dtg, a novel target gene of Dpp signaling pathway in the early Drosophila melanogaster embryo

In the early Drosophila melanogaster embryo, Dpp, a secreted molecule that belongs to the TGF-β superfamily of growth factors, activates a set of downstream genes to subdivide the dorsal region into amnioserosa and dorsal epidermis. Here, we examined the expression pattern and transcriptional regulation of Dtg, a new target gene of Dpp signaling pathway that is required for proper amnioserosa differentiation. We showed that the expression of Dtg was controlled by Dpp and characterized a 524-bp enhancer that mediated expression in the dorsal midline, as well as, in the differentiated amnioserosa in transgenic reporter embryos. This enhancer contained a highly conserved region of 48-bp in which bioinformatic predictions and in vitro assays identified three Mad binding motifs. Mutational analysis revealed that these three motifs were necessary for proper expression of a reporter gene in transgenic embryos, suggesting that short and highly conserved genomic sequences may be indicative of functional regulatory regions in D. melanogaster genes. Dtg orthologs were not detected in basal lineages of Dipterans, which unlike D. melanogaster develop two extra-embryonic membranes, amnion and serosa, nevertheless Dtg orthologs were identified in the transcriptome of Musca domestica, in which dorsal ectoderm patterning leads to the formation of a single extra-embryonic membrane. These results suggest that Dtg was recruited as a new component of the network that controls dorsal ectoderm patterning in the lineage leading to higher Cyclorrhaphan flies, such as D. melanogaster and M. domestica.

A feed-forward loop coupling extracellular BMP transport and morphogenesis in Drosophila wing

A variety of extracellular factors regulate morphogenesis during development. However, coordination between extracellular signaling and dynamic morphogenesis is largely unexplored. We address the fundamental question by studying posterior crossvein (PCV) development in Drosophila as a model, in which long-range BMP transport from the longitudinal veins plays a critical role during the pupal stages. Here, we show that RhoGAP Crossveinless-C (Cv-C) is induced at the PCV primordial cells by BMP signaling and mediates PCV morphogenesis cell-autonomously by inactivating members of the Rho-type small GTPases. Intriguingly, we find that Cv-C is also required non-cell-autonomously for BMP transport into the PCV region, while a long-range BMP transport is guided toward ectopic wing vein regions by loss of the Rho-type small GTPases. We present evidence that low level of ß-integrin accumulation at the basal side of PCV epithelial cells regulated by Cv-C provides an optimal extracellular environment for guiding BMP transport. These data suggest that BMP transport and PCV morphogenesis are tightly coupled. Our study reveals a feed-forward mechanism that coordinates the spatial distribution of extracellular instructive cues and morphogenesis. The coupling mechanism may be widely utilized to achieve precise morphogenesis during development and homeostasis.

It has been extensively studied how tissue morphogenesis is regulated by a variety of extracellular cues. Given that dynamic morphogenesis coincides with arrival of extracellular factors, there must be also mechanisms that coordinate extracellular signaling and intracellular morphogenesis. However, the coordination is largely unknown, due to the complexity of morphogenesis in vivo. We addressed the fundamental question by studying posterior crossvein (PCV) development in Drosophila as a model, in which a long-range transport of bone morphogenetic protein (BMP) type ligands from adjacent longitudinal veins plays a critical role during the pupal stages. Here, we first showed that RhoGAP Crossveinless-C (Cv-C) is induced at the PCV region by BMP signal and mediates PCV morphogenesis. By modulating wing vein morphogenesis, we then found that PCV morphogenesis is required for BMP transport, while ectopic wing vein morphogenesis sufficiently guides a long-range BMP transport. These data suggest a feed-forward mechanism that coordinates the spatial distribution of extracellular instructive cues and morphogenesis. The coupling mechanism may be widely utilized to achieve precise tissue morphogenesis and tissue homeostasis.

Activation and function of TGFβ signalling during Drosophila wing development and its interactions with the BMP pathway

The development of the Drosophila wing disc requires the activities of the BMP and TGFβ signalling pathways. BMP signalling is critical for the correct growth and patterning of the disc, whereas the related TGFβ pathway is mostly required for growth. The BMP and TGFβ pathways share a common co-receptor (Punt) and a nuclear effector (Medea), and consequently it is likely that these pathways can interfere with each other during normal development. In this work we focus on the spatial activation domains and requirements for TGFβ signalling during wing disc development. We found that the phosphorylation of Smad2, the specific transducer for TGFβ signalling, occurs in a generalised manner in the wing disc. It appears that the expression of the four candidate TGFβ ligands (Activinβ, Dawdle, Maverick and Myoglianin) in the wing disc is required to obtain normal levels of TGFβ signalling in this tissue. We show that Baboon, the specific receptor of the TGFβ pathway, can phosphorylate Mad, the specific transducer of the BMP pathway, in vivo. However, this activation only occurs in the wing disc when the receptor is constitutively activated in a background of reduced expression of Smad2. In the presence of Smad2, the normal situation during wing disc development, high levels of activated Baboon lead to a depletion in Mad phosphorylation and to BMP loss-of-function phenotypes. Although loss of either babo or Smad2 expression reduce growth in the wing blade in a similar manner, loss of Smad2 can also cause phenotypes related to ectopic BMP signalling, suggesting a physiological role for this transducer in the regulation of Mad spatial activation.

The Drosophila BMPRII, wishful thinking, is required for eggshell patterning

The Drosophila eggshell is an elaborate structure that is derived from a monolayer of follicular epithelium surrounding the developing oocyte within the female ovary. The bone morphogenetic protein (BMP) signaling pathway is essential for controlling the patterning and morphogenesis of the eggshell. During oogenesis, the roles of patterning and morphogenesis by the BMP type I receptor thickveins (tkv) have been studied extensively. However, signaling through this pathway requires both type I and II receptors, and the latter has yet to be established in oogenesis. We focus on wishful thinking (wit), the Drosophila homolog to the mammalian BMP type II receptor, BMPRII. We found that wit is expressed dynamically in the FCs of D. melanogaster in an evolutionary conserved pattern. The expression patterns are highly correlated with the dynamics of the BMP signaling, which is consistent with our finding that wit is a target of BMP signaling. Furthermore, we established that WIT is necessary for BMP signaling, and loss of WIT is associated with cell autonomous loss of BMP responses. Of importance, we demonstrated that perturbations in WIT led to changes in eggshell morphologies in domains that are patterned by BMP signaling. Previous studies have shown a role for WIT in BMP signaling during neurogenesis; however, our results reveal a role for WIT in epithelial cells' development.

Evolution of BMP signaling in Drosophila oogenesis: a receptor-based mechanism

The bone morphogenetic protein (BMP) signaling pathway is a conserved regulator of cellular and developmental processes in animals. The mechanisms underlying BMP signaling activation differ among tissues and mostly reflect changes in the expression of pathway components. BMP signaling is one of the major pathways responsible for the patterning of the Drosophila eggshell, a complex structure derived from a layer of follicle cells (FCs) surrounding the developing oocyte. Activation of BMP signaling in the FCs is dynamic. Initially, signaling is along the anterior-posterior (A/P) axis; later, signaling acquires dorsal-ventral (D/V) polarity. These dynamics are regulated by changes in the expression pattern of the type I BMP receptor thickveins (tkv). We recently found that signaling dynamics and TKV patterning are highly correlated in the FCs of multiple Drosophila species. In addition, we showed that signaling patterns are spatially different among species. Here, we use a mathematical model to simulate the dynamics and differences of BMP signaling in numerous species. This model predicts that qualitative and quantitative changes in receptor expression can lead to differences in the spatial pattern of BMP signaling. We tested these predications experimentally in three different Drosophila species and through genetic perturbations of BMP signaling in D. melanogaster. On the basis of our results, we concluded that changes in tkv patterning can account for the experimentally observed differences in the patterns of BMP signaling in multiple Drosophila species.

fussel (fuss)--A negative regulator of BMP signaling in Drosophila melanogaster

The TGF-β/BMP signaling cascades control a wide range of developmental and physiological functions in vertebrates and invertebrates. In Drosophila melanogaster, members of this pathway can be divided into a Bone Morphogenic Protein (BMP) and an Activin-ß (Act-ß) branch, where Decapentaplegic (Dpp), a member of the BMP family has been most intensively studied. They differ in ligands, receptors and transmitting proteins, but also share some components, such as the Co-Smad Medea (Med). The essential role of Med is to form a complex with one of the two activating Smads, mothers against decapentaplegic (Mad) or dSmad, and to translocate together to the nucleus where they can function as transcriptional regulators of downstream target genes. This signaling cascade underlies different mechanisms of negative regulation, which can be exerted by inhibitory Smads, such as daughters against decapentaplegic (dad), but also by the Ski-Sno family. In this work we identified and functionally analyzed a new member of the Ski/Sno-family, fussel (fuss), the Drosophila homolog of the human functional suppressing element 15 (fussel-15). fuss codes for two differentially spliced transcripts with a neuronal expression pattern. The proteins are characterized by a Ski-Sno and a SAND homology domain. Overexpression studies and genetic interaction experiments clearly reveal an interaction of fuss with members of the BMP pathway, leading to a strong repression of BMP-signaling. The protein interacts directly with Medea and seems to reprogram the Smad pathway through its influence upon the formation of functional Mad/Medea complexes. This leads amongst others to a repression of downstream target genes of the Dpp pathway, such as optomotor blind (omb). Taken together we could show that fuss exerts a pivotal role as an antagonist of BMP signaling in Drosophila melanogaster.

Hyperactive BMP signaling induced by ALK2(R206H) requires type II receptor function in a Drosophila model for classic fibrodysplasia ossificans progressiva

BACKGROUND

Fibrodysplasia Ossificans Progressiva (FOP) is an autosomal dominant disorder characterized by episodic deposition of heterotopic bone in place of soft connective tissue. All FOP-associated mutations map to the BMP type I receptor, ALK2, with the ALK2(R206H) mutant form found in the vast majority of patients. The mechanism(s) regulating the expressivity of hyperactive ALK2(R206H) signaling throughout a patient's life is not well understood.

RESULTS

In Drosophila, human ALK2(R206H) receptor induces hyperactive BMP signaling. As in vertebrates, elevated signaling associated with ALK2(R206H) in Drosophila is ligand-independent. We found that a key determinant for ALK2(R206H) hyperactivity is a functional type II receptor. Furthermore, our results indicate that like its Drosophila ortholog, Saxophone (Sax), wild-type ALK2 can antagonize, as well as promote, BMP signaling.

CONCLUSIONS

The dual function of ALK2 is of particular interest given the heterozygous nature of FOP, as the normal interplay between such disparate behaviors could be shifted by the presence of ALK2(R206H) receptors. Our studies provide a compelling example for Drosophila as a model organism to study the molecular underpinnings of complex human syndromes such as FOP.

The signaling and functions of heterodimeric bone morphogenetic proteins

Heterodimeric bone morphogenetic proteins (BMPs) consist of disulfide-linked dimeric monomers derived from different BMP members. Owing to this specific constitution pattern, they bear high affinity to both type I and type II BMP receptors simultaneously. Meanwhile, the antagonism efficiency of extracellular antagonists to heterodimeric BMPs is also significantly lower than that to homodimeric ones. All these specific properties confer heterodimeric BMPs with distinct signaling and bio-functions that are characterized by more speediness, lower concentration/dose threshold and higher efficiency than homodimeric BMPs. Consequently, heterodimeric BMPs bear promising application potential in inducing osteogenesis. In addition, they may play indispensible roles in organogenesis. In this review, we summarize the current knowledge of heterodimeric BMPs in their signaling pathways and bio-functions.

Drosophila TIEG is a modulator of different signalling pathways involved in wing patterning and cell proliferation

Acquisition of a final shape and size during organ development requires a regulated program of growth and patterning controlled by a complex genetic network of signalling molecules that must be coordinated to provide positional information to each cell within the corresponding organ or tissue. The mechanism by which all these signals are coordinated to yield a final response is not well understood. Here, I have characterized the Drosophila ortholog of the human TGF-β Inducible Early Gene 1 (dTIEG). TIEG are zinc-finger proteins that belong to the Krüppel-like factor (KLF) family and were initially identified in human osteoblasts and pancreatic tumor cells for the ability to enhance TGF-β response. Using the developing wing of Drosophila as “in vivo” model, the dTIEG function has been studied in the control of cell proliferation and patterning. These results show that dTIEG can modulate Dpp signalling. Furthermore, dTIEG also regulates the activity of JAK/STAT pathway suggesting a conserved role of TIEG proteins as positive regulators of TGF-β signalling and as mediators of the crosstalk between signalling pathways acting in a same cellular context.

The interaction between two TGF-beta type I receptors plays important roles in ligand binding, SMAD activation, and gradient formation

The goal of this report is to elucidate the contributions of the Drosophila TGF-beta type I receptors TKV and SAX to the activity gradient formed by the two BMP family members DPP and GBB that play important roles in growth and patterning of imaginal discs. Binding studies display preferential interactions of DPP and GBB with homodimers of TKV or SAX, respectively, but also low affinities of both ligands to heterodimers. Inside the cell, constitutively activated forms of both TKV and SAX can ectopically phosphorylate the SMAD transcription factor MAD. However, MAD phosphorylated by homodimers of activated SAX or certain mutant forms of TKV localizes to the nucleus without changing the expression of downstream genes. Differences in signaling between SAX and TKV can be localized to amino acid residues within an area that has been shown to influence complexes formation between type I and type II receptors. The finding that the type II receptor PUT but not activated forms of SAX can enhance signaling of a pseudo-activated MAD-SDVD, which cannot be phosphorylated at the C-terminus, suggests a model, where activation of SMADs requires the presence of type II receptors and a second activation step in addition to C-terminal phosphorylation. Complete activation of MAD can only occur in tetrameric complexes of type II receptors in combination with SAX-TKV heterodimers or TKV homodimers but not SAX homodimers. Since TKV is not distributed equally in wing discs, heterodimers of SAX and TKV play an important role in extending the BMP activity gradient by facilitating DPP diffusion and assisting GBB signaling through functional complexes with type II receptors.

Intricacies of BMP receptor assembly

The TGF-beta superfamily exhibits a feature making it distinct from many other growth factor families in that the inadequate number of ligands and receptors premises a high degree of promiscuity in ligand-receptor interaction. This highlights the importance of even small differences in affinities and specificities between different binding partners to maintain the broad spectrum of their well defined biological functions. Despite the promiscuous interactions recent data reveal differences in receptor recruitment, architectures of these assemblies and specific modulation by a multitude of extracellular as well as membrane-associated factors. These modulatory mechanisms might possibly add specificity towards defined biological functions despite the overlapping usage of receptors by various ligands.

The Drosophila Activin-like ligand Dawdle signals preferentially through one isoform of the Type-I receptor Baboon

How TGF-beta-type ligands achieve signaling specificity during development is only partially understood. Here, we show that Dawdle, one of four Activin-type ligands in Drosophila, preferentially signals through Babo(c), one of three isoforms of the Activin Type-I receptor that are expressed during development. In cell culture, Dawdle signaling is active in the presence of the Type-II receptor Punt but not Wit, demonstrating that the Type-II receptor also contributes to the specificity of the signaling complex. During development, different larval tissues express unique combinations of these receptors, and ectopic expression of Babo(c) in a tissue where it is not normally expressed at high levels can make that tissue sensitive to Dawdle signaling. These results reveal a mechanism by which distinct cell types can discriminate between different Activin-type signals during development as a result of differential expression of Type-I receptor isoforms.

Multiple modular promoter elements drive graded brinker expression in response to the Dpp morphogen gradient

Morphogen gradients play fundamental roles in patterning and cell specification during development by eliciting differential transcriptional responses in target cells. In Drosophila, Decapentaplegic (Dpp), the BMP2/4 homolog, downregulates transcription of the nuclear repressor brinker (brk) in a concentration-dependent manner to generate an inverse graded distribution. Both Dpp and Brk are crucial for directing Dpp target gene expression in defined domains and the consequent execution of distinct developmental programs. Thus, determining the mechanism by which the brk promoter interprets the Dpp activity gradient is essential for understanding both Dpp-dependent patterning and how graded signaling activity can generate different responses through transcriptional repression. We have uncovered key features of the brk promoter that suggest it uses a complex enhancer logic not represented in current models. First, we find that the regulatory region contains multiple compact modules that can independently drive brk-like expression patterns. Second, each module contains binding sites for the Schnurri/Mad/Medea (SMM) complex, which mediates Dpp-dependent repression, linked to regions that direct activation. Third, the SMM repression complex acts through a distance-dependent mechanism that probably uses the canonical co-repressor C-terminal Binding Protein (CtBP). Finally, our data suggest that inputs from multiple regulatory modules are integrated to generate the final pattern. This unusual promoter organization may be necessary for brk to respond to the Dpp gradient in a precise and robust fashion.

TGFbeta signaling in Tribolium: vertebrate-like components in a beetle

The cytokines of the TGFbeta superfamily are highly conserved in evolution and elicit a diverse range of cellular responses in all metazoa. In Drosophila, the signaling pathways of the two TGFbeta subfamilies, Activins and Bone Morphogenetic Proteins (BMPs), have been well studied. To address the question of whether the findings from Drosophila are representative of insects in general, we analyzed the components of TGFbeta-signaling present in the genome of the beetle Tribolium castaneum. We were able to identify orthologs of the BMPs Decapentaplegic and Glass bottom boat, of the Activins Activinbeta and Dawdle, as well as orthologs of the less well-known ligands Myoglianin and Maverick, together with orthologs of all TGFbeta receptors and cytoplasmic signal transducers present in Drosophila. This indicates that the diversity of TGFbeta signaling components is generally well conserved between Drosophila and Tribolium. However, the genome of the beetle-and of the bee Apis mellifera-lacks an ortholog of the Drosophila BMP Screw but does contain a vertebrate-like BMP10 homolog which is not found in Drosophila. Concerning BMP inhibitors, Tribolium displays an even more vertebrate-like ensemble of components. We found two orthologs of the vertebrate DAN family, Dan and Gremlin, and show embryonic expression of a vertebrate-like BAMBI ortholog, all of which are absent in Drosophila. This suggests that Tribolium might have retained a more ancestral composition of TGFbeta signaling components and that TGFbeta signaling underwent considerable change in the Drosophila lineage. Tribolium is an excellent model to study the function of these ancestral signaling components in insects.

Tiling of r7 axons in the Drosophila visual system is mediated both by transduction of an activin signal to the nucleus and by mutual repulsion

The organization of neuronal wiring into layers and columns is a common feature of both vertebrate and invertebrate brains. In the Drosophila visual system, each R7 photoreceptor axon projects within a single column to a specific layer of the optic lobe. We refer to the restriction of terminals to single columns as tiling. In a genetic screen based on an R7-dependent behavior, we identified the Activin receptor Baboon and the nuclear import adaptor Importin-alpha3 as being required to prevent R7 axon terminals from overlapping with the terminals of R7s in neighboring columns. This tiling function requires the Baboon ligand, dActivin, the transcription factor, dSmad2, and retrograde transport from the growth cone to the R7 nucleus. We propose that dActivin is an autocrine signal that restricts R7 growth cone motility, and we demonstrate that it acts in parallel with a paracrine signal that mediates repulsion between R7 terminals.

Drosophila Activin- and the Activin-like product Dawdle function redundantly to regulate proliferation in the larval brain

The Drosophila Activin-like ligands Activin-beta and Dawdle control several aspects of neuronal morphogenesis, including mushroom body remodeling, dorsal neuron morphogenesis and motoneuron axon guidance. Here we show that the same two ligands act redundantly through the Activin receptor Babo and its transcriptional mediator Smad2 (Smox), to regulate neuroblast numbers and proliferation rates in the developing larval brain. Blocking this pathway results in the development of larvae with small brains and aberrant photoreceptor axon targeting, and restoring babo function in neuroblasts rescued these mutant phenotypes. These results suggest that the Activin signaling pathway is required for producing the proper number of neurons to enable normal connection of incoming photoreceptor axons to their targets. Furthermore, as the Activin pathway plays a key role in regulating propagation of mouse and human embryonic stem cells, our observation that it also regulates neuroblast numbers and proliferation in Drosophila suggests that involvement of Activins in controlling stem cell propagation may be a common regulatory feature of this family of TGF-beta-type ligands.

A silent H-bond can be mutationally activated for high-affinity interaction of BMP-2 and activin type IIB receptor

Background

Bone morphogenetic proteins (BMPs) are key regulators in the embryonic development and postnatal tissue homeostasis in all animals. Loss of function or dysregulation of BMPs results in severe diseases or even lethality. Like transforming growth factors β (TGF-βs), activins, growth and differentiation factors (GDFs) and other members of the TGF-β superfamily, BMPs signal by assembling two types of serine/threonine-kinase receptor chains to form a hetero-oligomeric ligand-receptor complex. BMP ligand receptor interaction is highly promiscuous, i.e. BMPs bind more than one receptor of each subtype, and a receptor bind various ligands. The activin type II receptors are of particular interest, since they bind a large number of diverse ligands. In addition they act as high-affinity receptors for activins but are also low-affinity receptors for BMPs. ActR-II and ActR-IIB therefore represent an interesting example how affinity and specificity might be generated in a promiscuous background.

Results

Here we present the high-resolution structures of the ternary complexes of wildtype and a variant BMP-2 bound to its high-affinity type I receptor BMPR-IA and its low-affinity type II receptor ActR-IIB and compare them with the known structures of binary and ternary ligand-receptor complexes of BMP-2. In contrast to activin or TGF-β3 no changes in the dimer architecture of the BMP-2 ligand occur upon complex formation. Functional analysis of the ActR-IIB binding epitope shows that hydrophobic interactions dominate in low-affinity binding of BMPs; polar interactions contribute only little to binding affinity. However, a conserved H-bond in the center of the type II ligand-receptor interface, which does not contribute to binding in the BMP-2 – ActR-IIB interaction can be mutationally activated resulting in a BMP-2 variant with high-affinity for ActR-IIB. Further mutagenesis studies were performed to elucidate the binding mechanism allowing us to construct BMP-2 variants with defined type II receptor binding properties.

Conclusion

Binding specificity of BMP-2 for its three type II receptors BMPR-II, Act-RII and ActR-IIB is encoded on single amino acid level. Exchange of only one or two residues results in BMP-2 variants with a dramatically altered type II receptor specificity profile, possibly allowing construction of BMP-2 variants that address a single type II receptor. The structure-/function studies presented here revealed a new mechanism, in which the energy contribution of a conserved H-bond is modulated by surrounding intramolecular interactions to achieve a switch between low- and high-affinity binding.

Cg-TGF-beta, a TGF-beta/activin homologue in the Pacific Oyster Crassostrea gigas, is involved in immunity against Gram-negative microbial infection

Transforming growth factor-beta (TGF-beta) members represent a widespread protein superfamily in the animal kingdom, but few members have been characterised in lophotrochozoans, a major clade of invertebrates. Here, we report the identification of Crassostrea gigas-TGF-beta (Cg-TGF-beta), a homologue of vertebrate TGF-beta and activin, from the bivalve mollusc C. gigas. Phylogenetic analysis suggests an early ancestral origin of this subgroup of TGF-beta superfamily member. Investigation of the spatio-temporal expression of Cg-TGF-beta gene by real-time quantitative RT-PCR showed a ubiquitous pattern in all adult tissues. These findings imply that Cg-TGF-beta has multiple functions as described for its vertebrate counterparts. Moreover, Cg-TGF-beta was upregulated in haemocytes during infection by a Gram-negative bacterium, suggesting that it could act as a cytokine involved in immunity in molluscs.

Drosophila SnoN modulates growth and patterning by antagonizing TGF-beta signalling

Signalling by TGF-beta ligands through the Smad family of transcription factors is critical for developmental patterning and growth. Disruption of this pathway has been observed in various cancers. In vertebrates, members of the Ski/Sno protein family can act as negative regulators of TGF-beta signalling, interfering with the Smad machinery to inhibit the transcriptional output of this pathway. In some contexts ski/sno genes function as tumour suppressors, but they were originally identified as oncogenes, whose expression is up-regulated in many tumours. These growth regulatory effects and the normal physiological functions of Ski/Sno proteins have been proposed to result from changes in TGF-beta signalling. However, this model is controversial and may be over-simplified, because recent findings indicate that Ski/Sno proteins can affect other signalling pathways. To address this issue in an in vivo context, we have analyzed the function of the Drosophila Ski/Sno orthologue, SnoN. We found that SnoN inhibits growth when overexpressed, indicating a tumour suppressor role in flies. It can act in multiple tissues to selectively and cell autonomously antagonise signalling by TGF-beta ligands from both the BMP and Activin sub-families. By contrast, analysis of a snoN mutant indicates that the gene does not play a global role in TGF-beta-mediated functions, but specifically inhibits TGF-beta-induced wing vein formation. We propose that SnoN normally functions redundantly with other TGF-beta pathway antagonists to finely adjust signalling levels, but that it can behave as an extremely potent inhibitor of TGF-beta signalling when highly expressed, highlighting the significance of its deregulation in cancer cells.

Quantitative Models of Developmental Pattern Formation

Pattern formation in developing organisms can be regulated at a variety of levels, from gene sequence to anatomy. At this level of complexity, mechanistic models of development become essential for integrating data, guiding future experiments, and predicting the effects of genetic and physical perturbations. However, the formulation and analysis of quantitative models of development are limited by high levels of uncertainty in experimental measurements, a large number of both known and unknown system components, and the multiscale nature of development. At the same time, an expanding arsenal of experimental tools can constrain models and directly test their predictions, making the modeling efforts not only necessary, but feasible. Using a number of problems in fruit fly development, we discuss how models can be used to test the feasibility of proposed patterning mechanisms and characterize their systems-level properties.

DNA Recognition by the Brinker Repressor – An Extreme Case of Coupling Between Binding and Folding

The Brinker (Brk) nuclear repressor is a major element of the Drosophila Decapentaplegic morphogen signaling pathway. Its N-terminal part has weak homology to the Antennapedia homeodomain and binds to GC-rich DNA sequences. We have investigated the conformation and dynamics of the N-terminal 101 amino acid residues of Brk in the absence and in the presence of cognate DNA by solution NMR spectroscopy. In the absence of DNA, Brk is unfolded and highly flexible throughout the entire backbone. Addition of cognate DNA induces the formation of a well-folded structure for residues R46 to R95. This structure consists of four helices forming a helix-turn-helix motif that differs from homeodomains, but has similarities to the Tc3 transposase, the Pax-6 Paired domain, and the human centromere-binding protein. The GC-rich DNA recognition can be explained by specific major groove hydrogen bonds from the N-terminal end of helix alpha3. The transition from a highly flexible, completely unfolded conformation in the absence of DNA to a well-formed structure in the complex presents a very extreme case of the "coupling of binding and folding" phenomenon.

Shaping BMP morphogen gradients in the Drosophila embryo and pupal wing

In the early Drosophila embryo, BMP-type ligands act as morphogens to suppress neural induction and to specify the formation of dorsal ectoderm and amnioserosa. Likewise, during pupal wing development, BMPs help to specify vein versus intervein cell fate. Here, we review recent data suggesting that these two processes use a related set of extracellular factors, positive feedback, and BMP heterodimer formation to achieve peak levels of signaling in spatially restricted patterns. Because these signaling pathway components are all conserved, these observations should shed light on how BMP signaling is modulated in vertebrate development.

Identification of a follistatin-related protein from the tick Haemaphysalis longicornis and its effect on tick oviposition

The identification of ovary-associated molecules will lead to a better understanding of the physiology of tick reproduction and vector-pathogen interactions. A gene encoding a follistatin-related protein (FRP) was obtained by random sequencing from the ovary cDNA library of the tick Haemaphysalis longicornis. The full-length cDNA is 1157 bp, including an intact ORF encoding an expected protein with 289 amino acids. Three distinct domains were present in the deduced amino acids, namely, the follistatin-like domain, KAZAL, and two calcium-binding motifs, EFh. The sequence shows homology with the follistatin-related protein (FRP), which was thought to play some roles in the negative regulation of cellular growth. RT-PCR showed that the gene was expressed throughout the developing stages and mainly in the ovary as well as in fat body, hemocytes, salivary glands, and midgut. This gene was expressed in GST-fused recombinant protein with an expected size. The mouse antiserum against the recombinant protein recognized a 56-kDa native protein in both tick ovary and hemolymph. The recombinant proteins were found to have binding activity for both activin A and bone morphogenetic protein-2 (BMP-2). Silencing of FRP by RNAi showed a decrease in tick oviposition, which is consistent with the effect of a recombinant protein vaccine on the adult tick. These results showed that the tick FRP might be involved in tick oviposition. This is the first report of a member of follistatin family proteins in Chelicerata, which include ticks, spiders, and scorpions.

The BMP type II receptor is located in lipid rafts, including caveolae, of pulmonary endothelium in vivo and in vitro

Polymorphic mutations in the Bone Morphogenetic Protein type II receptor (BMPrII) gene have been implicated in the development of familial primary pulmonary hypertension (PPH) however, the role BMPrII mutations play in the development of PH has not yet been elucidated. Endothelial caveolae are an important domain of hemodynamics containing eNOS, the serotonin transporter, and endothelin receptors. In this study we show by standard immunohistochemistry (IHC) that BMPrII is widely distributed in the vasculature of the rat lung, and more specifically distributed to both apical and basal membranes of the arteriolar endothelium by fluorescent IHC. We also examined compartmentalization of BMPrII in lipid fractions of plasma membranes isolated by silica based extraction from human pulmonary artery endothelial cells and rat lung endothelium. Density gradient centrifugation demonstrated BMPrII in separate caveolin-1 (cav-1) and non-cav-1 lipid rich fractions. Electron microscopy co-localized cav-1 and BMPrII in flask shaped membrane fragments. Three-dimensional fluorescence microscopy demonstrated BMPrII in discrete membrane foci, a portion of which were co-localized with cav-1, as well as in Golgi. Our findings indicate that BMPrII is located within lipid-dense fractions of pulmonary endothelial cell membranes with a portion present in caveolae suggesting potential dynamic regulatory structural relationships.

TheDrosophilatype II receptor, Wishful thinking, binds BMP and myoglianin to activate multiple TGFβ family signaling pathways

Wishful thinking (Wit) is a Drosophila transforming growth factor-beta (TGFbeta) superfamily type II receptor most related to the mammalian bone morphogenetic protein (BMP) type II receptor, BMPRII. To better understand its function, we undertook a biochemical approach to establish the ligand binding repertoire and downstream signaling pathway. We observed that BMP4 and BMP7, bound to receptor complexes comprised of Wit and the type I receptor thickveins and saxophone to activate a BMP-like signaling pathway. Further we demonstrated that both myoglianin and its most closely related mammalian ligand, myostatin, interacted with a Wit and Baboon (Babo) type II-type I receptor complex to activate TGFbeta/activin-like signaling pathways. These results thereby demonstrate that Wit binds multiple ligands to activate both BMP and TGFbeta-like signaling pathways. Given that myoglianin is expressed in muscle and glial-derived cells, these results also suggest that Wit may mediate myoglianin-dependent signals in the nervous system.

Characterization of eastern oyster (Crassostrea virginica Gmelin) chromosomes by fluorescence in situ hybridization with bacteriophage P1 clones

Chromosome identification is an essential step in genomic research, which so far has not been possible in oysters. We tested bacteriophage P1 clones for chromosomal identification in the eastern oyster Crassostrea virginica, using fluorescence in situ hybridization (FISH). P1 clones were labeled with digoxigenin-11-dUTP using nick translation. Hybridization was detected with fluorescein-isothiocyanate-labeled anti-digoxigenin antibodies and amplified with 2 layers of antibodies. Nine of the 21 P1 clones tested produced clear and consistent FISH signals when Cot-1 DNA was used as a blocking agent against repetitive sequences. Karyotypic analysis and cohybridization positively assigned the 9 P1 clones to 7 chromosomes. The remaining 3 chromosomes can be separated by size and arm ratio. Five of the 9 P1 clones were sequenced at both ends, providing sequence-tagged sites that can be used to integrate linkage and cytogenetic maps. One sequence is part of the bone morphogenetic protein type 1b receptor, a member of the transforming growth factor superfamily, and mapped to the telomeric region of the long arm of chromosome 2. This study shows that large-insert clones such as P1 are useful as chromosome-specific FISH probes and for gene mapping in oysters.