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

A collection of split-Gal4 drivers targeting conserved signaling ligands in Drosophila

Communication between cells in metazoan organisms is mediated by a remarkably small number of highly conserved signaling pathways. Given this small number of signaling pathways, the existence of multiple related ligands for many of these pathways represents a key evolutionary innovation for encoding complexity into cell-cell signaling. Relatedly, crosstalk between pathways is another critical feature, which allows a modest number of pathways to ultimately generate an enormously diverse range of outcomes. It would thus be useful to have genetic tools to identify and manipulate not only those cells that express a given signaling ligand but also those cells that specifically coexpress pairs of signaling ligands. We present a collection of split-Gal4 knock-in lines targeting many of the ligands for highly conserved signaling pathways in Drosophila (Notch, Hedgehog, fibroblast growth factor (FGF), epidermal growth factor (EGF), transforming growth factor β (TGFβ), Janus kinase/signal transducer and activator of transcription (JAK/STAT), Jun kinase (JNK), and platelet-derived growth factor (PDGF)/vascular endothelial growth factor (VEGF)-related receptor (PVR). We demonstrate that these lines faithfully recapitulate the endogenous expression pattern of their targets and that they can be used to identify cells and tissues that coexpress pairs of ligands. As a proof of principle, we demonstrate that the 4th chromosome TGFβ ligands myoglianin and maverick are broadly coexpressed in muscles and other tissues of both larva and adults and that the JAK/STAT ligands upd2 and upd3 are partially coexpressed from cells of the midgut following gut damage. Together with our previously collection of split-Gal4 lines targeting the 7 Wnt ligands, this resource allows Drosophila researchers to identify and genetically manipulate cells that specifically express pairs of conserved ligands from nearly all the major intercellular signaling pathways.

A collection of split-Gal4 drivers targeting conserved signaling ligands in Drosophila

Communication between cells in metazoan organisms is mediated by a remarkably small number of highly conserved signaling pathways. Given the relatively small number of signaling pathways, the existence of multiple related ligands for many of these pathways is thought to represent a key evolutionary innovation for encoding complexity into cell-cell signaling. Relatedly, crosstalk and other interactions between pathways is another critical feature which allows a modest number pathways to ultimately generate an enormously diverse range of outcomes. It would thus be useful to have genetic tools to identify and manipulate not only those cells which express a given signaling ligand, but also those cells that specifically co-express pairs of signaling ligands. Here, we present a collection of split-Gal4 knock-in lines targeting many of the ligands for highly conserved signaling pathways in Drosophila (Notch, Hedgehog, FGF, EGF, TGFβ, JAK/STAT, JNK, and PVR). We demonstrate that these lines faithfully recapitulate the endogenous expression pattern of their targets, and that they can be used to specifically identify the cells and tissues that co-express pairs of signaling ligands. As a proof of principle, we demonstrate that the 4th chromosome TGFβ ligands myoglianin and maverick are broadly co-expressed in muscles and other tissues of both larva and adults, and that the JAK/STAT ligands upd2 and upd3 are partially co-expressed from cells of the midgut following gut damage. Together with our previously collection of split-Gal4 lines targeting the seven Wnt ligands, this resource allows Drosophila researchers to identify and genetically manipulate cells that specifically express pairs of conserved ligands from nearly all the major intercellular signaling pathways.

TGF-β Type II Receptor Punt Suppresses Antimicrobial Peptide Expression and Influences Development in Tribolium castaneum

The transforming growth factor-β (TGF-β) superfamily in insects regulated various physiological events, including immune response, growth and development, and metamorphosis. This complex network of signaling pathways involves conserved cell-surface receptors and signaling co-receptors that allow for precisely coordinated cellular events. However, the roles of TGF-β receptors, particularly the type II receptor Punt, in mediating the innate immunity in insects remains unclear. In this study, we used the red flour beetle, Tribolium castaneum, as a model species to investigate the role of TGF-β type II receptor Punt in mediating antimicrobial peptide (AMP) expression. Developmental and tissue-specific transcript profiles revealed Punt was constitutively expressed throughout development, with the highest transcript level in 1-day female pupae and the lowest transcript level in 18-day larvae. Tissue specific expression profiles showed the highest transcript level of Punt was observed in the Malpighian tubule and ovary in 18-day larvae and 1-day female adults, respectively, suggesting Punt might have distinct functions in larvae and adults. Further results indicated that Punt RNAi in the 18-day larvae led to increased transcript level of AMP genes through transcription factor Relish, leading to inhibition of Escherichia coli proliferation. Knockdown of Punt in larvae also led to splitting of adult elytra and abnormal compound eyes. Furthermore, knockdown of Punt during the female pupal stage resulted in increased transcript levels of AMP genes, as well as abnormal ovary, reduced fecundity, and failure of eggs to hatch. This study deepens our understanding of the biological significance of Punt in insect TGF-β signaling and lays the groundwork for further research of its role in insect immune response, development, and reproduction.

The extended analogy of extraembryonic development in insects and amniotes

It is fascinating that the amnion and serosa/chorion, two extraembryonic (EE) tissues that are characteristic of the amniote vertebrates (mammals, birds and reptiles), have also independently evolved in insects. In this review, we offer the first detailed, macroevolutionary comparison of EE development and tissue biology across these animal groups. Some commonalities represent independent solutions to shared challenges for protecting the embryo (environmental assaults, risk of pathogens) and supporting its development, including clear links between cellular properties (e.g. polyploidy) and physiological function. Further parallels encompass developmental features such as the early segregation of the serosa/chorion compared to later, progressive differentiation of the amnion and formation of the amniotic cavity from serosal-amniotic folds as a widespread morphogenetic mode across species. We also discuss common developmental roles for orthologous transcription factors and BMP signalling in EE tissues of amniotes and insects, and between EE and cardiac tissues, supported by our exploration of new resources for global and tissue-specific gene expression. This highlights the degree to which general developmental principles and protective tissue features can be deduced from each of these animal groups, emphasizing the value of broad comparative studies to reveal subtle developmental strategies and answer questions that are common across species. This article is part of the theme issue 'Extraembryonic tissues: exploring concepts, definitions and functions across the animal kingdom'.

Paralog Explorer: A resource for mining information about paralogs in common research organisms

Paralogs are genes which arose via gene duplication, and when such paralogs retain overlapping or redundant function, this poses a challenge to functional genetics research. Recent technological advancements have made it possible to systematically probe gene function for redundant genes using dual or multiplex gene perturbation, and there is a need for a simple bioinformatic tool to identify putative paralogs of a gene(s) of interest. We have developed Paralog Explorer (https://www.flyrnai.org/tools/paralogs/), an online resource that allows researchers to quickly and accurately identify candidate paralogous genes in the genomes of the model organisms D. melanogaster, C. elegans, D. rerio, M. musculus, and H. sapiens. Paralog Explorer deploys an effective between-species ortholog prediction software, DIOPT, to analyze within-species paralogs. Paralog Explorer allows users to identify candidate paralogs, and to navigate relevant databases regarding gene co-expression, protein-protein and genetic interaction, as well as gene ontology and phenotype annotations. Altogether, this tool extends the value of current ortholog prediction resources by providing sophisticated features useful for identification and study of paralogous genes.

Tachykinin-related peptides modulate immune-gene expression in the mealworm beetle Tenebrio molitor L

Tachykinins (TKs) are a group of conserved neuropeptides. In insects, tachykinin-related peptides (TRPs) are important modulators of several functions such as nociception and lipid metabolism. Recently, it has become clear that TRPs also play a role in regulating the insect immune system. Here, we report a transcriptomic analysis of changes in the expression levels of immune-related genes in the storage pest Tenebrio molitor after treatment with Tenmo-TRP-7. We tested two concentrations (10^-8 and 10^-6 M) at two time points, 6 and 24 h post-injection. We found significant changes in the transcript levels of a wide spectrum of immune-related genes. Some changes were observed 6 h after the injection of Tenmo-TRP-7, especially in relation to its putative anti-apoptotic action. Interestingly, 24 h after the injection of 10^-8 M Tenmo-TRP-7, most changes were related to the regulation of the cellular response. Applying 10^-6 M Tenmo-TRP-7 resulted in the downregulation of genes associated with humoral responses. Injecting Tenmo-TRP-7 did not affect beetle survival but led to a reduction in haemolymph lysozyme-like antibacterial activity, consistent with the transcriptomic data. The results confirmed the immunomodulatory role of TRP and shed new light on the functional homology between TRPs and TKs.

Evolution of the dorsoventral axis in insects: the changing role of Bone Morphogenetic Proteins

Embryonic dorsal-ventral (DV) patterning by Bone Morphogenetic Proteins (BMPs) is a conserved feature of Bilateria, based on graded BMP activity set up by diffusible BMP ligands and Chordin/Sog antagonists. In the fly Drosophila melanogaster BMP function is secondary to patterning by the Toll pathway, suggesting a more restricted role for BMPs in insects. With widespread genome sequencing technologies allied to functional analysis in a growing number of species, recent work has shown that BMP's role in DV patterning relative to Toll varies among insect orders. Further, the role of BMP antagonists to set up BMP gradients is also greatly diversified. Here we review the recent findings concerning the role of BMP in the DV patterning of insects and address the potential aspects that may have co-evolved with BMPs to attain this functional divergence.

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.

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.

Beetles as Model Organisms in Physiological, Biomedical and Environmental Studies - A Review

Model organisms are often used in biological, medical and environmental research. Among insects, Drosophila melanogaster, Galleria mellonella, Apis mellifera, Bombyx mori, Periplaneta americana, and Locusta migratoria are often used. However, new model organisms still appear. In recent years, an increasing number of insect species has been suggested as model organisms in life sciences research due to their worldwide distribution and environmental significance, the possibility of extrapolating research studies to vertebrates and the relatively low cost of rearing. Beetles are the largest insect order, with their representative - Tribolium castaneum - being the first species with a completely sequenced genome, and seem to be emerging as new potential candidates for model organisms in various studies. Apart from T. castaneum, additional species representing various Coleoptera families, such as Nicrophorus vespilloides, Leptinotarsa decemlineata, Coccinella septempunctata, Poecilus cupreus, Tenebrio molitor and many others, have been used. They are increasingly often included in two major research aspects: biomedical and environmental studies. Biomedical studies focus mainly on unraveling mechanisms of basic life processes, such as feeding, neurotransmission or activity of the immune system, as well as on elucidating the mechanism of different diseases (neurodegenerative, cardiovascular, metabolic, or immunological) using beetles as models. Furthermore, pharmacological bioassays for testing novel biologically active substances in beetles have also been developed. It should be emphasized that beetles are a source of compounds with potential antimicrobial and anticancer activity. Environmental-based studies focus mainly on the development and testing of new potential pesticides of both chemical and natural origin. Additionally, beetles are used as food or for their valuable supplements. Different beetle families are also used as bioindicators. Another important research area using beetles as models is behavioral ecology studies, for instance, parental care. In this paper, we review the current knowledge regarding beetles as model organisms and their practical application in various fields of life science.

Epigenetic modifications acetylation and deacetylation play important roles in juvenile hormone action

BACKGROUND

Epigenetic modifications including DNA methylation and post-translational modifications of histones are known to regulate gene expression. Antagonistic activities of histone acetyltransferases (HATs) and histone deacetylases (HDACs) mediate transcriptional reprogramming during insect development as shown in Drosophila melanogaster and other insects. Juvenile hormones (JH) play vital roles in the regulation of growth, development, metamorphosis, reproduction and other physiological processes. However, our current understanding of epigenetic regulation of JH action is still limited. Hence, we studied the role of CREB binding protein (CBP, contains HAT domain) and Trichostatin A (TSA, HDAC inhibitor) on JH action.

RESULTS

Exposure of Tribolium castaneum cells (TcA cells) to JH or TSA caused an increase in expression of Kr-h1 (a known JH-response gene) and 31 or 698 other genes respectively. Knockdown of the gene coding for CBP caused a decrease in the expression of 456 genes including Kr-h1. Interestingly, the expression of several genes coding for transcription factors, nuclear receptors, P450 and fatty acid synthase family members that are known to mediate JH action were affected by CBP knockdown or TSA treatment.

CONCLUSIONS

These data suggest that acetylation and deacetylation mediated by HATs and HDACs play an important role in JH action.

The dorsoventral patterning of Musca domestica embryos: insights into BMP/Dpp evolution from the base of the lower cyclorraphan flies

Background

In the last few years, accumulated information has indicated that the evolution of an extra-embryonic membrane in dipterans was accompanied by changes in the gene regulatory network controlled by the BMP/Dpp pathway, which is responsible for dorsal patterning in these insects. However, only comparative analysis of gene expression levels between distant species with two extra-embryonic membranes, like A. gambiae or C. albipunctata, and D. melanogaster, has been conducted. Analysis of gene expression in ancestral species, which evolved closer to the amnioserosa origin, could provide new insights into the evolution of dorsoventral patterning in dipterans.

Results

Here we describe the spatial expression of several key and downstream elements of the Dpp pathway and show the compared patterns of expression between Musca and Drosophila embryos, both dipterans with amnioserosa. Most of the analyzed gene showed a high degree of expression conservation, however, we found several differences in the gene expression pattern of M. domestica orthologs for sog and tolloid. Bioinformatics analysis of the promoter of both genes indicated that the variations could be related to the gain of several binding sites for the transcriptional factor Dorsal in the Md.tld promoter and Snail in the Md.sog enhancer. These altered expressions could explain the unclear formation of the pMad gradient in the M. domestica embryo, compared to the formation of the gradient in D. melanogaster.

Conclusion

Gene expression changes during the dorsal–ventral patterning in insects contribute to the differentiation of extra-embryonic tissues as a consequence of changes in the gene regulatory network controlled by BMP/Dpp. In this work, in early M. domestica embryos, we identified the expression pattern of several genes members involved in the dorsoventral specification of the embryo. We believe that these data can contribute to understanding the evolution of the BMP/Dpp pathway, the regulation of BMP ligands, and the formation of a Dpp gradient in higher cyclorraphan flies.

Electronic supplementary material

The online version of this article (10.1186/s13227-018-0102-5) contains supplementary material, which is available to authorized users.

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.

Myostatin-like proteins regulate synaptic function and neuronal morphology

Growth factors of the TGFβ superfamily play key roles in regulating neuronal and muscle function. Myostatin (or GDF8) and GDF11 are potent negative regulators of skeletal muscle mass. However, expression of myostatin and its cognate receptors in other tissues, including brain and peripheral nerves, suggests a potential wider biological role. Here, we show that Myoglianin (MYO), the Drosophila homolog of myostatin and GDF11, regulates not only body weight and muscle size, but also inhibits neuromuscular synapse strength and composition in a Smad2-dependent manner. Both myostatin and GDF11 affected synapse formation in isolated rat cortical neuron cultures, suggesting an effect on synaptogenesis beyond neuromuscular junctions. We also show that MYO acts in vivo to inhibit synaptic transmission between neurons in the escape response neural circuit of adult flies. Thus, these anti-myogenic proteins act as important inhibitors of synapse function and neuronal growth.

Summary: Myostatin-like proteins can modulate neuromuscular synapse strength as well as synaptogenesis beyond neuromuscular junctions, highlighting a key role for these proteins in synapse function and neuronal growth.

Transcriptome sequencing reveals maelstrom as a novel target gene of the terminal-system in the red flour beetle Tribolium castaneum

Terminal regions of the Drosophila embryo are patterned by the localized activation of the Torso-RTK pathway, which promotes the down-regulation of Capicua. In the short-germ beetle Tribolium, the function of the terminal system appears to be rather different, as the pathway promotes axis elongation and in addition, is required for patterning the extraembryonic serosa at the anterior. Here we show that Torso signalling induces gene expression by relieving CAPICUA-mediated repression also in Tribolium. Given that the majority of Torso target genes remain to be identified, we established a differential gene-expression screen. A subset of 50 putative terminal target genes was screened for functions in early embryonic patterning. Of those, 13 genes show early terminal expression domains and also phenotypes were related to terminal patterning. Among others, we found the PIWI-interacting RNA factor Maelstrom to be crucial for early embryonic polarization. Tc-mael is required for proper serosal size regulation and head morphogenesis. Moreover, Tc-mael promotes growth-zone formation and axis elongation. Our results suggest that posterior patterning by Torso may be realized through Maelstrom depended activation of posterior wnt-domains.

Deep, Staged Transcriptomic Resources for the Novel Coleopteran Models Atrachya menetriesi and Callosobruchus maculatus

Despite recent efforts to sample broadly across metazoan and insect diversity, current sequence resources in the Coleoptera do not adequately describe the diversity of the clade. Here we present deep, staged transcriptomic data for two coleopteran species, Atrachya menetriesi (Faldermann 1835) and Callosobruchus maculatus (Fabricius 1775). Our sampling covered key stages in ovary and early embryonic development in each species. We utilized this data to build combined assemblies for each species which were then analysed in detail. The combined A. menetriesi assembly consists of 228,096 contigs with an N50 of 1,598 bp, while the combined C. maculatus assembly consists of 128,837 contigs with an N50 of 2,263 bp. For these assemblies, 34.6% and 32.4% of contigs were identified using Blast2GO, and 97% and 98.3% of the BUSCO set of metazoan orthologs were present, respectively. We also carried out manual annotation of developmental signalling pathways and found that nearly all expected genes were present in each transcriptome. Our analyses show that both transcriptomes are of high quality. Lastly, we performed read mapping utilising our timed, stage specific RNA samples to identify differentially expressed contigs. The resources presented here will provide a firm basis for a variety of experimentation, both in developmental biology and in comparative genomic studies.

Adaptive protein divergence of BMP ligands takes place under developmental and evolutionary constraints

The bone morphogenetic protein (BMP) signaling network, comprising evolutionary conserved BMP2/4/Decapentaplegic (Dpp) and Chordin/Short gastrulation (Sog), is widely utilized for dorsal-ventral (DV) patterning during animal development. A similar network is required for posterior crossvein (PCV) formation in the Drosophila pupal wing. Although both transcriptional and post-transcriptional regulation of co-factors in the network gives rise to tissue-specific and species-specific properties, their mechanisms are incompletely understood. In Drosophila, BMP5/6/7/8-type ligands, Screw (Scw) and Glass bottom boat (Gbb), form heterodimers with Dpp for DV patterning and PCV development, respectively. Sequence analysis indicates that the Scw ligand contains two N-glycosylation motifs: one being highly conserved between BMP2/4- and BMP5/6/7/8-type ligands, and the other being Scw ligand specific. Our data reveal that N-glycosylation of the Scw ligand boosts BMP signaling both in cell culture and in the embryo. In contrast, N-glycosylation modifications of Gbb or Scw ligands reduce the consistency of PCV development. These results suggest that tolerance for structural changes of BMP5/6/7/8-type ligands is dependent on developmental constraints. Furthermore, gain and loss of N-glycosylation motifs in conserved signaling molecules under evolutionary constraints appear to constitute flexible modules to adapt to developmental processes.

A deuterostome origin of the Spemann organiser suggested by Nodal and ADMPs functions in Echinoderms

During development of chordates, establishment of the body plan relies on the activity of an organizing centre located on the dorsal side of the embryo that patterns the embryo and induces neural tissue. Intriguingly, the evolutionary origin of this crucial signalling centre remains unclear and whether analogous organizers regulate D/V patterning in other deuterostome or protostome phyla is not known. Here we provide evidence that the ventral ectoderm of the sea urchin embryo is a long-range organizing centre that shares several fundamental properties with the Spemann organizer: the ability to induce duplicated embryonic axes when ectopically induced, the ability to induce neural fate in neighbouring tissues and the ability to finely regulate the level of BMP signalling by using an autoregulatory expansion–repression mechanism. These findings suggest that the evolutionary origin of the Spemann organizer is more ancient than previously thought and that it may possibly be traced back to the common ancestor of deuterostomes.

Establishment of the body plan in chordates is determined by an organizing centre located on the dorsal side of the embryo. Here, the authors show that the ventral ectoderm of the sea urchin embryo is an organizing centre that shares several fundamental properties with the amphibian Spemann organizer.

Transcriptomic study of the red palm weevil Rhynchophorus ferrugineus embryogenesis

The red palm weevil (RPW), Rhynchophorus ferrugineus (Coleoptera: Curculionidae), is an invasive, concealed and destructive tissue borer, and it becomes a lethal pest of the palm family of plants and has been reported to attack 20 palm species around the globe. Here we report a systematic transcriptomic study on embryogenesis of RPW, where we analyze the transcriptomes across five developmental stages of RPW embryogenesis, involving four embryonic stages (E1, E2, E3 and E4) and one larval stage (L1). Using the RNA-seq and next-generation platforms, we generated 80 to 91 million reads for each library and assemble 22 532 genes that are expressed at different embryonic stages. Among the total transcripts from the five embryonic development stages, we found that 30.45 % are differentially expressed, 10.10 % show stage-specificity and even a larger fraction, 62.88 %, exhibit constitutive expression in all the stages. We also analyzes the expression dynamics of several conserved signaling pathways (such as Hedgehog, JAK-STAT, Notch, TGF-β, Ras/MAPK and Wnt), as well as key developmental genes, including those related to apoptosis, axis formation, Hox complex, neurogenesis and segmentation. The datasets provide an essential resource for gene annotation and RPW functional genomics, including studies by using tools and concepts from multiple disciplines, such as development, physiology, biochemistry, molecular biology and genetics.

Evidence for the plasticity of arthropod signal transduction pathways

Metazoans are known to contain a limited, yet highly conserved, set of signal transduction pathways that instruct early developmental patterning mechanisms. Genomic surveys that have compared gene conservation in signal transduction pathways between various insects and Drosophila support the conclusion that these pathways are conserved in evolution. However, the degree to which individual components of signal transduction pathways vary among more divergent arthropods is not known. Here, we report our results of a survey of the genome of the two-spotted spider mite Tetranychus urticae, using a set of 294 Drosophila orthologs of genes that function in signal transduction. We find a third of all genes surveyed absent from the spider mite genome. We also identify several novel duplications that have not been previously reported for a chelicerate. In comparison with previous insect surveys, Tetranychus contains a decrease in overall gene conservation, as well as an unusual ratio of ligands to receptors and other modifiers. These findings suggest that gene loss and duplication among components of signal transduction pathways are common among arthropods and suggest that signal transduction pathways in arthropods are more evolutionarily labile than previously hypothesized.

The first myriapod genome sequence reveals conservative arthropod gene content and genome organisation in the centipede Strigamia maritima

Myriapods (e.g., centipedes and millipedes) display a simple homonomous body plan relative to other arthropods. All members of the class are terrestrial, but they attained terrestriality independently of insects. Myriapoda is the only arthropod class not represented by a sequenced genome. We present an analysis of the genome of the centipede Strigamia maritima. It retains a compact genome that has undergone less gene loss and shuffling than previously sequenced arthropods, and many orthologues of genes conserved from the bilaterian ancestor that have been lost in insects. Our analysis locates many genes in conserved macro-synteny contexts, and many small-scale examples of gene clustering. We describe several examples where S. maritima shows different solutions from insects to similar problems. The insect olfactory receptor gene family is absent from S. maritima, and olfaction in air is likely effected by expansion of other receptor gene families. For some genes S. maritima has evolved paralogues to generate coding sequence diversity, where insects use alternate splicing. This is most striking for the Dscam gene, which in Drosophila generates more than 100,000 alternate splice forms, but in S. maritima is encoded by over 100 paralogues. We see an intriguing linkage between the absence of any known photosensory proteins in a blind organism and the additional absence of canonical circadian clock genes. The phylogenetic position of myriapods allows us to identify where in arthropod phylogeny several particular molecular mechanisms and traits emerged. For example, we conclude that juvenile hormone signalling evolved with the emergence of the exoskeleton in the arthropods and that RR-1 containing cuticle proteins evolved in the lineage leading to Mandibulata. We also identify when various gene expansions and losses occurred. The genome of S. maritima offers us a unique glimpse into the ancestral arthropod genome, while also displaying many adaptations to its specific life history.

Arthropods are the most abundant animals on earth. Among them, insects clearly dominate on land, whereas crustaceans hold the title for the most diverse invertebrates in the oceans. Much is known about the biology of these groups, not least because of genomic studies of the fruit fly Drosophila, the water flea Daphnia, and other species used in research. Here we report the first genome sequence from a species belonging to a lineage that has previously received very little attention—the myriapods. Myriapods were among the first arthropods to invade the land over 400 million years ago, and survive today as the herbivorous millipedes and venomous centipedes, one of which—Strigamia maritima—we have sequenced here. We find that the genome of this centipede retains more characteristics of the presumed arthropod ancestor than other sequenced insect genomes. The genome provides access to many aspects of myriapod biology that have not been studied before, suggesting, for example, that they have diversified receptors for smell that are quite different from those used by insects. In addition, it shows specific consequences of the largely subterranean life of this particular species, which seems to have lost the genes for all known light-sensing molecules, even though it still avoids light.

Toll signals regulate dorsal-ventral patterning and anterior-posterior placement of the embryo in the hemipteran Rhodnius prolixus

BACKGROUND

Insect embryonic dorso-ventral patterning depends greatly on two pathways: the Toll pathway and the Bone Morphogenetic Protein pathway. While the relative contribution of each pathway has been investigated in holometabolous insects, their role has not been explored in insects with a hemimetabolous type of development. The hemimetabolous insect Rhodnius prolixus, an important vector of Chagas disease in the Americas, develops from an intermediate germ band and displays complex movements during katatrepsis that are not observed in other orders. However, little is known about the molecular events that regulate its embryogenesis. Here we investigate the expression and function of genes potentially involved in the initial patterning events that establish the embryonic dorso-ventral axis in this hemipteran.

RESULTS

We establish a staging system for early embryogenesis that allows us to correlate embryo morphology with gene expression profiles. Using this system, we investigate the role of Toll pathway genes during embryogenesis. Detailed analyses of gene expression throughout development, coupled with functional analyses using parental RNA interference, revealed that maternal Toll is required to establish germ layers along the dorso-ventral axis and for embryo placement along the anterior-posterior axis. Interestingly, knockdown of the Toll pathway effector Rp-dorsal appears to regulate the expression of the Bone Morphogenetic Protein antagonist Rp-short-gastrulation.

CONCLUSIONS

Our results indicate that Toll signals are the initiating event in dorso-ventral patterning during Rhodnius embryogenesis, and this is the first report of a conserved role for Toll in a hemipteran. Furthermore, as Rp-dorsal RNA interference generates anteriorly misplaced embryos, our results indicate a novel role for Toll signals in establishment of the anterior-posterior axis in Rhodnius.

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.

Strategies for exploring TGF-β signaling in Drosophila

The TGF-β pathway is an evolutionarily conserved signal transduction module that mediates diverse biological processes in animals. In Drosophila, both the BMP and Activin branches are required for viability. Studies rooted in classical and molecular genetic approaches continue to uncover new developmental roles for TGF-β signaling. We present an overview of the secreted ligands, transmembrane receptors and cellular Smad transducer proteins that compose the core pathway in Drosophila. An assortment of tools have been developed to conduct tissue-specific loss- and gain-of-function experiments for these pathway components. We discuss the deployment of these reagents, with an emphasis on appropriate usage and limitations of the available tools. Throughout, we note reagents that are in need of further improvement or development, and signaling features requiring further study. A general theme is that comparison of phenotypes for ligands, receptors, and Smads can be used to map tissue interactions, and to separate canonical and non-canonical signaling activities. Core TGF-β signaling components are subject to multiple layers of regulation, and are coupled to context-specific inputs and outputs. In addition to fleshing out how TGF-β signaling serves the fruit fly, we anticipate that future studies will uncover new regulatory nodes and modes and will continue to advance paradigms for how TGF-β signaling regulates general developmental processes.

Cleavage of the Drosophila screw prodomain is critical for a dynamic BMP morphogen gradient in embryogenesis

Dorsoventral patterning of the Drosophila embryo is regulated by graded distribution of bone morphogenetic proteins (BMPs) composed of two ligands, decapentaplegic (Dpp) a BMP2/4 ortholog and screw (Scw) a BMP5/6/7/8 family member. scw(E1) encodes an unusual allele that was isolated as a dominant enhancer of partial loss-of-function mutations in dpp. However, the molecular mechanisms that underlie this genetic interaction remain to be addressed. Here we show that scw(E1) contains a mutation at the furin cleavage site within the prodomain that is crucial for ligand production. Furthermore, our data show that Scw(E1) preferentially forms heterodimers with Dpp rather than homotypic dimers, providing a possible explanation for the dominant negative phenotype of scw(E1) alleles. The unprocessed prodomain of Scw(E1) remains in a complex with the Dpp:Scw heterodimer, and thus could interfere with interaction of the ligand with the extracellular matrix, or the kinetics of processing/secretion of the ligand in vivo. These data reveal novel mechanisms by which post-translational regulation of Scw can modulate Dpp signaling activity.

Fine-tuned shuttles for bone morphogenetic proteins

Bone morphogenetic proteins (BMPs) are potent secreted signaling factors that trigger phosphorylation of Smad transcriptional regulators through receptor complex binding at the cell-surface. Resulting changes in target gene expression impact critical cellular responses during development and tissue homeostasis. BMP activity is tightly regulated in time and space by secreted modulators that control BMP extracellular distribution and availability for receptor binding. Such extracellular regulation is key for BMPs to function as morphogens and/or in the formation of morphogen activity gradients. Here, we review shuttling systems utilized to control the distribution of BMP ligands in tissue of various geometries, developing under different temporal constraints. We discuss the biological advantages for employing specific strategies for BMP shuttling and roles of varied ligand forms.

Dpp/BMP transport mechanism is required for wing venation in the sawfly Athalia rosae

The pattern of wing venation varies considerably among different groups of insects and has been used as a means of species-specific identification. However, little is known about how wing venation is established and diversified among insects. The decapentaplegic (Dpp)/bone morphogenetic protein (BMP) signaling pathway plays a critical role in wing vein formation during the pupal stages in Drosophila melanogaster. A key mechanism is BMP transport from the longitudinal veins (LVs) to the posterior crossvein (PCV) by the BMP-binding proteins, short gastrulation (Sog) and twisted gastrulation2/crossveinless (Tsg2/Cv). To investigate whether the BMP transport mechanism is utilized to specify insect wing vein patterns in other than Drosophila, we used the sawfly Athalia rosae as a model, which has distinct venation patterns in the fore- and hindwings. Here, we show that Ar-dpp is ubiquitously expressed in both the fore- and hindwings, but is required for localized BMP signaling that reflects distinct wing vein patterns between the fore- and hindwings. By isolating Ar-tsg/cv in the sawfly, we found that Ar-Tsg/Cv is also required for BMP signaling in wing vein formation and retains the ability to transport Dpp. These data suggest that the BMP transport system is widely used to redistribute Dpp to specify wing venation and may be a basal mechanism underlying diversified wing vein patterns among insects.

Evolution and expression of BMP genes in flies

Bone morphogenetic proteins (BMPs) play key roles in development. In Drosophila melanogaster, there are three BMP-encoding genes: decapentaplegic (dpp), glass bottom boat (gbb) and screw (scw). dpp and gbb are found in all groups of insects. In contrast, the origin of scw via duplication of an ancestral gbb homologue is more recent, with new evidence placing it within the Diptera. Recent studies show that scw appeared basal to the Schizophora, since scw orthologues exist in aschizan cyclorrhaphan flies. In order to further localise the origin of scw, we have utilised new genomic resources for the nematoceran moth midge Clogmia albipunctata (Psychodidae). We identified the BMP subclass members dpp and gbb from an early embryonic transcriptome and show that their expression patterns in the blastoderm differ considerably from those seen in cyclorrhaphan flies. Further searches of the genome of C. albipunctata were unable to identify a scw-like gbb duplicate, but confirm the presence of dpp and gbb. Our phylogenetic analysis shows these to be clear orthologues of dpp and gbb from other non-cyclorrhaphan insects, with C. albipunctata gbb branching ancestrally to the cyclorrhaphan gbb/scw split. Furthermore, our analysis suggests that scw is absent from all Nematocera, including the Bibionomorpha. We conclude that the gbb/scw duplication occurred between the separation of the lineage leading to Brachycera and the origin of cyclorrhaphan flies 200–150 Ma ago.

Lifestyle and host defense mechanisms of the dung beetle, Euoniticellus intermedius: the toll signaling pathway

The dung beetle, Euoniticellus intermedius (Reiche) (Coleoptera: Scarabaeidae) is an important ecological and agricultural agent. Their main activity, the burying of dung, improves quality of the soil and reduces pests that could cause illness in animals. E. intermedius are therefore important for agriculture and for good maintenance of the environment, and are regarded as effective biological control agents for parasites of the gastrointestinal tract in livestock. The ability of E. intermedius to co-exist comfortably with many microorganisms, some of which are important human pathogens, stimulated our interest in its host defense strategies. The aim of this study was to investigate the Toll signaling pathway, which is strongly activated by fungi. Gene expression associated with fungal infection was analyzed by using 2-D gel electrophoresis and mass spectroscopy. Furthermore, the partial adult transcriptome was investigated for the presence of known immune response genes by using high-throughput sequencing and bioinformatics. The results presented here suggest that E. intermedius responds to fungal challenge via the Toll signaling pathway.

Functional conservation and divergence of BMP ligands in limb development and lipid homeostasis of holometabolous insects

Bone morphogenetic protein (BMP) ligands play key roles in regulating morphological and physiological traits. To investigate how the functions of BMP ligands have evolved among insects, the roles of two key BMP ligands, decapentaplegic (dpp) and glass bottom boat (gbb), were studied in the flour beetle, Tribolium castaneum. RNA interference-mediated knockdown revealed that the role of dpp in establishing limb segmentation is conserved among insects. Based on the expression pattern of dpp in the presumptive leg tarsal segments, we propose that the function of dpp has evolved through heterochronic changes during the evolution of complete metamorphosis. Gbb1 was found to be necessary for sculpting the tarsal segment morphology characteristic of beetles. Knockdown of Dpp and Gbb1 expression also resulted in transparent larvae and reduced triglyceride levels, indicating their critical roles in maintaining lipid homeostasis. Both knockdown phenotypes were mediated by larval translucida. Because only Gbb regulates lipid metabolism in Drosophila, regulation of lipid homeostasis appears to have evolved by developmental systems drift. Thus, developmental systems drift may underlie evolution of both morphology and physiological processes.

BMP-dependent serosa and amnion specification in the scuttle fly Megaselia abdita

Bone morphogenetic protein (BMP) signaling is an essential factor in dorsoventral patterning of animal embryos but how BMP signaling evolved with fundamental changes in dorsoventral tissue differentiation is unclear. Flies experienced an evolutionary reduction of extra-embryonic tissue types from two (amniotic and serosal tissue) to one (amnionserosal tissue). BMP-dependent amnioserosa specification has been studied in Drosophila melanogaster. However, the mechanisms of serosal and amniotic tissue specification in less diverged flies remain unknown. To better understand potential evolutionary links between BMP signaling and extra-embryonic tissue specification, we examined the activity profile and function of BMP signaling in serosa and amnion patterning of the scuttle fly Megaselia abdita (Phoridae) and compared the BMP activity profiles between M. abdita and D. melanogaster. In blastoderm embryos of both species, BMP activity peaked at the dorsal midline. However, at the beginning of gastrulation, peak BMP activity in M. abdita shifted towards prospective amnion tissue. This transition correlated with the first signs of amnion differentiation laterally adjacent to the serosa anlage. Marker-assisted analysis of six BMP signaling components (dpp, gbb, scw, tkv, sax, sog) by RNA interference revealed that both serosa and amnion specification of M. abdita are dependent on BMP activity. Conversely, BMP gain-of-function experiments caused sharpened expression boundaries of extra-embryonic target genes indicative of positive feedback. We propose that changes in the BMP activity profile at the beginning of gastrulation might have contributed to the reduction of extra-embryonic tissue types during the radiation of cyclorrhaphan flies.

Position matters: variability in the spatial pattern of BMP modulators generates functional diversity

Bone morphogenetic proteins (BMPs) perform a variety of functions during development. Considering a single BMP, what enables its multiple roles in tissues of varied sizes and shapes? What regulates the spatial distribution and activity patterns of the BMP in these different developmental contexts? Some BMP functions require controlling spread of the BMP morphogen, while others require formation of localized, high concentration peaks of BMP activity. Here we review work in Drosophila that describes spatial regulation of the BMP encoded by decapentaplegic (dpp) in different developmental contexts. We concentrate on extracellular modulation of BMP function and discuss the mechanisms that generate concentrated peaks of Dpp activity, subdivide territories of different activity levels or regulate spread of the Dpp morphogen from a point source. We compare these findings with data from vertebrates and non-model organisms to discuss how changes in the regulation of Dpp distribution by extracellular modulators may lead to variability in dpp function in different species.

You're going to need a bigger (glass bottom) boat

Signaling molecules of the transforming growth factor (TGF)-β family are generated from proprotein precursors containing prodomain sequences that are typically removed to allow signaling by the mature ligands. A form of a TGF-β family ligand that remains covalently attached to its prodomain but retains signaling activity has been identified. Glass bottom boat (Gbb), a Drosophila homolog of the bone morphogenetic protein 5/6/7/8 subfamily, is active as a carboxyl-terminal fragment of the proprotein (Gbb15) that is generated by a conventional processing event common to TGF-β ligands. Unexpectedly, a larger form (Gbb38) produced by processing at a newly identified furin site in the prodomain is also secreted and active. Contrary to the present paradigm in which TGF-β ligands require dissociation of the entire prodomain for activity, Gbb38 is active in cell culture and in vivo without additional processing at conventional sites. The large form can restore the viability of gbb mutant animals but has distinct signaling properties compared with the conventional form. Production of multiple functional ligands from one proprotein is a potential mechanism to fine-tune TGF-β signaling outputs. Mutations in TGF-β family members have been linked to human diseases, several of which affect potential furin cleavage sites in prodomains. However, given the diversity of potential furin processing sites and prodomain functions, direct experimentation will be required to determine whether production of active jumbo ligands is a general feature of TGF-β superfamily members.

Coleopteran antimicrobial peptides: prospects for clinical applications

Antimicrobial peptides (AMPs) are activated in response to septic injury and have important roles in vertebrate and invertebrate immune systems. AMPs act directly against pathogens and have both wound healing and antitumor activities. Although coleopterans comprise the largest and most diverse order of eukaryotes and occupy an earlier branch than Drosophila in the holometabolous lineage of insects, their immune system has not been studied extensively. Initial research reports, however, indicate that coleopterans possess unique immune response mechanisms, and studies of these novel mechanisms may help to further elucidate innate immunity. Recently, the complete genome sequence of Tribolium was published, boosting research on coleopteran immunity and leading to the identification of Tribolium AMPs that are shared by Drosophila and mammals, as well as other AMPs that are unique. AMPs have potential applicability in the development of vaccines. Here, we review coleopteran AMPs, their potential impact on clinical medicine, and the molecular basis of immune defense.

Comparative genomic analysis of Drosophila melanogaster and vector mosquito developmental genes

Genome sequencing projects have presented the opportunity for analysis of developmental genes in three vector mosquito species: Aedes aegypti, Culex quinquefasciatus, and Anopheles gambiae. A comparative genomic analysis of developmental genes in Drosophila melanogaster and these three important vectors of human disease was performed in this investigation. While the study was comprehensive, special emphasis centered on genes that 1) are components of developmental signaling pathways, 2) regulate fundamental developmental processes, 3) are critical for the development of tissues of vector importance, 4) function in developmental processes known to have diverged within insects, and 5) encode microRNAs (miRNAs) that regulate developmental transcripts in Drosophila. While most fruit fly developmental genes are conserved in the three vector mosquito species, several genes known to be critical for Drosophila development were not identified in one or more mosquito genomes. In other cases, mosquito lineage-specific gene gains with respect to D. melanogaster were noted. Sequence analyses also revealed that numerous repetitive sequences are a common structural feature of Drosophila and mosquito developmental genes. Finally, analysis of predicted miRNA binding sites in fruit fly and mosquito developmental genes suggests that the repertoire of developmental genes targeted by miRNAs is species-specific. The results of this study provide insight into the evolution of developmental genes and processes in dipterans and other arthropods, serve as a resource for those pursuing analysis of mosquito development, and will promote the design and refinement of functional analysis experiments.

Gene expression in bryozoan larvae suggest a fundamental importance of pre-patterned blastemic cells in the bryozoan life-cycle

BACKGROUND

Bryozoa is a clade of aquatic protostomes. The bryozoan life cycle typically comprises a larval stage, which metamorphoses into a sessile adult that proliferates by asexual budding to form colonies. The homology of bryozoan larvae with other protostome larvae is enigmatic. Bryozoan larvae exhibit blastemic tissues that contribute to build the adult during morphogenesis. However, it remains unclear if the cells of these tissues are pre-determined according to their future fate or if the cells are undifferentiated, pluripotent stem cells. Gene expression studies can help to identify molecular patterning of larval and adult tissues and enlighten the evolution of bryozoan life cycle stages.

RESULTS

We investigated the spatial expression of 13 developmental genes in the larval stage of the gymnolaemate bryozoan Bugula neritina. We found most genes expressed in discrete regions in larval blastemic tissues that form definitive components of the adult body plan. Only two of the 13 genes, BnTropomyosin and BnFoxAB, were exclusively expressed in larval tissues that are discarded during metamorphosis.

CONCLUSIONS

Our results suggest that the larval blastemas in Bugula are pre-patterned according to their future fate in the adult. The gene expression patterns indicate that some of the bryozoan blastemas can be interpreted to correspond to homologous adult tissues of other animals. This study challenges an earlier proposed view that metazoan larvae share homologous undifferentiated "set-aside cells", and instead points to an independent origin of the bryozoan larval stage with respect to other lophotrochozoans.

BMP signaling components in embryonic transcriptomes of the hover fly Episyrphus balteatus (Syrphidae)

BACKGROUND

In animals, signaling of Bone Morphogenetic Proteins (BMPs) is essential for dorsoventral (DV) patterning of the embryo, but how BMP signaling evolved with changes in embryonic DV differentiation is largely unclear. Based on the extensive knowledge of BMP signaling in Drosophila melanogaster, the morphological diversity of extraembryonic tissues in different fly species provides a comparative system to address this question. The closest relatives of D. melanogaster with clearly distinct DV differentiation are hover flies (Diptera: Syrphidae). The syrphid Episyrphus balteatus is a commercial bio-agent against aphids and has been established as a model organism for developmental studies and chemical ecology. The dorsal blastoderm of E. balteatus gives rise to two extraembryonic tissues (serosa and amnion), whereas in D. melanogaster, the dorsal blastoderm differentiates into a single extraembryonic epithelium (amnioserosa). Recent studies indicate that several BMP signaling components of D. melanogaster, including the BMP ligand Screw (Scw) and other extracellular regulators, evolved in the dipteran lineage through gene duplication and functional divergence. These findings raise the question of whether the complement of BMP signaling components changed with the origin of the amnioserosa.

RESULTS

To search for BMP signaling components in E. balteatus, we generated and analyzed transcriptomes of freshly laid eggs (0-30 minutes) and late blastoderm to early germband extension stages (3-6 hours) using Roche/454 sequencing. We identified putative E. balteatus orthologues of 43% of all annotated D. melanogaster genes, including the genes of all BMP ligands and other BMP signaling components.

CONCLUSION

The diversification of several BMP signaling components in the dipteran linage of D. melanogaster preceded the origin of the amnioserosa.[Transcriptome sequence data from this study have been deposited at the NCBI Sequence Read Archive (SRP005289); individually assembled sequences have been deposited at GenBank (JN006969-JN006986).].

The evolution of dorsal-ventral patterning mechanisms in insects

The gene regulatory network (GRN) underpinning dorsal-ventral (DV) patterning of the Drosophila embryo is among the most thoroughly understood GRNs, making it an ideal system for comparative studies seeking to understand the evolution of development. With the emergence of widely applicable techniques for testing gene function, species with sequenced genomes, and multiple tractable species with diverse developmental modes, a phylogenetically broad and molecularly deep understanding of the evolution of DV axis formation in insects is feasible. Here, we review recent progress made in this field, compare our emerging molecular understanding to classical embryological experiments, and suggest future directions of inquiry.

Rapid evolution of a novel signalling mechanism by concerted duplication and divergence of a BMP ligand and its extracellular modulators

Gene duplication and divergence is widely considered to be a fundamental mechanism for generating evolutionary novelties. The Bone Morphogenetic Proteins (BMPs) are a diverse family of signalling molecules found in all metazoan genomes that have evolved by duplication and divergence from a small number of ancestral types. In the fruit fly Drosophila, there are three BMPs: Decapentaplegic (Dpp) and Glass bottom boat (Gbb), which are the orthologues of vertebrate BMP2/4 and BMP5/6/7/8, respectively, and Screw (Scw), which, at the sequence level, is equally divergent from Dpp and Gbb. It has recently been shown that Scw has arisen from a duplication of Gbb in the lineage leading to higher Diptera. We show that since this duplication event, Gbb has maintained the ancestral BMP5/6/7/8 functionality while Scw has rapidly diverged. The evolution of Scw was accompanied by duplication and divergence of a suite of extracellular regulators that continue to diverge together in the higher Diptera. In addition, Scw has become restricted in its receptor specificity: Gbb proteins can signal through the Type I receptors Thick veins (Tkv) and Saxophone (Sax), while Scw signals through Sax. Thus, in a relatively short span of evolutionary time, the duplication event that gave rise to Scw produced not only a novel ligand but also a novel signalling mode that is functionally distinct from the ancestral Gbb mode. Our results demonstrate the plasticity of the BMP pathway not only in evolving new family members and new functions but also new signalling modes by redeploying key regulators in the pathway.

A prominent requirement for single-minded and the ventral midline in patterning the dorsoventral axis of the crustacean Parhyale hawaiensis

In bilaterians, establishing the correct spatial positioning of structures along the dorsoventral (DV) axis is essential for proper embryonic development. Insects such as Drosophila rely on the Dorsal activity gradient and Bone morphogenetic protein (BMP) signaling to establish cell fates along the DV axis, leading to the distinction between tissues such as mesoderm, neurogenic ectoderm and dorsal ectoderm in the developing embryo. Subsequently, the ventral midline plays a more restricted role in DV patterning by establishing differential cell fates in adjacent regions of the neurogenic ectoderm. In this study, we examine the function of the ventral midline and the midline-associated gene single-minded (Ph-sim) in the amphipod crustacean Parhyale hawaiensis. Remarkably, we found that Ph-sim and the ventral midline play a central role in establishing proper fates along the entire DV axis in this animal; laser ablation of midline cells causes a failure to form neurogenic ectoderm and Ph-sim RNAi results in severely dorsalized embryos lacking both neurogenic ectoderm and the appendage-bearing lateral ectoderm. Furthermore, we hypothesize that this role of midline cells was present in the last common ancestor of crustaceans and insects. We predict that the transition to a Dorsal-dependent DV patterning system in the phylogenetically derived insect lineage leading to Drosophila has led to a more restricted role of the ventral midline in patterning the DV axis of these insects.

Bone morphogenetic proteins and articular cartilage: To serve and protect or a wolf in sheep clothing's?

OBJECTIVE

Alterations in chondrocyte differentiation and matrix remodeling play a central role in osteoarthritis (OA). Chondrocyte differentiation and remodeling are amongst others regulated by the so-called Bone Morphogenetic Proteins (BMPs). Although BMPs are considered protective for articular cartilage these factors can also be involved in chondrocyte hypertrophy and matrix degradation. This review is focused on these opposed roles of BMPs in OA development and progression.

METHODS

Peer reviewed publications published prior to August 2009 were searched in the Pubmed database. Articles that were relevant for the role of endogenous BMPs in OA were selected. Since good quality reviews on the application of BMP supplementation in cartilage tissue engineering have been described this subject has not been covered in this review.

RESULTS

BMPs can stimulate both chondrocyte matrix synthesis and chondrocyte terminal differentiation. The latter results in elevated matrix metalloproteinase-13 (MMP-13) production. Stimulation of matrix synthesis will be protective for cartilage while elevated MMP-13 activity will drive matrix degradation. What action of BMPs is dominant in OA is not yet elucidated and their role might be different in patient subgroups.

CONCLUSION

BMPs can be protective for articular cartilage but can, due to their effect on chondrocyte differentiation, have harmful effects on articular cartilage and contribute to OA progression.

Evolution of extracellular Dpp modulators in insects: The roles of tolloid and twisted-gastrulation in dorsoventral patterning of the Tribolium embryo

The formation of the BMP gradient which patterns the DV axis in flies and vertebrates requires several extracellular modulators like the inhibitory protein Sog/Chordin, the metalloprotease Tolloid (Tld), which cleaves Sog/Chordin, and the CR domain protein Twisted gastrulation (Tsg). While flies and vertebrates have only one sog/chordin gene they possess several paralogues of tld and tsg. A simpler and probably ancestral situation is observed in the short-germ beetle Tribolium castaneum (Tc), which possesses only one tld and one tsg gene. Here we show that in T. castaneum tld is required for early BMP signalling except in the head region and Tc-tld function is, as expected, dependent on Tc-sog. In contrast, Tc-tsg is required for all aspects of early BMP signalling and acts in a Tc-sog-independent manner. For comparison with Drosophila melanogaster we constructed fly embryos lacking all early Tsg activity (tsg;;srw double mutants) and show that they still establish a BMP signalling gradient. Thus, our results suggest that the role of Tsg proteins for BMP gradient formation has changed during insect evolution.

Distinct molecular evolutionary mechanisms underlie the functional diversification of the Wnt and TGFbeta signaling pathways

The canonical Wnt pathway is one of the oldest and most functionally diverse of animal intercellular signaling pathways. Though much is known about loss-of-function phenotypes for Wnt pathway components in several model organisms, the question of how this pathway achieved its current repertoire of functions has not been addressed. Our phylogenetic analyses of 11 multigene families from five species belonging to distinct phyla, as well as additional analyses employing the 12 Drosophila genomes, suggest frequent gene duplications affecting ligands and receptors as well as co-evolution of new ligand–receptor pairs likely facilitated the expansion of this pathway’s capabilities. Further, several examples of recent gene loss are visible in Drosophila when compared to family members in other phyla. By comparison the TGFβ signaling pathway is characterized by ancient gene duplications of ligands, receptors, and signal transducers with recent duplication events restricted to the vertebrate lineage. Overall, the data suggest that two distinct molecular evolutionary mechanisms can create a functionally diverse developmental signaling pathway. These are the recent dynamic generation of new genes and ligand–receptor interactions as seen in the Wnt pathway and the conservative adaptation of ancient pre-existing genes to new roles as seen in the TGFβ pathway. From a practical perspective, the former mechanism limits the investigator’s ability to transfer knowledge of specific pathway functions across species while the latter facilitates knowledge transfer.

Phylogenomic analyses reveal the evolutionary origin of the inhibin alpha-subunit, a unique TGFbeta superfamily antagonist

Transforming growth factor-beta (TGFβ) homologues form a diverse superfamily that arose early in animal evolution and control cellular function through membrane-spanning, conserved serine-threonine kinases (RII and RI receptors). Activin and inhibin are related dimers within the TGFβ superfamily that share a common β-subunit. The evolution of the inhibin α-subunit created the only antagonist within the TGFβ superfamily and the only member known to act as an endocrine hormone. This hormone introduced a new level of complexity and control to vertebrate reproductive function. The novel functions of the inhibin α-subunit appear to reflect specific insertion-deletion changes within the inhibin β-subunit that occurred during evolution. Using phylogenomic analysis, we correlated specific insertions with the acquisition of distinct functions that underlie the phenotypic complexity of vertebrate reproductive processes. This phylogenomic approach presents a new way of understanding the structure-function relationships between inhibin, activin, and the larger TGFβ superfamily.

The Drosophila DPP signal is produced by cleavage of its proprotein at evolutionary diversified furin-recognition sites

Maturation of bone morphogenetic proteins (BMPs) requires cleavage of their precursor proteins by furin-type proprotein convertases. Here, we find that cleavage sites of the BMP2/4/decapentaplegic (DPP) subfamily have been evolutionary diversified and can be categorized into 4 different types. Cnidaria BMP2/4/DPP is considered to be a prototype containing only 1 furin site. Bilateria BMP2/4/DPP acquired an additional cleavage site with either the combination of minimal-optimal or optimal-optimal furin sites. DPPs belonging to Diptera, such as Drosophila and mosquito, and Lepidoptera of silkworm contain a third cleavage site between the 2 optimal furin sites. We studied how the 3 furin sites (FSI-III) of Drosophila DPP coordinate maturation of ligands and contribute to signals in vivo. Combining mutational analysis of furin-recognition sites and RNAi experiments, we found that the Drosophila DPP precursor is initially cleaved at an upstream furin-recognition site (FSII), with consequent cleavages at 2 furin sites (FSI and FSIII). Both Dfurin1 and Dfurin2 are involved in the processing of DPP proproteins. Biochemical and genetic analyses using cleavage mutants of DPP suggest the first cleavage at FSII to be critical and sufficient for long-range DPP signaling. Our data suggest that the Drosophila DPP precursor is cleaved in a different manner from vertebrate BMP4 even though they are functional orthologs. This indicates that the furin-cleavage sites in BMP2/4/DPP precursors are tolerant to mutations acquired through evolution and have adapted to different systems in diversified species.