Cell and tissue dynamics during Tribolium embryogenesis revealed by versatile fluorescence labeling approaches

Embryogenesis in Myrmicine Ants Combines Features of Short Germ-Band Development With a Progressive Mode of Segmentation

Ants exhibit complex social organization, morphologically and functionally distinct castes, and the exploitation of diverse ecological niches. How these features have influenced embryonic development relative to other insects remains unclear. Insect embryogenesis has been classified into three modes: In long germ-band development, exemplified by the fruit fly Drosophila melanogaster, segments along the entire anterior-posterior axis of the embryonic primordium are established almost simultaneously, before gastrulation, with the initial embryonic primordium surrounding almost the entire volume of the egg. In short and intermediate germ-band modes, the embryonic primordium occupies a smaller proportion of the egg surface, with anterior segments initially specified, and remaining segments being added sequentially from a posterior growth zone. Here, we examine embryogenesis in three myrmicine ants, the fungus-gardening ants Atta texana and Mycocepurus smithii, and the red imported fire ant Solenopsis invicta. We find that these ant embryos combine features of short germ-band development with a newly characterized progressive pattern of segmentation that has been associated with some long germ-band-developing insects. Despite similarities in the size of ant and Drosophila eggs, embryogenesis in the three ant species is 10- to 20-fold longer than in Drosophila and is also significantly longer than in two other hymenopteran species that have been studied, the honeybee Apis mellifera and the jewel wasp Nasonia vitripennis. Moreover, the embryos produced by A. texana foundress queens develop to first instar larvae 25% faster than embryos produced by mature queens. We discuss these results in the context of the eusocial lifestyle of ants.

Vertex models capturing subcellular scales in epithelial tissues

Vertex models provide a robust theoretical framework for studying epithelial tissues as a network of cell boundaries. They have been pivotal in exploring properties such as cell packing geometry and rigidity transitions. Recently, extended vertex models have become instrumental in bridging the subcellular scales to the tissue scale. Here, we review extensions of the model aiming to capture experimentally observed subcellular features of epithelial tissues including heterogeneity in myosin activity across the tissue, non-uniform contractility structures, and mechanosensitive feedback loops. We discuss how these extensions change and challenge current perspectives on observables of macroscopic tissue properties. First, we find that extensions to the vertex model can change model properties significantly, impacting the critical threshold and in some cases even the existence of a rigidity transition. Second, we find that packing disorder can be explained by models employing different subcellular mechanisms, indicating a source of stochasticity and gradual local size changes as common mesoscopic motifs in the mechanics of tissue organization. We address complementary models and statistical inference, putting vertex models in a broader methodological context and we give a brief overview of software packages utilized in increasingly complex vertex model studies. Our review emphasizes the need for more comparative, systematic studies that identify specific classes of vertex models which share a set of well-defined properties, as well as a more in-depth discussion of modeling choices and their biological motivations.

The cryptonephridial/rectal complex: an evolutionary adaptation for water and ion conservation

Arthropods have integrated digestive and renal systems, which function to acquire and maintain homeostatically the substances they require for survival. The cryptonephridial complex (CNC) is an evolutionary novelty in which the renal organs and gut have been dramatically reorganised. Parts of the renal or Malpighian tubules (MpTs) form a close association with the surface of the rectum, and are surrounded by a novel tissue, the perinephric membrane, which acts to insulate the system from the haemolymph and thus allows tight regulation of ions and water into and out of the CNC. The CNC can reclaim water and solutes from the rectal contents and recycle these back into the haemolymph. Fluid flow in the MpTs runs counter to flow within the rectum. It is this countercurrent arrangement that underpins its powerful recycling capabilities, and represents one of the most efficient water conservation mechanisms in nature. CNCs appear to have evolved multiple times, and are present in some of the largest and most evolutionarily successful insect groups including the larvae of most Lepidoptera and in a major beetle lineage (Cucujiformia + Bostrichoidea), suggesting that the CNC is an important adaptation. Here we review the knowledge of this remarkable organ system gained over the past 200 years. We first focus on the CNCs of tenebrionid beetles, for which we have an in-depth understanding from physiological, structural and ultrastructural studies (primarily in Tenebrio molitor), which are now being extended by studies in Tribolium castaneum enabled by advances in molecular and microscopy approaches established for this species. These recent studies are beginning to illuminate CNC development, physiology and endocrine control. We then take a broader view of arthropod CNCs, phylogenetically mapping their reported occurrence to assess their distribution and likely evolutionary origins. We explore CNCs from an ecological viewpoint, put forward evidence that CNCs may primarily be adaptations for facing the challenges of larval life, and argue that their loss in many aquatic species could point to a primary function in conserving water in terrestrial species. Finally, by considering the functions of renal and digestive epithelia in insects lacking CNCs, as well as the typical architecture of these organs in relation to one another, we propose that ancestral features of these organs predispose them for the evolution of CNCs.

Tissue-Level Integration Overrides Gradations of Differentiating Cell Identity in Beetle Extraembryonic Tissue

During animal embryogenesis, one of the earliest specification events distinguishes extraembryonic (EE) from embryonic tissue fates: the serosa in the case of the insects. While it is well established that the homeodomain transcription factor Zen1 is the critical determinant of the serosa, the subsequent realization of this tissue's identity has not been investigated. Here, we examine serosal differentiation in the beetle Tribolium castaneum based on the quantification of morphological and morphogenetic features, comparing embryos from a Tc-zen1 RNAi dilution series, where complete knockdown results in amnion-only EE tissue identity. We assess features including cell density, tissue boundary morphology, and nuclear size as dynamic readouts for progressive tissue maturation. While some features exhibit an all-or-nothing outcome, other key features show dose-dependent phenotypic responses with trait-specific thresholds. Collectively, these findings provide nuance beyond the known status of Tc-Zen1 as a selector gene for serosal tissue patterning. Overall, our approach illustrates how the analysis of tissue maturation dynamics from live imaging extends but also challenges interpretations based on gene expression data, refining our understanding of tissue identity and when it is achieved.

Next generation marker-based vector concepts for rapid and unambiguous identification of single and double homozygous transgenic organisms

For diploid model organisms, the actual transgenesis processes require subsequent periods of transgene management, which are challenging in emerging model organisms due to the lack of suitable methodology. We used the red flour beetle Tribolium castaneum, a stored-grain pest, to perform a comprehensive functional evaluation of our AClashOfStrings (ACOS) and the combined AGameOfClones/AClashOfStrings (AGOC/ACOS) vector concepts, which use four clearly distinguishable markers to provide full visual control over up to two independent transgenes. We achieved comprehensive statistical validation of our approach by systematically creating seventeen novel single and double homozygous sublines intended for fluorescence live imaging, including several sublines in which the microtubule cytoskeleton is labeled. During the mating procedures, we genotyped more than 20,000 individuals in less than 80 working hours, which corresponds to about 10 to 15 s per individual. We also confirm the functionality of our combined concept in two double transgene special cases, i.e. integration of both transgenes in close proximity on the same chromosome and integration of one transgene on the X allosome. Finally, we discuss our vector concepts regarding performance, genotyping accuracy, throughput, resource saving potential, fluorescent protein choice, modularity, adaptation to other diploid model organisms and expansion capability.

Sculpting with stiffness: rigidity as a regulator of morphogenesis

From a physical perspective, morphogenesis of tissues results from interplay between their material properties and the mechanical forces exerted on them. The importance of mechanical forces in influencing cell behaviour is widely recognised, whereas the importance of tissue material properties in vivo, like stiffness, has only begun to receive attention in recent years. In this mini-review, we highlight key themes and concepts that have emerged related to how tissue stiffness, a fundamental material property, guides various morphogenetic processes in living organisms.

Nfinder: automatic inference of cell neighborhood in 2D and 3D using nuclear markers

BACKGROUND

In tissues and organisms, the coordination of neighboring cells is essential to maintain their properties and functions. Therefore, knowing which cells are adjacent is crucial to understand biological processes that involve physical interactions among them, e.g. cell migration and proliferation. In addition, some signaling pathways, such as Notch or extrinsic apoptosis, are highly dependent on cell-cell communication. While this is straightforward to obtain from membrane images, nuclei labelling is much more ubiquitous for technical reasons. However, there are no automatic and robust methods to find neighboring cells based only on nuclear markers.

RESULTS

In this work, we describe Nfinder, a method to assess the cell's local neighborhood from images with nuclei labeling. To achieve this goal, we approximate the cell-cell interaction graph by the Delaunay triangulation of nuclei centroids. Then, links are filtered by automatic thresholding in cell-cell distance (pairwise interaction) and the maximum angle that a pair of cells subtends with shared neighbors (non-pairwise interaction). We systematically characterized the detection performance by applying Nfinder to publicly available datasets from Drosophila melanogaster, Tribolium castaneum, Arabidopsis thaliana and C. elegans. In each case, the result of the algorithm was compared to a cell neighbor graph generated by manually annotating the original dataset. On average, our method detected 95% of true neighbors, with only 6% of false discoveries. Remarkably, our findings indicate that taking into account non-pairwise interactions might increase the Positive Predictive Value up to + 11.5%.

CONCLUSION

Nfinder is the first robust and automatic method for estimating neighboring cells in 2D and 3D based only on nuclear markers and without any free parameters. Using this tool, we found that taking non-pairwise interactions into account improves the detection performance significantly. We believe that using our method might improve the effectiveness of other workflows to study cell-cell interactions from microscopy images. Finally, we also provide a reference implementation in Python and an easy-to-use napari plugin.

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'.

Virtual spherical-shaped multicellular platform for simulating the morphogenetic processes of spider-like body axis formation

Remodeling of multicellular architecture is a critical developmental process for shaping the axis of a bilaterally symmetric animal body and involves coordinated cell–cell interactions and cell rearrangement. In arthropods, the early embryonic process that leads to the segmented body axis varies at the cellular and molecular levels depending on the species. Developmental studies using insect and spider model species have provided specific examples of these diversified mechanisms that regulate axis formation and segmentation in arthropod embryos. However, there are few theoretical models for how diversity in the early embryonic process occurred during evolution, in part because of a limited computational infrastructure. We developed a virtual spherical-shaped multicellular platform to reproduce body axis-forming processes. Each virtual cell behaves according to the cell vertex model, with the computational program organized in a hierarchical order from cells and tissues to whole embryos. Using an initial set of two different mechanical states for cell differentiation and global directional signals that are linked to the planar polarity of each cell, the virtual cell assembly exhibited morphogenetic processes similar to those observed in spider embryos. We found that the development of an elongating body axis is achieved through implementation of an interactive cell polarity parameter associated with edge tension at the cell–cell adhesion interface, with no local control of the cell division rate and direction. We also showed that modifying the settings can cause variation in morphogenetic processes. This platform also can embed a gene network that generates waves of gene expression in a virtual dynamic multicellular field. This study provides a computational platform for testing the development and evolution of animal body patterns.

The organizing role of Wnt signaling pathway during arthropod posterior growth

Wnt signaling pathways are recognized for having major roles in tissue patterning and cell proliferation. In the last years, remarkable progress has been made in elucidating the molecular and cellular mechanisms that underlie sequential segmentation and axial elongation in various arthropods, and the canonical Wnt pathway has emerged as an essential factor in these processes. Here we review, with a comparative perspective, the current evidence concerning the participation of this pathway during posterior growth, its degree of conservation among the different subphyla within Arthropoda and its relationship with the rest of the gene regulatory network involved. Furthermore, we discuss how this signaling pathway could regulate segmentation to establish this repetitive pattern and, at the same time, probably modulate different cellular processes precisely coupled to axial elongation. Based on the information collected, we suggest that this pathway plays an organizing role in the formation of the body segments through the regulation of the dynamic expression of segmentation genes, via controlling the caudal gene, at the posterior region of the embryo/larva, that is necessary for the correct sequential formation of body segments in most arthropods and possibly in their common segmented ancestor. On the other hand, there is insufficient evidence to link this pathway to axial elongation by controlling its main cellular processes, such as convergent extension and cell proliferation. However, conclusions are premature until more studies incorporating diverse arthropods are carried out.

The red flour beetle T. castaneum: elaborate genetic toolkit and unbiased large scale RNAi screening to study insect biology and evolution

The red flour beetle Tribolium castaneum has emerged as an important insect model system for a variety of topics. With respect to studying gene function, it is second only to the vinegar fly D. melanogaster. The RNAi response in T. castaneum is exceptionally strong and systemic, and it appears to target all cell types and processes. Uniquely for emerging model organisms, T. castaneum offers the opportunity of performing time- and cost-efficient large-scale RNAi screening, based on commercially available dsRNAs targeting all genes, which are simply injected into the body cavity. Well established transgenic and genome editing approaches are met by ease of husbandry and a relatively short generation time. Consequently, a number of transgenic tools like UAS/Gal4, Cre/Lox, imaging lines and enhancer trap lines are already available. T. castaneum has been a genetic experimental system for decades and now has become a workhorse for molecular and reverse genetics as well as in vivo imaging. Many aspects of development and general biology are more insect-typical in this beetle compared to D. melanogaster. Thus, studying beetle orthologs of well-described fly genes has allowed macro-evolutionary comparisons in developmental processes such as axis formation, body segmentation, and appendage, head and brain development. Transgenic approaches have opened new ways for in vivo imaging. Moreover, this emerging model system is the first choice for research on processes that are not represented in the fly, or are difficult to study there, e.g. extraembryonic tissues, cryptonephridial organs, stink gland function, or dsRNA-based pesticides.

Cellular basis of limb morphogenesis

How the size and shape of developing tissues is encoded in the genome has been a longstanding riddle for biologists. Constituent cells integrate several genetic and mechanical signals to decide whether to divide, die, change shape or position. We review here how morphogenetic cell behaviors contribute to leg formation from imaginal disc epithelia in the insect Drosophila melanogaster, as well as to direct embryonic limb outgrowths in the non-insect pancrustacean Parhyale hawaiensis. Considering the deep conservation of developmental programs for limb patterning among arthropods and other bilaterians, moving forward, it will be exciting to see how these genetic similarities reflect at the cellular and tissue mechanics level.

QuickPIV: Efficient 3D particle image velocimetry software applied to quantifying cellular migration during embryogenesis

BACKGROUND

The technical development of imaging techniques in life sciences has enabled the three-dimensional recording of living samples at increasing temporal resolutions. Dynamic 3D data sets of developing organisms allow for time-resolved quantitative analyses of morphogenetic changes in three dimensions, but require efficient and automatable analysis pipelines to tackle the resulting Terabytes of image data. Particle image velocimetry (PIV) is a robust and segmentation-free technique that is suitable for quantifying collective cellular migration on data sets with different labeling schemes. This paper presents the implementation of an efficient 3D PIV package using the Julia programming language-quickPIV. Our software is focused on optimizing CPU performance and ensuring the robustness of the PIV analyses on biological data.

RESULTS

QuickPIV is three times faster than the Python implementation hosted in openPIV, both in 2D and 3D. Our software is also faster than the fastest 2D PIV package in openPIV, written in C++. The accuracy evaluation of our software on synthetic data agrees with the expected accuracies described in the literature. Additionally, by applying quickPIV to three data sets of the embryogenesis of Tribolium castaneum, we obtained vector fields that recapitulate the migration movements of gastrulation, both in nuclear and actin-labeled embryos. We show normalized squared error cross-correlation to be especially accurate in detecting translations in non-segmentable biological image data.

CONCLUSIONS

The presented software addresses the need for a fast and open-source 3D PIV package in biological research. Currently, quickPIV offers efficient 2D and 3D PIV analyses featuring zero-normalized and normalized squared error cross-correlations, sub-pixel/voxel approximation, and multi-pass. Post-processing options include filtering and averaging of the resulting vector fields, extraction of velocity, divergence and collectiveness maps, simulation of pseudo-trajectories, and unit conversion. In addition, our software includes functions to visualize the 3D vector fields in Paraview.

Image Processing Filters for Grids of Cells Analogous to Filters Processing Grids of Pixels

Intra- and extra-cellular processes shape tissues together. For understanding how neighborhood relationships between cells play a role in this process, having image processing filters based on these relationships would be beneficial. Those operations are known and their application to microscopy image data typically requires programming skills. User-friendly general purpose tools for pursuing image processing on a level of neighboring cells were yet missing. In this manuscript I demonstrate image processing filters which process grids of cells on tissue level and the analogy to their better known counter parts processing grids of pixels. The tools are available as part of free and open source software in the ImageJ/Fiji and napari ecosystems and their application does not require any programming experience.

Patterning with clocks and genetic cascades: Segmentation and regionalization of vertebrate versus insect body plans

Oscillatory and sequential processes have been implicated in the spatial patterning of many embryonic tissues. For example, molecular clocks delimit segmental boundaries in vertebrates and insects and mediate lateral root formation in plants, whereas sequential gene activities are involved in the specification of regional identities of insect neuroblasts, vertebrate neural tube, vertebrate limb, and insect and vertebrate body axes. These processes take place in various tissues and organisms, and, hence, raise the question of what common themes and strategies they share. In this article, we review 2 processes that rely on the spatial regulation of periodic and sequential gene activities: segmentation and regionalization of the anterior-posterior (AP) axis of animal body plans. We study these processes in species that belong to 2 different phyla: vertebrates and insects. By contrasting 2 different processes (segmentation and regionalization) in species that belong to 2 distantly related phyla (arthropods and vertebrates), we elucidate the deep logic of patterning by oscillatory and sequential gene activities. Furthermore, in some of these organisms (e.g., the fruit fly Drosophila), a mode of AP patterning has evolved that seems not to overtly rely on oscillations or sequential gene activities, providing an opportunity to study the evolution of pattern formation mechanisms.

A deterministic genotyping workflow reduces waste of transgenic individuals by two-thirds

We present a deterministic workflow for genotyping single and double transgenic individuals directly upon nascence that prevents overproduction and reduces wasted animals by two-thirds. In our vector concepts, transgenes are accompanied by two of four clearly distinguishable transformation markers that are embedded in interweaved, but incompatible Lox site pairs. Following Cre-mediated recombination, the genotypes of single and double transgenic individuals were successfully identified by specific marker combinations in 461 scorings.

Functional analysis of sense organ specification in the Tribolium castaneum larva reveals divergent mechanisms in insects

Abstract

Insects and other arthropods utilise external sensory structures for mechanosensory, olfactory, and gustatory reception. These sense organs have characteristic shapes related to their function, and in many cases are distributed in a fixed pattern so that they are identifiable individually. In Drosophila melanogaster, the identity of sense organs is regulated by specific combinations of transcription factors. In other arthropods, however, sense organ subtypes cannot be linked to the same code of gene expression. This raises the questions of how sense organ diversity has evolved and whether the principles underlying subtype identity in D. melanogaster are representative of other insects. Here, we provide evidence that such principles cannot be generalised, and suggest that sensory organ diversification followed the recruitment of sensory genes to distinct sensory organ specification mechanism.

Results

We analysed sense organ development in a nondipteran insect, the flour beetle Tribolium castaneum, by gene expression and RNA interference studies. We show that in contrast to D. melanogaster, T. castaneum sense organs cannot be categorised based on the expression or their requirement for individual or combinations of conserved sense organ transcription factors such as cut and pox neuro, or members of the Achaete-Scute (Tc ASH, Tc asense), Atonal (Tc atonal, Tc cato, Tc amos), and neurogenin families (Tc tap). Rather, our observations support an evolutionary scenario whereby these sensory genes are required for the specification of sense organ precursors and the development and differentiation of sensory cell types in diverse external sensilla which do not fall into specific morphological and functional classes.

Conclusions

Based on our findings and past research, we present an evolutionary scenario suggesting that sense organ subtype identity has evolved by recruitment of a flexible sensory gene network to the different sense organ specification processes. A dominant role of these genes in subtype identity has evolved as a secondary effect of the function of these genes in individual or subsets of sense organs, probably modulated by positional cues.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12915-021-00948-y.

Spatiotemporal variation in cell proliferation patterns during arthropod axial elongation

An elongated and segmented body plan is a common morphological characteristic of all arthropods and is probably responsible for their high adaptation ability to diverse environments. Most arthropods form their bodies by progressively adding segments, resembling vertebrate somitogenesis. This sequential segmentation relies on a molecular clock that operates in the posterior region of the elongating embryo that combines dynamically with cellular behaviors and tissue rearrangements, allowing the extension of the developing body along its main embryonic axis. Even though the molecular mechanisms involved in elongation and segment formation have been found to be conserved in a considerable degree, cellular processes such as cell division are quite variable between different arthropods. In this study, we show that cell proliferation in the beetle Tribolium castaneum has a nonuniform spatiotemporal patterning during axial elongation. We found that dividing cells are preferentially oriented along the anterior-posterior axis, more abundant and posteriorly localized during thoracic segments formation and that this cell proliferation peak was triggered at the onset of axis elongation. This raise in cell divisions, in turn, was correlated with an increase in the elongation rate, but not with changes in cell density. When DNA synthesis was inhibited over this period, both the area and length of thoracic segments were significantly reduced but not of the first abdominal segment. We discuss the variable participation that different cell division patterns and cell movements may have on arthropod posterior growth and their evolutionary contribution.

Communal living: the role of polyploidy and syncytia in tissue biology

Multicellular organisms are composed of tissues with diverse cell sizes. Whether a tissue primarily consists of numerous, small cells as opposed to fewer, large cells can impact tissue development and function. The addition of nuclear genome copies within a common cytoplasm is a recurring strategy to manipulate cellular size within a tissue. Cells with more than two genomes can exist transiently, such as in developing germlines or embryos, or can be part of mature somatic tissues. Such nuclear collectives span multiple levels of organization, from mononuclear or binuclear polyploid cells to highly multinucleate structures known as syncytia. Here, we review the diversity of polyploid and syncytial tissues found throughout nature. We summarize current literature concerning tissue construction through syncytia and/or polyploidy and speculate why one or both strategies are advantageous.

Regionalized tissue fluidization is required for epithelial gap closure during insect gastrulation

Many animal embryos pull and close an epithelial sheet around the ellipsoidal egg surface during a gastrulation process known as epiboly. The ovoidal geometry dictates that the epithelial sheet first expands and subsequently compacts. Moreover, the spreading epithelium is mechanically stressed and this stress needs to be released. Here we show that during extraembryonic tissue (serosa) epiboly in the insect Tribolium castaneum, the non-proliferative serosa becomes regionalized into a solid-like dorsal region with larger non-rearranging cells, and a more fluid-like ventral region surrounding the leading edge with smaller cells undergoing intercalations. Our results suggest that a heterogeneous actomyosin cable contributes to the fluidization of the leading edge by driving sequential eviction and intercalation of individual cells away from the serosa margin. Since this developmental solution utilized during epiboly resembles the mechanism of wound healing, we propose actomyosin cable-driven local tissue fluidization as a conserved morphogenetic module for closure of epithelial gaps.

The mechanics of embryonic tissue spreading over spherical eggs is not fully understood. Here, the authors show that during gastrulation in the red flour beetle, extraembryonic tissue epiboly is facilitated by local actomyosin-mediated fluidization of the tissue at the leading edge.

Unexpected mutual regulation underlies paralogue functional diversification and promotes epithelial tissue maturation in Tribolium

Insect Hox3/zen genes represent an evolutionary hotspot for changes in function and copy number. Single orthologues are required either for early specification or late morphogenesis of the extraembryonic tissues, which protect the embryo. The tandemly duplicated zen paralogues of the beetle Tribolium castaneum present a unique opportunity to investigate both functions in a single species. We dissect the paralogues' expression dynamics (transcript and protein) and transcriptional targets (RNA-seq after RNAi) throughout embryogenesis. We identify an unexpected role of Tc-Zen2 in repression of Tc-zen1, generating a negative feedback loop that promotes developmental progression. Tc-Zen2 regulation is dynamic, including within co-expressed multigene loci. We also show that extraembryonic development is the major event within the transcriptional landscape of late embryogenesis and provide a global molecular characterization of the extraembryonic serosal tissue. Altogether, we propose that paralogue mutual regulation arose through multiple instances of zen subfunctionalization, leading to their complementary extant roles.

Elongation during segmentation shows axial variability, low mitotic rates, and synchronized cell cycle domains in the crustacean, Thamnocephalus platyurus

Background

Segmentation in arthropods typically occurs by sequential addition of segments from a posterior growth zone. However, the amount of tissue required for growth and the cell behaviors producing posterior elongation are sparsely documented.

Results

Using precisely staged larvae of the crustacean, Thamnocephalus platyurus, we systematically examine cell division patterns and morphometric changes associated with posterior elongation during segmentation. We show that cell division occurs during normal elongation but that cells in the growth zone need only divide ~ 1.5 times to meet growth estimates; correspondingly, direct measures of cell division in the growth zone are low. Morphometric measurements of the growth zone and of newly formed segments suggest tagma-specific features of segment generation. Using methods for detecting two different phases in the cell cycle, we show distinct domains of synchronized cells in the posterior trunk. Borders of cell cycle domains correlate with domains of segmental gene expression, suggesting an intimate link between segment generation and cell cycle regulation.

Conclusions

Emerging measures of cellular dynamics underlying posterior elongation already show a number of intriguing characteristics that may be widespread among sequentially segmenting arthropods and are likely a source of evolutionary variability. These characteristics include: the low rates of posterior mitosis, the apparently tight regulation of cell cycle at the growth zone/new segment border, and a correlation between changes in elongation and tagma boundaries.

From spiral cleavage to bilateral symmetry: the developmental cell lineage of the annelid brain

BACKGROUND

During early development, patterns of cell division-embryonic cleavage-accompany the gradual restriction of blastomeres to specific cell fates. In Spiralia, which include annelids, mollusks, and flatworms, "spiral cleavage" produces a highly stereotypic, spiral-like arrangement of blastomeres and swimming trochophore-type larvae with rotational (spiral) symmetry. However, starting at larval stages, spiralian larvae acquire elements of bilateral symmetry, before they metamorphose into fully bilateral juveniles. How this spiral-to-bilateral transition occurs is not known and is especially puzzling for the early differentiating brain and head sensory organs, which emerge directly from the spiral cleavage pattern. Here we present the developmental cell lineage of the Platynereis larval episphere.

RESULTS

Live-imaging recordings from the zygote to the mid-trochophore stage (~ 30 hpf) of the larval episphere of the marine annelid Platynereis dumerilii reveal highly stereotypical development and an invariant cell lineage of early differentiating cell types. The larval brain and head sensory organs develop from 11 pairs of bilateral founders, each giving rise to identical clones on the right and left body sides. Relating the origin of each bilateral founder pair back to the spiral cleavage pattern, we uncover highly divergent origins: while some founder pairs originate from corresponding cells in the spiralian lineage on each body side, others originate from non-corresponding cells, and yet others derive from a single cell within one quadrant. Integrating lineage and gene expression data for several embryonic and larval stages, we find that the conserved head patterning genes otx and six3 are expressed in bilateral founders representing divergent lineage histories and giving rise to early differentiating cholinergic neurons and head sensory organs, respectively.

CONCLUSIONS

We present the complete developmental cell lineage of the Platynereis larval episphere, and thus the first comprehensive account of the spiral-to-bilateral transition in a developing spiralian. The bilateral symmetry of the head emerges from pairs of bilateral founders, similar to the trunk; however, the head founders are more numerous and show striking left-right asymmetries in lineage behavior that we relate to differential gene expression.

Fog signaling has diverse roles in epithelial morphogenesis in insects

The Drosophila Fog pathway represents one of the best-understood signaling cascades controlling epithelial morphogenesis. During gastrulation, Fog induces apical cell constrictions that drive the invagination of mesoderm and posterior gut primordia. The cellular mechanisms underlying primordia internalization vary greatly among insects and recent work has suggested that Fog signaling is specific to the fast mode of gastrulation found in some flies. On the contrary, here we show in the beetle Tribolium, whose development is broadly representative for insects, that Fog has multiple morphogenetic functions. It modulates mesoderm internalization and controls a massive posterior infolding involved in gut and extraembryonic development. In addition, Fog signaling affects blastoderm cellularization, primordial germ cell positioning, and cuboidal-to-squamous cell shape transitions in the extraembryonic serosa. Comparative analyses with two other distantly related insect species reveals that Fog's role during cellularization is widely conserved and therefore might represent the ancestral function of the pathway.

Decoupling from yolk sac is required for extraembryonic tissue spreading in the scuttle fly Megaselia abdita

Extraembryonic tissues contribute to animal development, which often entails spreading over embryo or yolk. Apart from changes in cell shape, the requirements for this tissue spreading are not well understood. Here, we analyze spreading of the extraembryonic serosa in the scuttle fly Megaselia abdita. The serosa forms from a columnar blastoderm anlage, becomes a squamous epithelium, and eventually spreads over the embryo proper. We describe the dynamics of this process in long-term, whole-embryo time-lapse recordings, demonstrating that free serosa spreading is preceded by a prolonged pause in tissue expansion. Closer examination of this pause reveals mechanical coupling to the underlying yolk sac, which is later released. We find mechanical coupling prolonged and serosa spreading impaired after knockdown of M. abdita Matrix metalloprotease 1. We conclude that tissue–tissue interactions provide a critical functional element to constrain spreading epithelia.

Mechanics of epithelial tissue formation

A key process in the life of any multicellular organism is its development from a single egg into a full grown adult. The first step in this process often consists of forming a tissue layer out of randomly placed cells on the surface of the egg. We present a model for generating such a tissue, based on mechanical interactions between the cells, and find that the resulting cellular pattern corresponds to the Voronoi tessellation of the nuclei of the cells. Experimentally, we obtain the same result in both fruit flies and flour beetles, with a distribution of cell shapes that matches that of the model, without any adjustable parameters. Finally, we show that this pattern is broken when the cells grow at different rates.

A revised understanding of Tribolium morphogenesis further reconciles short and long germ development

In Drosophila melanogaster, the germband forms directly on the egg surface and solely consists of embryonic tissue. In contrast, most insect embryos undergo a complicated set of tissue rearrangements to generate a condensed, multilayered germband. The ventral side of the germband is embryonic, while the dorsal side is thought to be an extraembryonic tissue called the amnion. While this tissue organisation has been accepted for decades and has been widely reported in insects, its accuracy has not been directly tested in any species. Using live cell tracking and differential cell labelling in the short germ beetle Tribolium castaneum, I show that most of the cells previously thought to be amnion actually give rise to large parts of the embryo. This process occurs via the dorsal-to-ventral flow of cells and contributes to germband extension (GBE). In addition, I show that true 'amnion' cells in Tribolium originate from a small region of the blastoderm. Together, my findings show that development in the short germ embryos of Tribolium and the long germ embryos of Drosophila is more similar than previously proposed. Dorsal-to-ventral cell flow also occurs in Drosophila during GBE, and I argue that the flow is driven by a conserved set of underlying morphogenetic events in both species. Furthermore, the revised Tribolium fate map that I present is far more similar to that of Drosophila than the classic Tribolium fate map. Lastly, my findings show that there is no qualitative difference between the tissue structure of the cellularised blastoderm and the short/intermediate germ germband. As such, the same tissue patterning mechanisms could function continuously throughout the cellularised blastoderm and germband stages, and easily shift between them over evolutionary time.

Evo-Devo: Universal Toll Pass for the Extension Highway?

The distinction between long-germ and short-germ insects is a classic one in evo-devo, yet a common genetic mechanism may underlie germband extension in all insects, even all arthropods.

Double abdomen in a short-germ insect: Zygotic control of axis formation revealed in the beetle Tribolium castaneum

One of the first crucial steps of animal development is to distinguish the anterior versus the posterior pole of the embryo, i.e., the AP axis. If this process fails, embryos may develop two mirror image tails or heads. In the fly Drosophila, the mother provides the signals required for AP axis formation, while in vertebrates, gene activity of the embryo is required as well. We identified two genes whose knockdown leads to double-tail phenotypes in the beetle Tribolium, representing the insect-typical short-germ embryogenesis. Intriguingly, embryo polarity depends on zygotic gene activities and Wnt signaling. Hence, short-germ insect axis formation is more similar to vertebrates than the mechanism employed by Drosophila.

The distinction of anterior versus posterior is a crucial first step in animal embryogenesis. In the fly Drosophila, this axis is established by morphogenetic gradients contributed by the mother that regulate zygotic target genes. This principle has been considered to hold true for insects in general but is fundamentally different from vertebrates, where zygotic genes and Wnt signaling are required. We investigated symmetry breaking in the beetle Tribolium castaneum, which among insects represents the more ancestral short-germ embryogenesis. We found that maternal Tc-germ cell-less is required for anterior localization of maternal Tc-axin, which represses Wnt signaling and promotes expression of anterior zygotic genes. Both RNAi targeting Tc-germ cell-less or double RNAi knocking down the zygotic genes Tc-homeobrain and Tc-zen1 led to the formation of a second growth zone at the anterior, which resulted in double-abdomen phenotypes. Conversely, interfering with two posterior factors, Tc-caudal and Wnt, caused double-anterior phenotypes. These findings reveal that maternal and zygotic mechanisms, including Wnt signaling, are required for establishing embryo polarity and induce the segmentation clock in a short-germ insect.

The exon junction complex factor Y14 is dynamic in the nucleus of the beetle Tribolium castaneum during late oogenesis

Background

The oocyte chromosomes of the red flour beetle, Tribolium castaneum, are gathered into a knot, forming a karyosphere at the diplotene stage of meiotic prophase. Chromatin rearrangement, which is a characteristic feature of oocyte maturation, is well documented. The T. castaneum karyosphere is surrounded by a complex extrachromosomal structure termed the karyosphere capsule. The capsule contains the vast majority of oocyte RNA. We have previously shown using a BrUTP assay that oocyte chromosomes in T. castaneum maintain residual transcription up to the very end of oocyte maturation. Karyosphere transcription requires evidently not only transcription factors but also mRNA processing factors, including the components of the exon junction complex with its core component, the splicing factor Y14. We employed a gene engineering approach with injection of mRNA derived from the Myc-tagged Y14 plasmid-based construct in order to monitor the newly synthesized fusion protein in the oocyte nuclei.

Results

Our preliminary data have been presented as a brief correspondence elsewhere. Here, we provide a full-length article including immunoelectron-microscopy localization data on Y14–Myc distribution in the nucleus of previtellogenic and vitellogenic oocytes. The injections of the fusion protein Y14–Myc mRNA into the oocytes showed a dynamic pattern of the protein distribution. At the previtellogenic stage, there are two main locations for the protein: SC35 domains (the analogues of interchromatin granule clusters or nuclear speckles) and the karyosphere capsule. At the vitellogenic stage, SC35 domains were devoid of labels, and Y14–Myc was found in the perichromatin region of the karyosphere, presumably at the places of residual transcription. We show that karyosphere formation is accompanied by the movement of a nuclear protein while the residual transcription occurs during genome inactivation.

Conclusions

Our data indicate that the karyosphere capsule, being a destination site for a protein involved in mRNA splicing and export, is not only a specializes part of nuclear matrix separating the karyosphere from the products of chromosome activity, as believed previously, but represents a special nuclear compartment involved in the processes of gene expression in the case the karyosphere retains residual transcription activity.

Electronic supplementary material

The online version of this article (10.1186/s13039-017-0342-4) contains supplementary material, which is available to authorized users.

Contribution of cell proliferation to axial elongation in the red flour beetle Tribolium castaneum

Most arthropods generate their posterior bodies by adding segments periodically, as the embryo grows, from a posteriorly located region called the segment addition zone. This mode of segmentation is shared with vertebrates and relies on oscillatory mechanisms, where the temporal periodicity of a clock is translated into repetitive spatial patterns. This ordered anterior-to-posterior pattern is achieved at the same time as the tissue elongates, opening the question of the functional coordination between the mechanisms of segmental patterning and posterior growth. The study of these processes in different arthropods has played an important role in unravelling some of the molecular mechanisms of segment formation. However, the behavior of cells during elongation and how cellular processes affect this segmental patterning has been poorly studied. Cell proliferation together with cell rearrangements are presumed to be the major forces driving axis elongation in the red flour beetle Tribolium castaneum. However, there still no strong evidence about the role and distribution of cell proliferation within the embryo. In this study, we propose to address these questions by using whole embryo cultures and pharmacological manipulation. We show that considerable cell proliferation occurs during germband elongation, measured by incorporation of the nucleoside analog of thymidine 5-Ethynyl-2’-deoxyuridine, EdU. Moreover, proliferating cells appeared to be spread along the elongating embryo with a posterior bias at early segmentation. In addition, when we blocked cell division, treated germbands were always shorter than controls and in some cases not able to fully elongate, even when control embryos already started to retract and leg buds are evident. Finally, we found that the absence of cell proliferation has no apparent effect on segmental patterning, as evidenced by Tc-engrailed (Tc-en) gene expression.

Dynamic patterning by the Drosophila pair-rule network reconciles long-germ and short-germ segmentation

Drosophila segmentation is a well-established paradigm for developmental pattern formation. However, the later stages of segment patterning, regulated by the "pair-rule" genes, are still not well understood at the system level. Building on established genetic interactions, I construct a logical model of the Drosophila pair-rule system that takes into account the demonstrated stage-specific architecture of the pair-rule gene network. Simulation of this model can accurately recapitulate the observed spatiotemporal expression of the pair-rule genes, but only when the system is provided with dynamic "gap" inputs. This result suggests that dynamic shifts of pair-rule stripes are essential for segment patterning in the trunk and provides a functional role for observed posterior-to-anterior gap domain shifts that occur during cellularisation. The model also suggests revised patterning mechanisms for the parasegment boundaries and explains the aetiology of the even-skipped null mutant phenotype. Strikingly, a slightly modified version of the model is able to pattern segments in either simultaneous or sequential modes, depending only on initial conditions. This suggests that fundamentally similar mechanisms may underlie segmentation in short-germ and long-germ arthropods.

A novel role for Ets4 in axis specification and cell migration in the spider Parasteatoda tepidariorum

Organizers play important roles during the embryonic development of many animals. The most famous example is the Spemann organizer that sets up embryonic axes in amphibian embryos. In spiders, a group of BMP secreting mesenchymal cells (the cumulus) functions as an organizer of the dorsoventral axis. Similar to experiments performed with the Spemann organizer, transplantation of the cumulus is able to induce a secondary axis in spiders. Despite the importance of this structure, it is unknown which factors are needed to activate cumulus specific gene expression. To address this question, we performed a transcriptomic analysis of early embryonic development in the spider Parasteatoda tepidariorum. Through this work, we found that the transcription factor Pt-Ets4 is needed for cumulus integrity, dorsoventral patterning and for the activation of Pt-hunchback and Pt-twist expression. Furthermore, ectopic expression of Pt-Ets4 is sufficient to induce cell delamination and migration by inducing a mesoderm-like cell fate.

Evolution and multiple roles of the Pancrustacea specific transcription factor zelda in insects

Gene regulatory networks (GRNs) evolve as a result of the coevolutionary processes acting on transcription factors (TFs) and the cis-regulatory modules they bind. The zinc-finger TF zelda (zld) is essential for the maternal-to-zygotic transition (MZT) in Drosophila melanogaster, where it directly binds over thousand cis-regulatory modules to regulate chromatin accessibility. D. melanogaster displays a long germ type of embryonic development, where all segments are simultaneously generated along the whole egg. However, it remains unclear if zld is also involved in the MZT of short-germ insects (including those from basal lineages) or in other biological processes. Here we show that zld is an innovation of the Pancrustacea lineage, being absent in more distant arthropods (e.g. chelicerates) and other organisms. To better understand zld´s ancestral function, we thoroughly investigated its roles in a short-germ beetle, Tribolium castaneum, using molecular biology and computational approaches. Our results demonstrate roles for zld not only during the MZT, but also in posterior segmentation and patterning of imaginal disc derived structures. Further, we also demonstrate that zld is critical for posterior segmentation in the hemipteran Rhodnius prolixus, indicating this function predates the origin of holometabolous insects and was subsequently lost in long-germ insects. Our results unveil new roles of zld in different biological contexts and suggest that changes in expression of zld (and probably other major TFs) are critical in the evolution of insect GRNs.

Pioneer transcription factors (TFs) are considered the first regulators of chromatin accessibility in fruit flies and vertebrates, modulating the expression of a large number of target genes. In fruit flies, zelda resembles a pioneer TF, being essential during early embryogenesis. However, the evolutionary origins and ancestral functions of zelda remain largely unknown. Through a number of gene silencing, microscopy and evolutionary analysis, the present work shows that zelda is an innovation of the Pancrustacea lineage, governing not only the MZT in the short-germ insect Tribolium castaneum, but also posterior segmentation and post-embryonic patterning of imaginal disc derived structures such as wings, legs and antennae. Further, zelda regulation of posterior segmentation predates the origin of insects with complete metamorphosis (holometabolous), as supported by gene silencing experiments in the kissing bug Rhodnius prolixus. We hypothesize that the emergence of zelda contributed to the evolution of gene regulatory networks and new morphological structures of insects.

Linking gene regulation to cell behaviors in the posterior growth zone of sequentially segmenting arthropods

Virtually all arthropods all arthropods add their body segments sequentially, one by one in an anterior to posterior progression. That process requires not only segment specification but typically growth and elongation. Here we review the functions of some of the key genes that regulate segmentation: Wnt, caudal, Notch pathway, and pair-rule genes, and discuss what can be inferred about their evolution. We focus on how these regulatory factors are integrated with growth and elongation and discuss the importance and challenges of baseline measures of growth and elongation. We emphasize a perspective that integrates the genetic regulation of segment patterning with the cellular mechanisms of growth and elongation.

Light Sheet-based Fluorescence Microscopy of Living or Fixed and Stained Tribolium castaneum Embryos

The red flour beetle Tribolium castaneum has become an important insect model organism in developmental genetics and evolutionary developmental biology. The observation of Tribolium embryos with light sheet-based fluorescence microscopy has multiple advantages over conventional widefield and confocal fluorescence microscopy. Due to the unique properties of a light sheet-based microscope, three dimensional images of living specimens can be recorded with high signal-to-noise ratios and significantly reduced photo-bleaching as well as photo-toxicity along multiple directions over periods that last several days. With more than four years of methodological development and a continuous increase of data, the time seems appropriate to establish standard operating procedures for the usage of light sheet technology in the Tribolium community as well as in the insect community at large. This protocol describes three mounting techniques suitable for different purposes, presents two novel custom-made transgenic Tribolium lines appropriate for long-term live imaging, suggests five fluorescent dyes to label intracellular structures of fixed embryos and provides information on data post-processing for the timely evaluation of the recorded data. Representative results concentrate on long-term live imaging, optical sectioning and the observation of the same embryo along multiple directions. The respective datasets are provided as a downloadable resource. Finally, the protocol discusses quality controls for live imaging assays, current limitations and the applicability of the outlined procedures to other insect species. This protocol is primarily intended for developmental biologists who seek imaging solutions that outperform standard laboratory equipment. It promotes the continuous attempt to close the gap between the technically orientated laboratories/communities, which develop and refine microscopy methodologically, and the life science laboratories/communities, which require 'plug-and-play' solutions to technical challenges. Furthermore, it supports an axiomatic approach that moves the biological questions into the center of attention.

Dynamics of growth zone patterning in the milkweed bug Oncopeltus fasciatus

We describe the dynamic process of abdominal segment generation in the milkweed bug Oncopeltus fasciatus. We present detailed morphological measurements of the growing germband throughout segmentation. Our data are complemented by cell division profiles and expression patterns of key genes, including invected and even-skipped as markers for different stages of segment formation. We describe morphological and mechanistic changes in the growth zone and in nascent segments during the generation of individual segments and throughout segmentation, and examine the relative contribution of newly formed versus existing tissue to segment formation. Although abdominal segment addition is primarily generated through the rearrangement of a pool of undifferentiated cells, there is nonetheless proliferation in the posterior. By correlating proliferation with gene expression in the growth zone, we propose a model for growth zone dynamics during segmentation in which the growth zone is functionally subdivided into two distinct regions: a posterior region devoted to a slow rate of growth among undifferentiated cells, and an anterior region in which segmental differentiation is initiated and proliferation inhibited.

Summary: A detailed analysis of posterior segment addition in an insect reveals that the growth zone is divided into two functional domains responsible for growth and differentiation.

Genome Editing of Wnt-1, a Gene Associated with Segmentation, via CRISPR/Cas9 in the Pine Caterpillar Moth, Dendrolimus punctatus

The pine caterpillar moth, Dendrolimus punctatus, is a devastating forest pest. Genetic manipulation of this insect pest is limited due to the lack of genomic and functional genomic toolsets. Recently, CRISPR/Cas9 technology has been demonstrated to be a promising approach to modify the genome. To investigate gene functions during the embryogenesis, we introduced CRISPR/Cas9 system in D. punctatus to precisely and effectively manipulate gene expressions inmutant embryos. Compared to controls, knocking out of DpWnt-1, a gene well known for its role in the early body planning, led to high embryonic mortality. Among these mutants, 32.9% of the embryos and larvae showed an abnormal development. DpWnt-1 mutants predominantly exhibited abnormal posterior segments. In addition, multiple phenotypes were observed, including the loss of limbs and the head deformation, suggesting that DpWnt-1 signaling pathway is necessary for anterior segmentation and appendage development. Overall, our results demonstrate that CRISPR/Cas9 system is feasible and efficient in inducing mutations at a specific locus in D. punctatus. This study not only lays the foundation for characterizing gene functions in a non-model species, but also facilitates the future development of pest control alternatives for a major defoliator.

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.

Long-term fluorescence live imaging of Tribolium castaneum embryos: principles, resources, scientific challenges and the comparative approach

Light sheet-based fluorescence microscopy became an important tool in insect developmental biology due to its high acquisition speed, low photo-bleaching rate and the high survival probability of the specimens. Initially applied to document the embryogenesis of Drosophila melanogaster, it is now used to investigate the embryonic morphogenesis of emerging model organisms such as the red flour beetle Tribolium castaneum. Here, we discuss the principles of light sheet-based fluorescence microscopy and outline Tribolium as a model organism for developmental biology. We summarize labeling options and present two custom-made transgenic lines suitable for live imaging. Finally, we highlight studies on Tribolium that address scientific questions with fluorescence live imaging and discuss the comparative approach to investigate insect morphogenesis in an evolutionary context.

Formation of the germ-disc in spider embryos by a condensation-like mechanism

Background

Determination of the embryonic body axes is a crucial developmental process in all animals. The establishment of the embryonic axes of spiders has been best studied in the common-house-spider Parasteatoda tepidariorum. Here, anteroposterior (AP) polarity arises during germ disc formation; the centre of the germ-disc marks the future posterior pole, and the rim of the disc the future anterior pole of the spider embryo. The centre of the germ disc is also needed for the formation of the cumulus, a group of migratory cells needed to establish dorsoventral (DV) polarity. Thus, both body axes depend on proper germ disc formation and patterning. However, these processes have not been fully analysed at the cellular and molecular level.

Results

Here I present new techniques to stain the cell membranes/outlines in live and fixed spider embryos. I show that the germ-disc is formed from a regular and contiguous blastoderm and that co-ordinated cell shape changes, rather than migration of single cells, are required to drive germ-disc formation in P. tepidariorum embryos. Furthermore, I show that the rate of cell divisions within the embryonic and extra-embryonic region is not involved in the rapid establishment of the germ-disc. Finally, I show that the process of germ-disc formation is dependent on the initiation of zygotic transcription.

Conclusions

The presented data provide new insights in to the formation of the germ-disc in spider embryos. The establishment of the germ-disc in Parasteatoda embryos is a highly dynamic process that involves wide scale cell-shape changes. While most of the blastodermal cells become cuboidal to form the dense germ-disc, the remaining blastodermal cells stay squamous and develop into huge extra-embryonic, yolk rich cells. In addition, this study shows that the onset of zygotic transcription is needed to establish the germ-disc itself, and that the mid-blastula transition of Parasteatoda tepidariorum embryos is prior to any overt axis establishment.

Electronic supplementary material

The online version of this article (doi:10.1186/s12983-016-0166-9) contains supplementary material, which is available to authorized users.

Novel functions for Dorsocross in epithelial morphogenesis in the beetle Tribolium castaneum

Epithelial morphogenesis, the progressive restructuring of tissue sheets, is fundamental to embryogenesis. In insects, not only embryonic tissues but also extraembryonic (EE) epithelia play a crucial role in shaping the embryo. In Drosophila, the T-box transcription factor Dorsocross (Doc) is essential for EE tissue maintenance and therefore embryo survival. However, Drosophila possesses a single amnioserosa, whereas most insects have a distinct amnion and serosa. How does this derived situation compare with Doc function in the ancestral context of two EE epithelia? Here, we investigate the Doc orthologue in the red flour beetle, Tribolium castaneum, which is an excellent model for EE tissue complement and for functional, fluorescent live imaging approaches. Surprisingly, we find that Tc-Doc controls all major events in Tribolium EE morphogenesis without affecting EE tissue specification or maintenance. These macroevolutionary changes in function between Tribolium and Drosophila are accompanied by regulatory network changes, where BMP signaling and possibly the transcription factor Hindsight are downstream mediators. We propose that the ancestral role of Doc was to control morphogenesis and discuss how Tc-Doc could provide spatial precision for remodeling the amnion-serosa border.

Genome-wide identification of Tribolium dorsoventral patterning genes

The gene regulatory network controlling dorsoventral axis formation in insects has undergone drastic evolutionary changes. In Drosophila, a stable long-range gradient of Toll signalling specifies ventral cell fates and restricts BMP signalling to the dorsal half of the embryo. In Tribolium, however, Toll signalling is transient and only indirectly controls BMP signalling. In order to gain unbiased insights into the Tribolium network, we performed comparative transcriptome analyses of embryos with various dorsoventral pattering defects produced by parental RNAi for Toll and BMP signalling components. We also included embryos lacking the mesoderm (produced by Tc-twist RNAi) and characterized similarities and differences between Drosophila and Tribolium twist loss-of-function phenotypes. Using stringent conditions, we identified over 750 differentially expressed genes and analysed a subset with altered expression in more than one knockdown condition. We found new genes with localized expression and showed that conserved genes frequently possess earlier and stronger phenotypes than their Drosophila orthologues. For example, the leucine-rich repeat (LRR) protein Tartan, which has only a minor influence on nervous system development in Drosophila, is essential for early neurogenesis in Tribolium and the Tc-zinc-finger homeodomain protein 1 (Tc-zfh1), the orthologue of which plays a minor role in Drosophila muscle development, is essential for maintaining early Tc-twist expression, indicating an important function for mesoderm specification.

Morphogenetic functions of extraembryonic membranes in insects

Morphogenetic functions of the amnioserosa, the serosa, the amnion, and the yolk sac are reviewed on the basis of recent studies in flies (Drosophila, Megaselia), beetles (Tribolium), and hemipteran bugs (Oncopeltus). Three hypotheses are presented. First, it is suggested that the amnioserosa of Drosophila and the dorsal amnion of other fly species function in a similar manner. Second, it is proposed that in many species with an amniotic cavity, the amnion determines the site of serosa rupture, which, through interactions between the serosa and the amnion, enables the embryo to break free from the amniotic cavity and to close its backside. Finally, it is concluded that the yolk sac is likely an important player in insect morphogenesis.

The beetle amnion and serosa functionally interact as apposed epithelia

Unlike passive rupture of the human chorioamnion at birth, the insect extraembryonic (EE) tissues - the amnion and serosa - actively rupture and withdraw in late embryogenesis. Withdrawal is essential for development and has been a morphogenetic puzzle. Here, we use new fluorescent transgenic lines in the beetle Tribolium castaneum to show that the EE tissues dynamically form a basal-basal epithelial bilayer, contradicting the previous hypothesis of EE intercalation. We find that the EE tissues repeatedly detach and reattach throughout development and have distinct roles. Quantitative live imaging analyses show that the amnion initiates EE rupture in a specialized anterior-ventral cap. RNAi phenotypes demonstrate that the serosa contracts autonomously. Thus, apposition in a bilayer enables the amnion as 'initiator' to coordinate with the serosa as 'driver' to achieve withdrawal. This EE strategy may reflect evolutionary changes within the holometabolous insects and serves as a model to study interactions between developing epithelia.