The endoderm gene regulatory network in sea urchin embryos up to mid-blastula stage

A single-cell atlas of pig gastrulation as a resource for comparative embryology

Cell-fate decisions during mammalian gastrulation are poorly understood outside of rodent embryos. The embryonic disc of pig embryos mirrors humans, making them a useful proxy for studying gastrulation. Here we present a single-cell transcriptomic atlas of pig gastrulation, revealing cell-fate emergence dynamics, as well as conserved and divergent gene programs governing early porcine, primate, and murine development. We highlight heterochronicity in extraembryonic cell-types, despite the broad conservation of cell-type-specific transcriptional programs. We apply these findings in combination with functional investigations, to outline conserved spatial, molecular, and temporal events during definitive endoderm specification. We find early FOXA2 + /TBXT- embryonic disc cells directly form definitive endoderm, contrasting later-emerging FOXA2/TBXT+ node/notochord progenitors. Unlike mesoderm, none of these progenitors undergo epithelial-to-mesenchymal transition. Endoderm/Node fate hinges on balanced WNT and hypoblast-derived NODAL, which is extinguished upon endodermal differentiation. These findings emphasise the interplay between temporal and topological signalling in fate determination during gastrulation.

Echinobase: a resource to support the echinoderm research community

Echinobase (www.echinobase.org) is a model organism knowledgebase serving as a resource for the community that studies echinoderms, a phylum of marine invertebrates that includes sea urchins and sea stars. Echinoderms have been important experimental models for over 100 years and continue to make important contributions to environmental, evolutionary, and developmental studies, including research on developmental gene regulatory networks. As a centralized resource, Echinobase hosts genomes and collects functional genomic data, reagents, literature, and other information for the community. This third-generation site is based on the Xenbase knowledgebase design and utilizes gene-centric pages to minimize the time and effort required to access genomic information. Summary gene pages display gene symbols and names, functional data, links to the JBrowse genome browser, and orthology to other organisms and reagents, and tabs from the Summary gene page contain more detailed information concerning mRNAs, proteins, diseases, and protein-protein interactions. The gene pages also display 1:1 orthologs between the fully supported species Strongylocentrotus purpuratus (purple sea urchin), Lytechinus variegatus (green sea urchin), Patiria miniata (bat star), and Acanthaster planci (crown-of-thorns sea star). JBrowse tracks are available for visualization of functional genomic data from both fully supported species and the partially supported species Anneissia japonica (feather star), Asterias rubens (sugar star), and L. pictus (painted sea urchin). Echinobase serves a vital role by providing researchers with annotated genomes including orthology, functional genomic data aligned to the genomes, and curated reagents and data. The Echinoderm Anatomical Ontology provides a framework for standardizing developmental data across the phylum, and knowledgebase content is formatted to be findable, accessible, interoperable, and reusable by the research community.

Evolutionarily conserved Wnt/Sp5 signaling is critical for anterior-posterior axis patterning in sea urchin embryos

Studies across a diverse group of metazoan embryos indicate that Wnt signaling often activates the transcription factor Sp5, forming a signaling 'cassette' that plays critical roles in many developmental processes. This study explores the role of Wnt/Sp5 signaling during the specification and patterning of the primary germ layers during early anterior-posterior axis formation in the deuterostome sea urchin embryo. Our functional analyses show that Sp5 is critical for endomesoderm specification downstream of Wnt/β-catenin in posterior cells as well as anterior neuroectoderm patterning downstream of non-canonical Wnt/JNK signaling in anterior cells. Interestingly, expression and functional data comparisons show that Wnt/Sp5 signaling often plays similar roles in posterior endomesoderm as well as neuroectoderm patterning along the AP axis of several deuterostome embryos, including vertebrates. Thus, our findings provide strong support for the idea that Wnt-Sp5 signaling cassettes were critical for the establishment of early germ layers in the common deuterostome ancestor.

Widespread priming of transcriptional regulatory elements by incipient accessibility or RNA polymerase II pause in early embryos of the sea urchin Strongylocentrotus purpuratus

Transcriptional regulatory elements (TREs) are the primary nodes that control developmental gene regulatory networks. In embryo stages, larvae, and adult differentiated red spherule cells of the sea urchin Strongylocentrotus purpuratus, transcriptionally engaged TREs are detected by Precision Run-On Sequencing (PRO-seq), which maps genome-wide at base pair resolution the location of paused or elongating RNA polymerase II (Pol II). In parallel, TRE accessibility is estimated by the Assay for Transposase-Accessible Chromatin using Sequencing (ATAC-seq). Our analysis identifies surprisingly early and widespread TRE accessibility in 4-cell cleavage embryos that is not necessarily followed by concurrent or subsequent transcription. TRE transcriptional differences identified by PRO-seq provide more contrast among embryonic stages than ATAC-seq accessibility differences, in agreement with the apparent excess of accessible but inactive TREs during embryogenesis. Global TRE accessibility reaches a maximum around the 20-hour late blastula stage, which coincides with the consolidation of major embryo regionalizations and peak histone variant H2A.Z expression. A transcriptional potency model based on labile nucleosome TRE occupancy driven by DNA sequences and the prevalence of histone variants is proposed in order to explain the basal accessibility of transcriptionally inactive TREs during embryogenesis. However, our results would not reconcile well with labile nucleosome models based on simple A/T sequence enrichment. In addition, a large number of distal TREs become transcriptionally disengaged during developmental progression, in support of an early Pol II paused model for developmental gene regulation that eventually resolves in transcriptional activation or silencing. Thus, developmental potency in early embryos may be facilitated by incipient accessibility and transcriptional pause at TREs.

Identification of milk-related genes and regulatory networks in Bactrian camel either supplemented or under grazing

Using gene co-expression networks to understand dynamic characterizations in lactating animals becomes a common method. However, there are rarely reporters focusing on milk traits in Bactrian camel by high-throughput sequencing. We used RNA-seq to generate the camel transcriptome from the blood of 16 lactating Alxa Bactrian camel in different feeding groups. In total, we obtained 1185 milk-related genes correlated with milk yield, milk protein, milk fat, and milk lactose across the WGCNA analysis. Moreover, 364 milk-related genes were differentially expressed between supplementation and grazing feeding groups. The differential expression-camel milk-related genes CMRGs (DE-CMRGs) in supplement direct an intensive gene co-expression network to improve milk performance in lactating camels. This study provides a non-invasive method to identify the camel milk-related genes in camel blood for four primary milk traits and valuable theoretical basis and research ideas for the study of the milk performance regulation mechanism of camelid animals.

Feedback circuits are numerous in embryonic gene regulatory networks and offer a stabilizing influence on evolution of those networks

The developmental gene regulatory networks (dGRNs) of two sea urchin species, Lytechinus variegatus (Lv) and Strongylocentrotus purpuratus (Sp), have remained remarkably similar despite about 50 million years since a common ancestor. Hundreds of parallel experimental perturbations of transcription factors with similar outcomes support this conclusion. A recent scRNA-seq analysis suggested that the earliest expression of several genes within the dGRNs differs between Lv and Sp. Here, we present a careful reanalysis of the dGRNs in these two species, paying close attention to timing of first expression. We find that initial expression of genes critical for cell fate specification occurs during several compressed time periods in both species. Previously unrecognized feedback circuits are inferred from the temporally corrected dGRNs. Although many of these feedbacks differ in location within the respective GRNs, the overall number is similar between species. We identify several prominent differences in timing of first expression for key developmental regulatory genes; comparison with a third species indicates that these heterochronies likely originated in an unbiased manner with respect to embryonic cell lineage and evolutionary branch. Together, these results suggest that interactions can evolve even within highly conserved dGRNs and that feedback circuits may buffer the effects of heterochronies in the expression of key regulatory genes.

Early expression onset of tissue-specific effector genes during the specification process in sea urchin embryos

BACKGROUND

In the course of animal developmental processes, various tissues are differentiated through complex interactions within the gene regulatory network. As a general concept, differentiation has been considered to be the endpoint of specification processes. Previous works followed this view and provided a genetic control scheme of differentiation in sea urchin embryos: early specification genes generate distinct regulatory territories in an embryo to express a small set of differentiation driver genes; these genes eventually stimulate the expression of tissue-specific effector genes, which provide biological identity to differentiated cells, in each region. However, some tissue-specific effector genes begin to be expressed in parallel with the expression onset of early specification genes, raising questions about the simplistic regulatory scheme of tissue-specific effector gene expression and the current concept of differentiation itself.

RESULTS

Here, we examined the dynamics of effector gene expression patterns during sea urchin embryogenesis. Our transcriptome-based analysis indicated that many tissue-specific effector genes begin to be expressed and accumulated along with the advancing specification GRN in the distinct cell lineages of embryos. Moreover, we found that the expression of some of the tissue-specific effector genes commences before cell lineage segregation occurs.

CONCLUSIONS

Based on this finding, we propose that the expression onset of tissue-specific effector genes is controlled more dynamically than suggested in the previously proposed simplistic regulation scheme. Thus, we suggest that differentiation should be conceptualized as a seamless process of accumulation of effector expression along with the advancing specification GRN. This pattern of effector gene expression may have interesting implications for the evolution of novel cell types.

RNA localization to the mitotic spindle is essential for early development and is regulated by kinesin-1 and dynein

Mitosis is a fundamental and highly regulated process that acts to faithfully segregate chromosomes into two identical daughter cells. Localization of gene transcripts involved in mitosis to the mitotic spindle might be an evolutionarily conserved mechanism to ensure that mitosis occurs in a timely manner. We identified many RNA transcripts that encode proteins involved in mitosis localized at the mitotic spindles in dividing sea urchin embryos and mammalian cells. Disruption of microtubule polymerization, kinesin-1 or dynein results in lack of spindle localization of these transcripts in the sea urchin embryo. Furthermore, results indicate that the cytoplasmic polyadenylation element (CPE) within the 3'UTR of the Aurora B transcript, a recognition sequence for CPEB, is essential for RNA localization to the mitotic spindle in the sea urchin embryo. Blocking this sequence results in arrested development during early cleavage stages, suggesting that RNA localization to the mitotic spindle might be a regulatory mechanism of cell division that is important for early development.

Gene regulatory divergence amongst echinoderms underlies appearance of pigment cells in sea urchin development

Larvae of the sea urchin, Strongylocentrotus purpuratus, have pigmented migratory cells implicated in immune defense and gut patterning. The transcription factor SpGcm activates the expression of many pigment cell-specific genes, including those involved in pigment biosynthesis (SpPks1 and SpFmo3) and immune related genes (e.g. SpMif5). Despite the importance of this cell type in sea urchins, pigmented cells are absent in larvae of the sea star, Patiria miniata. In this study, we tested the premises that sea stars lack genes to synthesize echinochrome pigment, that the genes are present but are not expressed in the larvae, or rather that the homologous gene expression does not contribute to echinochrome synthesis. Our results show that orthologs of sea urchin pigment cell-specific genes (PmPks1, PmFmo3-1 and PmMifL1-2) are present in the sea star genome and expressed in the larvae. Although no cell lineage homologous to migratory sea urchin pigment cells is present, dynamic gene activation accomplishes a similar spatial and temporal expression profile. The mechanisms regulating the expression of these genes, though, is highly divergent. In sea stars, PmGcm lacks the central role in pigment gene expression since it is not expressed in PmPks1 and PmFmo3-1-positive cells, and knockdown of Gcm does not abrogate pigment gene expression. Pigment genes are instead expressed in the coelomic mesoderm early in development before later being expressed in the ectoderm. These findings were supported by in situ RNA hybridization and comparative scRNA-seq analyses. We conclude that simply the coexpression of Pks1 and Fmo3 orthologs in cells of the sea star is not sufficient to underlie the emergence of the larval pigment cell in the sea urchin.

An ancestral Wnt-Brachyury feedback loop in axial patterning and recruitment of mesoderm-determining target genes

Transcription factors are crucial drivers of cellular differentiation during animal development and often share ancient evolutionary origins. The T-box transcription factor Brachyury plays a pivotal role as an early mesoderm determinant and neural repressor in vertebrates; yet, the ancestral function and key evolutionary transitions of the role of this transcription factor remain obscure. Here, we present a genome-wide target-gene screen using chromatin immunoprecipitation sequencing in the sea anemone Nematostella vectensis, an early branching non-bilaterian, and the sea urchin Strongylocentrotus purpuratus, a representative of the sister lineage of chordates. Our analysis reveals an ancestral gene regulatory feedback loop connecting Brachyury, FoxA and canonical Wnt signalling involved in axial patterning that predates the cnidarian-bilaterian split about 700 million years ago. Surprisingly, we also found that part of the gene regulatory network controlling the fate of neuromesodermal progenitors in vertebrates was already present in the common ancestor of cnidarians and bilaterians. However, while several endodermal and neuronal Brachyury target genes are ancestrally shared, hardly any of the key mesodermal downstream targets in vertebrates are found in the sea anemone or the sea urchin. Our study suggests that a limited number of target genes involved in mesoderm formation were newly acquired in the vertebrate lineage, leading to a dramatic shift in the function of this ancestral developmental regulator.

Patterns of selection across gene regulatory networks

Gene regulatory networks (GRNs) are the core engine of organismal development. If we would like to understand the origin and diversification of phenotypes, it is necessary to consider the structure of GRNs in order to reconstruct the links between genetic mutations and phenotypic change. Much of the progress in evolutionary developmental biology, however, has occurred without a nuanced consideration of the evolution of functional relationships between genes, especially in the context of their broader network interactions. Characterizing and comparing GRNs across traits and species in a more detailed way will allow us to determine how network position influences what genes drive adaptive evolution. In this perspective paper, we consider the architecture of developmental GRNs and how positive selection strength may vary across a GRN. We then propose several testable models for these patterns of selection and experimental approaches to test these models.

Dynamics of Drosophila endoderm specification

During early Drosophila embryogenesis, a network of gene regulatory interactions orchestrates terminal patterning, playing a critical role in the subsequent formation of the gut. We utilized CRISPR gene editing at endogenous loci to create live reporters of transcription and light-sheet microscopy to monitor the individual components of the posterior gut patterning network across 90 min prior to gastrulation. We developed a computational approach for fusing imaging datasets of the individual components into a common multivariable trajectory. Data fusion revealed low intrinsic dimensionality of posterior patterning and cell fate specification in wild-type embryos. The simple structure that we uncovered allowed us to construct a model of interactions within the posterior patterning regulatory network and make testable predictions about its dynamics at the protein level. The presented data fusion strategy is a step toward establishing a unified framework that would explore how stochastic spatiotemporal signals give rise to highly reproducible morphogenetic outcomes.

Ecdysone signal pathway participates in shell formation in pearl oysters Pinctada fucata martensii

Ecdysone exists in arthropods, Mollusca and other invertebrates and plays vital roles in exoskeleton formation of Ecdysozoa. However, little is known about its functions in bivalve species. Herein, we identified ecdysone from the serum of pearl oyster Pinctada fucata martensii and obtained the coding sequence of ecdysone receptor (PmEcR) and homologue of its heterodimer protein retinoid X receptor (PmRXR). The deduced amino acid sequences of PmEcR and PmRXR contained a DNA-binding and ligand-binding domain and were very similar to the orthologs of other species. Moreover, PmEcR and PmRXR were located in the nuclei and cytoplasm of HEK-293T cells. PmEcR and PmRXR were highly expressed in early embryos and biomineralized mantle tissue. Moreover, the serum concentration of ecdysone significantly increased at 2, 4, 6, and 8 h post-shell notching. The expression of PmEcR in the mantle tissue was significantly induced at the corresponding time points, while that of PmRXR was significantly induced at 6 h. Ecdysone stimulation remarkably induced the expression of growth factors (BMP2 and BMP7), transcription factors (PmRunt and AP-1), and shell matrix protein genes (chitinase, lysine-rich matrix protein (KRMP), TYR2, and PmCOLVI), which indicated that ecdysone signaling plays important roles in shell repair. However, yeast two-hybrid assay and bimolecular fluorescence complementation showed that PmEcR and PmRXR did not form dimers, suggesting the different molecular interactions of EcR in bivalves. These findings provide insights into the function of ecdysone and its regulation pathway in bivalve species.

A single-cell RNA-seq analysis of Brachyury-expressing cell clusters suggests a morphogenesis-associated signal center of oral ectoderm in sea urchin embryos

Brachyury is a T-box family transcription factor and plays pivotal roles in morphogenesis. In sea urchin embryos, Brachyury, is expressed in the invaginating endoderm, and in the oral ectoderm of the invaginating mouth opening. The oral ectoderm is hypothesized to serve as a signaling center for oral (ventral)-aboral (dorsal) axis formation and to function as a ventral organizer. Our previous results of a single-cell RNA-seq (scRNA-seq) atlas of early Strongylocentrotus purpuratus embryos categorized the constituent cells into 22 clusters, in which the endoderm consists of three clusters and the oral ectoderm four clusters (Foster et al., 2020). Here we examined which clusters of cells expressed Brachyury in relation to the morphogenesis and the identity of the ventral organizer. Our results showed that cells of all three endoderm clusters expressed Brachyury in blastulae. Based on expression profiles of genes involved in the gene regulatory networks (GRNs) of sea urchin embryos, the three clusters are distinguishable, two likely derived from the Veg2 tier and one from the Veg1 tier. On the other hand, of the four oral-ectoderm clusters, cells of two clusters expressed Brachyury at the gastrula stage and genes that are responsible for the ventral organizer at the late blastula stage, but the other two clusters did not. At a single-cell level, most cells of the two oral-ectoderm clusters expressed organizer-related genes, nearly a half of which coincidently expressed Brachyury. This suggests that the ventral organizer contains Brachyury-positive cells which invaginate to form the stomodeum. This scRNA-seq study therefore highlights significant roles of Brachyury-expressing cells in body-plan formation of early sea urchin embryos, though cellular and molecular mechanisms for how Brachyury functions in these processes remain to be elucidated in future studies.

Pattern of Repetitive Element Transcription Segregate Cell Lineages during the Embryogenesis of Sea Urchin Strongylocentrotus purpuratus

Repetitive elements (REs) occupy a significant part of eukaryotic genomes and are shown to play diverse roles in genome regulation. During embryogenesis of the sea urchin, a large number of REs are expressed, but the role of these elements in the regulation of biological processes remains unknown. The aim of this study was to identify the RE expression at different stages of embryogenesis. REs occupied 44% of genomic DNA of Strongylocentrotus purpuratus. The most prevalent among these elements were the unknown elements-in total, they contributed 78.5% of REs (35% in total genome occupancy). It was revealed that the transcription pattern of genes and REs changes significantly during gastrulation. Using the de novo transcriptome assembly, we showed that the expression of RE is independent of its copy number in the genome. We also identified copies that are expressed. Only active RE copies were used for mapping and quantification of RE expression in the single-cell RNA sequencing data. REs expression was observed in all cell lineages and they were detected as population markers. Moreover, the primary mesenchyme cell (PMC) line had the greatest diversity of REs among the markers. Our data suggest a role for RE in the organization of developmental domains during the sea urchin embryogenesis at the single-cell resolution level.

Reprint of: Conditional specification of endomesoderm

Early in animal development many cells are conditionally specified based on observations that those cells can be directed toward alternate fates. The endomesoderm is so named because early specification produces cells that often have been observed to simultaneously express both early endoderm and mesoderm transcription factors. Experiments with these cells demonstrate that their progeny can directed entirely toward endoderm or mesoderm, whereas normally they establish both germ layers. This review examines the mechanisms that initiate the conditional endomesoderm state, its metastability, and the mechanisms that resolve that state into definitive endoderm and mesoderm.

Developmental single-cell transcriptomics in the Lytechinus variegatus sea urchin embryo

Using scRNA-seq coupled with computational approaches, we studied transcriptional changes in cell states of sea urchin embryos during development to the larval stage. Eighteen closely spaced time points were taken during the first 24 h of development of Lytechinus variegatus (Lv). Developmental trajectories were constructed using Waddington-OT, a computational approach to 'stitch' together developmental time points. Skeletogenic and primordial germ cell trajectories diverged early in cleavage. Ectodermal progenitors were distinct from other lineages by the 6th cleavage, although a small percentage of ectoderm cells briefly co-expressed endoderm markers that indicated an early ecto-endoderm cell state, likely in cells originating from the equatorial region of the egg. Endomesoderm cells also originated at the 6th cleavage and this state persisted for more than two cleavages, then diverged into distinct endoderm and mesoderm fates asynchronously, with some cells retaining an intermediate specification status until gastrulation. Seventy-nine out of 80 genes (99%) examined, and included in published developmental gene regulatory networks (dGRNs), are present in the Lv-scRNA-seq dataset and are expressed in the correct lineages in which the dGRN circuits operate.

Conditional specification of endomesoderm

Early in animal development many cells are conditionally specified based on observations that those cells can be directed toward alternate fates. The endomesoderm is so named because early specification produces cells that often have been observed to simultaneously express both early endoderm and mesoderm transcription factors. Experiments with these cells demonstrate that their progeny can directed entirely toward endoderm or mesoderm, whereas normally they establish both germ layers. This review examines the mechanisms that initiate the conditional endomesoderm state, its metastability, and the mechanisms that resolve that state into definitive endoderm and mesoderm.

FGF signalling plays similar roles in development and regeneration of the skeleton in the brittle star Amphiura filiformis

Regeneration as an adult developmental process is in many aspects similar to embryonic development. Although many studies point out similarities and differences, no large-scale, direct and functional comparative analyses between development and regeneration of a specific cell type or structure in one animal exist. Here, we use the brittle star Amphiura filiformis to characterise the role of the FGF signalling pathway during skeletal development in embryos and arm regeneration. In both processes, we find ligands expressed in ectodermal cells that flank underlying skeletal mesenchymal cells, which express the receptors. Perturbation of FGF signalling showed inhibited skeleton formation in both embryogenesis and regeneration, without affecting other key developmental processes. Differential transcriptome analysis finds mostly differentiation genes rather than transcription factors to be downregulated in both contexts. Moreover, comparative gene analysis allowed us to discover brittle star-specific differentiation genes. In conclusion, our results show that the FGF pathway is crucial for skeletogenesis in the brittle star, as in other deuterostomes, and provide evidence for the re-deployment of a developmental gene regulatory module during regeneration.

microRNA-31 regulates skeletogenesis by direct suppression of Eve and Wnt1

microRNAs (miRNAs) play a critical role in a variety of biological processes, including embryogenesis and the physiological functions of cells. Evolutionarily conserved microRNA-31 (miR-31) has been found to be involved in cancer, bone formation, and lymphatic development. We previously discovered that, in the sea urchin, miR-31 knockdown (KD) embryos have shortened dorsoventral connecting rods, mispatterned skeletogenic primary mesenchyme cells (PMCs) and shifted and expanded Vegf3 expression domain. Vegf3 itself does not contain miR-31 binding sites; however, we identified its upstream regulators Eve and Wnt1 to be directly suppressed by miR-31. Removal of miR-31's suppression of Eve and Wnt1 resulted in skeletal and PMC patterning defects, similar to miR-31 KD phenotypes. Additionally, removal of miR-31's suppression of Eve and Wnt1 results in an expansion and anterior shift in expression of Veg1 ectodermal genes, including Vegf3 in the blastulae. This indicates that miR-31 indirectly regulates Vegf3 expression through directly suppressing Eve and Wnt1. Furthermore, removing miR-31 suppression of Eve is sufficient to cause skeletogenic defects, revealing a novel regulatory role of Eve in skeletogenesis and PMC patterning. Overall, this study provides a proposed molecular mechanism of miR-31's regulation of skeletogenesis and PMC patterning through its cross-regulation of a Wnt signaling ligand and a transcription factor of the endodermal and ectodermal gene regulatory network.

Sea Urchin as a Universal Model for Studies of Gene Networks

The purple sea urchin Strongylocentrotus purpuratus has been used for over 150 years as a model organism in developmental biology. Using this model species, scientists have been able to describe, in detail, the mechanisms of cell cycle control and cell adhesion, fertilization, calcium signaling, cell differentiation, and death. Massive parallel sequencing of the sea urchin genome enabled the deciphering of the main components of gene regulatory networks during the activation of embryonic signaling pathways. This knowledge helped to extrapolate aberrations in somatic cells that may lead to diseases, including cancer in humans. Furthermore, since many, if not all, developmental signaling pathways were shown to be controlled by non-coding RNAs (ncRNAs), the sea urchin organism represents an attractive experimental model. In this review, we discuss the main discoveries in the genetics, genomics, and transcriptomics of sea urchins during embryogenesis with the main focus on the role of ncRNAs. This information may be useful for comparative studies between different organisms, and may help identify new regulatory networks controlled by ncRNAs.

A single cell RNA sequencing resource for early sea urchin development

Identifying cell states during development from their mRNA profiles provides insight into their gene regulatory network. Here, we leverage the sea urchin embryo for its well-established gene regulatory network to interrogate the embryo using single cell RNA sequencing. We tested eight developmental stages in Strongylocentrotus purpuratus, from the eight-cell stage to late in gastrulation. We used these datasets to parse out 22 major cell states of the embryo, focusing on key transition stages for cell type specification of each germ layer. Subclustering of these major embryonic domains revealed over 50 cell states with distinct transcript profiles. Furthermore, we identified the transcript profile of two cell states expressing germ cell factors, one we conclude represents the primordial germ cells and the other state is transiently present during gastrulation. We hypothesize that these cells of the Veg2 tier of the early embryo represent a lineage that converts to the germ line when the primordial germ cells are deleted. This broad resource will hopefully enable the community to identify other cell states and genes of interest to expose the underpinning of developmental mechanisms.

The role of core and variable Gene Regulatory Network modules in tooth development and evolution

Among the developmental processes that have been proposed to influence the direction of evolution, the modular organization of developmental gene regulatory networks (GRNs) has shown particular promise. In theory, GRNs have core modules comprised of essential, conserved circuits of genes, and sub-modules of downstream, secondary circuits of genes that are more susceptible to variation. While this idea has received considerable interest as of late, the field of evo-devo lacks the experimental systems needed to rigorously evaluate this hypothesis. Here, we introduce an experimental system, the vertebrate tooth, that has great potential as a model for testing this hypothesis. Tooth development and its associated GRN have been well studied and modeled in both model and non-model organisms. We propose that the existence of modules within the tooth GRN explains both the conservation of developmental mechanisms and the extraordinary diversity of teeth among vertebrates. Based on experimental data, we hypothesize that there is a conserved core module of genes that is absolutely necessary to ensure tooth or cusp initiation and development. In regard to tooth shape variation between species, we suggest that more relaxed sub-modules activated at later steps of tooth development, e.g., during the morphogenesis of the tooth and its cusps, control the different axes of tooth morphological variation.

Endodermal Maternal Transcription Factors Establish Super-Enhancers during Zygotic Genome Activation

Elucidation of the sequence of events underlying the dynamic interaction between transcription factors and chromatin states is essential. Maternal transcription factors function at the top of the regulatory hierarchy to specify the primary germ layers at the onset of zygotic genome activation (ZGA). We focus on the formation of endoderm progenitor cells and examine the interactions between maternal transcription factors and chromatin state changes underlying the cell specification process. Endoderm-specific factors Otx1 and Vegt together with Foxh1 orchestrate endoderm formation by coordinated binding to select regulatory regions. These interactions occur before the deposition of enhancer histone marks around the regulatory regions, and these TFs recruit RNA polymerase II, regulate enhancer activity, and establish super-enhancers associated with important endodermal genes. Therefore, maternal transcription factors Otx1, Vegt, and Foxh1 combinatorially regulate the activity of super-enhancers, which in turn activate key lineage-specifying genes during ZGA.

How do maternal transcription factors interact with chromatin regions to coordinate the endodermal gene regulatory program? Paraiso et al. demonstrate that combinatorial binding of maternal Otx1, Vegt, and Foxh1 to select enhancers and super-enhancers in the genome controls endodermal cell fate specification during zygotic gene activation.

Early Xenopus gene regulatory programs, chromatin states, and the role of maternal transcription factors

For decades, the early development of the Xenopus embryo has been an essential model system to study the gene regulatory mechanisms that govern cellular specification. At the top of the hierarchy of gene regulatory networks, maternally deposited transcription factors initiate this process and regulate the expression of zygotic genes that give rise to three distinctive germ layer cell types (ectoderm, mesoderm, and endoderm), and subsequent generation of organ precursors. The onset of germ layer specification is also closely coupled with changes associated with chromatin modifications. This review will examine the timing of maternal transcription factors initiating the zygotic genome activation, the epigenetic landscape of embryonic chromatin, and the network structure that governs the process.

Simulations of sea urchin early development delineate the role of oriented cell division in the morula-to-blastula transition

The sea urchin morula to blastula transition has long been thought to require oriented cell divisions and blastomere adherence to the enveloping hyaline layer. In a computer simulation model, cell divisions constrained by a surface plane division rule are adequate to effect morphological transition. The hyaline membrane acts as an enhancer but is not essential. The model is consistent with the orientation of micromere divisions and the open blastulae of direct developing species. The surface plane division rule precedes overt epithelization of surface cells and acts to organize the developing epithelium. It is a universal feature of early metazoan development and simulations of non-echinoid cleavage patterns support its role throughout Metazoa. The surface plane division rule requires only local cues and cells need not reference global positional information or embryonic axes.

The developmental transcriptome for Lytechinus variegatus exhibits temporally punctuated gene expression changes

Embryonic development is arguably the most complex process an organism undergoes during its lifetime, and understanding this complexity is best approached with a systems-level perspective. The sea urchin has become a highly valuable model organism for understanding developmental specification, morphogenesis, and evolution. As a non-chordate deuterostome, the sea urchin occupies an important evolutionary niche between protostomes and vertebrates. Lytechinus variegatus (Lv) is an Atlantic species that has been well studied, and which has provided important insights into signal transduction, patterning, and morphogenetic changes during embryonic and larval development. The Pacific species, Strongylocentrotus purpuratus (Sp), is another well-studied sea urchin, particularly for gene regulatory networks (GRNs) and cis-regulatory analyses. A well-annotated genome and transcriptome for Sp are available, but similar resources have not been developed for Lv. Here, we provide an analysis of the Lv transcriptome at 11 timepoints during embryonic and larval development. Temporal analysis suggests that the gene regulatory networks that underlie specification are well-conserved among sea urchin species. We show that the major transitions in variation of embryonic transcription divide the developmental time series into four distinct, temporally sequential phases. Our work shows that sea urchin development occurs via sequential intervals of relatively stable gene expression states that are punctuated by abrupt transitions.

Gastrulation in the sea urchin

Gastrulation is arguably the most important evolutionary innovation in the animal kingdom. This process provides the basic embryonic architecture, an inner layer separated from an outer layer, from which all animal forms arise. An extraordinarily simple and elegant process of gastrulation is observed in the sea urchin embryo. The cells participating in sea urchin gastrulation are specified early during cleavage. One outcome of that specification is the expression of transcription factors that control each of the many subsequent morphogenetic changes. The first of these movements is an epithelial-mesenchymal transition (EMT) of skeletogenic mesenchyme cells, then EMT of pigment cell progenitors. Shortly thereafter, invagination of the archenteron occurs. At the end of archenteron extension, a second wave of EMT occurs to release immune cells into the blastocoel and primordial germ cells that will home to the coelomic pouches. The archenteron then remodels to establish the three parts of the gut, and at the anterior end, the gut fuses with the stomodaeum to form the through-gut. As part of the anterior remodeling, mesodermal coelomic pouches bud off the lateral sides of the archenteron tip. Multiple cell biological processes conduct each of these movements and in some cases the upstream transcription factors controlling this process have been identified. Remarkably, each event seamlessly occurs at the right time to orchestrate formation of the primitive body plan. This review covers progress toward understanding many of the molecular mechanisms underlying this sequence of morphogenetic events.

Modelling gene regulation: (De)compositional and template-based strategies

Although the interdisciplinary nature of contemporary biological sciences has been addressed by philosophers, historians, and sociologists of science, the different ways in which engineering concepts and methods have been applied in biology have been somewhat neglected. We examine - using the mechanistic philosophy of science as an analytic springboard - the transfer of network methods from engineering to biology through the cases of two biology laboratories operating at the California Institute of Technology. The two laboratories study gene regulatory networks, but in remarkably different ways. The research strategy of the Davidson lab fits squarely into the traditional mechanist philosophy in its aim to decompose and reconstruct, in detail, gene regulatory networks of a chosen model organism. In contrast, the Elowitz lab constructs minimal models that do not attempt to represent any particular naturally evolved genetic circuits. Instead, it studies the principles of gene regulation through a template-based approach that is applicable to any kinds of networks, whether biological or not. We call for the mechanists to consider whether the latter approach can be accommodated by the mechanistic approach, and what kinds of modifications it would imply for the mechanistic paradigm of explanation, if it were to address modelling more generally.

Transcriptome analysis to characterize the genes related to gonad growth and fatty acid metabolism in the sea urchin Strongylocentrotus intermedius

BACKGROUND

Sea urchin gonads of both sexes, commonly termed "roe", are highly valued seafood delicacies, and Strongylocentrotus intermedius is considered one of the tastiest sea urchins. In order to produce high-quality gonads for consumption and clarify the mechanism of gonad growth and development of the sea urchin, more genetic information, especially at the transcriptome level, is needed.

OBJECTIVE

A more thorough understanding of sea urchin gonad growth and development in both sexes could enable regulation of these processes at several stages with the aim of suppressing gametogenesis in order to produce high-quality gonads for consumption.

METHODS

The adult sea urchins S. intermedius were cultured for 3 months, and were sampled for the gonadal transcriptome analysis which has been performed on the RNAs of three male and female adults of S. intermedius in each gonad development stage.

RESULTS

Illumina sequencing raw sequence data was deposited in the NCBI Sequence Read Archive (SRA) database (PRJNA532998). It generated 560,196,356 raw reads and 548,956,944 clean reads were acquired, which were assembled into 107,850 transcripts with 44,124 genes. Comparative analysis showed the differentially expressed genes (DEGs) from 114 to 2566. Gene Ontology (GO) and Kyoto Encyclopedia of Genes and Genomes (KEGG) pathway enrichment analyses were used to determine the functional significance of these DEGs. We have selected 9 genes related to growth and 12 genes related to fatty acid biosynthesis and metabolism in sea urchin gonads.

CONCLUSION

These data for sea urchins were intended to provide markers for gonad growth and development that can be accumulated for use in aquaculture applications.

Development and evolution of gut structures: from molecules to function

The emergence of a specialized system for food digestion and nutrient absorption was a crucial innovation for multicellular organisms. Digestive systems with different levels of complexity evolved in different animals, with the endoderm-derived one-way gut of most bilaterians to be the prevailing and more specialized form. While the molecular events regulating the early phases of embryonic tissue specification have been deeply investigated in animals occupying different phylogenetic positions, the mechanisms underlying gut patterning and gut-associated structures differentiation are still mostly obscure. In this review, we describe the main discoveries in gut and gut-associated structures development in echinoderm larvae (mainly for sea urchin and, when available, for sea star) and compare them with existing information in vertebrates. An impressive degree of conservation emerges when comparing the transcription factor toolkits recruited for gut cells and tissue differentiation in animals as diverse as echinoderms and vertebrates, thus suggesting that their function emerged in the deuterostome ancestor.

Transcriptional profiles of early stage red sea urchins (Mesocentrotus franciscanus) reveal differential regulation of gene expression across development

The red sea urchin, Mesocentrotus franciscanus, is an ecologically important kelp forest species that also serves as a valuable fisheries resource. In this study, we have assembled and annotated a developmental transcriptome for M. franciscanus that represents eggs and six stages of early development (8- to 16-cell, morula, hatched blastula, early gastrula, prism and early pluteus). Characterization of the transcriptome revealed distinct patterns of gene expression that corresponded to major developmental and morphological processes. In addition, the period during which maternally-controlled transcription was terminated and the zygotic genome was activated, the maternal-to-zygotic transition (MZT), was found to begin during early cleavage and persist through the hatched blastula stage, an observation that is similar to the timing of the MZT in other sea urchin species. The presented developmental transcriptome will serve as a useful resource for investigating, in both an ecological and fisheries context, how the early developmental stages of this species respond to environmental stressors.

How Does the Regulatory Genome Work?

The regulatory genome controls genome activity throughout the life of an organism. This requires that complex information processing functions are encoded in, and operated by, the regulatory genome. Although much remains to be learned about how the regulatory genome works, we here discuss two cases where regulatory functions have been experimentally dissected in great detail and at the systems level, and formalized by computational logic models. Both examples derive from the sea urchin embryo, but assess two distinct organizational levels of genomic information processing. The first example shows how the regulatory system of a single gene, endo16, executes logic operations through individual transcription factor binding sites and cis-regulatory modules that control the expression of this gene. The second example shows information processing at the gene regulatory network (GRN) level. The GRN controlling development of the sea urchin endomesoderm has been experimentally explored at an almost complete level. A Boolean logic model of this GRN suggests that the modular logic functions encoded at the single-gene level show compositionality and suffice to account for integrated function at the network level. We discuss these examples both from a biological-experimental point of view and from a computer science-informational point of view, as both illuminate principles of how the regulatory genome works.

Inference of Developmental Gene Regulatory Networks Beyond Classical Model Systems: New Approaches in the Post-genomic Era

The advent of high-throughput sequencing (HTS) technologies has revolutionized the way we understand the transformation of genetic information into morphological traits. Elucidating the network of interactions between genes that govern cell differentiation through development is one of the core challenges in genome research. These networks are known as developmental gene regulatory networks (dGRNs) and consist largely of the functional linkage between developmental control genes, cis-regulatory modules, and differentiation genes, which generate spatially and temporally refined patterns of gene expression. Over the last 20 years, great advances have been made in determining these gene interactions mainly in classical model systems, including human, mouse, sea urchin, fruit fly, and worm. This has brought about a radical transformation in the fields of developmental biology and evolutionary biology, allowing the generation of high-resolution gene regulatory maps to analyze cell differentiation during animal development. Such maps have enabled the identification of gene regulatory circuits and have led to the development of network inference methods that can recapitulate the differentiation of specific cell-types or developmental stages. In contrast, dGRN research in non-classical model systems has been limited to the identification of developmental control genes via the candidate gene approach and the characterization of their spatiotemporal expression patterns, as well as to the discovery of cis-regulatory modules via patterns of sequence conservation and/or predicted transcription-factor binding sites. However, thanks to the continuous advances in HTS technologies, this scenario is rapidly changing. Here, we give a historical overview on the architecture and elucidation of the dGRNs. Subsequently, we summarize the approaches available to unravel these regulatory networks, highlighting the vast range of possibilities of integrating multiple technical advances and theoretical approaches to expand our understanding on the global gene regulation during animal development in non-classical model systems. Such new knowledge will not only lead to greater insights into the evolution of molecular mechanisms underlying cell identity and animal body plans, but also into the evolution of morphological key innovations in animals.

Roles of β-catenin, TCF-4, and survivin in nasopharyngeal carcinoma: correlation with clinicopathological features and prognostic significance

Background

Nasopharyngeal carcinoma (NPC) is a common malignant tumor of the head and neck region with poorly understood progression and prognosis. The present study aims at exploring whether the expression of β-catenin, TCF-4, and survivin affects clinicopathological features and prognostic significance in NPC.

Methods

We enrolled 164 patients with NPC and 70 patients with chronic nasopharyngitis (CNP) in this study. Reverse transcription quantitative polymerase chain reaction (RT-qPCR) and immunohistochemistry (IHC) were conducted to evaluate the expression of β-catenin, TCF-4, and survivin. Spearman’s rank correlation analysis and Pearson correlation analysis were used to measure the correlation of β-catenin, TCF-4, and survivin. Risk factors for prognosis and survival conditions of NPC patients were analyzed by Cox proportional hazards model and Kaplan–Meier curves.

Results

The results obtained revealed that mRNA and protein expression of β-catenin, TCF-4, and survivin was higher in NPC tissues than in CNP tissues. Positive correlations amongst β-catenin, TCF-4, and survivin were identified by Spearman’s rank correlation analysis and Pearson correlation analysis. There was a significant correlation in expression of β-catenin, TCF-4, and survivin with EBV DNA, EBV-VCA-IgA, EBV-EA-IgA, T stage, N stage, and clinicopathological stages. Lower overall survival (OS), distant metastasis-free survival (DMFS), local recurrence-free survival (LRFS), and disease-free survival (DFS) rates were detected in NPC patients with positive expression of β-catenin, TCF-4, and survivin, in contrast to those with negative expression. Cox proportional hazards model demonstrated that β-catenin, TCF-4, and survivin protein positive expression were independent risk factors for OS and DFS of NPC prognosis; there was an evident correlation between clinicopathological stages, TCF-4, and EBV-EA-IgA and OS, DMFS, LRFS, and DFS of NPC.

Conclusions

The aforementioned results indicate that β-catenin, TCF-4, and survivin proteins are highly expressed in NPC, which can be used as factors to predict the malignancy of NPC. In addition, positive expression of β-catenin, TCF-4, and survivin are potential risk factors that lead to an unfavorable prognosis of OS and DFS in NPC patients.

Encounters across networks: Windows into principles of genomic regulation

Gene regulatory networks account for the ability of the genome to program development in complex multi-cellular organisms. Such networks are based on principles of gene regulation by combinations of transcription factors that bind to specific cis-regulatory DNA sites to activate transcription. These cis-regulatory regions mediate logic processing at each network node, enabling progressive increases in organismal complexity with development. Gene regulatory network explanations of development have been shown to account for patterning and cell type diversification in fly and sea urchin embryonic systems, where networks are characterized by fast coupling between transcriptional inputs and changes in target gene transcription rates, and crucial cis-regulatory elements are concentrated relatively close to the protein coding sequences of the target genes, thus facilitating their identification. Stem cell-based development in post-embryonic mammalian systems also depends on gene networks, but differs from the fly and sea urchin systems. First, the number of regulatory elements per gene and the distances between regulatory elements and the genes they control are considerably larger, forcing searches via genome-wide transcription factor binding surveys rather than functional assays. Second, the intrinsic timing of network state transitions can be slowed considerably by the need to undo stem-cell chromatin configurations, which presumably add stability to stem-cell states but retard responses to transcription factor changes during differentiation. The dispersed, partially redundant cis-regulatory systems controlling gene expression and the slow state transition kinetics in these systems already reveal new insights and opportunities to extend understanding of the repertoire of gene networks and regulatory system logic.

Conserved regulatory state expression controlled by divergent developmental gene regulatory networks in echinoids

Evolution of the animal body plan is driven by changes in developmental gene regulatory networks (GRNs), but how networks change to control novel developmental phenotypes remains, in most cases, unresolved. Here, we address GRN evolution by comparing the endomesoderm GRN in two echinoid sea urchins, Strongylocentrotus purpuratus and Eucidaris tribuloides, with at least 268 million years of independent evolution. We first analyzed the expression of twelve transcription factors and signaling molecules of the S. purpuratus GRN in E. tribuloides embryos, showing that orthologous regulatory genes are expressed in corresponding endomesodermal cell fates in the two species. However, perturbation of regulatory genes revealed that important regulatory circuits of the S. purpuratus GRN are significantly different in E. tribuloides For example, mesodermal Delta/Notch signaling controls exclusion of alternative cell fates in E. tribuloides but controls mesoderm induction and activation of a positive feedback circuit in S. purpuratus These results indicate that the architecture of the sea urchin endomesoderm GRN evolved by extensive gain and loss of regulatory interactions between a conserved set of regulatory factors that control endomesodermal cell fate specification.

Methods for the experimental and computational analysis of gene regulatory networks in sea urchins

The discovery of gene regulatory networks (GRNs) has opened a gate to access the genomic mechanisms controlling development. GRNs are systems of transcriptional regulatory circuits that control the differential specification of cell fates during development by regulating gene expression. The experimental analysis of GRNs involves a collection of methods, each revealing aspects of the overall control process. This review provides an overview of experimental and computational methods that have been successfully applied for solving developmental GRNs in the sea urchin embryo. The key in this approach is to obtain experimental evidence for functional interactions between transcription factors and regulatory DNA. In the second part of this review, a more generally applicable strategy is discussed that shows a path from experimental evidence to annotation of regulatory linkages to the generation of GRN models.

Inhibition of microRNA suppression of Dishevelled results in Wnt pathway-associated developmental defects in sea urchin

MicroRNAs (miRNAs) are highly conserved, small non-coding RNAs that regulate gene expressions by binding to the 3' untranslated region of target mRNAs thereby silencing translation. Some miRNAs are key regulators of the Wnt signaling pathways, which impact developmental processes. This study investigates miRNA regulation of different isoforms of Dishevelled (Dvl/Dsh), which encode a key component in the Wnt signaling pathway. The sea urchin Dvl mRNA isoforms have similar spatial distribution in early development, but one isoform is distinctively expressed in the larval ciliary band. We demonstrated that Dvl isoforms are directly suppressed by miRNAs. By blocking miRNA suppression of Dvl isoforms, we observed dose-dependent defects in spicule length, patterning of the primary mesenchyme cells, gut morphology, and cilia. These defects likely result from increased Dvl protein levels, leading to perturbation of Wnt-dependent signaling pathways and additional Dvl-mediated processes. We further demonstrated that overexpression of Dvl isoforms recapitulated some of the Dvl miRNATP-induced phenotypes. Overall, our results indicate that miRNA suppression of Dvl isoforms plays an important role in ensuring proper development and function of primary mesenchyme cells and cilia.

Notch signaling in the division of germ layers in bilaterian embryos

Bilaterian embryos are triploblastic organisms which develop three complete germ layers (ectoderm, mesoderm, and endoderm). While the ectoderm develops mainly from the animal hemisphere, there is diversity in the location from where the endoderm and the mesoderm arise in relation to the animal-vegetal axis, ranging from endoderm being specified between the ectoderm and mesoderm in echinoderms, and the mesoderm being specified between the ectoderm and the endoderm in vertebrates. A common feature is that part of the mesoderm segregates from an ancient bipotential endomesodermal domain. The process of segregation is noisy during the initial steps but it is gradually refined. In this review, we discuss the role of the Notch pathway in the establishment and refinement of boundaries between germ layers in bilaterians, with special focus on its interaction with the Wnt/β-catenin pathway.

Regulatory states in the developmental control of gene expression

A growing body of evidence shows that gene expression in multicellular organisms is controlled by the combinatorial function of multiple transcription factors. This indicates that not the individual transcription factors or signaling molecules, but the combination of expressed regulatory molecules, the regulatory state, should be viewed as the functional unit in gene regulation. Here, I discuss the concept of the regulatory state and its proposed role in the genome-wide control of gene expression. Recent analyses of regulatory gene expression in sea urchin embryos have been instrumental for solving the genomic control of cell fate specification in this system. Some of the approaches that were used to determine the expression of regulatory states during sea urchin embryogenesis are reviewed. Significant developmental changes in regulatory state expression leading to the distinct specification of cell fates are regulated by gene regulatory network circuits. How these regulatory state transitions are encoded in the genome is illuminated using the sea urchin endoderm-mesoderms cell fate decision circuit as an example. These observations highlight the importance of considering developmental gene regulation, and the function of individual transcription factors, in the context of regulatory states.

Assessing regulatory information in developmental gene regulatory networks

Gene regulatory networks (GRNs) provide a transformation function between the static genomic sequence and the primary spatial specification processes operating development. The regulatory information encompassed in developmental GRNs thus goes far beyond the control of individual genes. We here address regulatory information at different levels of network organization, from single node to subcircuit to large-scale GRNs and discuss how regulatory design features such as network architecture, hierarchical organization, and cis-regulatory logic contribute to the developmental function of network circuits. Using specific subcircuits from the sea urchin endomesoderm GRN, for which both circuit design and biological function have been described, we evaluate by Boolean modeling and in silico perturbations the import of given circuit features on developmental function. The examples include subcircuits encoding positive feedback, mutual repression, and coherent feedforward, as well as signaling interaction circuitry. Within the hierarchy of the endomesoderm GRN, these subcircuits are organized in an intertwined and overlapping manner. Thus, we begin to see how regulatory information encoded at individual nodes is integrated at all levels of network organization to control developmental process.

New inter-correlated genes targeted by diatom-derived polyunsaturated aldehydes in the sea urchin Paracentrotus lividus

The marine environment is continually subjected to the action of stressors (including natural toxins), which represent a constant danger for benthic communities. In the present work using network analysis we identified ten genes on the basis of associated functions (FOXA, FoxG, GFI-1, nodal, JNK, OneCut/Hnf6, TAK1, tcf4, TCF7, VEGF) in the sea urchin Paracentrotus lividus, having key roles in different processes, such as embryonic development and asymmetry, cell fate specification, cell differentiation and morphogenesis, and skeletogenesis. These genes are correlated with three HUB genes, Foxo, Jun and HIF1A. Real Time qPCR revealed that during sea urchin embryonic development the expression levels of these genes were modulated by three diatom-derived polyunsaturated aldehydes (PUAs), decadienal, heptadienal and octadienal. Our findings show how changes in gene expression levels may be used as an early indicator of stressful conditions in the marine environment. The identification of key genes and the molecular pathways in which they are involved represents a fundamental tool in understanding how marine organisms try to afford protection against toxicants, to avoid deleterious consequences and irreversible damages. The genes identified in this work as targets for PUAs can be considered as possible biomarkers to detect exposure to different environmental pollutants.

Mouth development

A mouth is present in all animals, and comprises an opening from the outside into the oral cavity and the beginnings of the digestive tract to allow eating. This review focuses on the earliest steps in mouth formation. In the first half, we conclude that the mouth arose once during evolution. In all animals, the mouth forms from ectoderm and endoderm. A direct association of oral ectoderm and digestive endoderm is present even in triploblastic animals, and in chordates, this region is known as the extreme anterior domain (EAD). Further support for a single origin of the mouth is a conserved set of genes that form a 'mouth gene program' including foxA and otx2. In the second half of this review, we discuss steps involved in vertebrate mouth formation, using the frog Xenopus as a model. The vertebrate mouth derives from oral ectoderm from the anterior neural ridge, pharyngeal endoderm and cranial neural crest (NC). Vertebrates form a mouth by breaking through the body covering in a precise sequence including specification of EAD ectoderm and endoderm as well as NC, formation of a 'pre-mouth array,' basement membrane dissolution, stomodeum formation, and buccopharyngeal membrane perforation. In Xenopus, the EAD is also a craniofacial organizer that guides NC, while reciprocally, the NC signals to the EAD to elicit its morphogenesis into a pre-mouth array. Human mouth anomalies are prevalent and are affected by genetic and environmental factors, with understanding guided in part by use of animal models. WIREs Dev Biol 2017, 6:e275. doi: 10.1002/wdev.275 For further resources related to this article, please visit the WIREs website.

A gene regulatory program controlling early Xenopus mesendoderm formation: Network conservation and motifs

Germ layer formation is among the earliest differentiation events in metazoan embryos. In triploblasts, three germ layers are formed, among which the endoderm gives rise to the epithelial lining of the gut tube and associated organs including the liver, pancreas and lungs. In frogs (Xenopus), where early germ layer formation has been studied extensively, the process of endoderm specification involves the interplay of dozens of transcription factors. Here, we review the interactions between these factors, summarized in a transcriptional gene regulatory network (GRN). We highlight regulatory connections conserved between frog, fish, mouse, and human endodermal lineages. Especially prominent is the conserved role and regulatory targets of the Nodal signaling pathway and the T-box transcription factors, Vegt and Eomes. Additionally, we highlight network topologies and motifs, and speculate on their possible roles in development.

Gene expression profiling during the embryo-to-larva transition in the giant red sea urchin Mesocentrotus franciscanus

In echinoderms, major morphological transitions during early development are attributed to different genetic interactions and changes in global expression patterns that shape the regulatory program for the specification of embryonic territories. In order more thoroughly to understand these biological and molecular processes, we examined the transcriptome structure and expression profiles during the embryo‐to‐larva transition of a keystone species, the giant red sea urchin Mesocentrotus franciscanus. Using a de novo assembly approach, we obtained 176,885 transcripts from which 60,439 (34%) had significant alignments to known proteins. From these transcripts, ~80% were functionally annotated allowing the identification of ~2,600 functional, structural, and regulatory genes involved in developmental process. Analysis of expression profiles between gastrula and pluteus stages of M. franciscanus revealed 791 differentially expressed genes with 251 GO overrepresented terms. For gastrula, up‐regulated GO terms were mainly linked to cell differentiation and signal transduction involved in cell cycle checkpoints. In the pluteus stage, major GO terms were associated with phosphoprotein phosphatase activity, muscle contraction, and olfactory behavior, among others. Our evolutionary comparative analysis revealed that several of these genes and functional pathways are highly conserved among echinoids, holothuroids, and ophiuroids.

Hematopoiesis and T-cell specification as a model developmental system

The pathway to generate T cells from hematopoietic stem cells guides progenitors through a succession of fate choices while balancing differentiation progression against proliferation, stage to stage. Many elements of the regulatory system that controls this process are known, but the requirement for multiple, functionally distinct transcription factors needs clarification in terms of gene network architecture. Here, we compare the features of the T-cell specification system with the rule sets underlying two other influential types of gene network models: first, the combinatorial, hierarchical regulatory systems that generate the orderly, synchronized increases in complexity in most invertebrate embryos; second, the dueling 'master regulator' systems that are commonly used to explain bistability in microbial systems and in many fate choices in terminal differentiation. The T-cell specification process shares certain features with each of these prevalent models but differs from both of them in central respects. The T-cell system is highly combinatorial but also highly dose-sensitive in its use of crucial regulatory factors. The roles of these factors are not always T-lineage-specific, but they balance and modulate each other's activities long before any mutually exclusive silencing occurs. T-cell specification may provide a new hybrid model for gene networks in vertebrate developmental systems.