Genomic inventory and expression of Sox and Fox genes in the cnidarian Nematostella vectensis

Genome-wide identification and expression analysis of the Sox gene family in bivalves

Since the discovery of the Sox gene family in 1990, research on its distribution, classification, characterization, and function across various species has been significantly deepened. However, the Sox gene family has not yet been systematically and comprehensively analyzed in bivalves. In this study, 254 Sox genes were identified in 51 bivalves (covering 20 orders and 37 families). The Sox gene numbers ranged from 1 and 10 in most bivalves but no Sox gene was identified in the transcriptomes of Poromya illevis (Poromyoidea), Thracia phaseolina (Thracioidea), Solen vaginoides (Solenoidea), Lamychaena hians (Gastrochaenoidea), and Limopsis sp. and Solemya velesiana (Limopsoidea). The phylogenetic analyses revealed that Sox genes in bivalves are divided into 7 primary groups: SoxB1, SoxB2, SoxC, SoxD, SoxE, SoxF, and SoxH, with different groups exhibiting distinct conserved motif patterns. Notably, SoxA and SoxG found in most vertebrates were not identified in bivalves. Moreover, through spatiotemporal expression profiling in 6 distinct bivalve species, it was determined that the SoxH genes exhibit male-biased expression mainly in non-hermaphroditic bivalves, while SoxB1 and SoxC genes demonstrate female-biased expression, and these two Sox genes may serve a pivotal role in embryonic development stage and SoxB2, SoxC and SoxE may play a significant impact in neural development in bivalves. Sox family members also appear to possess disparate functions across different species and tissues. Overall, this study may provide a basis for future investigations into the functions and evolution of Sox genes in bivalves, and offer new perspectives on their roles in development in bivalves.

LRRK2 kinase activity is necessary for development and regeneration in Nematostella vectensis

BACKGROUND

The starlet sea anemone, Nematostella vectensis, is an emerging model organism with a high regenerative capacity, which was recently found to possess an orthologue to the human Leucine Rich Repeat Kinase 2 (LRRK2) gene. Mutations in this gene are the most common cause of inherited Parkinson's Disease (PD), highlighting the importance of understanding its function. Despite two decades of research, however, the function of LRRK2 is not well established.

METHODS

To investigate the function of LRRKs in Nematostella vectensis, we applied small molecule inhibitors targeting the kinase activity of LRRK2 to examine its function in development, homeostasis and regeneration in Nematostella vectensis.

RESULTS

In vivo analyses inhibiting the kinase function of this enzyme demonstrated a role of nvLRRK2 in development and regeneration of N. vectensis. These findings implicate a developmental role of LRRK2 in Nematostella, adding to the expanding knowledge of its physiological function.

CONCLUSIONS

Our work introduces a new model organism with which to study LRRK biology. We report that LRRK kinase activity is necessary for the development and regeneration of Nematostella. Given the short generation time, genetic trackability and in vivo imaging capabilities, this work introduces Nematostella vectensis as a new model in which to study genes linked to neurodegenerative diseases such as Parkinson's.

Evolution of glial cells: a non-bilaterian perspective

Nervous systems of bilaterian animals generally consist of two cell types: neurons and glial cells. Despite accumulating data about the many important functions glial cells serve in bilaterian nervous systems, the evolutionary origin of this abundant cell type remains unclear. Current hypotheses regarding glial evolution are mostly based on data from model bilaterians. Non-bilaterian animals have been largely overlooked in glial studies and have been subjected only to morphological analysis. Here, we provide a comprehensive overview of conservation of the bilateral gliogenic genetic repertoire of non-bilaterian phyla (Cnidaria, Placozoa, Ctenophora, and Porifera). We overview molecular and functional features of bilaterian glial cell types and discuss their possible evolutionary history. We then examine which glial features are present in non-bilaterians. Of these, cnidarians show the highest degree of gliogenic program conservation and may therefore be crucial to answer questions about glial evolution.

Antibiotics alter development and gene expression in the model cnidarian Nematostella vectensis

Background

Antibiotics are commonly used for controlling microbial growth in diseased organisms. However, antibiotic treatments during early developmental stages can have negative impacts on development and physiology that could offset the positive effects of reducing or eliminating pathogens. Similarly, antibiotics can shift the microbial community due to differential effectiveness on resistant and susceptible bacteria. Though antibiotic application does not typically result in mortality of marine invertebrates, little is known about the developmental and transcriptional effects. These sublethal effects could reduce the fitness of the host organism and lead to negative changes after removal of the antibiotics. Here, we quantify the impact of antibiotic treatment on development, gene expression, and the culturable bacterial community of a model cnidarian, Nematostella vectensis.

Methods

Ampicillin, streptomycin, rifampicin, and neomycin were compared individually at two concentrations, 50 and 200 µg mL-1, and in combination at 50 µg mL-1 each, to assess their impact on N. vectensis. First, we determined the impact antibiotics have on larval development. Next Amplicon 16S rDNA gene sequencing was used to compare the culturable bacteria that persist after antibiotic treatment to determine how these treatments may differentially select against the native microbiome. Lastly, we determined how acute (3-day) and chronic (8-day) antibiotic treatments impact gene expression of adult anemones.

Results

Under most exposures, the time of larval settlement extended as the concentration of antibiotics increased and had the longest delay of 3 days in the combination treatment. Culturable bacteria persisted through a majority of exposures where we identified 359 amplicon sequence variants (ASVs). The largest proportion of bacteria belonged to Gammaproteobacteria, and the most common ASVs were identified as Microbacterium and Vibrio. The acute antibiotic exposure resulted in differential expression of genes related to epigenetic mechanisms and neural processes, while constant application resulted in upregulation of chaperones and downregulation of mitochondrial genes when compared to controls. Gene Ontology analyses identified overall depletion of terms related to development and metabolism in both antibiotic treatments.

Discussion

Antibiotics resulted in a significant increase to settlement time of N. vectensis larvae. Culturable bacterial species after antibiotic treatments were taxonomically diverse. Additionally, the transcriptional effects of antibiotics, and after their removal result in significant differences in gene expression that may impact the physiology of the anemone, which may include removal of bacterial signaling on anemone gene expression. Our research suggests that impacts of antibiotics beyond the reduction of bacteria may be important to consider when they are applied to aquatic invertebrates including reef building corals.

Expanded expression of pro-neurogenic factor SoxB1 during larval development of gastropod Lymnaea stagnalis suggests preadaptation to prolonged neurogenesis in Mollusca

Introduction

The remarkable diversity observed in the structure and development of the molluscan nervous system raises intriguing questions regarding the molecular mechanisms underlying neurogenesis in Mollusca. The expression of SoxB family transcription factors plays a pivotal role in neuronal development, thereby offering valuable insights into the strategies of neurogenesis.

Methods

In this study, we conducted gene expression analysis focusing on SoxB-family transcription factors during early neurogenesis in the gastropod Lymnaea stagnalis. We employed a combination of hybridization chain reaction in situ hybridization (HCR-ISH), immunocytochemistry, confocal microscopy, and cell proliferation assays to investigate the spatial and temporal expression patterns of LsSoxB1 and LsSoxB2 from the gastrula stage to hatching, with particular attention to the formation of central ring ganglia.

Results

Our investigation reveals that LsSoxB1 demonstrates expanded ectodermal expression from the gastrula to the hatching stage, whereas expression of LsSoxB2 in the ectoderm ceases by the veliger stage. LsSoxB1 is expressed in the ectoderm of the head, foot, and visceral complex, as well as in forming ganglia and sensory cells. Conversely, LsSoxB2 is mostly restricted to the subepithelial layer and forming ganglia cells during metamorphosis. Proliferation assays indicate a uniform distribution of dividing cells in the ectoderm across all developmental stages, suggesting the absence of distinct neurogenic zones with increased proliferation in gastropods.

Discussion

Our findings reveal a spatially and temporally extended pattern of SoxB1 expression in a gastropod representative compared to other lophotrochozoan species. This prolonged and widespread expression of SoxB genes may be interpreted as a form of transcriptional neoteny, representing a preadaptation to prolonged neurogenesis. Consequently, it could contribute to the diversification of nervous systems in gastropods and lead to an increase in the complexity of the central nervous system in Mollusca.

Fluorescence In Situ Hybridization as a Tool for Studying the Specification and Differentiation of Cell Types in Nematostella vectensis

The sea anemone Nematostella vectensis is a genetically tractable cnidarian species that has become a model organism for studying the evolution of developmental processes and genome regulation, resilience to fluctuations in environmental conditions, and the response to pollutants. Gene expression analyses are central to many of these studies, and in situ hybridization has been an important method for obtaining spatial information, in particular during embryonic development. Like other cnidarians, Nematostella embryos are of comparably low morphological complexity, but they possess many cell types that are dispersed throughout the tissue and originate from broad and overlapping areas. These features have made two-color fluorescence in situ hybridization an important method to determine potential co-expression of genes and to generate hypotheses for their functions in cell fate specification. We here share protocols for single and double fluorescence in situ hybridization in Nematostella and for the combination of fluorescence in situ hybridization and immunofluorescence.

LRRK2 kinase activity is necessary for development and regeneration in Nematostella vectensis

Background

The starlet sea anemone, Nematostella vectensis, is an emerging model organism with a high regenerative capacity, which was recently found to possess an orthologue to the human LRRK2 gene (nvLRRK2). The leucine rich repeat kinase 2 (LRRK2) gene, when mutated, is the most common cause of inherited Parkinson's Disease (PD). Its protein product (LRRK2) has implications in a variety of cellular processes, however, the full function of LRRK2 is not well established. Current research is focusing on understanding the function of LRRK2, including both its physiological role as well as its pathobiological underpinnings.

Methods

We used bioinformatics to determine the cross-species conservation of LRRK2, then applied drugs targeting the kinase activity of LRRK2 to examine its function in development, homeostasis and regeneration in Nematostella vectensis.

Results

An in-silico characterization and phylogenetic analysis of nvLRRK2 comparing it to human LRRK2 highlighted key conserved motifs and residues. In vivo analyses inhibiting the kinase function of this enzyme demonstrated a role of nvLRRK2 in development and regeneration of N. vectensis. These findings implicate a developmental role of LRRK2 in Nematostella, adding to the expanding knowledge of its physiological function.

Conclusions

Our work introduces a new model organism with which to study LRRK biology. We show a necessity for LRRK2 in development and regeneration. Given the short generation time, genetic trackability and in vivo imaging capabilities, this work introduces Nematostella vectensis as a new model in which to study genes linked to neurodegenerative diseases such as Parkinson's.

The Molecular Mechanisms Employed by the Parasite Myxobolus bejeranoi (Cnidaria: Myxozoa) from Invasion through Sporulation for Successful Proliferation in Its Fish Host

Myxozoa is a unique group of obligate endoparasites in the phylum Cnidaria that can cause emerging diseases in wild and cultured fish populations. Recently, we identified a new myxozoan species, Myxobolus bejeranoi, which infects the gills of cultured tilapia while suppressing host immunity. To uncover the molecular mechanisms underlying this successful parasitic strategy, we conducted transcriptomics analysis of M. bejeranoi throughout the infection. Our results show that histones, which are essential for accelerated cell division, are highly expressed even one day after invasion. As the infection progressed, conserved parasitic genes that are known to modulate the host immune reaction in different parasitic taxa were upregulated. These genes included energy-related glycolytic enzymes, as well as calreticulin, proteases, and miRNA biogenesis proteins. Interestingly, myxozoan calreticulin formed a distinct phylogenetic clade apart from other cnidarians, suggesting a possible function in parasite pathogenesis. Sporogenesis was in its final stages 20 days post-exposure, as spore-specific markers were highly expressed. Lastly, we provide the first catalog of transcription factors in a Myxozoa species, which is minimized compared to free-living cnidarians and is dominated by homeodomain types. Overall, these molecular insights into myxozoan infection support the concept that parasitic strategies are a result of convergent evolution.

NvPrdm14d-expressing neural progenitor cells contribute to non-ectodermal neurogenesis in Nematostella vectensis

Neurogenesis has been studied extensively in the ectoderm, from which most animals generate the majority of their neurons. Neurogenesis from non-ectodermal tissue is, in contrast, poorly understood. Here we use the cnidarian Nematostella vectensis as a model to provide new insights into the molecular regulation of non-ectodermal neurogenesis. We show that the transcription factor NvPrdm14d is expressed in a subpopulation of NvSoxB(2)-expressing endodermal progenitor cells and their NvPOU4-expressing progeny. Using a new transgenic reporter line, we show that NvPrdm14d-expressing cells give rise to neurons in the body wall and in close vicinity of the longitudinal retractor muscles. RNA-sequencing of NvPrdm14d::GFP-expressing cells and gene knockdown experiments provide candidate genes for the development and function of these neurons. Together, the identification of a population of endoderm-specific neural progenitor cells and of previously undescribed putative motoneurons in Nematostella provide new insights into the regulation of non-ectodermal neurogenesis.

Genome-Wide Identification, Evolutionary and Mutational Analysis of the Buffalo Sox Gene Family

The Sox gene family constitutes transcription factors with a conserved high mobility group box (HMG) that regulate a variety of developmental processes, including sex differentiation, neural, cartilage, and early embryonic development. In this study, we systematically analyzed and characterized the 20 Sox genes from the whole buffalo genome, using comparative genomic and evolutionary analyses. All the buffalo Sox genes were divided into nine sub-groups, and each gene had a specific number of exons and introns, which contributed to different gene structures. Molecular phylogeny revealed more sequence similarity of buffalo Sox genes with those of cattle. Furthermore, evolutionary analysis revealed that the HMG domain remained conserved in the all members of the Sox gene family. Similarly, all the genes are under strong purifying selection pressure; seven segmental duplications occurred from 9.65 to 21.41 million years ago (MYA), and four potential recombination breakpoints were also predicted. Mutational analysis revealed twenty non-synonymous mutations with potential effects on physiological functions, including embryonic development and cell differentiation in the buffalo. The present study provides insights into the genetic architecture of the Sox gene family in buffalo, highlights the significance of mutations, and provides their potential utility for marker-assisted selection for targeted genetic improvement in buffalo.

A new insight into potential roles of Spfoxl-2 in the testicular development of Scylla paramamosain by RNAi and transcriptome analysis

In our previous study, we found that the Spfoxl-2 transcript was highly expressed in gonads and explored its potential target genes in the ovary of Scylla paramamosain. In the current study, we primally analyzed its potential target genes in the testis through RNAi and RNA-Seq technology and compared with that in the ovary. The results showed that a total of 7892 unigenes were differentially expressed after Spfoxl-2 silencing in the testis, including plenty of conserved genes involved in testicular development, such as Dmrt family genes, Sox family genes, Caspase family genes, Cdk family genes, Kinesin family genes, Fox family genes and other genes. Further analysis revealed that these differentially expressed genes (DEGs) were enriched in crucial pathways involved in spermatogenesis, such as DNA replication, Cell cycle, Spliceosome, Homologous recombination, Meiosis and Apoptosis. The comparison results of potential target genes in the ovary and testis reveal 135 common potential target genes, including some genes involved in the immune response. According to our knowledge, the present work was the first to disclose the functions of foxl-2 in the testis of crustacean species using transcriptome analysis. It not only identifies key genes and pathways involved in the testicular development of S. paramamosain, but also reveals a new molecular-level understanding of the function of foxl-2 in testicular development.

Single-cell atavism reveals an ancient mechanism of cell type diversification in a sea anemone

Cnidocytes are the explosive stinging cells unique to cnidarians (corals, jellyfish, etc). Specialized for prey capture and defense, cnidocytes comprise a group of over 30 morphologically and functionally distinct cell types. These unusual cells are iconic examples of biological novelty but the developmental mechanisms driving diversity of the stinging apparatus are poorly characterized, making it challenging to understand the evolutionary history of stinging cells. Using CRISPR/Cas9-mediated genome editing in the sea anemone Nematostella vectensis, we show that a single transcription factor (NvSox2) acts as a binary switch between two alternative stinging cell fates. Knockout of NvSox2 causes a transformation of piercing cells into ensnaring cells, which are common in other species of sea anemone but appear to have been silenced in N. vectensis. These results reveal an unusual case of single-cell atavism and expand our understanding of the diversification of cell type identity.

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.

The Fox Gene Repertoire in the Annelid Owenia fusiformis Reveals Multiple Expansions of the foxQ2 Class in Spiralia

Fox genes are a large and conserved family of transcription factors involved in many key biological processes, including embryogenesis and body patterning. Although the role of Fox genes has been studied in an array of model systems, comprehensive comparative studies in Spiralia-a large clade of invertebrate animals including molluscs and annelids-are scarce but much needed to better understand the evolutionary history of this gene family. Here, we reconstruct and functionally characterize the Fox gene complement in the annelid Owenia fusiformis, a slow evolving species and member of the sister group to all remaining annelids. The genome of O. fusiformis contains at least a single ortholog for 20 of the 22 Fox gene classes that are ancestral to Bilateria, including an ortholog of the recently discovered foxT class. Temporal and spatial expression dynamics reveal a conserved role of Fox genes in gut formation, mesoderm patterning, and apical organ and cilia formation in Annelida and Spiralia. Moreover, we uncover an ancestral expansion of foxQ2 genes in Spiralia, represented by 11 paralogs in O. fusiformis. Notably, although all foxQ2 copies have apical expression in O. fusiformis, they show variable spatial domains and staggered temporal activation, which suggest cooperation and sub-functionalization among foxQ2 genes for the development of apical fates in this annelid. Altogether, our study informs the evolution and developmental roles of Fox genes in Annelida and Spiralia generally, providing the basis to explore how regulatory changes in Fox gene expression might have contributed to developmental and morphological diversification in Spiralia.

Gene activation of metazoan Fox transcription factors at the onset of metamorphosis in the marine demosponge Amphimedon queenslandica

Transcription factors encoded by the Forkhead (Fox) gene family have diverse, sometimes conserved, regulatory roles in eumetazoan development, immunity, and physiology. Although this gene family includes members that predate the origin of the animal kingdom, the majority of metazoan Fox genes evolved after the divergence of animals and choanoflagellates. Here, we characterize the composition, structure, and expression of Fox genes in the marine demosponge Amphimedon queenslandica to better understand the origin and evolution of this family. The Fox gene repertoire in A. queenslandica appears to be similar to the ancestral metazoan Fox gene family. All 17 A. queenslandica Fox genes are differentially expressed during development and in adult cell types. Remarkably, eight of these, all of which appear to be metazoan-specific, are induced within just 1 h of larval settlement and commencement of metamorphosis. Gene co-expression analyses suggest that these eight Fox genes regulate developmental and physiological processes similar to their roles in other animals. These findings are consistent with Fox genes playing deeply ancestral roles in animal development and physiology, including in response to changes in the external environment.

Single-cell transcriptomics identifies conserved regulators of neuroglandular lineages

Communication in bilaterian nervous systems is mediated by electrical and secreted signals; however, the evolutionary origin and relation of neurons to other secretory cell types has not been elucidated. Here, we use developmental single-cell RNA sequencing in the cnidarian Nematostella vectensis, representing an early evolutionary lineage with a simple nervous system. Validated by transgenics, we demonstrate that neurons, stinging cells, and gland cells arise from a common multipotent progenitor population. We identify the conserved transcription factor gene SoxC as a key upstream regulator of all neuroglandular lineages and demonstrate that SoxC knockdown eliminates both neuronal and secretory cell types. While in vertebrates and many other bilaterians neurogenesis is largely restricted to early developmental stages, we show that in the sea anemone, differentiation of neuroglandular cells is maintained throughout all life stages, and follows the same molecular trajectories from embryo to adulthood, ensuring lifelong homeostasis of neuroglandular cell lineages.

A comprehensive study of arthropod and onychophoran Fox gene expression patterns

Fox genes represent an evolutionary old class of transcription factor encoding genes that evolved in the last common ancestor of fungi and animals. They represent key-components of multiple gene regulatory networks (GRNs) that are essential for embryonic development. Most of our knowledge about the function of Fox genes comes from vertebrate research, and for arthropods the only comprehensive gene expression analysis is that of the fly Drosophila melanogaster. For other arthropods, only selected Fox genes have been investigated. In this study, we provide the first comprehensive gene expression analysis of arthropod Fox genes including representative species of all main groups of arthropods, Pancrustacea, Myriapoda and Chelicerata. We also provide the first comprehensive analysis of Fox gene expression in an onychophoran species. Our data show that many of the Fox genes likely retained their function during panarthropod evolution highlighting their importance in development. Comparison with published data from other groups of animals shows that this high degree of evolutionary conservation often dates back beyond the last common ancestor of Panarthropoda.

Comparative Transcriptome Reveals the Potential Modulation Mechanisms of Spfoxl-2 Affecting Ovarian Development of Scylla paramamosain

Previously, we reported the identification, tissue distribution and confirmed the roles of Spfoxl-2 in regulating vitellogenin (vtg) expression in Scylla paramamosain. Here, we primally analyzed its potential target genes in the ovary with RNAi and RNA-Seq technology. By comparing the transcriptome data of two groups (ovaries that injected with EGFP and Foxl-2 siRNA, respectively), we found 645 DEGs (differentially expressed genes), including several conserved crucial genes involved in ovarian development, such as vtg, vitellogenin receptor (vtgR), adenylate cyclase (AC), cyclinB, and cell division cycle 2 (cdc2). In addition, these DEGs were also enriched in pathways related to ovary development, including relaxin signaling pathway, ovarian steroidogenesis, and progesterone-mediated oocyte maturation. Moreover, several genes were selected for qRT-PCR to validate the accuracy of the bioinformatic result. To the best of our knowledge, the current study was the first report about foxl-2 function through comparative transcriptome analysis in crustacean species, which identified not only relevant genes and pathways involved in ovarian development of S. paramamosain, but also provided new insights into the regulatory mechanisms of foxl-2 at the molecular level in crustacean.

Phylogenetic analysis of forkhead transcription factors in the Panarthropoda

Fox genes encode transcription factors that contain a DNA binding domain, the forkhead domain, and are known from diverse animal species. The exact homology of the Fox genes of different species is debated and this makes inferences about the evolution of the Fox genes, and their duplications and losses difficult. We have performed phylogenetic analyses of the Fox gene complements of 32 panarthropod species. Our results confirm an ancestral complement of FoxA, FoxB, FoxC, FoxD, FoxF, FoxG, FoxJ1, FoxJ2/3, FoxK, FoxL1, FoxL2, FoxN1/4, FoxN2/3, FoxO, FoxP, and FoxQ2 in the Arthropoda, and additionally FoxH and FoxQ1 in the Panarthropoda (including tardigrades and onychophorans). We identify a novel Fox gene sub-family, that we designate as FoxT that includes two genes in Drosophila melanogaster, Circadianly Regulated Gene (Crg-1) and forkhead domain 3F (fd3F). In a very recent paper, the same new Fox gene sub-family was identified in insects (Lin et al. 2021). Our analysis confirms the presence of FoxT and shows that its members are present throughout Panarthropoda. We show that the hitherto unclassified gene CG32006 from the fly Drosophila melanogaster belongs to FoxJ1. We also detect gene losses: FoxE and FoxM were lost already in the panarthropod ancestor, whereas the loss of FoxH occurred in the arthropod ancestor. Finally, we find an ortholog of FoxQ1 in the bark scorpion Centruroides sculpturatus, confirmed not only by phylogenetic analysis, but also by forming an evolutionarily conserved gene cluster with FoxF, FoxC, and FoxL1. This suggests that FoxQ1 belongs to the ancestral Fox gene complement in panarthropods and also in chelicerates, but has been lost at the base of the mandibulate arthropods.

A cellular and molecular analysis of SoxB-driven neurogenesis in a cnidarian

Neurogenesis is the generation of neurons from stem cells, a process that is regulated by SoxB transcription factors (TFs) in many animals. Although the roles of these TFs are well understood in bilaterians, how their neural function evolved is unclear. Here, we use Hydractinia symbiolongicarpus, a member of the early-branching phylum Cnidaria, to provide insight into this question. Using a combination of mRNA in situ hybridization, transgenesis, gene knockdown, transcriptomics, and in vivo imaging, we provide a comprehensive molecular and cellular analysis of neurogenesis during embryogenesis, homeostasis, and regeneration in this animal. We show that SoxB genes act sequentially at least in some cases. Stem cells expressing Piwi1 and Soxb1, which have broad developmental potential, become neural progenitors that express Soxb2 before differentiating into mature neural cells. Knockdown of SoxB genes resulted in complex defects in embryonic neurogenesis. Hydractinia neural cells differentiate while migrating from the aboral to the oral end of the animal, but it is unclear whether migration per se or exposure to different microenvironments is the main driver of their fate determination. Our data constitute a rich resource for studies aiming at addressing this question, which is at the heart of understanding the origin and development of animal nervous systems.

Stem Cells and Innate Immunity in Aquatic Invertebrates: Bridging Two Seemingly Disparate Disciplines for New Discoveries in Biology

The scopes related to the interplay between stem cells and the immune system are broad and range from the basic understanding of organism's physiology and ecology to translational studies, further contributing to (eco)toxicology, biotechnology, and medicine as well as regulatory and ethical aspects. Stem cells originate immune cells through hematopoiesis, and the interplay between the two cell types is required in processes like regeneration. In addition, stem and immune cell anomalies directly affect the organism's functions, its ability to cope with environmental changes and, indirectly, its role in ecosystem services. However, stem cells and immune cells continue to be considered parts of two branches of biological research with few interconnections between them. This review aims to bridge these two seemingly disparate disciplines towards much more integrative and transformative approaches with examples deriving mainly from aquatic invertebrates. We discuss the current understanding of cross-disciplinary collaborative and emerging issues, raising novel hypotheses and comments. We also discuss the problems and perspectives of the two disciplines and how to integrate their conceptual frameworks to address basic equations in biology in a new, innovative way.

The cytochrome P450 (CYP) superfamily in cnidarians

The cytochrome P450 (CYP) superfamily is a diverse and important enzyme family, playing a central role in chemical defense and in synthesis and metabolism of major biological signaling molecules. The CYPomes of four cnidarian genomes (Hydra vulgaris, Acropora digitifera, Aurelia aurita, Nematostella vectensis) were annotated; phylogenetic analyses determined the evolutionary relationships amongst the sequences and with existing metazoan CYPs. 155 functional CYPs were identified and 90 fragments. Genes were from 24 new CYP families and several new subfamilies; genes were in 9 of the 12 established metazoan CYP clans. All species had large expansions of clan 2 diversity, with H. vulgaris having reduced diversity for both clan 3 and mitochondrial clan. We identified potential candidates for xenobiotic metabolism and steroidogenesis. That each genome contained multiple, novel CYP families may reflect the large evolutionary distance within the cnidarians, unique physiology in the cnidarian classes, and/or different ecology of the individual species.

Variance in expression and localization of sex-related genes CgDsx, CgBHMG1 and CgFoxl2 during diploid and triploid Pacific oyster Crassostrea gigas gonad differentiation

Several evolutionarily conserved classes of transcriptional regulators were involved in diverse sex determination and differentiation pathways across taxa, whereas their roles in most mollusks is still limited. The Pacific oyster Crassostrea gigas, a dioecious bivalve with sex reversal, could be an ideal model for this issue because of its complex sexuality and potential disruption of sex differentiation in triploid individuals. Here, two mRNA splicing isoforms of a DM domain gene CgDsx and two isoforms of a novel sex-related CgBHMG1 (ortholog of BHMG1 in mammals) were identified in C. gigas. Real time PCR showed that two isoforms of CgDsx and one isoform of CgBHMG1 displayed male-specific expression in diploid oysters, opposite with the female-specific CgFoxl2 (a potential factor of female gonadic differentiation). Interestingly, the four sex-specific transcripts in diploid oyster were expressed in triploid oysters with opposite sex, triploid hermaphrodites and individuals at stage I that sex could not be determined. Subsequent in situ hybridization analysis on gonads of diploid oysters revealed predominant expression of CgDsx in spermatogonia of testes, CgBHMG1 in spermatocytes of testes and follicle cells of ovaries, and CgFoxl2 in follicle cells of ovaries and some male germ cells in testes. And aberrant co-expression of the three genes in triploid oysters was localized in gonadal tubules of gonads at stage I, ovarian follicle cells and undetermined gonial cells in nontypical hermaphroditic gonads with rare female materials. From the above, temporal and spatial expression of sex-related genes in diploid and triploid gonads indicated that CgDsx and CgFoxl2 might mainly function in C. gigas sex differentiation, and CgBHMG1 appeared as a factor involved in meiosis. This work will help to illuminate the gene network of sex differentiation in bivalves and provides new sight on this issue from comparison between diploid and triploid individuals.

NvPOU4/Brain3 Functions as a Terminal Selector Gene in the Nervous System of the Cnidarian Nematostella vectensis

Terminal selectors are transcription factors that control the morphological, physiological, and molecular features that characterize distinct cell types. Here, we show that, in the sea anemone Nematostella vectensis, NvPOU4 is expressed in post-mitotic cells that give rise to a diverse set of neural cell types, including cnidocytes and NvElav1-expressing neurons. Morphological analyses of NvPOU4 mutants crossed to transgenic reporter lines show that the loss of NvPOU4 does not affect the initial specification of neural cells. Transcriptomes derived from the mutants and from different neural cell populations reveal that NvPOU4 is required for the execution of the terminal differentiation program of these neural cells. These findings suggest that POU4 genes have ancient functions as terminal selectors for morphologically and functionally disparate types of neurons and they provide experimental support for the relevance of terminal selectors for understanding the evolution of cell types.

Systematic identification and expression analysis of the Sox gene family in spotted sea bass (Lateolabrax maculatus)

The Sox gene family encodes a set of transcription factors characterized by a conserved Sry-related high mobility group (HMG)-box domain, which performs a series of essential biological functions in diverse tissues and developmental processes. In this study, the Sox gene family was systematically characterized in spotted sea bass (Lateolabrax maculatus). A total of 26 Sox genes were identified and classified into eight subfamilies, namely, SoxB1, SoxB2, SoxC, SoxD, SoxE, SoxF, SoxH and SoxK. The phylogenetic relationship, exon-intron and domain structure analyses supported their annotation and classification. Comparison of gene copy numbers and chromosome locations among different species indicated that except tandem duplicated paralogs of Sox17/Sox32, duplicated Sox genes in spotted sea bass were generated from teleost-specific whole genome duplication during evolution. In addition, qRT-PCR was performed to detect the expression profiles of Sox genes during development and adulthood. The results showed that the expression of 16 out of 26 Sox genes was induced dramatically at different starting points after the multicellular stage, which is consistent with embryogenesis. At the early stage of sex differentiation, 9 Sox genes exhibited sexually dimorphic expression patterns, among which Sox3, Sox19 and Sox6b showed the most significant ovary-biased expression. Moreover, the distinct expression pattern of Sox genes was observed in different adult tissues. Our results provide a fundamental resource for further investigating the functions of Sox genes in embryonic processes, sex determination and differentiation as well as controlling the homeostasis of adult tissues in spotted sea bass.

Analysis of Fox genes in Schmidtea mediterranea reveals new families and a conserved role of Smed-foxO in controlling cell death

The forkhead box (Fox) genes encode transcription factors that control several key aspects of development. Present in the ancestor of all eukaryotes, Fox genes underwent several duplications followed by loss and diversification events that gave rise to the current 25 families. However, few Fox members have been identified from the Lophotrochozoa clade, and specifically from planarians, which are a unique model for understanding development, due to the striking plasticity of the adult. The aim of this study was to identify and perform evolutionary and functional studies of the Fox genes of lophotrochozoan species and, specifically, of the planarian Schmidtea mediterranea. Generating a pipeline for identifying Forkhead domains and using phylogenetics allowed us the phylogenetic reconstruction of Fox genes. We corrected the annotation for misannotated genes and uncovered a new family, the QD, present in all metazoans. According to the new phylogeny, the 27 Fox genes found in Schmidtea mediterranea were classified into 12 families. In Platyhelminthes, family losses were accompanied by extensive gene diversification and the appearance of specific families, the A(P) and N(P). Among the newly identified planarian Fox genes, we found a single copy of foxO, which shows an evolutionary conserved role in controlling cell death.

Ectopic activation of GABAB receptors inhibits neurogenesis and metamorphosis in the cnidarian Nematostella vectensis

The metabotropic gamma-aminobutyric acid B receptor (GABA_BR) is a G protein-coupled receptor that mediates neuronal inhibition by the neurotransmitter GABA. While GABA_BR-mediated signalling has been suggested to play central roles in neuronal differentiation and proliferation across evolution, it has mostly been studied in the mammalian brain. Here, we demonstrate that ectopic activation of GABA_BR signalling affects neurogenic functions in the sea anemone Nematostella vectensis. We identified four putative Nematostella GABA_BR homologues presenting conserved three-dimensional extracellular domains and residues needed for binding GABA and the GABA_BR agonist baclofen. Moreover, sustained activation of GABA_BR signalling reversibly arrests the critical metamorphosis transition from planktonic larva to sessile polyp life stage. To understand the processes that underlie the developmental arrest, we combined transcriptomic and spatial analyses of control and baclofen-treated larvae. Our findings reveal that the cnidarian neurogenic programme is arrested following the addition of baclofen to developing larvae. Specifically, neuron development and neurite extension were inhibited, resulting in an underdeveloped and less organized nervous system and downregulation of proneural factors including NvSoxB(2), NvNeuroD1 and NvElav1. Our results thus point to an evolutionarily conserved function of GABA_BR in neurogenesis regulation and shed light on early cnidarian development.

Cadherin preserves cohesion across involuting tissues during C. elegans neurulation

The internalization of the central nervous system, termed neurulation in vertebrates, is a critical step in embryogenesis. Open questions remain regarding how force propels coordinated tissue movement during the process, and little is known as to how internalization happens in invertebrates. We show that in C. elegans morphogenesis, apical constriction in the retracting pharynx drives involution of the adjacent neuroectoderm. HMR-1/cadherin mediates this process via inter-tissue attachment, as well as cohesion within the neuroectoderm. Our results demonstrate that HMR-1 is capable of mediating embryo-wide reorganization driven by a centrally located force generator, and indicate a non-canonical use of cadherin on the basal side of an epithelium that may apply to vertebrate neurulation. Additionally, we highlight shared morphology and gene expression in tissues driving involution, which suggests that neuroectoderm involution in C. elegans is potentially homologous with vertebrate neurulation and thus may help elucidate the evolutionary origin of the brain.

The Diversity of Muscles and Their Regenerative Potential across Animals

Cells with contractile functions are present in almost all metazoans, and so are the related processes of muscle homeostasis and regeneration. Regeneration itself is a complex process unevenly spread across metazoans that ranges from full-body regeneration to partial reconstruction of damaged organs or body tissues, including muscles. The cellular and molecular mechanisms involved in regenerative processes can be homologous, co-opted, and/or evolved independently. By comparing the mechanisms of muscle homeostasis and regeneration throughout the diversity of animal body-plans and life cycles, it is possible to identify conserved and divergent cellular and molecular mechanisms underlying muscle plasticity. In this review we aim at providing an overview of muscle regeneration studies in metazoans, highlighting the major regenerative strategies and molecular pathways involved. By gathering these findings, we wish to advocate a comparative and evolutionary approach to prompt a wider use of “non-canonical” animal models for molecular and even pharmacological studies in the field of muscle regeneration.

Identification and expression profiles of Fox transcription factors in the Yesso scallop (Patinopecten yessoensis)

The forkhead box (Fox) gene family is a family of transcription factors that play important roles in a variety of biological processes in vertebrates, including early development and cell proliferation and differentiation. However, at present, studies on the mollusk Fox family are relatively lacking. In the present study, the Fox gene family of the Yesso scallop (Patinopecten yessoensis) was systematically identified. In addition, the expression profiles of the Fox gene family in early development and adult tissues were analyzed. The results showed that there were 26 Fox genes in P. yessoensis. Of the 26 genes, 24 belonged to 20 subfamilies. The Fox genes belonging to the I, Q1, R and S subfamilies were absent in P. yessoensis. The other 2 genes formed 2 independent clades with the Fox genes of other mollusks and protostomes. They might be new members of the Fox family and were named FoxY and FoxZ. P. yessoensis contained a FoxC-FoxL1 gene cluster similar in structure to that of Branchiostoma floridae, suggesting that the cluster might already exist in the ancestors of bilaterally symmetrical animals. The gene expression analysis of Fox showed that most of the genes were continuously expressed in multiple stages of early development, suggesting that Fox genes might be widely involved in the regulation of embryo and larval development of P. yessoensis. Nine Fox genes were specifically expressed in certain tissues, such as the nerve ganglia, foot, ovary, testis, and gills. For the 9 genes that were differentially expressed between the testis and ovary, their expression levels were analyzed during the 4 developmental stages of gonads. The results showed that FoxL2, FoxE and FoxY were highly expressed in the ovary during all developmental stages, while FoxZ was highly expressed in the testis during all developmental stages. The results suggested that these genes might play an important role in sex maintenance or gametogenesis. The present study could provide a reference for evolutionary and functional studies of the Fox family in metazoans.

Expression Analysis of Cnidarian-Specific Neuropeptides in a Sea Anemone Unveils an Apical-Organ-Associated Nerve Net That Disintegrates at Metamorphosis

Neuropeptides are ancient neuronal signaling molecules that have diversified across Cnidaria (e.g., jellyfish, corals, and sea anemones) and its sister group Bilateria (e.g., vertebrates, insects, and worms). Over the course of neuropeptide evolution emerged lineage-specific neuropeptides, but their roles in the evolution and diversification of nervous system function remain enigmatic. As a step toward filling in this knowledge gap, we investigated the expression pattern of a cnidarian-specific neuropeptide-RPamide-during the development of the starlet sea anemone Nematostella vectensis, using in situ hybridization and immunohistochemistry. We show that RPamide precursor transcripts first occur during gastrulation in scattered epithelial cells of the aboral ectoderm. These RPamide-positive epithelial cells exhibit a spindle-shaped, sensory-cell-like morphology, and extend basal neuronal processes that form a nerve net in the aboral ectoderm of the free-swimming planula larva. At the aboral end, RPamide-positive sensory cells become integrated into the developing apical organ that forms a bundle of long cilia referred to as the apical tuft. Later during planula development, RPamide expression becomes evident in sensory cells in the oral ectoderm of the body column and pharynx, and in the developing endodermal nervous system. At metamorphosis into a polyp, the RPamide-positive sensory nerve net in the aboral ectoderm degenerates by apoptosis, and RPamide expression begins in ectodermal sensory cells of growing oral tentacles. In addition, we find that the expression pattern of RPamide in planulae differs from that of conserved neuropeptides that are shared across Cnidaria and Bilateria, indicative of distinct functions. Our results not only provide the anatomical framework necessary to analyze the function of the cnidarian-specific neuropeptides in future studies, but also reveal previously unrecognized features of the sea anemone nervous system-the apical organ neurons of the planula larva, and metamorphosis-associated reorganization of the ectodermal nervous system.

Transcriptome Characterization of Reverse Development in Turritopsis dohrnii (Hydrozoa, Cnidaria)

Medusae of Turritopsis dohrnii undergo reverse development in response to physical damage, adverse environmental conditions, or aging. Senescent, weakened or damaged medusae transform into a cluster of poorly differentiated cells (known as the cyst stage), which metamorphose back into a preceding life cycle stage, the polyp. During the metamorphosis, cell transdifferentiation occurs. The cyst represents the intermediate stage between a reverting medusa and a healthy polyp, during which cell transdifferentiation and tissue reorganization take place. Here we characterize and compare the transcriptomes of the polyp and newborn medusa stages of T. dohrnii with that of the cyst, to identify biological networks potentially involved in the reverse development and transdifferentiation processes. The polyp, medusa and cyst of T. dohrnii were sequenced through Illumina RNA-sequencing and assembled using a de novo approach, resulting in 92,569, 74,639 and 86,373 contigs, respectively. The transcriptomes were annotated and comparative analyses among the stages identified biological networks that were significantly over-and under-expressed in the cyst as compared to the polyp and medusa stages. Biological processes that occur at the cyst stage such as telomerase activity, regulation of transposable elements and DNA repair systems, and suppression of cell signaling pathways, mitotic cell division and cellular differentiation and development may be involved in T. dohrnii's reverse development and transdifferentiation. Our results are the first attempt to understand T. dohrnii's life-cycle reversal at the genetic level, and indicate possible avenues of future research on developmental strategies, cell transdifferentiation, and aging using T. dohrnii as a non-traditional in vivo system.

Systematic Identification and Evolution Analysis of Sox Genes in Coturnix japonica Based on Comparative Genomics

Coturnix japonica (Japanese quail) has been extensively used as a model animal for biological studies. The Sox gene family, which was systematically characterized by a high-mobility group (HMG-box) in many animal species, encodes transcription factors that play central roles during multiple developmental processes. However, genome-wide investigations on the Sox gene family in birds are scarce. In the current study, we first performed a genome-wide study to explore the Sox gene family in galliform birds. Based on available genomic sequences retrieved from the NCBI database, we focused on the global identification of the Sox gene family in C. japonica and other species in Galliformes, and the evolutionary relationships of Sox genes. In our result, a total of 35 Sox genes in seven groups were identified in the C. japonica genome. Our results also revealed that dispersed gene duplications contributed the most to the expansion of the Sox gene family in Galliform birds. Evolutionary analyses indicated that Sox genes are an ancient gene family, and strong purifying selections played key roles in the evolution of CjSox genes of C. japonica. More interestingly, we observed that most Sox genes exhibited highly embryo-specific expression in both gonads. Our findings provided new insights into the molecular function and phylogeny of Sox gene family in birds.

The cadherin-catenin complex is necessary for cell adhesion and embryogenesis in Nematostella vectensis

The cadherin-catenin complex is a conserved, calcium-dependent cell-cell adhesion module that is necessary for normal development and the maintenance of tissue integrity in bilaterian animals. Despite longstanding evidence of a deep ancestry of calcium-dependent cell adhesion in animals, the requirement of the cadherin-catenin complex to coordinate cell-cell adhesion has not been tested directly in a non-bilaterian organism. Here, we provide the first analysis of classical cadherins and catenins in the Starlet Sea Anemone, Nematostella vectensis. Gene expression, protein localization, siRNA-mediated knockdown of α-catenin, and calcium-dependent cell aggregation assays provide evidence that a bonafide cadherin-catenin complex is present in the early embryo, and that α-catenin is required for normal embryonic development and the formation of cell-cell adhesions between cells dissociated from whole embryos. Together these results support the hypothesis that the cadherin-catenin complex was likely a complete and functional cell-cell adhesion module in the last common cnidarian-bilaterian ancestor. SUMMARY STATEMENT: Embryonic manipulations and ex vivo adhesion assays in the sea anemone, Nematostella vectensis, indicate that the necessity of the cadherin-catenin complex for mediating cell-cell adhesion is deeply conserved in animal evolution.

Temporal and spatial expression of the Fox gene family in the Leech Helobdella austinensis

The Forkhead box (Fox) gene family is an evolutionarily ancient gene family named after the Drosophila melanogaster forkhead gene (fkh). Fox genes are highly conserved transcription factors critical for embryogenesis and carcinogenesis. In the current study, we report a whole-genome survey of Fox genes and their expression patterns in the leech Helobdella austienesis. Phylogenetic analysis suggests that some Fox genes of leeches are correlated with other Lophotrochozoa and vertebrate Fox genes. Here we have performed semiquantitative reverse transcription polymerase chain reaction and whole-mount in situ hybridization of Fox genes throughout the embryonic development of H. austinensis. We found that each one of the leech Fox genes (FoxA1, FoxA3, FoxC, FoxL2, FoxO1, and FoxO2) is expressed in a specific set of cells or tissue type. From Stages 9-11, Hau-FoxA1 was expressed in the foregut of the anterior region, and Hau-FoxL2 was expressed in mesodermal muscle fiber. Hau-FoxA3 was temporally expressed in the ventral neuroectoderm. At Stage 11, Hau-FoxC was expressed in the foregut. Hau-FoxO genes have a ubiquitous expression. Our results provide more insight on the evolutionary linkage and role of the Fox gene function in Bilateria.

Understanding Animal Evolution: The Added Value of Sponge Transcriptomics and Genomics: The disconnect between gene content and body plan evolution

Sponges are important but often-neglected organisms. The absence of classical animal traits (nerves, digestive tract, and muscles) makes sponges challenging for non-specialists to work with and has delayed getting high quality genomic data compared to other invertebrates. Yet analyses of sponge genomes and transcriptomes currently available have radically changed our understanding of animal evolution. Sponges are of prime evolutionary importance as one of the best candidates to form the sister group of all other animals, and genomic data are essential to understand the mechanisms that control animal evolution and diversity. Here we review the most significant outcomes of current genomic and transcriptomic analyses of sponges, and discuss limitations and future directions of sponge transcriptomic and genomic studies.

Embryonic expression patterns and phylogenetic analysis of panarthropod sox genes: insight into nervous system development, segmentation and gonadogenesis

BACKGROUND

Sox (Sry-related high-mobility-group box) genes represent important factors in animal development. Relatively little, however, is known about the embryonic expression patterns and thus possible function(s) of Sox genes during ontogenesis in panarthropods (Arthropoda+Tardigrada+Onychophora). To date, studies have been restricted exclusively to higher insects, including the model system Drosophila melanogaster, with no comprehensive data available for any other arthropod group, or any tardigrade or onychophoran.

RESULTS

This study provides a phylogenetic analysis of panarthropod Sox genes and presents the first comprehensive analysis of embryonic expression patterns in the flour beetle Tribolium castaneum (Hexapoda), the pill millipede Glomeris marginata (Myriapoda), and the velvet worm, Euperipatoides kanangrensis (Onychophora). 24 Sox genes were identified and investigated: 7 in Euperipatoides, 8 in Glomeris, and 9 in Tribolium. Each species possesses at least one ortholog of each of the five expected Sox gene families, B, C, D, E, and F, many of which are differentially expressed during ontogenesis.

CONCLUSION

Sox gene expression (and potentially function) is highly conserved in arthropods and their closest relatives, the onychophorans. Sox B, C and D class genes appear to be crucial for nervous system development, while the Sox B genes Dichaete (D) and Sox21b likely play an additional conserved role in panarthropod segmentation. The Sox B gene Sox21a likely has a conserved function in foregut and Malpighian tubule development, at least in Hexapoda. The data further suggest that Sox D and E genes are involved in mesoderm differentiation, and that Sox E genes are involved in gonadal development. The new data expand our knowledge about the expression and implied function of Sox genes to Mandibulata (Myriapoda+Pancrustacea) and Panarthropoda (Arthropoda+Onychophora).

Unipotent progenitors contribute to the generation of sensory cell types in the nervous system of the cnidarian Nematostella vectensis

Nervous systems often consist of a large number of different types of neurons which are generated from neural stem and progenitor cells by a series of symmetric and asymmetric divisions. The origin and early evolution of these neural progenitor systems is not well understood. Here we use a cnidarian model organism, Nematostella vectensis, to gain insight into the generation of neural cell type diversity in a non-bilaterian animal. We identify NvFoxQ2d as a transcription factor that is expressed in a population of spatially restricted, proliferating ectodermal cells that are derived from NvSoxB(2)-expressing neural progenitor cells. Using a transgenic reporter line we show that the NvFoxQ2d cells undergo a terminal, symmetric division to generate a morphologically homogeneous population of putative sensory cells. The abundance of these cells, but not their proliferation status is affected by treatment with the γ-secretase inhibitor DAPT, suggesting regulation by Notch signalling. Our data suggest that intermediate progenitor cells and symmetric divisions contribute to the formation of the seemingly simple nervous system of a sea anemone.

Diversity of Cnidarian Muscles: Function, Anatomy, Development and Regeneration

The ability to perform muscle contractions is one of the most important and distinctive features of eumetazoans. As the sister group to bilaterians, cnidarians (sea anemones, corals, jellyfish, and hydroids) hold an informative phylogenetic position for understanding muscle evolution. Here, we review current knowledge on muscle function, diversity, development, regeneration and evolution in cnidarians. Cnidarian muscles are involved in various activities, such as feeding, escape, locomotion and defense, in close association with the nervous system. This variety is reflected in the large diversity of muscle organizations found in Cnidaria. Smooth epithelial muscle is thought to be the most common type, and is inferred to be the ancestral muscle type for Cnidaria, while striated muscle fibers and non-epithelial myocytes would have been convergently acquired within Cnidaria. Current knowledge of cnidarian muscle development and its regeneration is limited. While orthologs of myogenic regulatory factors such as MyoD have yet to be found in cnidarian genomes, striated muscle formation potentially involves well-conserved myogenic genes, such as twist and mef2. Although satellite cells have yet to be identified in cnidarians, muscle plasticity (e.g., de- and re-differentiation, fiber repolarization) in a regenerative context and its potential role during regeneration has started to be addressed in a few cnidarian systems. The development of novel tools to study those organisms has created new opportunities to investigate in depth the development and regeneration of cnidarian muscle cells and how they contribute to the regenerative process.

Inducing Complete Polyp Regeneration from the Aboral Physa of the Starlet Sea Anemone Nematostella vectensis

Cnidarians, and specifically Hydra, were the first animals shown to regenerate damaged or severed structures, and indeed such studies arguably launched modern biological inquiry through the work of Trembley more than 250 years ago. Presently the study of regeneration has seen a resurgence using both "classic" regenerative organisms, such as the Hydra, planaria and Urodeles, as well as a widening spectrum of species spanning the range of metazoa, from sponges through mammals. Besides its intrinsic interest as a biological phenomenon, understanding how regeneration works in a variety of species will inform us about whether regenerative processes share common features and/or species or context-specific cellular and molecular mechanisms. The starlet sea anemone, Nematostella vectensis, is an emerging model organism for regeneration. Like Hydra, Nematostella is a member of the ancient phylum, cnidaria, but within the class anthozoa, a sister clade to the hydrozoa that is evolutionarily more basal. Thus aspects of regeneration in Nematostella will be interesting to compare and contrast with those of Hydra and other cnidarians. In this article, we present a method to bisect, observe and classify regeneration of the aboral end of the Nematostella adult, which is called the physa. The physa naturally undergoes fission as a means of asexual reproduction, and either natural fission or manual amputation of the physa triggers re-growth and reformation of complex morphologies. Here we have codified these simple morphological changes in a Nematostella Regeneration Staging System (the NRSS). We use the NRSS to test the effects of chloroquine, an inhibitor of lysosomal function that blocks autophagy. The results show that the regeneration of polyp structures, particularly the mesenteries, is abnormal when autophagy is inhibited.

The cellular and molecular basis of cnidarian neurogenesis

Neurogenesis initiates during early development and it continues through later developmental stages and in adult animals to enable expansion, remodeling, and homeostasis of the nervous system. The generation of nerve cells has been analyzed in detail in few bilaterian model organisms, leaving open many questions about the evolution of this process. As the sister group to bilaterians, cnidarians occupy an informative phylogenetic position to address the early evolution of cellular and molecular aspects of neurogenesis and to understand common principles of neural development. Here we review studies in several cnidarian model systems that have revealed significant similarities and interesting differences compared to neurogenesis in bilaterian species, and between different cnidarian taxa. Cnidarian neurogenesis is currently best understood in the sea anemone Nematostella vectensis, where it includes epithelial neural progenitor cells that express transcription factors of the soxB and atonal families. Notch signaling regulates the number of these neural progenitor cells, achaete‐scute and dmrt genes are required for their further development and Wnt and BMP signaling appear to be involved in the patterning of the nervous system. In contrast to many vertebrates and Drosophila, cnidarians have a high capacity to generate neurons throughout their lifetime and during regeneration. Utilizing this feature of cnidarian biology will likely allow gaining new insights into the similarities and differences of embryonic and regenerative neurogenesis. The use of different cnidarian model systems and their expanding experimental toolkits will thus continue to provide a better understanding of evolutionary and developmental aspects of nervous system formation. WIREs Dev Biol 2017, 6:e257. doi: 10.1002/wdev.257

This article is categorized under: 1

Gene Expression and Transcriptional Hierarchies > Cellular Differentiation

2

Signaling Pathways > Cell Fate Signaling

3

Comparative Development and Evolution > Organ System Comparisons Between Species

Xenacoelomorpha: a case of independent nervous system centralization?

Centralized nervous systems (NSs) and complex brains are among the most important innovations in the history of life on our planet. In this context, two related questions have been formulated: How did complex NSs arise in evolution, and how many times did this occur? As a step towards finding an answer, we describe the NS of several representatives of the Xenacoelomorpha, a clade whose members show different degrees of NS complexity. This enigmatic clade is composed of three major taxa: acoels, nemertodermatids and xenoturbellids. Interestingly, while the xenoturbellids seem to have a rather 'simple' NS (a nerve net), members of the most derived group of acoel worms clearly have ganglionic brains. This interesting diversity of NS architectures (with different degrees of compaction) provides a unique system with which to address outstanding questions regarding the evolution of brains and centralized NSs. The recent sequencing of xenacoelomorph genomes gives us a privileged vantage point from which to analyse neural evolution, especially through the study of key gene families involved in neurogenesis and NS function, such as G protein-coupled receptors, helix-loop-helix transcription factors and Wnts. We finish our manuscript proposing an adaptive scenario for the origin of centralized NSs (brains).

A transcriptional time-course analysis of oral vs. aboral whole-body regeneration in the Sea anemone Nematostella vectensis

Background

The ability of regeneration is essential for the homeostasis of all animals as it allows the repair and renewal of tissues and body parts upon normal turnover or injury. The extent of this ability varies greatly in different animals with the sea anemone Nematostella vectensis, a basal cnidarian model animal, displaying remarkable whole-body regeneration competence.

Results

In order to study this process in Nematostella we performed an RNA-Seq screen wherein we analyzed and compared the transcriptional response to bisection in the wound-proximal body parts undergoing oral (head) or aboral (tail) regeneration at several time points up to the initial restoration of the basic body shape. The transcriptional profiles of regeneration responsive genes were analyzed so as to define the temporal pattern of differential gene expression associated with the tissue-specific oral and aboral regeneration. The identified genes were characterized according to their GO (gene ontology) assignations revealing groups that were enriched in the regeneration process with particular attention to their affiliation to the major developmental signaling pathways. While some of the genes and gene groups thus analyzed were previously known to be active in regeneration, we have also revealed novel and surprising candidate genes such as cilia-associated genes that likely participate in this important developmental program.

Conclusions

This work highlighted the main groups of genes which showed polarization upon regeneration, notably the proteinases, multiple transcription factors and the Wnt pathway genes that were highly represented, all displaying an intricate temporal balance between the two sides. In addition, the evolutionary comparison performed between regeneration in different animal model systems may reveal the basic mechanisms playing a role in this fascinating process.

Electronic supplementary material

The online version of this article (doi:10.1186/s12864-016-3027-1) contains supplementary material, which is available to authorized users.

Evolution of eumetazoan nervous systems: insights from cnidarians

Cnidarians, the sister group to bilaterians, have a simple diffuse nervous system. This morphological simplicity and their phylogenetic position make them a crucial group in the study of the evolution of the nervous system. The development of their nervous systems is of particular interest, as by uncovering the genetic programme that underlies it, and comparing it with the bilaterian developmental programme, it is possible to make assumptions about the genes and processes involved in the development of ancestral nervous systems. Recent advances in sequencing methods, genetic interference techniques and transgenic technology have enabled us to get a first glimpse into the molecular network underlying the development of a cnidarian nervous system—in particular the nervous system of the anthozoan Nematostella vectensis. It appears that much of the genetic network of the nervous system development is partly conserved between cnidarians and bilaterians, with Wnt and bone morphogenetic protein (BMP) signalling, and Sox genes playing a crucial part in the differentiation of neurons. However, cnidarians possess some specific characteristics, and further studies are necessary to elucidate the full regulatory network. The work on cnidarian neurogenesis further accentuates the need to study non-model organisms in order to gain insights into processes that shaped present-day lineages during the course of evolution.

Gastric pouches and the mucociliary sole: setting the stage for nervous system evolution

Prerequisite for tracing nervous system evolution is understanding of the body plan, feeding behaviour and locomotion of the first animals in which neurons evolved. Here, a comprehensive scenario is presented for the diversification of cell types in early metazoans, which enhanced feeding efficiency and led to the emergence of larger animals that were able to move. Starting from cup-shaped, gastraea-like animals with outer and inner choanoflagellate-like cells, two major innovations are discussed that set the stage for nervous system evolution. First, the invention of a mucociliary sole entailed a switch from intra- to extracellular digestion and increased the concentration of nutrients flowing into the gastric cavity. In these animals, an initial nerve net may have evolved via division of labour from mechanosensory-contractile cells in the lateral body wall, enabling coordinated movement of the growing body that involved both mucociliary creeping and changes of body shape. Second, the inner surface of the animals folded into metameric series of gastric pouches, which optimized nutrient resorption and allowed larger body sizes. The concomitant acquisition of bilateral symmetry may have allowed more directed locomotion and, with more demanding coordinative tasks, triggered the evolution of specialized nervous subsystems. Animals of this organizational state would have resembled Ediacarian fossils such as Dickinsonia and may have been close to the cnidarian–bilaterian ancestor. In the bilaterian lineage, the mucociliary sole was used mostly for creeping, or frequently lost. One possible remnant is the enigmatic Reissner's fibre in the ventral neural tube of cephalochordates and vertebrates.

MAPK signaling is necessary for neurogenesis in Nematostella vectensis

Background

The nerve net of Nematostella is generated using a conserved cascade of neurogenic transcription factors. For example, NvashA, a homolog of the achaete-scute family of basic helix-loop-helix transcription factors, is necessary and sufficient to specify a subset of embryonic neurons. However, positive regulators required for the expression of neurogenic transcription factors remain poorly understood.

Results

We show that treatment with the MEK/MAPK inhibitor U0126 severely reduces the expression of known neurogenic genes, Nvath-like, NvsoxB(2), and NvashA, and known markers of differentiated neurons, suggesting that MAPK signaling is necessary for neural development. Interestingly, ectopic NvashA fails to rescue the expression of neural markers in U0126-treated animals. Double fluorescence in situ hybridization and transgenic analysis confirmed that NvashA targets represent both unique and overlapping populations of neurons. Finally, we used a genome-wide microarray to identify additional patterning genes downstream of MAPK that might contribute to neurogenesis. We identified 18 likely neural transcription factors, and surprisingly identified ~40 signaling genes and transcription factors that are expressed in either the aboral domain or animal pole that gives rise to the endomesoderm at late blastula stages.

Conclusions

Together, our data suggest that MAPK is a key early regulator of neurogenesis, and that it is likely required at multiple steps. Initially, MAPK promotes neurogenesis by positively regulating expression of NvsoxB(2), Nvath-like, and NvashA. However, we also found that MAPK is necessary for the activity of the neurogenic transcription factor NvashA. Our forward molecular approach provided insight about the mechanisms of embryonic neurogenesis. For instance, NvashA suppression of Nvath-like suggests that inhibition of progenitor identity is an active process in newly born neurons, and we show that downstream targets of NvashA reflect multiple neural subtypes rather than a uniform neural fate. Lastly, analysis of the MAPK targets in the early embryo suggests that MAPK signaling is critical not only to neurogenesis, but also endomesoderm formation and aboral patterning.

Electronic supplementary material

The online version of this article (doi:10.1186/s12915-016-0282-1) contains supplementary material, which is available to authorized users.

Analyses of Sox-B and Sox-E Family Genes in the Cephalopod Sepia officinalis: Revealing the Conserved and the Unusual

Cephalopods provide an unprecedented opportunity for comparative studies of the developmental genetics of organ systems that are convergent with analogous vertebrate structures. The Sox-family of transcription factors is an important class of DNA-binding proteins that are known to be involved in many aspects of differentiation, but have been largely unstudied in lophotrochozoan systems. Using a degenerate primer strategy we have isolated coding sequence for three members of the Sox family of transcription factors from a cephalopod mollusk, the European cuttlefish Sepia officinalis: Sof-SoxE, Sof-SoxB1, and Sof-SoxB2. Analyses of their expression patterns during organogenesis reveals distinct spatial and temporal expression domains. Sof-SoxB1 shows early ectodermal expression throughout the developing epithelium, which is gradually restricted to presumptive sensory epithelia. Expression within the nervous system appears by mid-embryogenesis. Sof-SoxB2 expression is similar to Sof-SoxB1 within the developing epithelia in early embryogenesis, however appears in largely non-overlapping expression domains within the central nervous system and is not expressed in the maturing sensory epithelium. In contrast, Sof-SoxE is expressed throughout the presumptive mesodermal territories at the onset of organogenesis. As development proceeds, Sof-SoxE expression is elevated throughout the developing peripheral circulatory system. This expression disappears as the circulatory system matures, but expression is maintained within undifferentiated connective tissues throughout the animal, and appears within the nervous system near the end of embryogenesis. SoxB proteins are widely known for their role in neural specification in numerous phylogenetic lineages. Our data suggests that Sof-SoxB genes play similar roles in cephalopods. In contrast, Sof-SoxE appears to be involved in the early stages of vasculogenesis of the cephalopod closed circulatory system, a novel role for a member of this gene family.

From nerve net to nerve ring, nerve cord and brain--evolution of the nervous system

The puzzle of how complex nervous systems emerged remains unsolved. Comparative studies of neurodevelopment in cnidarians and bilaterians suggest that this process began with distinct integration centres that evolved on opposite ends of an initial nerve net. The 'apical nervous system' controlled general body physiology, and the 'blastoporal nervous system' coordinated feeding movements and locomotion. We propose that expansion, integration and fusion of these centres gave rise to the bilaterian nerve cord and brain.