Adoption of conserved developmental genes in development and origin of the medusa body plan

A whole-body atlas of BMP signaling activity in an adult sea anemone

BACKGROUND

BMP signaling is responsible for the second body axis patterning in Bilateria and in the bilaterally symmetric members of the bilaterian sister clade Cnidaria-corals and sea anemones. However, medusozoan cnidarians (jellyfish, hydroids) are radially symmetric, and yet their genomes contain BMP signaling components. This evolutionary conservation suggests that BMP signaling must have other functions not related to axial patterning, which keeps BMP signaling components under selective pressure.

RESULTS

To find out what these functions might be, we generated a detailed whole-body atlas of BMP activity in the sea anemone Nematostella. In the adult polyp, we discover an unexpected diversity of domains with BMP signaling activity, which is especially prominent in the head, as well as across the neuro-muscular and reproductive parts of the gastrodermis. In accordance, analysis of two medusozoan species, the true jellyfish Aurelia and the box jellyfish Tripedalia, revealed similarly broad and diverse BMP activity.

CONCLUSIONS

Our study reveals multiple, distinct domains of BMP signaling in Anthozoa and Medusozoa, supporting the versatile nature of the BMP pathway across Cnidaria. Most prominently, BMP signaling appears to be involved in tentacle formation, neuronal development, and gameto- or gonadogenesis.

Changes of cell-type diversity in the polyp-to-medusa metagenesis of the scyphozoan jellyfish Aurelia coerulea (formerly sp.1)

The life cycle of most medusozoan cnidarians is marked by the metagenesis from the asexually reproducing sessile polyp and the sexually reproducing motile medusa. At present it is unknown to what extent this drastic morphological transformation is accompanied by changes in the cell type composition. Here, we provide a single cell transcriptome atlas of the cosmopolitan scyphozoan Aurelia coerulea focussing on changes in cell-type composition during the transition from polyp to medusa. Notably, this transition marked by an increase in cell type diversity, including an expansion of neural subtypes. We find that two families of neuronal lineages are specified by homologous transcription factors in the sea anemone Nematostella vectensis and Aurelia coerulea , suggesting an origin in the common ancestor of medusozoans and anthozoans about 500 Myr ago. Our analysis suggests that gene duplications might be drivers for the increase of cellular complexity during the evolution of cnidarian neuroglandular lineages. One key medusozoan-specific cell type is the striated muscle in the subumbrella. Analysis of muscle fiber anatomy and gene expression raises the possibility that the striated muscles arise from a population of smooth muscle cells during strobilation. Although smooth and striated muscles are phenotypically distinct, both have a similar contractile complex, in contrast to bilaterian smooth and striated muscles. This suggests that in Aurelia , smooth and striated muscle cells may derive from the same progenitor cells.

Teaser

Single cell transcriptome atlas across the jellyfish life cycle reveals emergence of novel medusa-specific cell types is associated with expression of medusa-specific paralogs.

The microbiome of a Pacific moon jellyfish Aurelia coerulea

The impact of microbiome in animal physiology is well appreciated, but characterization of animal-microbe symbiosis in marine environments remains a growing need. This study characterizes the microbial communities associated with the moon jellyfish Aurelia coerulea, first isolated from the East Pacific Ocean and has since been utilized as an experimental system. We find that the microbiome of this Pacific Aurelia culture is dominated by two taxa, a Mollicutes and Rickettsiales. The microbiome is stable across life stages, although composition varies. Mining the host sequencing data, we assembled the bacterial metagenome-assembled genomes (MAGs). The bacterial MAGs are highly reduced, and predict a high metabolic dependence on the host. Analysis using multiple metrics suggest that both bacteria are likely new species. We therefore propose the names Ca. Mariplasma lunae (Mollicutes) and Ca. Marinirickettsia aquamalans (Rickettsiales). Finally, comparison with studies of Aurelia from other geographical populations suggests the association with Ca. Mariplasma lunae occurs in Aurelia from multiple geographical locations. The low-diversity microbiome of Aurelia provides a relatively simple system to study host-microbe interactions.

A macroscopic free-swimming medusa from the middle Cambrian Burgess Shale

Cnidarians are regarded as one of the earliest-diverging animal phyla. One of the hallmarks of the cnidarian body plan is the evolution of a free-swimming medusa in some medusozoan classes, but the origin of this innovation remains poorly constrained by the fossil record and molecular data. Previously described macrofossils, putatively representing medusa stages of crown-group medusozoans from the Cambrian of Utah and South China, are here reinterpreted as ctenophore-grade organisms. Other putative Ediacaran to Cambrian medusozoan fossils consist mainly of microfossils and tubular forms. Here we describe Burgessomedusa phasmiformis gen. et sp. nov., the oldest unequivocal macroscopic free-swimming medusa in the fossil record. Our study is based on 182 exceptionally preserved body fossils from the middle Cambrian Burgess Shale (Raymond Quarry, British Columbia, Canada). Burgessomedusa possesses a cuboidal umbrella up to 20 cm high and over 90 short, finger-like tentacles. Phylogenetic analysis supports a medusozoan affinity, most likely as a stem group to Cubozoa or Acraspeda (a group including Staurozoa, Cubozoa and Scyphozoa). Burgessomedusa demonstrates an ancient origin for the free-swimming medusa life stage and supports a growing number of studies showing an early evolutionary diversification of Medusozoa, including of the crown group, during the late Precambrian-Cambrian transition.

Coevolution of the Tlx homeobox gene with medusa development (Cnidaria: Medusozoa)

Cnidarians display a wide diversity of life cycles. Among the main cnidarian clades, only Medusozoa possesses a swimming life cycle stage called the medusa, alternating with a benthic polyp stage. The medusa stage was repeatedly lost during medusozoan evolution, notably in the most diverse medusozoan class, Hydrozoa. Here, we show that the presence of the homeobox gene Tlx in Cnidaria is correlated with the presence of the medusa stage, the gene having been lost in clades that ancestrally lack a medusa (anthozoans, endocnidozoans) and in medusozoans that secondarily lost the medusa stage. Our characterization of Tlx expression indicate an upregulation of Tlx during medusa development in three distantly related medusozoans, and spatially restricted expression patterns in developing medusae in two distantly related species, the hydrozoan Podocoryna carnea and the scyphozoan Pelagia noctiluca. These results suggest that Tlx plays a key role in medusa development and that the loss of this gene is likely linked to the repeated loss of the medusa life cycle stage in the evolution of Hydrozoa.

The evolutionary history of Brachyury genes in Hydrozoa involves duplications, divergence, and neofunctionalization

Brachyury, a member of T-box gene family, is widely known for its major role in mesoderm specification in bilaterians. It is also present in non-bilaterian metazoans, such as cnidarians, where it acts as a component of an axial patterning system. In this study, we present a phylogenetic analysis of Brachyury genes within phylum Cnidaria, investigate differential expression and address a functional framework of Brachyury paralogs in hydrozoan Dynamena pumila. Our analysis indicates two duplication events of Brachyury within the cnidarian lineage. The first duplication likely appeared in the medusozoan ancestor, resulting in two copies in medusozoans, while the second duplication arose in the hydrozoan ancestor, resulting in three copies in hydrozoans. Brachyury1 and 2 display a conservative expression pattern marking the oral pole of the body axis in D. pumila. On the contrary, Brachyury3 expression was detected in scattered presumably nerve cells of the D. pumila larva. Pharmacological modulations indicated that Brachyury3 is not under regulation of cWnt signaling in contrast to the other two Brachyury genes. Divergence in expression patterns and regulation suggest neofunctionalization of Brachyury3 in hydrozoans.

Past, present and future of Clytia hemisphaerica as a laboratory jellyfish

The hydrozoan species Clytia hemisphaerica was selected in the mid-2000s to address the cellular and molecular basis of body axis specification in a cnidarian, providing a reliable daily source of gametes and building on a rich foundation of experimental embryology. The many practical advantages of this species include genetic uniformity of laboratory jellyfish, derived clonally from easily-propagated polyp colonies. Phylogenetic distance from other laboratory models adds value in providing an evolutionary perspective on many biological questions. Here we outline the current state of the art regarding available experimental approaches and in silico resources, and illustrate the contributions of Clytia to understanding embryo patterning mechanisms, oogenesis and regeneration. Looking forward, the recent establishment of transgenesis methods is now allowing gene function and imaging studies at adult stages, making Clytia particularly attractive for whole organism biology studies across fields and extending its scientific impact far beyond the original question of interest.

Binding Pattern Reconstructions of FGF-FGFR Budding-Inducing Signaling in Reef-Building Corals

Reef-building corals play an important role in marine ecosystems. However, owing to climate change, ocean acidification, and predation by invasive crown-of-thorns starfish, these corals are declining. As marine animals comprise polyps, reproduction by asexual budding is pivotal in scleractinian coral growth. The fibroblast growth factor (FGF) signaling pathway is essential in coral budding morphogenesis. Here, we sequenced the full-length transcriptomes of four common and frequently dominant reef-building corals and screened out the budding-related FGF and FGFR genes. Thereafter, three-dimensional (3D) models of FGF and FGFR proteins as well as FGF-FGFR binding models were reconstructed. Based on our findings, the FGF8-FGFR3 binding models in Pocillopora damicornis, Montipora capricornis, and Acropora muricata are typical receptor tyrosine kinase-signaling pathways that are similar to the Kringelchen (FGFR) in hydra. However, in P. verrucosa, FGF8 is not the FGFR3 ligand, which is found in other hydrozoan animals, and its FGFR3 must be activated by other tyrosine kinase-type ligands. Overall, this study provides background on the potentially budding propagation signaling pathway activated by the applications of biological agents in reef-building coral culture that could aid in the future restoration of coral reefs.

From apolar gastrula to polarized larva: Embryonic development of a marine hydroid, Dynamena pumila

BACKGROUND

In almost all metazoans examined to this respect, the axial patterning system based on canonical Wnt (cWnt) signaling operates throughout the course of development. In most metazoans, gastrulation is polar, and embryos develop morphological landmarks of axial polarity, such as blastopore under control/regulation from cWnt signaling. However, in many cnidarian species, gastrulation is morphologically apolar. The question remains whether сWnt signaling providing the establishment of a body axis controls morphogenetic processes involved in apolar gastrulation.

RESULTS

In this study, we focused on the embryonic development of Dynamena pumila, a cnidarian species with apolar gastrulation. We thoroughly described cell behavior, proliferation, and ultrastructure and examined axial patterning in the embryos of this species. We revealed that the first signs of morphological polarity appear only after the end of gastrulation, while molecular prepatterning of the embryo does exist during gastrulation. We have shown experimentally that in D. pumila, the direction of the oral-aboral axis is highly robust against perturbations in cWnt activity.

CONCLUSIONS

Our results suggest that morphogenetic processes are uncoupled from molecular axial patterning during gastrulation in D. pumila. Investigation of D. pumila might significantly expand our understanding of the ways in which morphological polarization and axial molecular patterning are linked in Metazoa.

сWnt signaling modulation results in a change of the colony architecture in a hydrozoan

At the polyp stage, most hydrozoan cnidarians form highly elaborate colonies with a variety of branching patterns, which makes them excellent models for studying the evolutionary mechanisms of body plan diversification. At the same time, molecular mechanisms underlying the robust patterning of the architecturally complex hydrozoan colonies remain unexplored. Using non-model hydrozoan Dynamena pumila we showed that the key components of the Wnt/β-catenin (cWnt) pathway (β-catenin, TCF) and the cWnt-responsive gene, brachyury 2, are involved in specification and patterning of the developing colony shoots. Strikingly, pharmacological modulation of the cWnt pathway leads to radical modification of the monopodially branching colony of Dynamena which acquire branching patterns typical for colonies of other hydrozoan species. Our results suggest that modulation of the cWnt signaling is the driving force promoting the evolution of the vast variety of the body plans in hydrozoan colonies and offer an intriguing possibility that the involvement of the cWnt pathway in the regulation of branching morphogenesis might represent an ancestral feature predating the cnidarian-bilaterian split.

Box, stalked, and upside-down? Draft genomes from diverse jellyfish (Cnidaria, Acraspeda) lineages: Alatina alata (Cubozoa), Calvadosia cruxmelitensis (Staurozoa), and Cassiopea xamachana (Scyphozoa)

BACKGROUND

Anthozoa, Endocnidozoa, and Medusozoa are the 3 major clades of Cnidaria. Medusozoa is further divided into 4 clades, Hydrozoa, Staurozoa, Cubozoa, and Scyphozoa-the latter 3 lineages make up the clade Acraspeda. Acraspeda encompasses extraordinary diversity in terms of life history, numerous nuisance species, taxa with complex eyes rivaling other animals, and some of the most venomous organisms on the planet. Genomes have recently become available within Scyphozoa and Cubozoa, but there are currently no published genomes within Staurozoa and Cubozoa.

FINDINGS

Here we present 3 new draft genomes of Calvadosia cruxmelitensis (Staurozoa), Alatina alata (Cubozoa), and Cassiopea xamachana (Scyphozoa) for which we provide a preliminary orthology analysis that includes an inventory of their respective venom-related genes. Additionally, we identify synteny between POU and Hox genes that had previously been reported in a hydrozoan, suggesting this linkage is highly conserved, possibly dating back to at least the last common ancestor of Medusozoa, yet likely independent of vertebrate POU-Hox linkages.

CONCLUSIONS

These draft genomes provide a valuable resource for studying the evolutionary history and biology of these extraordinary animals, and for identifying genomic features underlying venom, vision, and life history traits in Acraspeda.

Medusozoan genomes inform the evolution of the jellyfish body plan

Cnidarians are astonishingly diverse in body form and lifestyle, including the presence of a jellyfish stage in medusozoans and its absence in anthozoans. Here, we sequence the genomes of Aurelia aurita (a scyphozoan) and Morbakka virulenta (a cubozoan) to understand the molecular mechanisms responsible for the origin of the jellyfish body plan. We show that the magnitude of genetic differences between the two jellyfish types is equivalent, on average, to the level of genetic differences between humans and sea urchins in the bilaterian lineage. About one-third of Aurelia genes with jellyfish-specific expression have no matches in the genomes of the coral and sea anemone, indicating that the polyp-to-jellyfish transition requires a combination of conserved and novel, medusozoa-specific genes. While no genomic region is specifically associated with the ability to produce a jellyfish stage, the arrangement of genes involved in the development of a nematocyte-a phylum-specific cell type-is highly structured and conserved in cnidarian genomes; thus, it represents a phylotypic gene cluster.

The genome of the jellyfish Clytia hemisphaerica and the evolution of the cnidarian life-cycle

Jellyfish (medusae) are a distinctive life-cycle stage of medusozoan cnidarians. They are major marine predators, with integrated neurosensory, muscular and organ systems. The genetic foundations of this complex form are largely unknown. We report the draft genome of the hydrozoan jellyfish Clytia hemisphaerica and use multiple transcriptomes to determine gene use across life-cycle stages. Medusa, planula larva and polyp are each characterized by distinct transcriptome signatures reflecting abrupt life-cycle transitions and all deploy a mixture of phylogenetically old and new genes. Medusa-specific transcription factors, including many with bilaterian orthologues, associate with diverse neurosensory structures. Compared to Clytia, the polyp-only hydrozoan Hydra has lost many of the medusa-expressed transcription factors, despite similar overall rates of gene content evolution and sequence evolution. Absence of expression and gene loss among Clytia orthologues of genes patterning the anthozoan aboral pole, secondary axis and endomesoderm support simplification of planulae and polyps in Hydrozoa, including loss of bilateral symmetry. Consequently, although the polyp and planula are generally considered the ancestral cnidarian forms, in Clytia the medusa maximally deploys the ancestral cnidarian-bilaterian transcription factor gene complement.

Mechanisms of cnidocyte development in the moon jellyfish Aurelia

Stinging cells called cnidocytes are a defining trait of the cnidarians (sea anemones, corals, jellyfish, and their relatives). In hydrozoan cnidarians such as Hydra, cnidocytes develop from interstitial stem cells set aside in the ectoderm. It is less clear how cnidocytes develop outside the Hydrozoa, as other cnidarians appear to lack interstitial stem cells. We addressed this question by studying cnidogenesis in the moon jellyfish (Aurelia) through the visualization of minicollagen-a protein associated with cnidocyte development-as well as transmission electron microscopy. We discovered that developing cnidoblasts are rare or absent in feeding structures rich in mature cnidocytes, such as tentacles and lappets. Using transmission electron microscopy, we determined that the progenitors of cnidocytes have traits consistent with epitheliomuscular cells. Our data suggests a dynamic where cnidocytes develop at high concentrations in the epithelium of more proximal regions, and subsequently migrate to more distal regions where they exhibit high usage and turnover. Similar to some anthozoans, cnidocytes in Aurelia do not appear to be generated by interstitial stem cells; instead, epitheliomuscular cells appear to be the progenitor cell type. This observation polarizes the evolution of cnidogenesis, and raises the question of how interstitial stem cells came to regulate cnidogenesis in hydrozoans.

The genome of the jellyfish Aurelia and the evolution of animal complexity

We present the genome of the moon jellyfish Aurelia, a genome from a cnidarian with a medusa life stage. Our analyses suggest that gene gain and loss in Aurelia is comparable to what has been found in its morphologically simpler relatives-the anthozoan corals and sea anemones. RNA sequencing analysis does not support the hypothesis that taxonomically restricted (orphan) genes play an oversized role in the development of the medusa stage. Instead, genes broadly conserved across animals and eukaryotes play comparable roles throughout the life cycle. All life stages of Aurelia are significantly enriched in the expression of genes that are hypothesized to interact in protein networks found in bilaterian animals. Collectively, our results suggest that increased life cycle complexity in Aurelia does not correlate with an increased number of genes. This leads to two possible evolutionary scenarios: either medusozoans evolved their complex medusa life stage (with concomitant shifts into new ecological niches) primarily by re-working genetic pathways already present in the last common ancestor of cnidarians, or the earliest cnidarians had a medusa life stage, which was subsequently lost in the anthozoans. While we favour the earlier hypothesis, the latter is consistent with growing evidence that many of the earliest animals were more physically complex than previously hypothesized.

Transcriptome analysis of scyphozoan jellyfish Rhopilema esculentum from polyp to medusa identifies potential genes regulating strobilation

Strobilation is a unique asexual reproduction mode of scyphozoan jellyfish, through which benthic polyp develops into pelagic medusa. It is an orderly metamorphosis process triggered by environmental signals. However, the knowledges of molecular mechanisms under the drastic morphological and physiological changes are still limited. In this study, the transcriptomes from polyps to juvenile medusae at different stages were characterized by RNA-seq in scyphozoan jellyfish Rhopilema esculentum. Among 96,076 de novo assembled unigenes, 7090 differentially expressed genes (DEGs) were identified during the developmental stages. The co-expression pattern analysis of DEGs yielded 15 clusters with different expression patterns. Among them, a cluster with 388 unigenes was related to strobila. In this specific cluster, the GO terms related to "sequence-specific DNA binding transcription factor activity" and "sequence-specific DNA binding" were significantly enriched. Transcription factors, including segmentation protein even-skipped-like, segmentation polarity protein engrailed-like, homeobox proteins Otx-like, Twist-like and Cnox2-Pc-like, as well as genes such as RxR-like and Dmrtf-like, were identified to be potentially involved in strobilation. Their expression patterns and the other 11 TFs/genes involved in strobilation were confirmed with qRT-PCR methods. The present study pointed out the role of transcription factors in strobilation and produced a list of novel candidate genes for further studies. It could provide valuable information for understanding the molecular mechanisms of jellyfish strobilation.

Hox and Wnt pattern the primary body axis of an anthozoan cnidarian before gastrulation

Hox gene transcription factors are important regulators of positional identity along the anterior–posterior axis in bilaterian animals. Cnidarians (e.g., sea anemones, corals, and hydroids) are the sister group to the Bilateria and possess genes related to both anterior and central/posterior class Hox genes. Here we report a previously unrecognized domain of Hox expression in the starlet sea anemone, Nematostella vectensis, beginning at early blastula stages. We explore the relationship of two opposing Hox genes (NvAx6/NvAx1) expressed on each side of the blastula during early development. Functional perturbation reveals that NvAx6 and NvAx1 not only regulate their respective expression domains, but also interact with Wnt genes to pattern the entire oral–aboral axis. These findings suggest an ancient link between Hox/Wnt patterning during axis formation and indicate that oral–aboral domains are likely established during blastula formation in anthozoan cnidarians.

Hox genes regulate anterior–posterior axis formation but their role in cnidarians is unclear. Here, the authors disrupt Hox genes NvAx1 and NvAx6 in the starlet sea anemone, Nematostella vectensis, showing antagonist function in patterning the oral–aboral axis and a link to Wnt signaling.

Medusa: A Review of an Ancient Cnidarian Body Form

Medusae (aka jellyfish) have multiphasic life cycles and a propensity to adapt to, and proliferate in, a plethora of aquatic habitats, connecting them to a number of ecological and societal issues. Now, in the midst of the genomics era, affordable next-generation sequencing (NGS) platforms coupled with publically available bioinformatics tools present the much-anticipated opportunity to explore medusa taxa as potential model systems. Genome-wide studies of medusae would provide a remarkable opportunity to address long-standing questions related to the biology, physiology, and nervous system of some of the earliest pelagic animals. Furthermore, medusae have become key targets in the exploration of marine natural products, in the development of marine biomarkers, and for their application to the biomedical and robotics fields. Presented here is a synopsis of the current state of medusa research, highlighting insights provided by multi-omics studies, as well as existing knowledge gaps, calling upon the scientific community to adopt a number of medusa taxa as model systems in forthcoming research endeavors.

Indoles induce metamorphosis in a broad diversity of jellyfish, but not in a crown jelly (Coronatae)

Many animals go through one or more metamorphoses during their lives, however, the molecular underpinnings of metamorphosis across diverse species are not well understood. Medusozoa (Cnidaria) is a clade of animals with complex life cycles, these life cycles can include a polyp stage that metamorphoses into a medusa (jellyfish). Medusae are produced through a variety of different developmental mechanisms-in some species polyps bud medusae (Hydrozoa), in others medusae are formed through polyp fission (Scyphozoa), while in others medusae are formed through direct transformation of the polyp (Cubozoa). To better understand the molecular mechanisms that may coordinate these different forms of metamorphosis, we tested two compounds first identified to induce metamorphosis in the moon jellyfish Aurelia aurita (indomethacin and 5-methoxy-2-methylindole) on a broad diversity of medusozoan polyps. We discovered that indole-containing compounds trigger metamorphosis across a broad diversity of species. All tested discomedusan polyps metamorphosed in the presence of both compounds, including species representatives of several major lineages within the clade (Pelagiidae, Cyaneidae, both clades of Rhizostomeae). In a cubozoan, low levels of 5-methoxy-2-methylindole reliably induced complete and healthy metamorphosis. In contrast, neither compound induced medusa metamorphosis in a coronate scyphozoan, or medusa production in either hydrozoan tested. Our results support the hypothesis that metamorphosis is mediated by a conserved induction pathway within discomedusan scyphozoans, and possibly cubozoans. However, failure of these compounds to induce metamorphosis in a coronate suggests this induction mechanism may have been lost in this clade, or is convergent between Scyphozoa and Cubozoa.

A safer, urea-based in situ hybridization method improves detection of gene expression in diverse animal species

In situ hybridization is a widely employed technique allowing spatial visualization of gene expression in fixed specimens. It has greatly advanced our understanding of biological processes, including developmental regulation. In situ protocols are today routinely followed in numerous laboratories, and although details might change, they all include a hybridization step, where specific antisense RNA or DNA probes anneal to the target nucleic acid sequence. This step is generally carried out at high temperatures and in a denaturing solution, called hybridization buffer, commonly containing 50% (v/v) formamide - a hazardous chemical. When applied to the soft-bodied hydrozoan medusa Clytia hemisphaerica, we found that this traditional hybridization approach was not fully satisfactory, causing extensive deterioration of morphology and tissue texture which compromised our observation and interpretation of results. We thus tested alternative solutions for in situ detection of gene expression and, inspired by optimized protocols for Northern and Southern blot analysis, we substituted the 50% formamide with an equal volume of 8M urea solution in the hybridization buffer. Our new protocol not only yielded better morphologies and tissue consistency, but also notably improved the resolution of the signal, allowing more precise localization of gene expression and reducing aspecific staining associated with problematic areas. Given the improved results and reduced manipulation risks, we tested the urea protocol on other metazoans, two brachiopod species (Novocrania anomala and Terebratalia transversa) and the priapulid worm Priapulus caudatus, obtaining a similar reduction of aspecific probe binding. Overall, substitution of formamide by urea during in situ hybridization offers a safer alternative, potentially of widespread use in research, medical and teaching contexts. We encourage other workers to test this approach on their study organisms, and hope that they will also obtain better sample preservation, more precise expression patterns and fewer problems due to aspecific staining, as we report here for Clytia medusae and Novocrania and Terebratalia developing larvae.

Gut-like ectodermal tissue in a sea anemone challenges germ layer homology

Cnidarians (for example, sea anemones and jellyfish) develop from an outer ectodermal and inner endodermal germ layer, whereas bilaterians (for example, vertebrates and flies) additionally have a mesodermal layer as intermediate germ layer. Currently, cnidarian endoderm (that is, 'mesendoderm') is considered homologous to both bilaterian endoderm and mesoderm. Here we test this hypothesis by studying the fate of germ layers, the localization of gut cell types, and the expression of numerous 'endodermal' and 'mesodermal' transcription factor orthologues in the anthozoan sea anemone Nematostella vectensis. Surprisingly, we find that the developing pharyngeal ectoderm and its derivatives display a transcription-factor expression profile (foxA, hhex, islet, soxB1, hlxB9, tbx2/3, nkx6 and nkx2.2) and cell-type combination (exocrine and insulinergic) reminiscent of the developing bilaterian midgut, and, in particular, vertebrate pancreatic tissue. Endodermal derivatives, instead, display cell functions and transcription-factor profiles similar to bilaterian mesoderm derivatives (for example, somatic gonad and heart). Thus, our data supports an alternative model of germ layer homologies, where cnidarian pharyngeal ectoderm corresponds to bilaterian endoderm, and the cnidarian endoderm is homologous to bilaterian mesoderm.

Molecular characterization of aspartylglucosaminidase, a lysosomal hydrolase upregulated during strobilation in the moon jellyfish, Aurelia aurita

The life cycle of the moon jellyfish, Aurelia aurita, alternates between a benthic asexual polyp stage and a planktonic sexual medusa (jellyfish) stage. Transition from polyp to medusa is called strobilation. To investigate the molecular mechanisms of strobilation, we screened for genes that are upregulated during strobilation using the differential display method and we identified aspartylglucosaminidase (AGA), which encodes a lysosomal hydrolase. Similar to AGAs from other species, Aurelia AGA possessed an N-terminal signal peptide and potential N-glycosylation sites. The genomic region of Aurelia AGA was approximately 9.8 kb in length and contained 12 exons and 11 introns. Quantitative RT-PCR analysis revealed that AGA expression increased during strobilation, and was then decreased in medusae. To inhibit AGA function, we administered the lysosomal acidification inhibitors, chloroquine or bafilomycin A1, to animals during strobilation. Both inhibitors disturbed medusa morphogenesis at the oral end, suggesting involvement of lysosomal hydrolases in strobilation.

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.

Cell tracking supports secondary gastrulation in the moon jellyfish Aurelia

The moon jellyfish Aurelia exhibits a dramatic reorganization of tissue during its metamorphosis from planula larva to polyp. There are currently two competing hypotheses regarding the fate of embryonic germ layers during this metamorphosis. In one scenario, the original endoderm undergoes apoptosis and is replaced by a secondary endoderm derived from ectodermal cells. In the second scenario, both ectoderm and endoderm remain intact through development. In this study, we performed a pulse-chase experiment to trace the fate of larval ectodermal cells. We observed that prior to metamorphosis, ectodermal cells that proliferated early in larval development concentrate at the future oral end of the polyp. During metamorphosis, these cells migrate into the endoderm, extending all the way to the aboral portion of the gut. We therefore reject the hypothesis that larval endoderm remains intact during metamorphosis and provide additional support for the "secondary gastrulation" hypothesis. Aurelia appears to offer the first and only described case where a cnidarian derives its endoderm twice during normal development, adding to a growing body of evidence that germ layers can be dramatically reorganized in cnidarian life cycles.

Genomics and development of Nematostella vectensis and other anthozoans

Due to their rather simple body plan with only few organs and a low number of cell types, cnidarians have long been recognized as an important animal group for evolutionary comparisons of animal body plans. Recent studies have shown, however, that the genomes of cnidarians and their epigenetic and posttranscriptional regulation are more complex than their morphology might suggest. How these complex genomes are deployed during embryonic development is an open question. With a focus on the sea anemone Nematostella vectensis we describe new findings about the development of the nervous system from neural progenitor cells and how Wnt and BMP signalling control axial patterning. These studies show that beyond evolutionary comparisons, cnidarian model organisms can provide new insights into generic questions in developmental biology.