Animal pole determinants define oral–aboral axis polarity and endodermal cell-fate in hydrozoan jellyfish Podocoryne carnea

Animal evolution at the ocean's water-air interface

Innovation is a key to evolutionary success and entrance into novel ecosystems.1 Species that float freely at the ocean's surface, termed obligate neuston (also called pleuston, here referred to simply as neuston), include highly specialized taxa from distinct evolutionary lineages that evolved floating morphologies.2 In 1958, Soviet scientist, A.I. Savilov,3 stated that floating animal species are derived from benthic ancestors, rather than species from the adjacent pelagic zone, and that floating morphologies are homologous to benthic attachment structures. To test Savilov's hypothesis, we constructed molecular phylogenies and ancestral states for all major floating groups for which molecular data were available. Our results reveal that four of the five clades examined arose directly from a substrate-attached ancestor, although that substrate was not necessarily the benthos, as Savilov stated, and instead included epibiotic and rafting ancestors. Despite their diverse evolutionary origins, floating animals use gas-trapping mechanisms to remain at the surface,4,5,6 and many of these gas-trapping structures appear to be homologous to substrate attachment structures. We also reconstruct the trophic habits of floating mollusks and their sister species, revealing that prey preference remains conserved upon entering the ocean's surface ecosystem. Colonization of the ocean's surface seems to have occurred through successive evolutionary steps from the seafloor. Our results suggest that these steps often included transitions through epibiotic (where species attach to other living organisms) or rafting (where species attach to floating debris) habits. The water-air interface, despite its unique properties, may, in some ways, be just another substrate.

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.

Studying of Molecular Regulation of Developmental Processes of Lower Metazoans Exemplified by Cnidaria Using High-Throughput Sequencing

A unique set of features and characteristics of species of the Cnidaria phylum is the one reason that makes them a model for a various studies. The plasticity of a life cycle and the processes of cell differentiation and development of an integral multicellular organism associated with it are of a specific scientific interest. A new stage of development of molecular genetic methods, including methods for high-throughput genome, transcriptome, and epigenome sequencing, both at the level of the whole organism and at the level of individual cells, makes it possible to obtain a detailed picture of the development of these animals. This review examines some modern approaches and advances in the reconstruction of the processes of ontogenesis of cnidarians by studying the regulatory signal transduction pathways and their interactions.

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.

Gastrulation and germ layer formation in the sea anemone Nematostella vectensis and other cnidarians

Among the basally branching metazoans, cnidarians display well-defined gastrulation processes leading to a diploblastic body plan, consisting of an endodermal and an ectodermal cell layer. As the outgroup to all Bilateria, cnidarians are an interesting group to investigate ancestral developmental mechanisms. Interestingly, all known gastrulation mechanisms known in Bilateria are already found in different species of Cnidaria. Here I review the morphogenetic processes found in different Cnidaria and focus on the investigation of the cellular and molecular mechanisms in the sea anemone Nematostella vectensis, which has been a major model organism among cnidarians for evolutionary developmental biology. Many of the genes involved in germ layer specification and morphogenetic processes in Bilateria are also found active during gastrulation of Nematostella and other cnidarians, suggesting an ancestral role of this process. The molecular analyses indicate a tight link between gastrulation and axis patterning processes by Wnt and FGF signaling. Interestingly, the endodermal layer displays many features of the mesodermal layer in Bilateria, while the pharyngeal ectoderm has an endodermal expression profile. Comparative analyses as well as experimental studies using embryonic aggregates suggest that minor differences in the gene regulatory networks allow the embryo to transition relatively easily from one mode of gastrulation to another.

A cell-based boundary model of gastrulation by unipolar ingression in the hydrozoan cnidarian Clytia hemisphaerica

In Cnidaria, modes of gastrulation to produce the two body layers vary greatly between species. In the hydrozoan species Clytia hemisphaerica gastrulation involves unipolar ingression of presumptive endoderm cells from an oral domain of the blastula, followed by migration of these cells to fill the blastocoel with concomitant narrowing of the gastrula and elongation along the oral-aboral axis. We developed a 2D computational boundary model capable of simulating the morphogenetic changes during embryonic development from early blastula stage to the end of gastrulation. Cells are modeled as polygons with elastic membranes and cytoplasm, colliding and adhering to other cells, and capable of forming filopodia. With this model we could simulate compaction of the embryo preceding gastrulation, bottle cell formation, ingression, and intercalation between cells of the ingressing presumptive endoderm. We show that embryo elongation is dependent on the number of endodermal cells, low endodermal cell-cell adhesion, and planar cell polarity (PCP). When the strength of PCP is reduced in our model, resultant embryo morphologies closely resemble those reported previously following morpholino-mediated knockdown of the core PCP proteins Strabismus and Frizzled. Based on our results, we postulate that cellular processes of apical constriction, compaction, ingression, and then reduced cell-cell adhesion and mediolateral intercalation in the presumptive endoderm, are required and when combined, sufficient for Clytia gastrulation.

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.

Germ-layer commitment and axis formation in sea anemone embryonic cell aggregates

Robust morphogenetic events are pivotal for animal embryogenesis. However, comparison of the modes of development of different members of a phylum suggests that the spectrum of developmental trajectories accessible for a species might be far broader than can be concluded from the observation of normal development. Here, by using a combination of microsurgery and transgenic reporter gene expression, we show that, facing a new developmental context, the aggregates of dissociated embryonic cells of the sea anemone Nematostella vectensis take an alternative developmental trajectory. The self-organizing aggregates rely on Wnt signals produced by the cells of the original blastopore lip organizer to form body axes but employ morphogenetic events typical for normal development of distantly related cnidarians to re-establish the germ layers. The reaggregated cells show enormous plasticity including the capacity of the ectodermal cells to convert into endoderm. Our results suggest that new developmental trajectories may evolve relatively easily when highly plastic embryonic cells face new constraints.

Coral individuality - confluence of change physical splitting and developmental ability of embryos

Previous studies have suggested that blastomeres from the 2-, 4-, or 8-cell stage of corals have the ability to develop into normal primary polyps. However, it is still not known which developmental stage's blastomere produces which juvenile. In this study, we demonstrated that only the blastomeres with animal hemispheres have the capacity to develop into normal primary polyps. Individuality was evaluated using blastomeres isolated from the corals Acropora digitifera, A. intermedia, Dipsastraea lizardensis, and Favites chinensis. On commencement of embryo cleavage, the animal pole was marked using Neutral red staining, and at the 2-, 4-, and 8-cell stages, embryos were divided into individual blastomeres using glass needles. We found that the survival rate and percentage metamorphosis were higher in the larger-sized blastomeres with animal hemispheres. The vegetal hemisphere alone is incapable of developing into a normal primary polyp; however, a ball-shaped embryo with incomplete mesenteries and no pharynx developed in some cases. These results indicate that the animal hemisphere is needed for corals to develop into normal primary polyps, and that the individuality of corals is possibly determined by a combination of the chance physical splitting of embryos by waves and their innate developmental ability.

Differential responses to Wnt and PCP disruption predict expression and developmental function of conserved and novel genes in a cnidarian

We have used Digital Gene Expression analysis to identify, without bilaterian bias, regulators of cnidarian embryonic patterning. Transcriptome comparison between un-manipulated Clytia early gastrula embryos and ones in which the key polarity regulator Wnt3 was inhibited using morpholino antisense oligonucleotides (Wnt3-MO) identified a set of significantly over and under-expressed transcripts. These code for candidate Wnt signaling modulators, orthologs of other transcription factors, secreted and transmembrane proteins known as developmental regulators in bilaterian models or previously uncharacterized, and also many cnidarian-restricted proteins. Comparisons between embryos injected with morpholinos targeting Wnt3 and its receptor Fz1 defined four transcript classes showing remarkable correlation with spatiotemporal expression profiles. Class 1 and 3 transcripts tended to show sustained expression at "oral" and "aboral" poles respectively of the developing planula larva, class 2 transcripts in cells ingressing into the endodermal region during gastrulation, while class 4 gene expression was repressed at the early gastrula stage. The preferential effect of Fz1-MO on expression of class 2 and 4 transcripts can be attributed to Planar Cell Polarity (PCP) disruption, since it was closely matched by morpholino knockdown of the specific PCP protein Strabismus. We conclude that endoderm and post gastrula-specific gene expression is particularly sensitive to PCP disruption while Wnt-/β-catenin signaling dominates gene regulation along the oral-aboral axis. Phenotype analysis using morpholinos targeting a subset of transcripts indicated developmental roles consistent with expression profiles for both conserved and cnidarian-restricted genes. Overall our unbiased screen allowed systematic identification of regionally expressed genes and provided functional support for a shared eumetazoan developmental regulatory gene set with both predicted and previously unexplored members, but also demonstrated that fundamental developmental processes including axial patterning and endoderm formation in cnidarians can involve newly evolved (or highly diverged) genes.

Ectopic activation of the canonical wnt signaling pathway affects ectodermal patterning along the primary axis during larval development in the anthozoan Nematostella vectensis

The primary axis of cnidarians runs from the oral pole to the apical tuft and defines the major body axis of both the planula larva and adult polyp. In the anthozoan cnidarian Nematostella vectensis, the primary oral-aboral (O-Ab) axis first develops during the early embryonic stage. Here, we present evidence that pharmaceutical activators of canonical wnt signaling affect molecular patterning along the primary axis of Nematostella. Although not overtly morphologically complex, molecular investigations in Nematostella reveal that the O-Ab axis is demarcated by the expression of differentially localized signaling molecules and transcription factors that may serve roles in establishing distinct ectodermal domains. We have further characterized the larval epithelium by determining the position of a nested set of molecular boundaries, utilizing several newly characterized as well as previously reported epithelial markers along the primary axis. We have assayed shifts in their position in control embryos and in embryos treated with the pharmacological agents alsterpaullone and azakenpaullone, Gsk3β inhibitors that act as canonical wnt agonists, and the Wnt antagonist iCRT14, following gastrulation. Agonist drug treatments result in an absence of aboral markers, a shift in the expression boundaries of oral markers toward the aboral pole, and changes in the position of differentially localized populations of neurons in a dose-dependent manner, while antagonist treatment had the opposite effect. These experiments are consistent with canonical wnt signaling playing a role in an orally localized wnt signaling center. These findings suggest that in Nematostella, wnt signaling mediates O-Ab ectodermal patterning across a surprisingly complex epithelium in planula stages following gastrulation in addition to previously described roles for the wnt signaling pathway in endomesoderm specification during gastrulation and overall animal-vegetal patterning at earlier stages of anthozoan development.

The bilaterian head patterning gene six3/6 controls aboral domain development in a cnidarian

The origin of the bilaterian head is a fundamental question for the evolution of animal body plans. The head of bilaterians develops at the anterior end of their primary body axis and is the site where the brain is located. Cnidarians, the sister group to bilaterians, lack brain-like structures and it is not clear whether the oral, the aboral, or none of the ends of the cnidarian primary body axis corresponds to the anterior domain of bilaterians. In order to understand the evolutionary origin of head development, we analysed the function of conserved genetic regulators of bilaterian anterior development in the sea anemone Nematostella vectensis. We show that orthologs of the bilaterian anterior developmental genes six3/6, foxQ2, and irx have dynamic expression patterns in the aboral region of Nematostella. Functional analyses reveal that NvSix3/6 acts upstream of NvFoxQ2a as a key regulator of the development of a broad aboral territory in Nematostella. NvSix3/6 initiates an autoregulatory feedback loop involving positive and negative regulators of FGF signalling, which subsequently results in the downregulation of NvSix3/6 and NvFoxQ2a in a small domain at the aboral pole, from which the apical organ develops. We show that signalling by NvFGFa1 is specifically required for the development of the apical organ, whereas NvSix3/6 has an earlier and broader function in the specification of the aboral territory. Our functional and gene expression data suggest that the head-forming region of bilaterians is derived from the aboral domain of the cnidarian-bilaterian ancestor.

A conserved function for Strabismus in establishing planar cell polarity in the ciliated ectoderm during cnidarian larval development

Functional and morphological planar cell polarity (PCP) oriented along the oral-aboral body axis is clearly evident in the ectoderm of torpedo-shaped planula larvae of hydrozoan cnidarians such as Clytia hemisphaerica. Ectodermal epithelial cells bear a single motile cilium the beating of which is coordinated between cells, causing directional swimming towards the blunt, aboral pole. We have characterised PCP during Clytia larval development and addressed its molecular basis. PCP is first detectable in ectodermal cells during gastrulation as coordinated basal body positioning, the ciliary root becoming consistently positioned on the oral side of the apical surface of the cell. At later stages, more pronounced structural polarity develops around the base of each cilium in relation to the cilia beating direction, including a characteristic asymmetric cortical actin organisation. Morpholino antisense oligonucleotide and mRNA injection studies showed that PCP development requires the Clytia orthologues of the core Fz-PCP pathway components Strabismus (CheStbm), Frizzled (CheFz1) and Dishevelled (CheDsh). Morpholinos targeting any of these components prevented ectodermal PCP, disrupted ciliogenesis and inhibited embryo elongation during gastrulation, which involves cell intercalation. We show that YFP-tagged CheStbm adopts a polarised intracellular distribution, localising preferentially to the aboral boundary of each cell, as has been demonstrated in Drosophila and some vertebrate PCP studies. Our findings in a cnidarian strongly suggest that the Fz-PCP pathway is a highly conserved and evolutionary ancient metazoan feature that is probably widely responsible for oriented swimming and/or feeding in relation to body axis in the many ciliated larval types found throughout the animal kingdom.

Nervous systems of the sea anemone Nematostella vectensis are generated by ectoderm and endoderm and shaped by distinct mechanisms

As a sister group to Bilateria, Cnidaria is important for understanding early nervous system evolution. Here we examine neural development in the anthozoan cnidarian Nematostella vectensis in order to better understand whether similar developmental mechanisms are utilized to establish the strikingly different overall organization of bilaterian and cnidarian nervous systems. We generated a neuron-specific transgenic NvElav1 reporter line of N. vectensis and used it in combination with immunohistochemistry against neuropeptides, in situ hybridization and confocal microscopy to analyze nervous system formation in this cnidarian model organism in detail. We show that the development of neurons commences in the ectoderm during gastrulation and involves interkinetic nuclear migration. Transplantation experiments reveal that sensory and ganglion cells are autonomously generated by the ectoderm. In contrast to bilaterians, neurons are also generated throughout the endoderm during planula stages. Morpholino-mediated gene knockdown shows that the development of a subset of ectodermal neurons requires NvElav1, the ortholog to bilaterian neural elav1 genes. The orientation of ectodermal neurites changes during planula development from longitudinal (in early-born neurons) to transverse (in late-born neurons), whereas endodermal neurites can grow in both orientations at any stage. Our findings imply that elav1-dependent ectodermal neurogenesis evolved prior to the divergence of Cnidaria and Bilateria. Moreover, they suggest that, in contrast to bilaterians, almost the entire ectoderm and endoderm of the body column of Nematostella planulae have neurogenic potential and that the establishment of connectivity in its seemingly simple nervous system involves multiple neurite guidance systems.

Evolutionary crossroads in developmental biology: Cnidaria

There is growing interest in the use of cnidarians (corals, sea anemones, jellyfish and hydroids) to investigate the evolution of key aspects of animal development, such as the formation of the third germ layer (mesoderm), the nervous system and the generation of bilaterality. The recent sequencing of the Nematostella and Hydra genomes, and the establishment of methods for manipulating gene expression, have inspired new research efforts using cnidarians. Here, we present the main features of cnidarian models and their advantages for research, and summarize key recent findings using these models that have informed our understanding of the evolution of the developmental processes underlying metazoan body plan formation.

Preparation techniques for transmission electron microscopy of Hydra

Hydra is a classical model organism in developmental and cell biology with a simple body plan reminiscent of a gastrula with one body axis and a limited number of cell types. This rather simple organism exhibits a regeneration capacity that is unique among all eumetazoans and is largely dependent on the stem cell properties of its epithelial stem cell population. Molecular work in the past few years has revealed an unexpected genetic complexity of these simple animals, making them an interesting model for studying the generation of animal form and regeneration. In addition, Hydra has an interstitial stem cell system with a unique population of nematocytes, neuronal cells that are characterized by an explosive exocytotic discharge. Here, we compare classical and modern transmission electron microscopy (TEM) fixation protocols including protocols for TEM immunocytochemistry (post-embedding immunogold labeling). We presume that TEM studies will become an important tool to analyze cell-cell interactions as well as cell matrix interrelationships in Hydra in the future.

A developmental perspective: changes in the position of the blastopore during bilaterian evolution

Progress in resolving the phylogenetic relationships among animals and the expansion of molecular developmental studies to a broader variety of organisms has provided important insights into the evolution of developmental programs. These new studies make it possible to reevaluate old hypotheses about the evolution of animal body plans and to elaborate new ones. Here, we review recent studies that shed light on the transition from a radially organized ancestor to the last common ancestor of the Bilateria ("Urbilaterian") and present an integrative hypothesis about plausible developmental scenarios for the evolution of complex multicellular animals.

Immortality and the base of multicellular life: Lessons from cnidarian stem cells

Cnidarians are phylogenetically basal members of the animal kingdom (>600 million years old). Together with plants they share some remarkable features that cannot be found in higher animals. Cnidarians and plants exhibit an almost unlimited regeneration capacity and immortality. Immortality can be ascribed to the asexual mode of reproduction that requires cells with an unlimited self-renewal capacity. We propose that the basic properties of animal stem cells are tightly linked to this archaic mode of reproduction. The cnidarian stem cells can give rise to a number of differentiated cell types including neuronal and germ cells. The genomes of Hydra and Nematostella, representatives of two major cnidarian classes indicate a surprising complexity of both genomes, which is in the range of vertebrates. Recent work indicates that highly conserved signalling pathways control Hydra stem cell differentiation. Furthermore, the availability of genomic resources and novel technologies provide approaches to analyse these cells in vivo. Studies of stem cells in cnidarians will therefore open important insights into the basic mechanisms of stem cell biology. Their critical phylogenetic position at the base of the metazoan branch in the tree of life makes them an important link in unravelling the common mechanisms of stem cell biology between animals and plants.

Three distinct RNA localization mechanisms contribute to oocyte polarity establishment in the cnidarian Clytia hemisphaerica

Egg animal-vegetal polarity in cnidarians is less pronounced than in most bilaterian species, and its normal alignment with the future embryonic axis can be disturbed by low-speed centrifugation. We have analyzed the development of oocyte polarity within the transparent and autonomously functioning gonads of Clytia medusae, focusing on the localization of three recently identified maternal mRNAs coding for axis-directing Wnt pathway regulators. Animal-vegetal polarity was first detectable in oocytes committed to their final growth phase, as the oocyte nucleus (GV) became positioned at the future animal pole. In situ hybridization analyses showed that during this first, microtubule-dependent polarization event, CheFz1 RNA adopts a graded cytoplasmic distribution, most concentrated around the GV. CheFz3 and CheWnt3 RNAs adopt their polarized cortical localizations later, during meiotic maturation. Vegetal localization of CheFz3 RNA was found to require both microtubules and an intact gonad structure, while animal localization of CheWnt3 RNA was microtubule independent and oocyte autonomous. The cortical distribution of both these RNAs was sensitive to microfilament-disrupting drugs. Thus, three temporally and mechanistically distinct RNA localization pathways contribute to oocyte polarity in Clytia. Unlike the two cortical RNAs, CheFz1 RNA was displaced in fertilized eggs upon centrifugation, potentially explaining how this treatment re-specifies the embryonic axis.

Wnt signaling in cnidarians

Cnidarians are an ancient group of animals at the base of metazoan evolution. They exhibit a simple body plan with only one well-defined body axis and a small number of cell types. Cnidarians are also well known for their enormous regeneration capacity. Recent work in the freshwater polyp Hydra and in the sea anemone Nematostella has identified an unexpectedly high level of genetic complexity of wnt genes. Canonical Wnt signaling acts in pattern formation and regeneration of Hydra and also in gastrulation and early embryogenesis of Nematostella. Vertebrate-specific Wnt-antagonists were also identified from cnidarians and exhibit similar conserved functions. The simple cnidarian body plan and the now available genomes from Hydra and Nematostella, together with new functional approaches, make these animals an attractive model for studying the basic functions of canonical and non-canonical Wnt signaling.

Two oppositely localised frizzled RNAs as axis determinants in a cnidarian embryo

In phylogenetically diverse animals, including the basally diverging cnidarians, “determinants” localised within the egg are responsible for directing development of the embryonic body plan. Many such determinants are known to regulate the Wnt signalling pathway, leading to regionalised stabilisation of the transcriptional coregulator β-catenin; however, the only strong molecular candidate for a Wnt-activating determinant identified to date is the ligand Wnt11 in Xenopus. We have identified embryonic “oral–aboral” axis determinants in the cnidarian Clytia hemisphaerica in the form of RNAs encoding two Frizzled family Wnt receptors, localised at opposite poles of the egg. Morpholino-mediated inhibition of translation showed that CheFz1, localised at the animal pole, activates the canonical Wnt pathway, promotes oral fates including gastrulation, and may also mediate global polarity in the ectoderm. CheFz3, whose RNA is localised at the egg vegetal cortex, was found to oppose CheFz1 function and to define an aboral territory. Active downregulation mechanisms maintained the reciprocal localisation domains of the two RNAs during early development. Importantly, ectopic expression of either CheFz1 or CheFz3 was able to redirect axis development. These findings identify Frizzled RNAs as axis determinants in Clytia, and have implications for the evolution of embryonic patterning mechanisms, notably that diverse Wnt pathway regulators have been adopted to initiate asymmetric Wnt pathway activation.

How do different animal body parts form in the correct arrangement during development? Often, the explanation is provided by “determinant” molecules, prepositioned in the egg cell before it is fertilised. These determinant molecules initiate spatially localized programmes of gene expression, causing the various body parts to form in the appropriate place. Many determinants work by activating the Wnt signalling pathway; however, few concrete examples of determinant molecules have yet been discovered. We have found a new example of such a molecule by studying embryos of a jellyfish called Clytia. This molecule, found on one side of the egg, belongs to the “Frizzled” group of membrane proteins that activate Wnt signalling. Unexpectedly, we also found a second type of Frizzled molecule on the other side of the egg, which has a counterbalancing role in the embryo. Comparison of our findings in Clytia with those in other animals suggests that the molecular mechanisms responsible for body patterning via asymmetric Wnt pathway activation have not been tightly constrained during evolution.

The axis of a cnidarian is specified by the location of two maternal Frizzled mRNAs, revising the view that cnidarian axes are specified by the first cleavage initiation site.

Asymmetric developmental potential along the animal–vegetal axis in the anthozoan cnidarian, Nematostella vectensis, is mediated by Dishevelled

The relationship between egg polarity and the adult body plan is well understood in many bilaterians. However, the evolutionary origins of embryonic polarity are not known. Insight into the evolution of polarity will come from understanding the ontogeny of polarity in non-bilaterian forms, such as cnidarians. We examined how the axial properties of the starlet sea anemone, Nematostella vectensis (Anthozoa, Cnidaria), are established during embryogenesis. Egg-cutting experiments and sperm localization show that Nematostella eggs are only fertilized at the animal pole. Vital marking experiments demonstrate that the egg animal pole corresponds to the sites of first cleavage and gastrulation, and the oral pole of the adult. Embryo separation experiments demonstrate an asymmetric segregation of developmental potential along the animal-vegetal axis prior to the 8-cell stage. We demonstrate that Dishevelled (Dsh) plays an important role in mediating this asymmetric segregation of developmental fate. Although NvDsh mRNA is ubiquitously expressed during embryogenesis, the protein is associated with the female pronucleus at the animal pole in the unfertilized egg, becomes associated with the unipolar first cleavage furrow, and remains enriched in animal pole blastomeres. Our results suggest that at least one mechanism for Dsh enrichment at the animal pole is through its degradation at the vegetal pole. Functional studies reveal that NvDsh is required for specifying embryonic polarity and endoderm by stabilizing beta-catenin in the canonical Wnt signaling pathway. The localization of Dsh to the animal pole in Nematostella and two other anthozoan cnidarians (scleractinian corals) provides a possible explanation for how the site of gastrulation has changed in bilaterian evolution while other axial components of development have remained the same and demonstrates that modifications of the Wnt signaling pathway have been used to pattern a wide variety of metazoan embryos.

Embryonic development in two species of scleractinian coral embryos: Symbiodinium localization and mode of gastrulation

Reef-building scleractinian corals widely engage in symbiotic relationships with Symbiodinium dinoflagellates (zooxanthellae), which reside inside cells of the gastrodermis. In most cases, sexually produced larvae acquire their symbionts from the environment in the early developmental stages preceding settlement; however, some scleractinian corals maternally "seed" their oocytes with symbionts, and these symbionts are reported to be restricted to the gastrodermis at the time of its formation (gastrulation). A precise mechanism for how Symbiodinium are translocated to endoderm in these seeded species was previously unknown. In order to examine the process of endoderm formation and Symbiodinium localization during gastrulation, we have examined two species of "robust" clade scleractinians: Fungia scutaria (nonseeded) and Pocillopora meandrina (maternally seeded). We determined that both species, independent of whether or not they are seeded, undergo a "nutritive" stage before gastrulation, wherein lipid-rich cells (F. scutaria) or membrane-bound cellular fragments (P. meandrina) are passed to the blastocoel where they are subsequently taken up by the definitive endoderm. This emergent property of anthozoan development has been co-opted to facilitate the movement of Symbiodinium to the blastocoel (future site of endoderm), in the seeded species, where they are later phagocytosed by the newly formed definitive endoderm. Additionally, both species of robust clade scleractinians examined gastrulate by way of invagination, as do the majority of anthozoans. This invagination differs from the prawn chip-type gastrulation seen in the complex clade corals and provides evidence for a possible linkage between gastrulation type and phylogenetic history.

Early development and axis specification in the sea anemone Nematostella vectensis

We investigated the early development of the sea anemone Nematostella vectensis, an emerging model system of the Cnidaria. Early cleavage stages are characterized by substantial variability from embryo to embryo, yet invariably lead to the formation of a coeloblastula. The coeloblastula undergoes a series of unusual broad invaginations-evaginations which can be blocked by cell cycle inhibitors suggesting a causal link of the invagination cycles to the synchronized cell divisions. Blastula invagination cycles stop as cell divisions become asynchronous. Marking experiments show a clear correspondence of the animal-vegetal axis of the egg to the oral-aboral axis of the embryo. The animal pole gives rise to the concave side of the blastula and later to the blastopore of the gastrula, and hence the oral pole of the future polyp. Asymmetric distribution of granules in the unfertilized egg suggest an animal-vegetal asymmetry in the egg in addition to the localized position of the pronucleus. To determine whether this asymmetry reflects asymmetrically distributed determinants along the animal-vegetal axis, we carried out blastomere isolations and embryonic divisions at various stages. Our data strongly indicate that normal primary polyps develop only if cellular material from the animal hemisphere is included, whereas the vegetal hemisphere alone is incapable to differentiate an oral pole. Molecular marker analysis suggests that also the correct patterning of the aboral pole depends on signals from the oral half. This suggests that in Nematostella embryos the animal hemisphere contains organizing activity to form a normal polyp.