Cnidarian hair cell development illuminates an ancient role for the class IV POU transcription factor in defining mechanoreceptor identity.

Although specialized mechanosensory cells are found across animal phylogeny, early evolutionary histories of mechanoreceptor development remain enigmatic. Cnidaria (e.g. sea anemones and jellyfishes) is the sister group to well-studied Bilateria (e.g. flies and vertebrates), and has two mechanosensory cell types - a lineage-specific sensory effector known as the cnidocyte, and a classical mechanosensory neuron referred to as the hair cell. While developmental genetics of cnidocytes is increasingly understood, genes essential for cnidarian hair cell development are unknown. Here, we show that the class IV POU homeodomain transcription factor (POU-IV) - an indispensable regulator of mechanosensory cell differentiation in Bilateria and cnidocyte differentiation in Cnidaria - controls hair cell development in the sea anemone cnidarian Nematostella vectensis. N. vectensis POU-IV is postmitotically expressed in tentacular hair cells, and is necessary for development of the apical mechanosensory apparatus, but not of neurites, in hair cells. Moreover, it binds to deeply conserved DNA recognition elements, and turns on a unique set of effector genes - including the transmembrane receptor-encoding gene polycystin 1 - specifically in hair cells. Our results suggest that POU-IV directs differentiation of cnidarian hair cells and cnidocytes via distinct gene regulatory mechanisms, and support an evolutionarily ancient role for POU-IV in defining the mature state of mechanosensory neurons.

Chromatin Dynamics and Gene Expression Response to Heat Exposure in Field-Conditioned versus Laboratory-Cultured Nematostella vectensis.

Organisms' survival is associated with the ability to respond to natural or anthropogenic environmental stressors. Frequently, these responses involve changes in gene regulation and expression, consequently altering physiology, development, or behavior. Here, we present modifications in response to heat exposure that mimics extreme summertime field conditions of lab-cultured and field-conditioned Nematostella vectensis. Using ATAC-seq and RNA-seq data, we found that field-conditioned animals had a more concentrated reaction to short-term thermal stress, expressed as enrichment of the DNA repair mechanism pathway. By contrast, lab animals had a more diffuse reaction that involved a larger number of differentially expressed genes and enriched pathways, including amino acid metabolism. Our results demonstrate that pre-conditioning affects the ability to respond efficiently to heat exposure in terms of both chromatin accessibility and gene expression and reinforces the importance of experimentally addressing ecological questions in the field.

Microtubule organization of vertebrate sensory neurons in vivo.

Dorsal root ganglion (DRG) neurons are the predominant cell type that innervates the vertebrate skin. They are typically described as pseudounipolar cells that have central and peripheral axons branching from a single root exiting the cell body. The peripheral axon travels within a nerve to the skin, where free sensory endings can emerge and branch into an arbor that receives and integrates information. In some immature vertebrates, DRG neurons are preceded by Rohon-Beard (RB) neurons. While the sensory endings of RB and DRG neurons function like dendrites, we use live imaging in zebrafish to show that they have axonal plus-end-out microtubule polarity at all stages of maturity. Moreover, we show both cell types have central and peripheral axons with plus-end-out polarity. Surprisingly, in DRG neurons these emerge separately from the cell body, and most cells never acquire the signature pseudounipolar morphology. Like another recently characterized cell type that has multiple plus-end-out neurites, ganglion cells in Nematostella, RB and DRG neurons maintain a somatic microtubule organizing center even when mature. In summary, we characterize key cellular and subcellular features of vertebrate sensory neurons as a foundation for understanding their function and maintenance.

Mesoglea Extracellular Matrix Reorganization during Regenerative Process in Anemonia viridis (Forskål, 1775).

Given the anatomical simplicity and the extraordinary ability to regenerate missing parts of the body, Cnidaria represent an excellent model for the study of the mechanisms regulating regenerative processes. They possess the mesoglea, an amorphous and practically acellular extracellular matrix (ECM) located between the epidermis and the gastrodermis of the body and tentacles and consists of the same molecules present in the ECM of vertebrates, such as collagen, laminin, fibronectin and proteoglycans. This feature makes cnidarians anthozoans valid models for understanding the ECM role during regenerative processes. Indeed, it is now clear that its role in animal tissues is not just tissue support, but instead plays a key role during wound healing and tissue regeneration. This study aims to explore regenerative events after tentacle amputation in the Mediterranean anemone Anemonia viridis, focusing in detail on the reorganization of the ECM mesoglea. In this context, both enzymatic, biometric and histological experiments reveal how this gelatinous connective layer plays a fundamental role in the correct restoration of the original structures by modifying its consistency and stiffness. Indeed, through the deposition of collagen I, it might act as a scaffold and as a guide for the reconstruction of missing tissues and parts, such as amputated tentacles.

Hedgehog signaling is required for endomesodermal patterning and germ cell development in the sea anemone Nematostella vectensis.

Two distinct mechanisms for primordial germ cell (PGC) specification are observed within Bilatera: early determination by maternal factors or late induction by zygotic cues. Here we investigate the molecular basis for PGC specification in Nematostella, a representative pre-bilaterian animal where PGCs arise as paired endomesodermal cell clusters during early development. We first present evidence that the putative PGCs delaminate from the endomesoderm upon feeding, migrate into the gonad primordia, and mature into germ cells. We then show that the PGC clusters arise at the interface between hedgehog1 and patched domains in the developing mesenteries and use gene knockdown, knockout and inhibitor experiments to demonstrate that Hh signaling is required for both PGC specification and general endomesodermal patterning. These results provide evidence that the Nematostella germline is specified by inductive signals rather than maternal factors, and support the existence of zygotically-induced PGCs in the eumetazoan common ancestor.

Feeding-dependent tentacle development in the sea anemone Nematostella vectensis.

In cnidarians, axial patterning is not restricted to embryogenesis but continues throughout a prolonged life history filled with unpredictable environmental changes. How this developmental capacity copes with fluctuations of food availability and whether it recapitulates embryonic mechanisms remain poorly understood. Here we utilize the tentacles of the sea anemone Nematostella vectensis as an experimental paradigm for developmental patterning across distinct life history stages. By analyzing over 1000 growing polyps, we find that tentacle progression is stereotyped and occurs in a feeding-dependent manner. Using a combination of genetic, cellular and molecular approaches, we demonstrate that the crosstalk between Target of Rapamycin (TOR) and Fibroblast growth factor receptor b (Fgfrb) signaling in ring muscles defines tentacle primordia in fed polyps. Interestingly, Fgfrb-dependent polarized growth is observed in polyp but not embryonic tentacle primordia. These findings show an unexpected plasticity of tentacle development, and link post-embryonic body patterning with food availability.

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.

Application of a behavioural and biochemical endpoint in ecotoxicity testing with Exaiptasia pallida.

Exaiptasia pallida has been applied as a cnidarian model to assess the toxicity of various contaminants using endpoints related to growth, reproduction and mortality. However, increasingly accepted behavioural and biochemical endpoints are underrepresented in ecotoxicity testing with cnidarian species. The aim of this study was to assess the suitability of tentacle retraction and superoxide dismutase activity as behavioural and biochemical endpoints for ecotoxicity testing with E. pallida. A concentration-dependent, tentacle retraction response was found in sub-lethal toxicity testing for anemones exposed to 1-65 µg L-1 Cu and 2-630 µg L-1 Zn for 24 and 96 h. Semi-quantitative and quantitative approaches to tentacle retraction analysis showed a difference in response sensitivity, however, both methods resulted in similar 24- and 96-h EC50 values for Cu and Zn. Additionally, tentacle retraction analysis provided the benefit of identifying recovery in anemones previously exposed to 359 µg L-1 Zn following a 96-h recovery period. Conversely, no significant difference in superoxide dismutase activity was detected in anemones exposed to the Cu and Zn solutions compared with controls, after either 24- or 96-h exposures. These findings support the ease of application and sensitivity of tentacle retraction as an endpoint in ecotoxicity testing with E. pallida and recommend its suitability for use in acute, sub-lethal toxicity testing. Moreover, evidence of recovery in E. pallida following exposure suggests that recovery should be incorporated into future toxicity assessments.

Plastic pellets trigger feeding responses in sea anemones.

Multiple mechanisms for plastic consumption by marine animals have been proposed based on the feeding cues and behavior of the animal studied. We investigated plastic consumption in sea anemones. We found that anemones readily consumed pristine National Institute of Standards and Technology low-density polyethylene and high-density polyethylene II and III pre-production pellets. Anemone weight, crown area, and number of tentacles were measured before and after 12 days of daily pellet consumption. Crown area significantly increased for control anemones only. Fresh anemones were then sequentially fed consumed and egested pellets from two of the earlier daily trials to measure feeding retention time, which decreased over three to four feedings. The concentrations of elements in anemones (zinc, iron, arsenic, manganese, chromium, copper, vanadium, selenium, nickel, cadmium, and cobalt) were similar to control anemones that were not exposed to pellets. Lead concentrations were significantly higher in anemones fed HDPE III pellets as compared to control. Plastic consumption by marine animals might be reduced by reducing the amount of plastic that enters the ocean and understanding the chemical triggers underlying plastic consumption.

Generating Transgenic Reporter Lines for Studying Nervous System Development in the Cnidarian Nematostella vectensis.

Neurons often display complex morphologies with long and fine processes that can be difficult to visualize, in particular in living animals. Transgenic reporter lines in which fluorescent proteins are expressed in defined populations of neurons are important tools that can overcome these difficulties. By using membrane-attached fluorescent proteins, such reporter transgenes can identify the complete outline of subsets of neurons or they can highlight the subcellular localization of fusion proteins, for example at pre- or postsynaptic sites. The relative stability of fluorescent proteins furthermore allows the tracing of the progeny of cells over time and can therefore provide information about potential roles of the gene whose regulatory elements are controlling the expression of the fluorescent protein. Here we describe the generation of transgenic reporter lines in the sea anemone Nematostella vectensis, a cnidarian model organism for studying the evolution of developmental processes. We also provide an overview of existing transgenic Nematostella lines that have been used to study conserved and derived aspects of nervous system development.

Genotyping of Sea Anemone during Early Development.

Described here is a PCR-based protocol to genotype the gastrula stage embryo of the anthozoan cnidarian Nematostella vectensis without sacrificing the life of the animal. Following in vitro fertilization and de-jellying, zygotes are allowed to develop for 24 h at room temperature to reach the early- to mid-gastrula stage. The gastrula embryos are then placed on an agarose gel bed in a Petri dish containing seawater. Under the dissecting microscope, a tungsten needle is used to surgically separate an aboral tissue fragment from each embryo. Post-surgery embryos are then allowed to heal and continue development. Genomic DNA is extracted from the isolated tissue fragment and used as a template for locus-specific PCR. The genotype can be determined based on the size of PCR products or presence/absence of allele-specific PCR products. Post-surgery embryos are then sorted according to the genotype. The duration of the entire genotyping process depends on the number of embryos to be screened, but it minimally requires 4-5 h. This method can be used to identify knockout mutants from a genetically heterogeneous population of embryos and enables analyses of phenotypes during development.

Cnidom variation through distinct developmental stages in the sea anemone Aulactinia marplatensis (Zamponi, 1977) (Cnidaria: Actiniaria).

The cnidae are the exclusive diagnostic structures of phylum Cnidaria. The inventory of all cnidae types of a particular species is called the cnidom. The study of cnidae has been widely addressed in all classes of cnidarians. Particularly in the order Actiniaria (sea anemones), the study of the composition, size and distribution of cnidae is essential to the identification and description of species. In the present study, we examine the cnidom of the sea anemone Aulactinia marplatensis in three different stages of development throughout its life cycle. We found that the composition and abundance patterns are very similar between the adult and juvenile stages, although significant differences in the size capsules were found between both stages and in all cnidae types observed, being bigger those from the adult forms. The planula larvae stage presents a less diverse cnidom in comparison to the juvenile and adult stages; however, it presents an exclusive cnidae type (the mesobasic p-mastigophore) which is the biggest in size of all the cnidae types observed in the species. These results highlight the importance of considering the stage of development when cnidae is used as a diagnostic character, and the particular relevance of the study of the cnidom in larval stages.

An axial Hox code controls tissue segmentation and body patterning in Nematostella vectensis.

Hox genes encode conserved developmental transcription factors that govern anterior-posterior (A-P) pattering in diverse bilaterian animals, which display bilateral symmetry. Although Hox genes are also present within Cnidaria, these simple animals lack a definitive A-P axis, leaving it unclear how and when a functionally integrated Hox code arose during evolution. We used short hairpin RNA (shRNA)-mediated knockdown and CRISPR-Cas9 mutagenesis to demonstrate that a Hox-Gbx network controls radial segmentation of the larval endoderm during development of the sea anemone Nematostella vectensis. Loss of Hox-Gbx activity also elicits marked defects in tentacle patterning along the directive (orthogonal) axis of primary polyps. On the basis of our results, we propose that an axial Hox code may have controlled body patterning and tissue segmentation before the evolution of the bilaterian A-P axis.

Cnidae sizes in the two morphotypes of the giant Caribbean anemone Condylactis gigantea (Actiniaria: Actiniidae) / Tamaños de cnidae en dos morfotipos de la anémona gigante del Caribe Condylactis gigantea (Actiniaria: Actiniidae)

Abstract The sea anemone Condylactis gigantea is an ecologically important member of the benthic community in coral reefs of the tropical Atlantic, and displays two morphotypes with respect to the color in their tentacular tips: the green tip morphotype and the pink/purple tip morphotype. Although some molecular and ecological differences have been found between these morphotypes, no other morphological distinctions have been reported, and currently both are still considered a single taxonomic species. In the present study, we perform an exploration on the variability in the size of cnidae between these two morphotypes and performed statistical analyses to compare the 10 categories of cnidae from specimens hosted in the Cnidarian Collection of Gulf of Mexico and Mexican Caribbean, of the Universidad Nacional Autónoma de México, which were previously collected in several coral reefs localities of the Yucatán Peninsula. Results reveal no significant variation in cnidae size between the two morphotypes, but significant variations were found within each morphotype. In addition, we update the composition of the cnidom of C. gigantea, and the utility of the size of cnidae to distinguish between morphotypes or closely related species is discussed. Rev. Biol. Trop. 66(3): 1055-1064. Epub 2018 September 01.

Resumen La anémona Condylactis gigantea es un miembro ecológicamente importante de la comunidad bentónica en arrecifes de coral del Atlántico tropical, y exhibe dos morfotipos con respecto al color de las puntas de sus tentáculos: el morfotipo de puntas verdes y el morfotipo de puntas rosadas/púrpuras. Aunque se han encontrado algunas diferencias moleculares y ecológicas entre estos morfotipos, no se han reportado otras distinciones morfológicas, y actualmente ambos siguen siendo considerados una sola especie taxonómica. En el presente estudio, realizamos una exploración sobre la variabilidad en el tamaño de los cnidocistos entre estos dos morfotipos y realizamos un análisis estadístico de 10 categorías de cnidocistos a partir de especímenes albergados en la Colección de cnidarios del Golfo de México y Caribe Mexicano, de la Universidad Nacional Autónoma de México, los cuales fueron previamente recolectados en varias localidades arrecifales de la Península de Yucatán. Los resultados no revelan variación significativa en el tamaño de los cnidocistos entre los dos morfotipos, aunque fueron encontradas variaciones significativas dentro de cada morfotipo. Adicionalmente, actualizamos la composición del cnidoma de C. gigantea, y discutimos sobre la utilidad de la talla de los cnidocistos para distinguir entre morfotipos o entre especies estrechamente relacionadas.

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.

Temperature-Dependent Growth and Fission Rate Plasticity Drive Seasonal and Geographic Changes in Body Size in a Clonal Sea Anemone.

The temperature-size rule is a commonly observed pattern where adult body size is negatively correlated with developmental temperature. In part, this may occur as a consequence of allometric scaling, where changes in the ratio of surface area to mass limit oxygen diffusion as body size increases. As oxygen demand increases with temperature, a smaller body should be favored as temperature increases. For clonal animals, small changes in growth and/or fission rate can rapidly alter the average body size of clonal descendants. Here I test the hypothesis that the clonal sea anemone Diadumene lineata is able to track an optimal body size through seasonal temperature changes using fission rate plasticity. Individuals from three regions (Florida, Georgia, and Massachusetts) across the species' latitudinal range were grown in a year-long reciprocal common garden experiment mimicking seasonal temperature changes at three sites. Average body size was found to be smaller and fission rates higher in warmer conditions, consistent with the temperature-size rule pattern. However, seasonal size and fission patterns reflect a complex interaction between region-specific thermal reaction norms and the local temperature regime. These details provide insight into both the range of conditions required for oxygen limitation to contribute to a negative correlation between body size and temperature and the role that fission rate plasticity can play in tracking a rapidly changing optimal phenotype.

High-throughput method for extracting and visualizing the spatial gene expressions from in situ hybridization images: A case study of the early development of the sea anemone Nematostella vectensis.

Studying the spatial gene expression profiles from in situ hybridization images of the embryo is one of the first steps toward the comprehensive understanding of gene interactions in an organism. In the case of N. vectensis, extracting and collecting these data is a challenging task due to the difficulty of detecting the cell layer through the transparent body plan and changing morphology during the blastula and gastrula stages. Here, first, we introduce a method to algorithmically identify and track the cell layer in N. vectensis embryo from the late blastula to the late gastrula stage. With this, we will be able to extract spatial expression profiles of genes alongside the cell layer and consequently reconstructing the 1D representation of gene expression profiles. Furthermore, we use the morphological configurations of the embryo extracted from confocal images, to model the dynamics of embryos morphology during the gastrulation process in 2D. Ultimately, we provide a visualization tool for studying and comparing the extracted spatial gene expression profiles over the simulated embryo. We anticipate that our method of extraction and visualization to be a starting point for quantifying and collecting more in situ images from various sources, which can potentially accelerate our understanding of gene interactions in the early development of N. vectensis. The method allows researchers to visualize and compare the different gene expressions from different in situ images or different experiments. As an example, we were able to show the complementary expression of NvFoxA-NvSnailA and NvBra-NvErg in the central domain and central/external rings during the development which suggests the possible repression effects between each pair; as it has been discovered by functional analysis.

The impact of the sea anemone Actinothoe sphyrodeta on Mytilus galloprovincialis mussel cultivation (Galicia, Spain).

Marine mussel aggregations act as a substratum and refuge for many fouling species. Mussel cultivation in Galicia, Spain, is carried out on hanging ropes in subtidal systems. The fauna associated with this cultivation includes a large number of invertebrates that compete for space or food with the mussels, or use their clusters as a refuge from predators or water turbulence. Outbreaks of the epibiont anemone Actinothoe sphyrodeta have been reported in cultivated Galician mussels since 2013, but their impact has not been investigated rigorously. Here, the temporal and spatial variability of Actinothoe sphyrodeta on mussel shells throughout one year is presented. Sampling of mussel size, weight and byssus attachment strength allowed mussel tenacity (attachment strength relative to size) to be calculated. A higher presence of Actinothoe sphyrodeta correlated with lower mussel tenacity and greater biomass losses, suggesting that this species could be an economically important biofouling component.

Sea anemone model has a single Toll-like receptor that can function in pathogen detection, NF-κB signal transduction, and development.

In organisms from insects to vertebrates, Toll-like receptors (TLRs) are primary pathogen detectors that activate downstream pathways, specifically those that direct expression of innate immune effector genes. TLRs also have roles in development in many species. The sea anemone Nematostella vectensis is a useful cnidarian model to study the origins of TLR signaling because its genome encodes a single TLR and homologs of many downstream signaling components, including the NF-κB pathway. We have characterized the single N. vectensis TLR (Nv-TLR) and demonstrated that it can activate canonical NF-κB signaling in human cells. Furthermore, we show that the intracellular Toll/IL-1 receptor (TIR) domain of Nv-TLR can interact with the human TLR adapter proteins MAL and MYD88. We demonstrate that the coral pathogen Vibrio coralliilyticus causes a rapidly lethal disease in N. vectensis and that heat-inactivated V. coralliilyticus and bacterial flagellin can activate a reconstituted Nv-TLR-to-NF-κB pathway in human cells. By immunostaining of anemones, we show that Nv-TLR is expressed in a subset of cnidocytes and that many of these Nv-TLR-expressing cells also express Nv-NF-κB. Additionally, the nematosome, which is a Nematostella-specific multicellular structure, expresses Nv-TLR and many innate immune pathway homologs and can engulf V. coralliilyticus Morpholino knockdown indicates that Nv-TLR also has an essential role during early embryonic development. Our characterization of this primitive TLR and identification of a bacterial pathogen for N. vectensis reveal ancient TLR functions and provide a model for studying the molecular basis of cnidarian disease and immunity.

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.