Toxin-like neuropeptides in the sea anemone Nematostella unravel recruitment from the nervous system to venom.

The sea anemone Nematostella vectensis (Anthozoa, Cnidaria) is a powerful model for characterizing the evolution of genes functioning in venom and nervous systems. Although venom has evolved independently numerous times in animals, the evolutionary origin of many toxins remains unknown. In this work, we pinpoint an ancestral gene giving rise to a new toxin and functionally characterize both genes in the same species. Thus, we report a case of protein recruitment from the cnidarian nervous to venom system. The ShK-like1 peptide has a ShKT cysteine motif, is lethal for fish larvae and packaged into nematocysts, the cnidarian venom-producing stinging capsules. Thus, ShK-like1 is a toxic venom component. Its paralog, ShK-like2, is a neuropeptide localized to neurons and is involved in development. Both peptides exhibit similarities in their functional activities: They provoke contraction in Nematostella polyps and are toxic to fish. Because ShK-like2 but not ShK-like1 is conserved throughout sea anemone phylogeny, we conclude that the two paralogs originated due to a Nematostella-specific duplication of a ShK-like2 ancestor, a neuropeptide-encoding gene, followed by diversification and partial functional specialization. ShK-like2 is represented by two gene isoforms controlled by alternative promoters conferring regulatory flexibility throughout development. Additionally, we characterized the expression patterns of four other peptides with structural similarities to studied venom components and revealed their unexpected neuronal localization. Thus, we employed genomics, transcriptomics, and functional approaches to reveal one venom component, five neuropeptides with two different cysteine motifs, and an evolutionary pathway from nervous to venom system in Cnidaria.

Never, Ever Make an Enemy Out of an Anemone: Transcriptomic Comparison of Clownfish Hosting Sea Anemone Venoms.

Sea anemones are predatory marine invertebrates and have diverse venom arsenals. Venom is integral to their biology, and is used in competition, defense, and feeding. Three lineages of sea anemones are known to have independently evolved symbiotic relationships with clownfish, however the evolutionary impact of this relationship on the venom composition of the host is still unknown. Here, we investigate the potential of this symbiotic relationship to shape the venom profiles of the sea anemones that host clownfish. We use transcriptomic data to identify differences and similarities in venom profiles of six sea anemone species, representing the three known clades of clownfish-hosting sea anemones. We recovered 1121 transcripts matching verified toxins across all species, and show that hemolytic and hemorrhagic toxins are consistently the most dominant and diverse toxins across all species examined. These results are consistent with the known biology of sea anemones, provide foundational data on venom diversity of these species, and allow for a review of existing hierarchical structures in venomic studies.

Some like it hot: population-specific adaptations in venom production to abiotic stressors in a widely distributed cnidarian.

BACKGROUND: In cnidarians, antagonistic interactions with predators and prey are mediated by their venom, whose synthesis may be metabolically expensive. The potentially high cost of venom production has been hypothesized to drive population-specific variation in venom expression due to differences in abiotic conditions. However, the effects of environmental factors on venom production have been rarely demonstrated in animals. Here, we explore the impact of specific abiotic stresses on venom production of distinct populations of the sea anemone Nematostella vectensis (Actiniaria, Cnidaria) inhabiting estuaries over a broad geographic range where environmental conditions such as temperatures and salinity vary widely. RESULTS: We challenged Nematostella polyps with heat, salinity, UV light stressors, and a combination of all three factors to determine how abiotic stressors impact toxin expression for individuals collected across this species' range. Transcriptomics and proteomics revealed that the highly abundant toxin Nv1 was the most downregulated gene under heat stress conditions in multiple populations. Physiological measurements demonstrated that venom is metabolically costly to produce. Strikingly, under a range of abiotic stressors, individuals from different geographic locations along this latitudinal cline modulate differently their venom production levels. CONCLUSIONS: We demonstrate that abiotic stress results in venom regulation in Nematostella. Together with anecdotal observations from other cnidarian species, our results suggest this might be a universal phenomenon in Cnidaria. The decrease in venom production under stress conditions across species coupled with the evidence for its high metabolic cost in Nematostella suggests downregulation of venom production under certain conditions may be highly advantageous and adaptive. Furthermore, our results point towards local adaptation of this mechanism in Nematostella populations along a latitudinal cline, possibly resulting from distinct genetics and significant environmental differences between their habitats.

The Birth and Death of Toxins with Distinct Functions: A Case Study in the Sea Anemone Nematostella.

The cnidarian Nematostella vectensis has become an established lab model, providing unique opportunities for venom evolution research. The Nematostella venom system is multimodal: involving both nematocytes and ectodermal gland cells, which produce a toxin mixture whose composition changes throughout the life cycle. Additionally, their modes of interaction with predators and prey vary between eggs, larvae, and adults, which is likely shaped by the dynamics of the venom system. Nv1 is a major component of adult venom, with activity against arthropods (through specific inhibition of sodium channel inactivation) and fish. Nv1 is encoded by a cluster of at least 12 nearly identical genes that were proposed to be undergoing concerted evolution. Surprisingly, we found that Nematostella venom includes several Nv1 paralogs escaping a pattern of general concerted evolution, despite belonging to the Nv1-like family. Here, we show two of these new toxins, Nv4 and Nv5, are lethal for zebrafish larvae but harmless to arthropods, unlike Nv1. Furthermore, unlike Nv1, the newly identified toxins are expressed in early life stages. Using transgenesis and immunostaining, we demonstrate that Nv4 and Nv5 are localized to ectodermal gland cells in larvae. The evolution of Nv4 and Nv5 can be described either as neofunctionalization or as subfunctionalization. Additionally, the Nv1-like family includes several pseudogenes being an example of nonfunctionalization and venom evolution through birth-and-death mechanism. Our findings reveal the evolutionary history for a toxin radiation and point toward the ecological function of the novel toxins constituting a complex cnidarian venom.

Transcriptomic Analysis of Four Cerianthid (Cnidaria, Ceriantharia) Venoms.

Tube anemones, or cerianthids, are a phylogenetically informative group of cnidarians with complex life histories, including a pelagic larval stage and tube-dwelling adult stage, both known to utilize venom in stinging-cell rich tentacles. Cnidarians are an entirely venomous group that utilize their proteinaceous-dominated toxins to capture prey and defend against predators, in addition to several other ecological functions, including intraspecific interactions. At present there are no studies describing the venom for any species within cerianthids. Given their unique development, ecology, and distinct phylogenetic-placement within Cnidaria, our objective is to evaluate the venom-like gene diversity of four species of cerianthids from newly collected transcriptomic data. We identified 525 venom-like genes between all four species. The venom-gene profile for each species was dominated by enzymatic protein and peptide families, which is consistent with previous findings in other cnidarian venoms. However, we found few toxins that are typical of sea anemones and corals, and furthermore, three of the four species express toxin-like genes closely related to potent pore-forming toxins in box jellyfish. Our study is the first to provide a survey of the putative venom composition of cerianthids and contributes to our general understanding of the diversity of cnidarian toxins.

A RNA-seq approach to identify putative toxins from acrorhagi in aggressive and non-aggressive Anthopleura elegantissima polyps.

BACKGROUND: The use of venom in intraspecific aggression is uncommon and venom-transmitting structures specifically used for intraspecific competition are found in few lineages of venomous taxa. Next-generation transcriptome sequencing allows robust characterization of venom diversity and exploration of functionally unique tissues. Using a tissue-specific RNA-seq approach, we investigate the venom composition and gene ontology diversity of acrorhagi, specialized structures used in intraspecific competition, in aggressive and non-aggressive polyps of the aggregating sea anemone Anthopleura elegantissima (Cnidaria: Anthozoa: Hexacorallia: Actiniaria: Actiniidae). RESULTS: Collectively, we generated approximately 450,000 transcripts from acrorhagi of aggressive and non-aggressive polyps. For both transcriptomes we identified 65 candidate sea anemone toxin genes, representing phospholipase A2s, cytolysins, neurotoxins, and acrorhagins. When compared to previously characterized sea anemone toxin assemblages, each transcriptome revealed greater within-species sequence divergence across all toxin types. The transcriptome of the aggressive polyp had a higher abundance of type II voltage gated potassium channel toxins/Kunitz-type protease inhibitors and type II acrorhagins. Using toxin-like proteins from other venomous taxa, we also identified 612 candidate toxin-like transcripts with signaling regions, potentially unidentified secretory toxin-like proteins. Among these, metallopeptidases and cysteine rich (CRISP) candidate transcripts were in high abundance. Furthermore, our gene ontology analyses identified a high prevalence of genes associated with "blood coagulation" and "positive regulation of apoptosis", as well as "nucleoside: sodium symporter activity" and "ion channel binding". The resulting assemblage of expressed genes may represent synergistic proteins associated with toxins or proteins related to the morphology and behavior exhibited by the aggressive polyp. CONCLUSION: We implement a multifaceted approach to investigate the assemblage of expressed genes specifically within acrorhagi, specialized structures used only for intraspecific competition. By combining differential expression, phylogenetic, and gene ontology analyses, we identify several candidate toxins and other potentially important proteins in acrorhagi of A. elegantissima. Although not all of the toxins identified are used in intraspecific competition, our analysis highlights some candidates that may play a vital role in intraspecific competition. Our findings provide a framework for further investigation into components of venom used exclusively for intraspecific competition in acrorhagi-bearing sea anemones and potentially other venomous animals.

Venom trade-off shapes interspecific interactions, physiology, and reproduction.

The ability of an animal to effectively capture prey and defend against predators is pivotal for survival. Venom is often a mixture of many components including toxin proteins that shape predator-prey interactions. Here, we used the sea anemone Nematostella vectensis to test the impact of toxin genotypes on predator-prey interactions. We developed a genetic manipulation technique to demonstrate that both transgenically deficient and a native Nematostella strain lacking a major neurotoxin (Nv1) have a reduced ability to defend themselves against grass shrimp, a native predator. In addition, secreted Nv1 can act indirectly in defense by attracting mummichog fish, which prey on grass shrimp. Here, we provide evidence at the molecular level of an animal-specific tritrophic interaction between a prey, its antagonist, and a predator. Last, this study reveals an evolutionary trade-off, as the reduction of Nv1 levels allows for faster growth and increased reproductive rates.

Camelina seed transcriptome: a tool for meal and oil improvement and translational research.

Camelina (Camelina sativa), a Brassicaceae oilseed, has received recent interest as a biofuel crop and production platform for industrial oils. Limiting wider production of camelina for these uses is the need to improve the quality and content of the seed protein-rich meal and oil, which is enriched in oxidatively unstable polyunsaturated fatty acids that are deleterious for biodiesel. To identify candidate genes for meal and oil quality improvement, a transcriptome reference was built from 2047 Sanger ESTs and more than 2 million 454-derived sequence reads, representing genes expressed in developing camelina seeds. The transcriptome of approximately 60K transcripts from 22 597 putative genes includes camelina homologues of nearly all known seed-expressed genes, suggesting a high level of completeness and usefulness of the reference. These sequences included candidates for 12S (cruciferins) and 2S (napins) seed storage proteins (SSPs) and nearly all known lipid genes, which have been compiled into an accessible database. To demonstrate the utility of the transcriptome for seed quality modification, seed-specific RNAi lines deficient in napins were generated by targeting 2S SSP genes, and high oleic acid oil lines were obtained by targeting FATTY ACID DESATURASE 2 (FAD2) and FATTY ACID ELONGASE 1 (FAE1). The high sequence identity between Arabidopsis thaliana and camelina genes was also exploited to engineer high oleic lines by RNAi with Arabidopsis FAD2 and FAE1 sequences. It is expected that these transcriptomic data will be useful for breeding and engineering of additional camelina seed traits and for translating findings from the model Arabidopsis to an oilseed crop.

Exploring Toxin Genes of Myanmar Russell's Viper, Daboia siamensis, through De Novo Venom Gland Transcriptomics.

The Russell's viper (Daboia siamensis) is a medically important venomous snake in Myanmar. Next-generation sequencing (NGS) shows potential to investigate the venom complexity, giving deeper insights into snakebite pathogenesis and possible drug discoveries. mRNA from venom gland tissue was extracted and sequenced on the Illumina HiSeq platform and de novo assembled by Trinity. The candidate toxin genes were identified via the Venomix pipeline. Protein sequences of identified toxin candidates were compared with the previously described venom proteins using Clustal Omega to assess the positional homology among candidates. Candidate venom transcripts were classified into 23 toxin gene families including 53 unique full-length transcripts. C-type lectins (CTLs) were the most highly expressed, followed by Kunitz-type serine protease inhibitors, disintegrins and Bradykinin potentiating peptide/C-type natriuretic peptide (BPP-CNP) precursors. Phospholipase A2, snake venom serine proteases, metalloproteinases, vascular endothelial growth factors, L-amino acid oxidases and cysteine-rich secretory proteins were under-represented within the transcriptomes. Several isoforms of transcripts which had not been previously reported in this species were discovered and described. Myanmar Russell's viper venom glands displayed unique sex-specific transcriptome profiles which were correlated with clinical manifestation of envenoming. Our results show that NGS is a useful tool to comprehensively examine understudied venomous snakes.

Micro and macroevolution of sea anemone venom phenotype.

Venom is a complex trait with substantial inter- and intraspecific variability resulting from strong selective pressures acting on the expression of many toxic proteins. However, understanding the processes underlying toxin expression dynamics that determine the venom phenotype remains unresolved. By interspecific comparisons we reveal that toxin expression in sea anemones evolves rapidly and that in each species different toxin family dictates the venom phenotype by massive gene duplication events. In-depth analysis of the sea anemone, Nematostella vectensis, revealed striking variation of the dominant toxin (Nv1) diploid copy number across populations (1-24 copies) resulting from independent expansion/contraction events, which generate distinct haplotypes. Nv1 copy number correlates with expression at both the transcript and protein levels with one population having a near-complete loss of Nv1 production. Finally, we establish the dominant toxin hypothesis which incorporates observations in other venomous lineages that animals have convergently evolved a similar strategy in shaping their venom.

Simultaneous delimitation of species and quantification of interspecific hybridization in Amazonian peacock cichlids (genus cichla) using multi-locus data.

BACKGROUND: Introgression likely plays a significant role in evolution, but understanding the extent and consequences of this process requires a clear identification of species boundaries in each focal group. The delimitation of species, however, is a contentious endeavor. This is true not only because of the inadequacy of current tools to identify species lineages, but also because of the inherent ambiguity between natural populations and species paradigms. The result has been a debate about the supremacy of various species concepts and criteria. Here, we utilized multiple separate sources of molecular data, mtDNA, nuclear sequences, and microsatellites, to delimit species under a polytypic species concept (PTSC) and estimate the frequency and genomic extent of introgression in a Neotropical genus of cichlid fishes (Cichla). We compared our inferences of species boundaries and introgression under this paradigm to those when species are identified under a diagnostic species concept (DSC). RESULTS: We find that, based on extensive molecular data and an inclusive species concept, 8 separate biological entities should be recognized rather than the 15 described species of Cichla. Under the PTSC, fewer individuals are expected to exhibit hybrid ancestry than under the DSC (~2% vs. ~12%), but a similar number of the species exhibit introgression from at least one other species (75% vs. 60%). Under either species concept, the phylogenetic breadth of introgression in this group is notable, with both sister species and species from different major mtDNA clades exhibiting introgression. CONCLUSIONS: Introgression was observed to be a widespread phenomenon for delimited species in this group. While several instances of introgressive hybridization were observed in anthropogenically altered habitats, most were found in undisturbed natural habitats, suggesting that introgression is a natural but ephemeral part of the evolution of many tropical species. Nevertheless, even transient introgression may facilitate an increase in genetic diversity or transfer of adaptive mutations that have important consequences in the evolution of tropical biodiversity.

Linear Mitochondrial Genome in Anthozoa (Cnidaria): A Case Study in Ceriantharia.

Sequences and structural attributes of mitochondrial genomes have played a critical role in the clarification of relationships among Cnidaria, a key phylum of early-diverging animals. Among the major lineages of Cnidaria, Ceriantharia ("tube anemones") remains one of the most enigmatic in terms of its phylogenetic position. We sequenced the mitochondrial genomes of two ceriantharians to see whether the complete organellar genome would provide more support for the phylogenetic placement of Ceriantharia. For both Isarachnanthus nocturnus and Pachycerianthus magnus, the mitochondrial gene sequences could not be assembled into a single circular genome. Instead, our analyses suggest that both species have mitochondrial genomes consisting of multiple linear fragments. Linear mitogenomes are characteristic of members of Medusozoa, one of the major lineages of Cnidaria, but are unreported for Anthozoa, which includes the Ceriantharia. The inferred number of fragments and variation in gene order between species is much greater within Ceriantharia than among the lineages of Medusozoa. We identify origins of replication for each of the five putative chromosomes of the Isarachnanthus nocturnus mitogenome and for each of the eight putative chromosomes of the Pachycerianthus magnus mitogenome. At 80,923 bp, I. nocturnus now holds the record for the largest animal mitochondrial genome reported to date. The novelty of the mitogenomic structure in Ceriantharia highlights the distinctiveness of this lineage but, because it appears to be both unique to and diverse within Ceriantharia, it is uninformative about the phylogenetic position of Ceriantharia relative to other Anthozoa. The presence of tRNAMet and tRNATrp in both ceriantharian mitogenomes supports a closer relationship between Ceriantharia and Hexacorallia than between Ceriantharia and any other cnidarian lineage, but phylogenetic analysis of the genes contained in the mitogenomes suggests that Ceriantharia is sister to a clade containing Octocorallia + Hexacorallia indicating a possible suppression of tRNATrp in Octocorallia.

Venomix: a simple bioinformatic pipeline for identifying and characterizing toxin gene candidates from transcriptomic data.

The advent of next-generation sequencing has resulted in transcriptome-based approaches to investigate functionally significant biological components in a variety of non-model organism. This has resulted in the area of "venomics": a rapidly growing field using combined transcriptomic and proteomic datasets to characterize toxin diversity in a variety of venomous taxa. Ultimately, the transcriptomic portion of these analyses follows very similar pathways after transcriptome assembly often including candidate toxin identification using BLAST, expression level screening, protein sequence alignment, gene tree reconstruction, and characterization of potential toxin function. Here we describe the Python package Venomix, which streamlines these processes using common bioinformatic tools along with ToxProt, a publicly available annotated database comprised of characterized venom proteins. In this study, we use the Venomix pipeline to characterize candidate venom diversity in four phylogenetically distinct organisms, a cone snail (Conidae; Conus sponsalis), a snake (Viperidae; Echis coloratus), an ant (Formicidae; Tetramorium bicarinatum), and a scorpion (Scorpionidae; Urodacus yaschenkoi). Data on these organisms were sampled from public databases, with each original analysis using different approaches for transcriptome assembly, toxin identification, or gene expression quantification. Venomix recovered numerically more candidate toxin transcripts for three of the four transcriptomes than the original analyses and identified new toxin candidates. In summary, we show that the Venomix package is a useful tool to identify and characterize the diversity of toxin-like transcripts derived from transcriptomic datasets. Venomix is available at: https://bitbucket.org/JasonMacrander/Venomix/.

Transcriptome sequencing and characterization of Symbiodinium muscatinei and Elliptochloris marina, symbionts found within the aggregating sea anemone Anthopleura elegantissima.

There is a growing body of literature using transcriptomic data to study how tropical cnidarians and their photosynthetic endosymbionts respond to environmental stressors and participate in metabolic exchange. Despite these efforts, our understanding of how essential genes function to facilitate symbiosis establishment and maintenance remains limited. The inclusion of taxonomically and ecologically diverse endosymbionts will enhance our understanding of these interactions. Here we characterize the transcriptomes of two very different symbionts found within the temperate sea anemone Anthopleura elegantissima: the chlorophyte Elliptochloris marina and the dinoflagellate Symbiodinium muscatinei. We use a multi-level approach to assess the diversity of genes found across S. muscatinei and E. marina transcriptomes, and compare their overall protein domains with other dinoflagellates and chlorophytes. Our analysis identified several genes that are potentially involved in mitigating stress response (e.g., heat shock proteins pathways for mediating reactive oxygen species) and metabolic exchange (e.g., ion transporters). Finally, we show that S. muscatinei and other Symbiodinium strains are equipped with a high salt peridinin-chl-protein (HSPCP) gene previously identified only in free-living dinoflagellates. The addition of these transcriptomes to the cnidarian-symbiont molecular toolkit will aid in understanding how these vitally important symbiotic relationships are established and maintained across a variety of environmental conditions.

Conservation of DNA and ligand binding properties of retinoid X receptor from the placozoan Trichoplax adhaerens to human.

Nuclear receptors are a superfamily of transcription factors restricted to animals. These transcription factors regulate a wide variety of genes with diverse roles in cellular homeostasis, development, and physiology. The origin and specificity of ligand binding within lineages of nuclear receptors (e.g., subfamilies) continues to be a focus of investigation geared toward understanding how the functions of these proteins were shaped over evolutionary history. Among early-diverging animal lineages, the retinoid X receptor (RXR) is first detected in the placozoan, Trichoplax adhaerens. To gain insight into RXR evolution, we characterized ligand- and DNA-binding activity of the RXR from T. adhaerens (TaRXR). Like bilaterian RXRs, TaRXR specifically bound 9-cis-retinoic acid, which is consistent with a recently published result and supports a conclusion that the ancestral RXR bound ligand. DNA binding site specificity of TaRXR was determined through protein binding microarrays (PBMs) and compared with human RXRɑ. The binding sites for these two RXR proteins were broadly conserved (∼85% shared high-affinity sequences within a targeted array), suggesting evolutionary constraint for the regulation of downstream genes. We searched for predicted binding motifs of the T. adhaerens genome within 1000 bases of annotated genes to identify potential regulatory targets. We identified 648 unique protein coding regions with predicted TaRXR binding sites that had diverse predicted functions, with enriched processes related to intracellular signal transduction and protein transport. Together, our data support hypotheses that the original RXR protein in animals bound a ligand with structural similarity to 9-cis-retinoic acid; the DNA motif recognized by RXR has changed little in more than 1 billion years of evolution; and the suite of processes regulated by this transcription factor diversified early in animal evolution.

Dynamics of venom composition across a complex life cycle.

Little is known about venom in young developmental stages of animals. The appearance of toxins and stinging cells during early embryonic stages in the sea anemone Nematostella vectensis suggests that venom is already expressed in eggs and larvae of this species. Here, we harness transcriptomic, biochemical and transgenic tools to study venom production dynamics in Nematostella. We find that venom composition and arsenal of toxin-producing cells change dramatically between developmental stages of this species. These findings can be explained by the vastly different interspecific interactions of each life stage, as individuals develop from a miniature non-feeding mobile planula to a larger sessile polyp that predates on other animals and interact differently with predators. Indeed, behavioral assays involving prey, predators and Nematostella are consistent with this hypothesis. Further, the results of this work suggest a much wider and dynamic venom landscape than initially appreciated in animals with a complex life cycle.

Structure, folding and stability of a minimal homologue from Anemonia sulcata of the sea anemone potassium channel blocker ShK.

Peptide toxins elaborated by sea anemones target various ion-channel sub-types. Recent transcriptomic studies of sea anemones have identified several novel candidate peptides, some of which have cysteine frameworks identical to those of previously reported sequences. One such peptide is AsK132958, which was identified in a transcriptomic study of Anemonia sulcata and has a cysteine framework similar to that of ShK from Stichodactyla helianthus, but is six amino acid residues shorter. We have determined the solution structure of this novel peptide using NMR spectroscopy. The disulfide connectivities and structural scaffold of AsK132958 are very similar to those of ShK but the structure is more constrained. Toxicity assays were performed using grass shrimp (Palaemonetes sp) and Artemia nauplii, and patch-clamp electrophysiology assays were performed to assess the activity of AsK132958 against a range of voltage-gated potassium (KV) channels. AsK132958 showed no activity against grass shrimp, Artemia nauplii, or any of the KV channels tested, owing partly to the absence of a functional Lys-Tyr dyad. Three AsK132958 analogues, each containing a Tyr in the vicinity of Lys19, were therefore generated in an effort to restore binding, but none showed activity against any of KV channels tested. However, AsK132958 and its analogues are less susceptible to proteolysis than that of ShK. Our structure suggests that Lys19, which might be expected to occupy the pore of the channel, is not sufficiently accessible for binding, and therefore that AsK132958 must have a distinct functional role that does not involve KV channels.

Evolution of the Cytolytic Pore-Forming Proteins (Actinoporins) in Sea Anemones.

Sea anemones (Cnidaria, Anthozoa, and Actiniaria) use toxic peptides to incapacitate and immobilize prey and to deter potential predators. Their toxin arsenal is complex, targeting a variety of functionally important protein complexes and macromolecules involved in cellular homeostasis. Among these, actinoporins are one of the better characterized toxins; these venom proteins form a pore in cellular membranes containing sphingomyelin. We used a combined bioinformatic and phylogenetic approach to investigate how actinoporins have evolved across three superfamilies of sea anemones (Actinioidea, Metridioidea, and Actinostoloidea). Our analysis identified 90 candidate actinoporins across 20 species. We also found clusters of six actinoporin-like genes in five species of sea anemone (Nematostella vectensis, Stomphia coccinea, Epiactis japonica, Heteractis crispa, and Diadumene leucolena); these actinoporin-like sequences resembled actinoporins but have a higher sequence similarity with toxins from fungi, cone snails, and Hydra. Comparative analysis of the candidate actinoporins highlighted variable and conserved regions within actinoporins that may pertain to functional variation. Although multiple residues are involved in initiating sphingomyelin recognition and membrane binding, there is a high rate of replacement for a specific tryptophan with leucine (W112L) and other hydrophobic residues. Residues thought to be involved with oligomerization were variable, while those forming the phosphocholine (POC) binding site and the N-terminal region involved with cell membrane penetration were highly conserved.

Evidence for an Alternative Mechanism of Toxin Production in the Box Jellyfish Alatina alata.

Cubozoans (box jellyfish) have a reputation as the most venomous animals on the planet. Herein, we provide a review of cubozoan prey capture and digestion informed by the scientific literature. Like all cnidarians, box jellyfish envenomation originates from structures secreted within nematocyte post-Golgi vesicles called nematocysts. When tentacles come in contact with prey or would-be predators, a cocktail of toxins is rapidly deployed from nematocysts via a long spiny tubule that serves to immobilize the target organism. The implication has long been that toxin peptides and proteins making up the venom within the nematocyst capsule are secreted directly by nematocytes during nematogenesis. However, our combined molecular and morphological analysis of the venomous box jellyfish Alatina alata suggests that gland cells with possible dual roles in secreting toxins and toxic-like enzymes are found in the gastric cirri. These putative gland cell assemblages might be functionally important internally (digestion of prey) as well as externally (envenomation) in cubozoans. Despite the absence of nematocysts in the gastric cirri of mature A. alata medusae, this area of the digestive system appears to be the region of the body where venom-implicated gene products are found in highest abundance, challenging the idea that in cnidarians venom is synthesized exclusively in, or nearby, nematocysts. In an effort to uncover evidence for a central area enriched in gland cells associated with the gastric cirri we provide a comparative description of the morphology of the digestive structures of A. alata and Carybdea box jellyfish species. Finally, we conduct a multi-faceted analysis of the gene ontology terms associated with venom-implicated genes expressed in the tentacle/pedalium and gastric cirri, with a particular emphasis on zinc metalloprotease homologs and genes encoding other bioactive proteins that are abundant in the A. alata transcriptome.

Tissue-Specific Venom Composition and Differential Gene Expression in Sea Anemones.

Cnidarians represent one of the few groups of venomous animals that lack a centralized venom transmission system. Instead, they are equipped with stinging capsules collectively known as nematocysts. Nematocysts vary in abundance and type across different tissues; however, the venom composition in most species remains unknown. Depending on the tissue type, the venom composition in sea anemones may be vital for predation, defense, or digestion. Using a tissue-specific RNA-seq approach, we characterize the venom assemblage in the tentacles, mesenterial filaments, and column for three species of sea anemone (Anemonia sulcata, Heteractis crispa, and Megalactis griffithsi). These taxa vary with regard to inferred venom potency, symbiont abundance, and nematocyst diversity. We show that there is significant variation in abundance of toxin-like genes across tissues and species. Although the cumulative toxin abundance for the column was consistently the lowest, contributions to the overall toxin assemblage varied considerably among tissues for different toxin types. Our gene ontology (GO) analyses also show sharp contrasts between conserved GO groups emerging from whole transcriptome analysis and tissue-specific expression among GO groups in our differential expression analysis. This study provides a framework for future characterization of tissue-specific venom and other functionally important genes in this lineage of simple bodied animals.