Innate immunity and cnidarian-Symbiodiniaceae mutualism

Transcriptomic and physiological analyses reveal the toxic effects of inorganic filters (nZnO and nTiO2) on scleractinian coral Galaxea fascicularis

The effects of sunscreen on scleractinian corals have garnered widespread attention; however, the toxic effects and mechanisms remain unclear. This study investigated the toxicological effects of two common inorganic filters used in sunscreens, nano zinc oxide and titanium dioxide (nZnO and nTiO₂), on the reef-building coral Galaxea fascicularis, focusing on the phenotypic, physiological, and transcriptomic responses. The results showed that after exposure to 0.8 mg/L of nZnO and 30 mg/L of nTiO₂ for 48 h, all coral polyps exhibited retraction. Zn and Ti ions were detected in coral tissues at concentrations of 67.18 and 24.87 μg/g, respectively, indicating the accumulation of nZnO and nTiO2 in coral tissues. The zooxanthellae density, Fv/Fm, and chlorophyll-a content decreased significantly. The activity of antioxidant enzymes showed an increasing trend. Meanwhile, glutamine synthetase and glutamate dehydrogenase activities exhibited a decreasing trend. The health status of corals was impacted as a result of nZnO and nTiO2 stress. Transcriptomic analysis showed that the toxicity mechanisms of nZnO and nTiO2 differed in corals. Following exposure to nZnO, differentially expressed genes (DEGs) in corals were mainly enriched in signaling pathways related to immune response. The genes related to innate immunity, such as MASP1, MUC5AC, TLRs, and C2, were significantly upregulated, indicating that nZnO exposure induces an innate immune response in corals. Meanwhile, following nTiO2 exposure, the upregulated DEGs were mainly enriched in signaling pathways related to transporter activity. In contrast, the downregulated DEGs were mainly enriched in energy metabolism pathways, indicating that nTiO2 disrupted the energy supply of corals, thereby leading to an increased demand for nutrient transport. This study reveals the toxic effects of nZnO and nTiO2, and their mechanisms of action on scleractinian corals, providing a reference for further assessing the toxicity of sunscreen on corals.

Integrative transcriptomic analysis reveals a broad range of toxic effects of triclosan on coral Porites lutea

Triclosan (TCS) is an antimicrobial agent commonly used in personal care products. However, little is known about its toxicity to corals. Here, we examined the acute toxic effects (96 h) of TCS at different levels to the coral Porites lutea. Results showed that the bioaccumulation factors (BAFs) of TCS in Porites lutea decreased with increasing TCS exposure levels. Exposure to TCS at the level up to 100 μg/L did not induce bleaching of Porites lutea. However, by the end of the experiment, both the density and chlorophyll a content of the symbiotic zooxanthellae were 19-52 % and 19.9-45.6 % lower in the TCS treatment groups than in the control, respectively. For the coral host, its total antioxidant capacity (T-AOC), superoxide dismutase (SOD) and catalase (CAT) activities were all significantly lower in the TCS treatment groups than the control. Transcriptome analysis showed that 942 and 1077 differentially expressed genes (DEGs) were identified in the coral host in the 0.5 and 100 μg/L TCS treatment groups, respectively. Meanwhile, TCS can interfere with pathways related to immune system and reproductive system in coral host. Overall, our results suggest that environmentally relevant concentrations of TCS can impact both the coral host and the symbiotic zooxanthellae.

Saving coral reefs: significance and biotechnological approaches for coral conservation

Coral reefs are highly productive ecosystems that provide valuable services to coastal communities worldwide. However, both local and global anthropogenic stressors, threaten the coral-algal symbiosis that enables reef formation. This breakdown of the symbiotic relationship, known as bleaching, is often triggered by cumulative cell damage. UV and heat stress are commonly implicated in bleaching, but other anthropogenic factors may also play a role. To address coral loss, active restoration is already underway in many critical regions. Additionally, coral researchers are exploring assisted evolution methods for greater coral resilience to projected climate change. This review provides an overview of the symbiotic relationship, the mechanisms underlying coral bleaching in response to stressors, and the strategies being pursued to address coral loss. Despite the necessity of ongoing research in all aspects of this field, action on global climate change remains crucial for the long-term survival of coral reefs.

Photosymbiont Density Is Correlated with Constitutive and Induced Immunity in the Facultatively Symbiotic Coral, Astrangia poculata

Scleractinian corals, essential ecosystem engineers that form the base of coral reef ecosystems, have faced unprecedented mortality in recent decades due to climate change-related stressors, including disease outbreaks. Despite this emergent threat to corals, many questions still remain regarding mechanisms underlying observed variation in disease susceptibility. Recent data suggest at least some degree of variation in disease response may be linked to variability in the relationship between host corals and their algal photosymbionts (Family Symbiodiniaceae). Still, the nuances of connections between symbiosis and immunity in cnidarians, including scleractinian corals, remain poorly understood. Here, we leveraged an emergent model species, the facultatively symbiotic, temperate, scleractinian coral Astrangia poculata, to investigate associations between symbiont density and both constitutive and induced immunity. We used a combination of controlled immune challenges with heat-inactivated pathogens and transcriptomic analyses. Our results demonstrate that A. poculata mounts a robust initial response to pathogenic stimuli that is highly similar to responses documented in tropical corals. We document positive associations between symbiont density and both constitutive and induced immune responses, in agreement with recent preliminary studies in A. poculata. A suite of immune genes, including those coding for antioxidant peroxiredoxin biosynthesis, are positively associated with symbiont density in A. poculata under constitutive conditions. Furthermore, variation in symbiont density is associated with distinct patterns of immune response; low symbiont density corals induce preventative immune mechanisms, whereas high symbiont density corals mobilize energetic resources to fuel humoral immune responses. In summary, our study reveals the need for more nuanced study of symbiosis-immune interplay across diverse scleractinian corals, preferably including quantitative energy budget analysis for full disentanglement of these complex associations and their effects on host pathogen susceptibility.

Prior heat stress increases pathogen susceptibility in the model cnidarian Exaiptasia diaphana

Anthropogenic climate change has significantly altered terrestrial and marine ecosystems globally, often in the form of climate-related events such as thermal anomalies and disease outbreaks. Although the isolated effects of these stressors have been well documented, a growing body of literature suggests that stressors often interact, resulting in complex effects on ecosystems. This includes coral reefs where sequential associations between heat stress and disease have had profound impacts. Here we used the model cnidarian Exaiptasia diaphana to investigate mechanisms linking prior heat stress to increased disease susceptibility. We examined anemone pathogen susceptibility and physiology (symbiosis, immunity, and energetics) following recovery from heat stress. We observed significantly increased pathogen susceptibility in anemones previously exposed to heat stress. Notably, prior heat stress reduced anemone energetic reserves (carbohydrate concentration), and activity of multiple immune components. Minimal effects of prior heat stress on symbiont density were observed. Together, results suggest changes in energetic availability might have the strongest effect on pathogen susceptibility and immunity following heat stress. The results presented here provide critical insight regarding the interplay between heat stress recovery and pathogen susceptibility in cnidarians and are an important first step towards understanding temporal associations between these stressors.

Trade-off between photosymbiosis and innate immunity influences cnidarian's response to pathogenic bacteria

Mutualistic relationships with photosynthetic organisms are common in cnidarians, which form an intracellular symbiosis with dinoflagellates in the family Symbiodiniaceae. The establishment and maintenance of these symbionts are associated with the suppression of key host immune factors. Because of this, there are potential trade-offs between the nutrition that cnidarian hosts gain from their symbionts and their ability to successfully defend themselves from pathogens. To investigate these potential trade-offs, we utilized the facultatively symbiotic polyps of the upside-down jellyfish Cassiopea xamachana and exposed aposymbiotic and symbiotic polyps to the pathogen Serratia marcescens. Symbiotic polyps had a lower probability of survival following S. marcescens exposure. Gene expression analyses 24 hours following pathogen exposure indicate that symbiotic animals mounted a more damaging immune response, with higher levels of inflammation and oxidative stress likely resulting in more severe disruptions to cellular homeostasis. Underlying this more damaging immune response may be differences in constitutive and pathogen-induced expression of immune transcription factors between aposymbiotic and symbiotic polyps rather than broadscale immune suppression during symbiosis. Our findings indicate that in facultatively symbiotic polyps, hosting symbionts limits C. xamachana's ability to survive pathogen exposure, indicating a trade-off between symbiosis and immunity that has potential implications for coral disease research.

Triggers, cascades, and endpoints: connecting the dots of coral bleaching mechanisms

The intracellular coral-dinoflagellate symbiosis is the engine that underpins the success of coral reefs, one of the most diverse ecosystems on the planet. However, the breakdown of the symbiosis and the loss of the microalgal symbiont (i.e. coral bleaching) due to environmental changes are resulting in the rapid degradation of coral reefs globally. There is an urgent need to understand the cellular physiology of coral bleaching at the mechanistic level to help develop solutions to mitigate the coral reef crisis. Here, at an unprecedented scope, we present novel models that integrate putative mechanisms of coral bleaching within a common framework according to the triggers (initiators of bleaching, e.g. heat, cold, light stress, hypoxia, hyposalinity), cascades (cellular pathways, e.g. photoinhibition, unfolded protein response, nitric oxide), and endpoints (mechanisms of symbiont loss, e.g. apoptosis, necrosis, exocytosis/vomocytosis). The models are supported by direct evidence from cnidarian systems, and indirectly through comparative evolutionary analyses from non-cnidarian systems. With this approach, new putative mechanisms have been established within and between cascades initiated by different bleaching triggers. In particular, the models provide new insights into the poorly understood connections between bleaching cascades and endpoints and highlight the role of a new mechanism of symbiont loss, i.e. 'symbiolysosomal digestion', which is different from symbiophagy. This review also increases the approachability of bleaching physiology for specialists and non-specialists by mapping the vast landscape of bleaching mechanisms in an atlas of comprehensible and detailed mechanistic models. We then discuss major knowledge gaps and how future research may improve the understanding of the connections between the diverse cascade of cellular pathways and the mechanisms of symbiont loss (endpoints).

Photosynthesis and other factors affecting the establishment and maintenance of cnidarian-dinoflagellate symbiosis

Coral growth depends on the partnership between the animal hosts and their intracellular, photosynthetic dinoflagellate symbionts. In this study, we used the sea anemone Aiptasia, a laboratory model for coral biology, to investigate the poorly understood mechanisms that mediate symbiosis establishment and maintenance. We found that initial colonization of both adult polyps and larvae by a compatible algal strain was more effective when the algae were able to photosynthesize and that the long-term maintenance of the symbiosis also depended on photosynthesis. In the dark, algal cells were taken up into host gastrodermal cells and not rapidly expelled, but they seemed unable to reproduce and thus were gradually lost. When we used confocal microscopy to examine the interaction of larvae with two algal strains that cannot establish stable symbioses with Aiptasia, it appeared that both pre- and post-phagocytosis mechanisms were involved. With one strain, algae entered the gastric cavity but appeared to be completely excluded from the gastrodermal cells. With the other strain, small numbers of algae entered the gastrodermal cells but appeared unable to proliferate there and were slowly lost upon further incubation. We also asked if the exclusion of either incompatible strain could result simply from their cells' being too large for the host cells to accommodate. However, the size distributions of the compatible and incompatible strains overlapped extensively. Moreover, examination of macerates confirmed earlier reports that individual gastrodermal cells could expand to accommodate multiple algal cells. This article is part of the theme issue 'Sculpting the microbiome: how host factors determine and respond to microbial colonization'.

Tracking the early events of photosymbiosis evolution

Oxygenic photosynthesis evolved in cyanobacteria around 3.2 giga-annum (Ga) ago and was acquired by eukaryotes starting around 1.8 Ga ago by endosymbiosis. Photosymbiosis results either from integration of a photosynthetic bacteria by heterotrophic eukaryotes (primary photosymbiosis) or by successive integration of photosymbiotic eukaryotes by heterotrophic eukaryotes (secondary photosymbiosis). Primary endosymbiosis is thought to have been a rare event, whereas secondary and higher-order photosymbiosis evolved multiple times independently in different taxa. Despite its recurrent evolution, the molecular and cellular mechanisms underlying photosymbiosis are unknown. In this opinion, we discuss the primary events leading to the establishment of photosymbiosis, and we present recent research suggesting that, in some cases, domestication occurred instead of symbiosis, and how oxygen and host immunity can be involved in symbiont maintenance.

Genes possibly related to symbiosis in early life stages of Acropora tenuis inoculated with Symbiodinium microadriaticum

Due to the ecological importance of mutualism between reef-building corals and symbiotic algae (Family Symbiodiniaceae), various transcriptomic studies on coral-algal symbiosis have been performed; however, molecular mechanisms, especially genes essential to initiate and maintain these symbioses remain unknown. We investigated transcriptomic responses of Acropora tenuis to inoculation with the native algal symbiont, Symbiodinium microadriaticum, during early life stages, and identified possible symbiosis-related genes. Genes involved in immune regulation, protection against oxidative stress, and metabolic interactions between partners are particularly important for symbiosis during Acropora early life stages. In addition, molecular phylogenetic analysis revealed that some possible symbiosis-related genes originated by gene duplication in the Acropora lineage, suggesting that gene duplication may have been the driving force to establish stable mutualism in Acropora, and that symbiotic molecular mechanisms may vary among coral lineages.

Role of the bicarbonate transporter SLC4γ in stony-coral skeleton formation and evolution

Coral reefs are highly diverse ecosystems of immense ecological, economic, and aesthetic importance built on the calcium-carbonate-based skeletons of stony corals. The formation of these skeletons is threatened by increasing ocean temperatures and acidification, and a deeper understanding of the molecular mechanisms involved may assist efforts to mitigate the effects of such anthropogenic stressors. In this study, we focused on the role of the predicted bicarbonate transporter SLC4γ, which was suggested in previous studies to be a product of gene duplication and to have a role in coral-skeleton formation. Our comparative-genomics study using 30 coral species and 15 outgroups indicates that SLC4γ is present throughout the stony corals, but not in their non-skeleton-forming relatives, and apparently arose by gene duplication at the onset of stony-coral evolution. Our expression studies show that SLC4γ, but not the closely related and apparently ancestral SLC4β, is highly upregulated during coral development coincident with the onset of skeleton deposition. Moreover, we show that juvenile coral polyps carrying CRISPR/Cas9-induced mutations in SLC4γ are defective in skeleton formation, with the severity of the defect in individual animals correlated with their frequencies of SLC4γ mutations. Taken together, the results suggest that the evolution of the stony corals involved the neofunctionalization of the newly arisen SLC4γ for a unique role in the provision of concentrated bicarbonate for calcium-carbonate deposition. The results also demonstrate the feasibility of reverse-genetic studies of ecologically important traits in adult corals.

Pervasive tandem duplications and convergent evolution shape coral genomes

BACKGROUND

Over the last decade, several coral genomes have been sequenced allowing a better understanding of these symbiotic organisms threatened by climate change. Scleractinian corals are reef builders and are central to coral reef ecosystems, providing habitat to a great diversity of species.

RESULTS

In the frame of the Tara Pacific expedition, we assemble two coral genomes, Porites lobata and Pocillopora cf. effusa, with vastly improved contiguity that allows us to study the functional organization of these genomes. We annotate their gene catalog and report a relatively higher gene number than that found in other public coral genome sequences, 43,000 and 32,000 genes, respectively. This finding is explained by a high number of tandemly duplicated genes, accounting for almost a third of the predicted genes. We show that these duplicated genes originate from multiple and distinct duplication events throughout the coral lineage. They contribute to the amplification of gene families, mostly related to the immune system and disease resistance, which we suggest to be functionally linked to coral host resilience.

CONCLUSIONS

At large, we show the importance of duplicated genes to inform the biology of reef-building corals and provide novel avenues to understand and screen for differences in stress resilience.

Transfer of knowledge from model organisms to evolutionarily distant non-model organisms: The coral Pocillopora damicornis membrane signaling receptome

With the ease of gene sequencing and the technology available to study and manipulate non-model organisms, the extension of the methodological toolbox required to translate our understanding of model organisms to non-model organisms has become an urgent problem. For example, mining of large coral and their symbiont sequence data is a challenge, but also provides an opportunity for understanding functionality and evolution of these and other non-model organisms. Much more information than for any other eukaryotic species is available for humans, especially related to signal transduction and diseases. However, the coral cnidarian host and human have diverged over 700 million years ago and homologies between proteins in the two species are therefore often in the gray zone, or at least often undetectable with traditional BLAST searches. We introduce a two-stage approach to identifying putative coral homologues of human proteins. First, through remote homology detection using Hidden Markov Models, we identify candidate human homologues in the cnidarian genome. However, for many proteins, the human genome alone contains multiple family members with similar or even more divergence in sequence. In the second stage, therefore, we filter the remote homology results based on the functional and structural plausibility of each coral candidate, shortlisting the coral proteins likely to have conserved some of the functions of the human proteins. We demonstrate our approach with a pipeline for mapping membrane receptors in humans to membrane receptors in corals, with specific focus on the stony coral, P. damicornis. More than 1000 human membrane receptors mapped to 335 coral receptors, including 151 G protein coupled receptors (GPCRs). To validate specific sub-families, we chose opsin proteins, representative GPCRs that confer light sensitivity, and Toll-like receptors, representative non-GPCRs, which function in the immune response, and their ability to communicate with microorganisms. Through detailed structure-function analysis of their ligand-binding pockets and downstream signaling cascades, we selected those candidate remote homologues likely to carry out related functions in the corals. This pipeline may prove generally useful for other non-model organisms, such as to support the growing field of synthetic biology.

Help Me, Symbionts, You're My Only Hope: Approaches to Accelerate our Understanding of Coral Holobiont Interactions

Tropical corals construct the three-dimensional framework for one of the most diverse ecosystems on the planet, providing habitat to a plethora of species across taxa. However, these ecosystem engineers are facing unprecedented challenges, such as increasing disease prevalence and marine heatwaves associated with anthropogenic global change. As a result, major declines in coral cover and health are being observed across the world's oceans, often due to the breakdown of coral-associated symbioses. Here, we review the interactions between the major symbiotic partners of the coral holobiont-the cnidarian host, algae in the family Symbiodiniaceae, and the microbiome-that influence trait variation, including the molecular mechanisms that underlie symbiosis and the resulting physiological benefits of different microbial partnerships. In doing so, we highlight the current framework for the formation and maintenance of cnidarian-Symbiodiniaceae symbiosis, and the role that immunity pathways play in this relationship. We emphasize that understanding these complex interactions is challenging when you consider the vast genetic variation of the cnidarian host and algal symbiont, as well as their highly diverse microbiome, which is also an important player in coral holobiont health. Given the complex interactions between and among symbiotic partners, we propose several research directions and approaches focused on symbiosis model systems and emerging technologies that will broaden our understanding of how these partner interactions may facilitate the prediction of coral holobiont phenotype, especially under rapid environmental change.

Heat stress increases immune cell function in Hexacorallia

Climate change induced heat stress has increased coral bleaching events worldwide. Differentially regulated immune genes are one of the primary responses to heat stress suggesting that immune activation is critical. However, the cellular immune mechanisms of coral bleaching is currently unknown, and it is still not known if the immune response documented during heat stress is a consequence of bleaching or is directly caused by the heat stress itself. To address this question, we have used two model system sea anemones (Order: Actiniaria): Exaiptasia diaphana and Nematostella vectensis. E. diaphana is an established sea anemone model for algal symbiont interaction, while N. vectensis is an established sea anemone model that lacks the algal symbiont. Here, we examined the effect of increased temperature on phagocytic activity, as an indication of immune function. Our data shows that immune cell activity increases during heat stress, while small molecule pinocytosis remains unaffected. We observed an increase in cellular production of reactive oxygen species with increasing temperatures. We also found that the cellular immune activity was not affected by the presence of the Symbiodiniaceae. Our results suggest that the immune activity observed in heat-stress induced bleaching in corals is a fundamental and basic response independent of the bleaching effect. These results establish a foundation for improving our understanding of hexacorallian immune cell biology, and its potential role in coral bleaching.

Variation in symbiont density is linked to changes in constitutive immunity in the facultatively symbiotic coral, Astrangia poculata

Scleractinian corals are essential ecosystem engineers, forming the basis of coral reef ecosystems. However, these organisms are in decline globally, in part due to rising disease prevalence. Most corals are dependent on symbiotic interactions with single-celled algae from the family Symbiodiniaceae to meet their nutritional needs, however, suppression of host immunity may be essential to this relationship. To explore immunological consequences of algal symbioses in scleractinian corals, we investigated constitutive immune activity in the facultatively symbiotic coral, Astrangia poculata. We compared immune metrics (melanin synthesis, antioxidant production and antibacterial activity) between coral colonies of varying symbiont density. Symbiont density was positively correlated to both antioxidant activity and melanin concentration, likely as a result of the dual roles of these pathways in immunity and symbiosis regulation. Our results confirm the complex nature of relationships between algal symbiosis and host immunity and highlight the need for nuanced approaches when considering these relationships.

Indications of symbiotic state influencing melanin-synthesis immune response in the facultative coral Astrangia poculata

Increased ocean warming is causing detrimental impacts to tropical corals worldwide. Compounding the effects of heat stress, incidences of tropical coral disease have risen concurrently. While tropical coral responses to these impacts are well studied, temperate coral responses remain largely unknown. The present study focused on the immune response of the temperate coral Astrangia poculata to increased temperature and disease. Symbiotic and aposymbiotic A. poculata were collected from Narragansett Bay, Rhode Island (USA) in summer and winter seasons and exposed to control (18°C) versus elevated temperatures (26°C) in the presence of an immune stimulant (i.e. lipopolysaccharide) for a 12 h period. Prophenoloxidase (PPO) and melanin concentrations from the melanin-synthesis pathway were assessed via spectrophotometry to examine immune responses. While PPO measurements were higher on average in symbiotic corals compared with aposymbiotic corals, temperature and season did not significantly affect this metric. Melanin was significantly higher in symbiotic compared to aposymbiotic corals, implying that symbiotic state may be important for melanin-synthesis response. Conversely, melanin as an immune response may be of less importance in aposymbiotic A. poculata due to the potential capacity of other immune responses in this species. In addition, differences in resource allocation to immune investment as a result of symbiosis is plausible given melanin production observed within the present study. However, thermal stressors may reduce the overall influence of symbiosis on melanin production. Future studies should build upon these results to further understand the entirety of innate immunity responses in temperate coral species.

Genetic structure of dinoflagellate symbionts in coral recruits differs from that of parental or local adults

The symbiotic relationship between dinoflagellate algae in the family Symbiodiniaceae and scleractinian corals forms the base of the tropical reef ecosystem. In scleractinian corals, recruits acquire symbionts either "vertically" from the maternal colony or initially lack symbionts and acquire them "horizontally" from the environment. Regardless of the mode of acquisition, coral species and individual colonies harbor only a subset of the highly diverse complex of species/taxa within the Symbiodiniaceae. This suggests a genetic basis for specificity, but local environmental conditions and/or symbiont availability may also play a role in determining which symbionts within the Symbiodiniaceae are initially taken up by the host. To address the relative importance of genetic and environmental drivers of symbiont uptake/establishment, we examined the acquisition of these dinoflagellate symbionts in one to three-month-old recruits of Orbicella faveolata to compare symbiont types present in recruits to those of parental populations versus co-occurring adults in their destination reef. Variation in chloroplast 23S ribosomal DNA and in three polymorphic microsatellite loci was examined. We found that, in general, symbiont communities within adult colonies differed between reefs, suggesting that endemism is common among symbiont populations of O. faveolata on a local scale. Among recruits, initial symbiont acquisition was selective. O. faveolata recruits only acquired a subset of locally available symbionts, and these generally did not reflect symbiont populations in adults at either the parental or the outplant reef. Instead, symbiont communities within new recruits at a given outplant site and region tended to be similar to each other, regardless of parental source population. These results suggest temporal variation in the local symbiont source pool, although other possible drivers behind the distinct difference between symbionts within O. faveolata adults and new generations of recruits may include different ontogenetic requirements and/or reduced host selectivity in early ontogeny.

Cryopreservation of six Symbiodiniaceae genera and assessment of fatty acid profiles in response to increased salinity treatments

Symbiodiniaceae are a diverse group of dinoflagellates, the majority of which are free-living and/or associated with a variety of protists and other invertebrate hosts. Maintenance of isolated cultures is labour-intensive and expensive, and cryopreservation provides an excellent avenue for their long-term storage. We aimed to cryopreserve 15 cultured isolates from six Symbiodiniaceae genera using dimethyl sulfoxide (DMSO) as the cryoprotectant agent (CPA). Under 15% DMSO, 10 isolates were successfully cryopreserved using either rapid freezing or controlled-rate freezing. Cultures that failed or had low survival, were subjected to (1) a reduction of CPA to 10%, or (2) increased salinity treatment before freezing. At 10% DMSO, three further isolates were successfully cryopreserved. At 15% DMSO there were high cell viabilities in Symbiodinium pilosum treated with 44 parts per thousand (ppt) and 54 ppt culture medium. An isolate of Fugacium sp. successfully cryopreserved after salinity treatments of 54 ppt and 64 ppt. Fatty acid (FA) analyses of S. pilosum after 54 ppt salinity treatment showed increased saturated FA levels, whereas Fugacium sp. had low poly-unsaturated FAs compared to normal salinity (34 ppt). Understanding the effects of salinity and roles of FAs in cryopreservation will help in developing protocols for these ecologically important taxa.

Adaptive Variation in Homolog Number Within Transcript Families Promotes Expression Divergence in Reef-Building Coral

Gene expression, especially in multi-species experiments, is used to gain insight into the genetic basis of how organisms adapt and respond to changing environments. However, evolutionary processes which can influence gene expression patterns between species such as the presence of paralogs which arise from gene duplication events are rarely accounted for. Paralogous transcripts can alter the transcriptional output of a gene and thus exclusion of these transcripts can obscure important biological differences between species. To address this issue, we investigated how differences in transcript family size is associated with divergent gene expression patterns in five species of Caribbean reef-building corals. We demonstrate that transcript families that are rapidly evolving in terms of size have increased levels of expression divergence. Additionally, these rapidly evolving transcript families are enriched for multiple biological processes, with genes involved in the coral innate immune system demonstrating pronounced variation in homolog number between species. Overall, this investigation demonstrates the importance of incorporating paralogous transcripts when comparing gene expression across species by influencing both transcriptional output and the number of transcripts within biological processes. As this investigation was based on transcriptome assemblies, additional insights into the relationship between gene duplications and expression patterns will likely emergence once more genome assemblies are available for study.

Implications of bleaching on cnidarian venom ecology

Cnidarian bleaching research often focuses on the effects on a cnidarian's physiological health and fitness, whilst little focus has been towards the impacts of these events on their venom ecology. Given the importance of a cnidarian's venom to their survival and the increasing threat of bleaching events, it is important to understand the effects that this threat may have on this important aspect of their ecology as it may have unforeseen impacts on their ability to catch prey and defend themselves. This review aims to explore evidence that suggests that bleaching may impact on each of the key aspects of a cnidarians' venom ecology: cnidae, venom composition, and venom toxicity. Additionally, the resulting energy deficit, compensatory heterotrophic feeding, and increased defensive measures have been highlighted as possible ecological factors driving these changes. Suggestions are also made to guide the success of research in this field into the future, specifically in regards to selecting a study organism, the importance of accurate symbiont and cnidae identification, use of appropriate bleaching methods, determination of bleaching, and animal handling. Ultimately, this review highlights a significant and important gap in our knowledge into how cnidarians are, and will, continue to be impacted by bleaching stress.

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Information on the effects of bleaching on cnidarian venom ecology is limited.

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There is evidence to suggest nematocysts, venom composition and venom toxicity may each be impacted by bleaching.

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Bleaching may result in depleted energy, increased heterotrophy and/or the need for stronger defensive strategies.

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To fully understand how cnidarians may be impacted by bleaching stress further research in this field is needed.

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Future studies should consider the model organism and methodologies, thereby minimising indirect confounding effects.

Larval transcriptomic responses of a stony coral, Acropora tenuis, during initial contact with the native symbiont, Symbiodinium microadriaticum

Although numerous dinoflagellate species (Family Symbiodiniaceae) are present in coral reef environments, Acropora corals tend to select a single species, Symbiodinium microadriaticum, in early life stages, even though this species is rarely found in mature colonies. In order to identify molecular mechanisms involved in initial contact with native symbionts, we analyzed transcriptomic responses of Acropora tenuis larvae at 1, 3, 6, 12, and 24 h after their first contact with S. microadriaticum, as well as with non-native symbionts, including the non-symbiotic S. natans and the occasional symbiont, S. tridacnidorum. Some gene expression changes were detected in larvae inoculated with non-native symbionts at 1 h post-inoculation, but those returned to baseline levels afterward. In contrast, when larvae were exposed to native symbionts, we found that the number of differentially expressed genes gradually increased in relation to inoculation time. As a specific response to native symbionts, upregulation of pattern recognition receptor-like and transporter genes, and suppression of cellular function genes related to immunity and apoptosis, were exclusively observed. These findings indicate that coral larvae recognize differences between symbionts, and when the appropriate symbionts infect, they coordinate gene expression to establish stable mutualism.

Decreased Photosynthetic Efficiency in Response to Site Translocation and Elevated Temperature Is Mitigated with LPS Exposure in Porites astreoides Symbionts

Coral reefs have been detrimentally impacted causing health issues due to elevated ocean temperatures as a result of increased greenhouse gases. Extreme temperatures have also exacerbated coral diseases in tropical reef environments. Numerous studies have outlined the impacts of thermal stress and disease on coral organisms, as well as understanding the influence of site-based characteristics on coral physiology. However, few have discussed the interaction of all three. Laboratory out-planting restoration projects have been of importance throughout impacted areas such as the Caribbean and southern Florida in order to increase coral cover in these areas. This study analyzes photosynthetic efficiency of Porites astreoides from the lower Florida Keys after a two-year reciprocal transplant study at inshore (Birthday reef) and offshore (Acer24 reef) sites to understand acclimation capacity of this species. Laboratory experiments subjected these colonies to one of three treatments: control conditions, increases in temperature, and increases in temperature plus exposure to an immune stimulant (lipopolysaccharide (LPS)) to determine their influence on photosynthetic efficiency and how stress events impact these measurements. In addition, this study is a continuation of previous studies from this group. Here, we aim to understand if these results are static or if an acclimation capacity could be found. Overall, we observed site-specific influences from the Acer24 reef site, which had significant decreases in photosynthetic efficiencies in 32 °C treatments compared to Birthday reef colonies. We suggest that high irradiance and lack of an annual recovery period from the Acer24 site exposes these colonies to significant photoinhibition. In addition, we observed significant increases in photosynthetic efficiencies from LPS exposure. We suggest host-derived antioxidants can mitigate the negative impacts of increased thermal stress. Further research is required to understand the full complexity of host immunity and symbiont photosynthetic interactions.

Why and how do protective symbionts impact immune priming with pathogens in invertebrates?

Growing evidence demonstrates that invertebrates display adaptive-like immune abilities, commonly known as "immune priming". Immune priming is a process by which a host improves its immune defences following an initial pathogenic exposure, leading to better protection after a subsequent infection with the same - or different - pathogens. Nevertheless, beneficial symbionts can enhance similar immune priming processes in hosts, such as when they face repeated infections with pathogens. This "symbiotic immune priming" protects the host against pathogenic viruses, bacteria, fungi, or eukaryotic parasites. In this review, we explore the extent to which protective symbionts interfere and impact immune priming against pathogens from both a mechanical (proximal) and an evolutionary (ultimate) point of view. We highlight that the immune priming of invertebrates is the cornerstone of the tripartite interaction of hosts/symbionts/pathogens. The main shared mechanism of immune priming (induced by symbionts or pathogens) is the sustained immune response at the beginning of host-microbial interactions. However, the evolutionary outcome of immune priming leads to a specific discrimination, which provides enhanced tolerance or resistance depending on the type of microbe. Based on several studies testing immune priming against pathogens in the presence or absence of protective symbionts, we observed that both types of immune priming could overlap and affect each other inside the same hosts. As protective symbionts could be an evolutionary force that influences immune priming, they may help us to better understand the heterogeneity of pathogenic immune priming across invertebrate populations and species.

Comparison of NF-κB from the protists Capsaspora owczarzaki and Acanthoeca spectabilis reveals extensive evolutionary diversification of this transcription factor

We provide a functional characterization of transcription factor NF-κB in protists and provide information about the evolution and diversification of this biologically important protein. We characterized NF-κB in two protists using phylogenetic, cellular, and biochemical techniques. NF-κB of the holozoan Capsaspora owczarzaki (Co) has an N-terminal DNA-binding domain and a C-terminal Ankyrin repeat (ANK) domain, and its DNA-binding specificity is more similar to metazoan NF-κB proteins than to Rel proteins. Removal of the ANK domain allows Co-NF-κB to enter the nucleus, bind DNA, and activate transcription. However, C-terminal processing of Co-NF-κB is not induced by IκB kinases in human cells. Overexpressed Co-NF-κB localizes to the cytoplasm in Co cells. Co-NF-κB mRNA and DNA-binding levels differ across three Capsaspora life stages. RNA-sequencing and GO analyses identify possible gene targets of Co-NF-κB. Three NF-κB-like proteins from the choanoflagellate Acanthoeca spectabilis (As) contain conserved Rel Homology domain sequences, but lack C-terminal ANK repeats. All three As-NF-κB proteins constitutively enter the nucleus of cells, but differ in their DNA-binding abilities, transcriptional activation activities, and dimerization properties. These results provide a basis for understanding the evolutionary origins of this key transcription factor and could have implications for the origins of regulated immunity in higher taxa.

Will coral reefs survive by adaptive bleaching?

Some reef-building corals form symbioses with multiple algal partners that differ in ecologically important traits like heat tolerance. Coral bleaching and recovery can drive symbiont community turnover toward more heat-tolerant partners, and this 'adaptive bleaching' response can increase future bleaching thresholds by 1-2°C, aiding survival in warming oceans. However, this mechanism of rapid acclimatization only occurs in corals that are compatible with multiple symbionts, and only when the disturbance regime and competitive dynamics among symbionts are sufficient to bring about community turnover. The full scope of coral taxa and ecological scenarios in which symbiont shuffling occurs remains poorly understood, though its prevalence is likely to increase as warming oceans boost the competitive advantage of heat-tolerant symbionts, increase the frequency of bleaching events, and strengthen metacommunity feedbacks. Still, the constraints, limitations, and potential tradeoffs of symbiont shuffling suggest it will not save coral reef ecosystems; however, it may significantly improve the survival trajectories of some, or perhaps many, coral species. Interventions to manipulate coral symbionts and symbiont communities may expand the scope of their adaptive potential, which may boost coral survival until climate change is addressed.

microRNAs facilitate comprehensive responses of Bathymodiolinae mussel against symbiotic and nonsymbiotic bacteria stimulation

Bathymodiolinae mussels are dominant species in cold seeps and hydrothermal vents and could harbor endosymbionts in gill bacteriocytes. However, mechanisms underlying the symbiosis have remained largely undisclosed for years. In the present study, the global expression pattern of immune-related genes and miRNAs were surveyed in Gigantidas platifrons during bacterial challenges using enriched symbiotic methane oxidation bacteria MOBs or nonsymbiotic Vibrio. As a result, multiple pattern recognition receptors were found differentially expressed at 12 h and 24 h post bacteria challenges and distinctly clustered between stimulations. Dozens of immune effectors along with signal transducers were also modulated simultaneously during MOB or Vibrio challenge. A total of 459 miRNAs were identified in the gill while some were differentially expressed post MOB or nonsymbiotic bacteria challenge. A variety of immune-related genes were annotated as target genes of aforesaid differentially expressed miRNAs. As a result, biological processes including the immune recognition, lysosome activity and bacteria engulfment were suggested to be dynamically modulated by miRNAs in either symbiotic or nonsymbiotic bacteria challenge. It was suggested that G. platifrons mussels could maintain a robust immune response against invading pathogens while establishing symbiosis with chemosynthetic bacteria with the orchestra of immune-related genes and miRNAs.

Transgenerational Effects on the Coral Pocillopora damicornis Microbiome Under Ocean Acidification

Reef-building corals are inhabited by functionally diverse microorganisms which play important roles in coral health and persistence in the Anthropocene. However, our understanding of the complex associations within coral holobionts is largely limited, particularly transgenerational exposure to environmental stress, like ocean acidification. Here we investigated the microbiome development of an ecologically important coral Pocillopora damicornis following transgenerational exposure to moderate and high pCO₂ (partial pressure of CO₂) levels, using amplicon sequencing and analysis. Our results showed that the Symbiodiniaceae community structures in adult and juvenile had similar patterns, all of which were dominated by Durusdinium spp., previously known as clade D. Conversely, prokaryotic communities varied between adults and juveniles, possibly driven by the effect of host development. Surprisingly, there were no significant changes in both Symbiodiniaceae and prokaryotic communities with different pCO₂ treatments, which was independent of the life history stage. This study shows that ocean acidification has no significant effect on P. damicornis microbiome, and warrants further research to test whether transgenerational acclimation exists in coral holobiont to projected future climate change.

Cnidarian Pattern Recognition Receptor Repertoires Reflect Both Phylogeny and Life History Traits

Pattern recognition receptors (PRRs) are evolutionarily ancient and crucial components of innate immunity, recognizing danger-associated molecular patterns (DAMPs) and activating host defenses. Basal non-bilaterian animals such as cnidarians must rely solely on innate immunity to defend themselves from pathogens. By investigating cnidarian PRR repertoires we can gain insight into the evolution of innate immunity in these basal animals. Here we utilize the increasing amount of available genomic resources within Cnidaria to survey the PRR repertoires and downstream immune pathway completeness within 15 cnidarian species spanning two major cnidarian clades, Anthozoa and Medusozoa. Overall, we find that anthozoans possess prototypical PRRs, while medusozoans appear to lack these immune proteins. Additionally, anthozoans consistently had higher numbers of PRRs across all four classes relative to medusozoans, a trend largely driven by expansions in NOD-like receptors and C-type lectins. Symbiotic, sessile, and colonial cnidarians also have expanded PRR repertoires relative to their non-symbiotic, mobile, and solitary counterparts. Interestingly, cnidarians seem to lack key components of mammalian innate immune pathways, though similar to PRR numbers, anthozoans possess more complete immune pathways than medusozoans. Together, our data indicate that anthozoans have greater immune specificity than medusozoans, which we hypothesize to be due to life history traits common within Anthozoa. Overall, this investigation reveals important insights into the evolution of innate immune proteins within these basal animals.

Dinoflagellate symbionts escape vomocytosis by host cell immune suppression

Alveolata comprises diverse taxa of single-celled eukaryotes, many of which are renowned for their ability to live inside animal cells. Notable examples are apicomplexan parasites and dinoflagellate symbionts, the latter of which power coral reef ecosystems. Although functionally distinct, they evolved from a common, free-living ancestor and must evade their host's immune response for persistence. Both the initial cellular events that gave rise to this intracellular lifestyle and the role of host immune modulation in coral-dinoflagellate endosymbiosis are poorly understood. Here, we use a comparative approach in the cnidarian endosymbiosis model Aiptasia, which re-establishes endosymbiosis with free-living dinoflagellates every generation. We find that uptake of microalgae is largely indiscriminate, but non-symbiotic microalgae are expelled by vomocytosis, while symbionts induce host cell innate immune suppression and form a lysosomal-associated membrane protein 1-positive niche. We demonstrate that exogenous immune stimulation results in symbiont expulsion and, conversely, inhibition of canonical Toll-like receptor signalling enhances infection of host animals. Our findings indicate that symbiosis establishment is dictated by local innate immune suppression, to circumvent expulsion and promote niche formation. This work provides insight into the evolution of the cellular immune response and key steps involved in mediating endosymbiotic interactions.

Physiological processes and lipidome dynamics in the soft coral Sinularia heterospiculata under experimental bleaching

Coral polyps host intracellular symbiotic dinoflagellates (SD). The loss of SD (referred as bleaching) under stressful environmental conditions is the main reason of coral reef destruction, and therefore, intensively studied over the world. Lipids are the structural base of biomembranes and energy reserve of corals and are directly involved in the coral bleaching. In order to establish a relationship between coral tissue morphology, physiological processes and lipidome dynamics during bleaching, the soft coral Sinularia heterospiculata was exposed to experimental heat stress (33 °C) for 72 h. A chlorophyll content, structure of cells, the level of reactive oxygen species (ROS), and molecular species of storage and structural lipids were analyzed. After 24 h of heat exposure, the level of ROS-positive SD cells did not increase, but the host tissues lost a significant part of SD. The removal of SD cells by exocytosis were suggested. Exocytosis was presumed to prevail at earlier stages of the soft coral bleaching. Symbiophagosomes with degenerative SD were observed in the stressed coral host cells. After 24 h, the content of phosphatidylinositols, which involved in apoptosis and autophagy, was significantly decreased. The innate immune response was triggered, and SD were digested by the coral host. After 48 h, a degradation of SD chloroplasts and a decrease in the specific monogalactosyldiacylglycerol molecular species were detected that confirmed a disruption of lipid biosynthesis in chloroplasts. At the end of coral bleaching, the appearance of oxidized phosphatidylethanolamines, indicating damage to the host membranes, and the degradation of the coral tissues were simultaneously observed. Thus, a switch between dominant mechanisms of the SD loss during bleaching of S. heterospiculata was found and proved by certain variations of the lipidomic profile. Lipidomic parameters may become indicators of physiological processes occurring in the symbiotic coral organism and may be used for assessing anthropogenic or natural destructive effects on coral reefs.

Limitations of Using Cultured Algae to Study Cnidarian-Algal Symbioses and Suggestions for Future Studies

Much of our understanding of the cellular mechanisms underlying cnidarian-algal symbiosis comes from studying the biological differences between the partners when they are engaged in symbiosis and when they are isolated from one another. When comparing the in hospite and ex hospite states in Symbiodiniaceae, the in hospite state is represented by algae sampled from hosts, and the ex hospite state is commonly represented by cultured algae. The use of cultured algae in this comparison may introduce nutrition as a confounding variable because, while hosts are kept in nutrient-depleted conditions, culture media is nutrient rich and designed to facilitate algal growth. In this perspective, we reexamine how nutrition may be a confounding variable in studies that compare the biology of Symbiodiniaceae in hospite and in culture. We also suggest several innovations in experimental design to strengthen the comparison of the two lifestyles, including the adoption of nutritional controls, alternatives to culture for the representation of Symbiodiniaceae ex hospite, and the adoption of several proteomic approaches to find novel Symbiodiniaceae genes important for symbiosis.

Insights into coral bleaching under heat stress from analysis of gene expression in a sea anemone model system

Loss of endosymbiotic algae ("bleaching") under heat stress has become a major problem for reef-building corals worldwide. To identify genes that might be involved in triggering or executing bleaching, or in protecting corals from it, we used RNAseq to analyze gene-expression changes during heat stress in a coral relative, the sea anemone Aiptasia. We identified >500 genes that showed rapid and extensive up-regulation upon temperature increase. These genes fell into two clusters. In both clusters, most genes showed similar expression patterns in symbiotic and aposymbiotic anemones, suggesting that this early stress response is largely independent of the symbiosis. Cluster I was highly enriched for genes involved in innate immunity and apoptosis, and most transcript levels returned to baseline many hours before bleaching was first detected, raising doubts about their possible roles in this process. Cluster II was highly enriched for genes involved in protein folding, and most transcript levels returned more slowly to baseline, so that roles in either promoting or preventing bleaching seem plausible. Many of the genes in clusters I and II appear to be targets of the transcription factors NFκB and HSF1, respectively. We also examined the behavior of 337 genes whose much higher levels of expression in symbiotic than aposymbiotic anemones in the absence of stress suggest that they are important for the symbiosis. Unexpectedly, in many cases, these expression levels declined precipitously long before bleaching itself was evident, suggesting that loss of expression of symbiosis-supporting genes may be involved in triggering bleaching.

Gene expression kinetics of Exaiptasia pallida innate immune response to Vibrio parahaemolyticus infection

BACKGROUND

Recent sequencing projects on early-diverging metazoans such as cnidarians, have unveiled a rich innate immunity gene repertoire; however, little is known about immunity gene regulation in the host's early response against marine bacterial pathogens over time. Here, we used RNA-seq on the sea anemone Exaiptasia pallida (Ep) strain CC7 as a model to depict the innate immune response during the onset of infection with the marine pathogenic bacteria Vibrio parahaemolyticus (Vp) clinical strain O3:K6, and lipopolysaccharides (LPS) exposure. Pairwise and time series analyses identified the genes responsive to infection as well as the kinetics of innate immune genes over time. Comparisons between the responses to live Vp and purified LPS was then performed.

RESULTS

Gene expression and functional analyses detected hundreds to thousands of genes responsive to the Vp infection after 1, 3, 6 and 12 h, including a few shared with the response to LPS. Our results bring to light the first indications that non-canonical cytoplasmic pattern recognition receptors (PRRs) such as NOD-like and RIG-I-like receptor homologs take part in the immune response of Ep. Over-expression of several members of the lectin-complement pathways in parallel with novel transmembrane and Ig containing ficolins (CniFLs) suggest an active defense against the pathogen. Although lacking typical Toll-like receptors (TLRs), Ep activates a TLR-like pathway including the up-regulation of MyD88, TRAF6, NF-κB and AP-1 genes, which are not induced under LPS treatment and therefore suggest an alternative ligand-to-PRR trigger. Two cytokine-dependent pathways involving Tumor necrosis factor receptors (TNFRs) and several other potential downstream signaling genes likely lead to inflammation and/or apoptosis. Finally, both the extrinsic and intrinsic apoptotic pathways were strongly supported by over-expression of effector and executioner genes.

CONCLUSIONS

To our knowledge, this pioneering study is first to follow the kinetics of the innate immune response in a cnidarian during the onset of infection with a bacterial pathogen. Overall, our findings reveal the involvement of both novel immune gene candidates such as NLRs, RLRs and CniFLs, and previously identified TLR-like and apoptotic pathways in anthozoan innate immunity with a large amount of transcript-level evidence.

Identification of scavenger receptors and thrombospondin-type-1 repeat proteins potentially relevant for plastid recognition in Sacoglossa

Functional kleptoplasty is a photosymbiotic relationship, in which photosynthetically active chloroplasts serve as an intracellular symbiont for a heterotrophic host. Among Metazoa, functional kleptoplasty is only found in marine sea slugs belonging to the Sacoglossa and recently described in Rhabdocoela worms. Although functional kleptoplasty has been intensively studied in Sacoglossa, the fundamentals of the specific recognition of the chloroplasts and their subsequent incorporation are unknown. The key to ensure the initiation of any symbiosis is the ability to specifically recognize the symbiont and to differentiate a symbiont from a pathogen. For instance, in photosymbiotic cnidarians, several studies have shown that the host innate immune system, in particular scavenger receptors (SRs) and thrombospondin-type-1 repeat (TSR) protein superfamily, is playing a major role in the process of recognizing and differentiating symbionts from pathogens. In the present study, SRs and TSRs of three Sacoglossa sea slugs, Elysia cornigera, Elysia timida, and Elysia chlorotica, were identified by translating available transcriptomes into potential proteins and searching for receptor specific protein and/or transmembrane domains. Both receptors classes are highly diverse in the slugs, and many new domain arrangements for each receptor class were found. The analyses of the gene expression of these three species provided a set of species-specific candidate genes, that is, SR-Bs, SR-Es, C-type lectins, and TSRs, that are potentially relevant for the recognition of kleptoplasts. The results set the base for future experimental studies to understand if and how these candidate receptors are indeed involved in chloroplast recognition.

Caspases from scleractinian coral show unique regulatory features

Coral reefs are experiencing precipitous declines around the globe with coral diseases and temperature-induced bleaching being primary drivers of these declines. Regulation of apoptotic cell death is an important component in the coral stress response. Although cnidaria are known to contain complex apoptotic signaling pathways, similar to those in vertebrates, the mechanisms leading to cell death are largely unexplored. We identified and characterized two caspases each from Orbicella faveolata, a disease-sensitive reef-building coral, and Porites astreoides, a disease-resistant reef-building coral. The caspases are predicted homologs of the human executioner caspases-3 and -7, but OfCasp3a (Orbicella faveolata caspase-3a) and PaCasp7a (Porites astreoides caspase-7a), which we show to be DXXDases, contain an N-terminal caspase activation/recruitment domain (CARD) similar to human initiator/inflammatory caspases. OfCasp3b (Orbicella faveolata caspase-3b) and PaCasp3 (Porites astreoides caspase-3), which we show to be VXXDases, have short pro-domains, like human executioner caspases. Our biochemical analyses suggest a mechanism in coral which differs from that of humans, where the CARD-containing DXXDase is activated on death platforms but the protease does not directly activate the VXXDase. The first X-ray crystal structure of a coral caspase, of PaCasp7a determined at 1.57 Å resolution, reveals a conserved fold and an N-terminal peptide bound near the active site that may serve as a regulatory exosite. The binding pocket has been observed in initiator caspases of other species. These results suggest mechanisms for the evolution of substrate selection while maintaining common activation mechanisms of CARD-mediated dimerization.

Evolution of animal immunity in the light of beneficial symbioses

Immune system processes serve as the backbone of animal defences against pathogens and thus have evolved under strong selection and coevolutionary dynamics. Most microorganisms that animals encounter, however, are not harmful, and many are actually beneficial. Selection should act on hosts to maintain these associations while preventing exploitation of within-host resources. Here, we consider how several key aspects of beneficial symbiotic associations may shape host immune system evolution. When host immunity is used to regulate symbiont populations, there should be selection to evolve and maintain targeted immune responses that recognize symbionts and suppress but not eliminate symbiont populations. Associating with protective symbionts could relax selection on the maintenance of redundant host-derived immune responses. Alternatively, symbionts could facilitate the evolution of host immune responses if symbiont-conferred protection allows for persistence of host populations that can then adapt. The trajectory of immune system evolution will likely differ based on the type of immunity involved, the symbiont transmission mode and the costs and benefits of immune system function. Overall, the expected influence of beneficial symbiosis on immunity evolution depends on how the host immune system interacts with symbionts, with some interactions leading to constraints while others possibly relax selection on immune system maintenance. This article is part of the theme issue 'The role of the microbiome in host evolution'.

Cell Biology of Coral Symbiosis: Foundational Study Can Inform Solutions to the Coral Reef Crisis

Coral reefs are faced with almost complete destruction by the end of the century due to global warming unless humanity can cap global temperature rise. There is now a race to develop a diverse set of solutions to save coral reefs. In this perspective, a case is made for understanding the cell biology of coral–dinoflagellate symbiosis to help inform development of solutions for saving reefs. Laboratory model systems for the study of coral symbiosis, including the sea anemone Exaiptasia pallida, are featured as valuable tools in the fight to save corals. The roles of host innate immunity and inter-partner nutrient dynamics in the onset, ongoing maintenance, and dysregulation of symbiosis are reviewed and discussed. Key innate immune genes and pathways, such as glycan–lectin interactions, the sphingosine rheostat, and the cytokine transforming growth factor beta are shown to modulate a host immune response in the symbiotic state. An upset in the homeostatic inorganic nutrient balance during heat stress and high exogenous nutrient availability is credited with driving the partnership toward dysregulation and coral bleaching. Specific examples are given where knowledge of the cell biology of symbiosis is informing the development of solutions, including studies showing clear limitations in the value of partner switching and acclimatization protocols. Finally, emphasis is placed on rapid advancement of knowledge to try to meet the urgent need for solutions. This includes real-time open communication with colleagues on successes and failures, sharing of resources and information, and working together in the spirit of a collective mission to save coral reefs.