Endosymbiosis drives transcriptomic adjustements and genomic adapatations in cnidarians

To decipher inter-partner signaling within the cnidarian-dinoflagellate endosymbiosis, we developed genomic resources (cDNA library and microarrays) for the symbiotic sea anemone Anemonia viridis. Differential gene expression was quantified during thermal stress, with and without UV radiation, between symbiotic vs aposymbiotic specimens and gastroderm vs epidermis tissues. During stress time-course experiments, each stress showed a specific gene expression profile with very little overlap. We show that the major response to thermal stress is rapid (24 hours) but returns to the baseline levels after 2 days. UVR alone has little effect but potentiates thermal stress, as expression of a second set of genes becomes differentially expressed at day 5. Analysis of genes differentially expressed between symbiotic vs bleached and symbiotic vs stressed specimens defined a restricted subset of genes (Kern). Tissue specific expression mapping of Kern genes showed that many were specifically enhanced in the symbiotic cells (gastroderm). Altogether, these data define the Kern genes as major molecular components of the symbiotic interaction. Functional annotations highlighted several pathways including collagen fibrillogenesis, vesicular trafficking, lipid metabolism, calcium signaling, inorganic carbon transfer and cell death, that were modified by stress. Phylogenomic investigations of several Kern genes (calumenin, NPC2, SYM32, dermatopontin, and Rhbg) demonstrate that these issued from cnidarian specific duplication events, with the Kern member being preferentially expressed in the gastroderm and specifically responding to stress. Such host specific genes subfunctionalizations suggest both genomic and transcriptomic adaptations driven by the physiological constraints of endosymbiosis

Adaptations to Endosymbiosis in a Cnidarian-Dinoflagellate Association: Differential Gene Expression and Specific Gene Duplications

Trophic endosymbiosis between anthozoans and photosynthetic dinoflagellates forms the key foundation of reef ecosystems. Dysfunction and collapse of symbiosis lead to bleaching (symbiont expulsion), which is responsible for the severe worldwide decline of coral reefs. Molecular signals are central to the stability of this partnership and are therefore closely related to coral health. To decipher inter-partner signaling, we developed genomic resources (cDNA library and microarrays) from the symbiotic sea anemone Anemonia viridis. Here we describe differential expression between symbiotic (also called zooxanthellate anemones) or aposymbiotic (also called bleached) A. viridis specimens, using microarray hybridizations and qPCR experiments. We mapped, for the first time, transcript abundance separately in the epidermal cell layer and the gastrodermal cells that host photosynthetic symbionts. Transcriptomic profiles showed large inter-individual variability, indicating that aposymbiosis could be induced by different pathways. We defined a restricted subset of 39 common genes that are characteristic of the symbiotic or aposymbiotic states. We demonstrated that transcription of many genes belonging to this set is specifically enhanced in the symbiotic cells (gastroderm). A model is proposed where the aposymbiotic and therefore heterotrophic state triggers vesicular trafficking, whereas the symbiotic and therefore autotrophic state favors metabolic exchanges between host and symbiont. Several genetic pathways were investigated in more detail: i) a key vitamin K–dependant process involved in the dinoflagellate-cnidarian recognition; ii) two cnidarian tissue-specific carbonic anhydrases involved in the carbon transfer from the environment to the intracellular symbionts; iii) host collagen synthesis, mostly supported by the symbiotic tissue. Further, we identified specific gene duplications and showed that the cnidarian-specific isoform was also up-regulated both in the symbiotic state and in the gastroderm. Our results thus offer new insight into the inter-partner signaling required for the physiological mechanisms of the symbiosis that is crucial for coral health

The Tara Pacific expedition—A pan-ecosystemic approach of the “-omics” complexity of coral reef holobionts across the Pacific Ocean

Coral reefs are the most diverse habitats in the marine realm. Their productivity, structural complexity, and biodiversity critically depend on ecosystem services provided by corals that are threatened because of climate change effects—in particular, ocean warming and acidification. The coral holobiont is composed of the coral animal host, endosymbiotic dinoflagellates, associated viruses, bacteria, and other microeukaryotes. In particular, the mandatory photosymbiosis with microalgae of the family Symbiodiniaceae and its consequences on the evolution, physiology, and stress resilience of the coral holobiont have yet to be fully elucidated. The functioning of the holobiont as a whole is largely unknown, although bacteria and viruses are presumed to play roles in metabolic interactions, immunity, and stress tolerance. In the context of climate change and anthropogenic threats on coral reef ecosystems, the Tara Pacific project aims to provide a baseline of the “-omics” complexity of the coral holobiont and its ecosystem across the Pacific Ocean and for various oceanographically distinct defined areas. Inspired by the previous Tara Oceans expeditions, the Tara Pacific expedition (2016–2018) has applied a pan-ecosystemic approach on coral reefs throughout the Pacific Ocean, drawing an east–west transect from Panama to Papua New Guinea and a south–north transect from Australia to Japan, sampling corals throughout 32 island systems with local replicates. Tara Pacific has developed and applied state-of-the-art technologies in very-high-throughput genetic sequencing and molecular analysis to reveal the entire microbial and chemical diversity as well as functional traits associated with coral holobionts, together with various measures on environmental forcing. This ambitious project aims at revealing a massive amount of novel biodiversity, shedding light on the complex links between genomes, transcriptomes, metabolomes, organisms, and ecosystem functions in coral reefs and providing a reference of the biological state of modern coral reefs in the Anthropocene

Symbiodinium isolation by NaOH treatment

International audienceThe presence of photosynthetic zooxanthellae (dinoflagellates) in the tissue of many cnidarians is the main reason for their ecological success (i.e. coral reefs). It could also be the main cause of their demise, as the worldwide bleaching of reef-building coral is nothing less than the breakdown of this symbiotic association. The stability of this relationship is the principal marker for the biomonitoring of cnidarian health. We have therefore developed a new, simple method to isolate zooxanthellae in a few steps using NaOH solution. The protocol was validated in three symbiotic cnidarian species: a sea anemone, a gorgonian and a coral. Our method allows the isolation of intact and viable zooxanthellae with better yields than classic methods, especially for species with a calcareous skeleton. Moreover, the isolated zooxanthellae were free of host nucleic contaminants, facilitating subsequent specific molecular analyses

Adaptation d'Eunicella singularis en milieu perturbé (symbiose et structuration génétique)

La symbiose entre Cnidaires et Symbiodinium (Dinobionte photosynthétique) est relativement souple. Les Cnidaires généralistes sont capables de s adapter à un changement environnemental en modifiant leur population symbiotique. Les hôtes spécialistes, a priori privés d un tel mécanisme d adaptation, peuvent-ils malgré tout faire face aux modifications environnementales ? C est dans ce cadre que nous avons étudié la symbiose chez un hôte spécialiste en milieu variable : la gorgone méditerranéenne Eunicella singularis. En effet, la diversité symbiotique se limite à un seul clade ribosomique de Symbiodinium en Méditerranée. Des études de génétiques réalisées à l aide de microsatellites sur le bassin Méditerranéen Nord-Occidental ont révélé une diversité symbiotique intra-clade. Nous avons montré que les symbiotes se distribuent indépendamment de leurs hôtes et de la profondeur. Un échantillonnage des mêmes populations naturelles à 6 ans intervalle a mis en évidence des acquisitions horizontales massives de symbiotes chez des hôtes adultes. Des transplantations réciproques ainsi qu un suivi de colonies maintenues en aquarium semblent indiquer que de telles modifications nécessitent une variation environnementale forte. Par ailleurs, nous avons étudié le comportement de l association en absence d apport autotrophe et/ou hétérotrophe. Il en ressort que l hôte semble contrôler l association et qu il apparaît prioritaire quant au partage des ressources. Même chez un hôte dit spécialiste, l association avec Symbiodinium peut donc se révéler dynamique. Cela pourrait faciliter l adaptation de ces spécialistes, pour peu qu à la diversité génétique symbiotique corresponde une diversité fonctionnelle.The symbiosis between Cnidaria and Symbiodinium (photosynthetic Dinoflagelate) is relatively flexible. Generalist Cnidaria are able to adapt themselves to environmental variations by modifying their symbiont population. Are specialists hosts, a priori lacking this adaptive mechanism, able to confront environmental changes ? In this context, we studied the symbiosis n a specialist host : the Mediterranean sea whip Eunicella singularis. Indeed, symbiont diversity is limited to a single Symbiodinium ribosomal clade in Mediterranean Sea. Genetic studies based on microsatellites loci revealed a with-in independently from their host and from depth. A re-sampling of natural populations brought to light massive horizontal acquisitions of symbionts in adult colonies. Reciprocal transplants and monitoring of colonies maintained in aquarium seems to indicate that strong environmental variations are required for such modifications of symbiont populations. In addition we studied the behavior of this association when deprived of autotrophic and/or heterotrophic resources. We conclude that the host seems able to control the association and that he appears to have priority in resources sharing. Therefore, even for a specialist host, association with Symbiodinium can be a dynamic. This could make adaptation easier for this specialist, but only of genetic diversity is correlate with a functional diversity.NICE-BU Sciences (060882101) / SudocSudocFranceF

The antioxidant responses differ between Symbiodinium strains from different geographic origins

Studies conducted these last two decades have revealed that high sea surface temperatures accompanied by high levels of solar irradiance are responsible for an over production of reactive oxygen species (ROS) leading to the disruption of the symbiosis between cnidarians and their symbiotic Symbiodinium. But, all coral species do not show the same sensitivity to stress. In this context we examined how the antioxidant network of different Symbiodinium species responds to oxidative stress. We bypassed the various thermal and light tolerances existing among the genus Symbiodinium by using a chemical approach, i.e., a treatment with menadione. ROS produced during this oxidative burst reduced photosynthesis by 30 to 50% and significantly decreased the activity of superoxide dismutase. In addition, the low level of lipid peroxidation concomitantly with the decrease in the concentration of diatoxanthin and other carotenoids during the oxidative stress confirms their function of antioxidants and their role in the stabilization of membrane lipids. The analysis of the cellular damages also indicates that proteins were damaged and most likely eliminated by the ubiquitin-proteasome pathway. Finally, caspase-like activity decreased suggesting that cell death mechanisms are not initiated at the early stage of the stress. Although, the mechanisms at play seem to be the same, we found that the temperate Symbiodinium strain (A1) was less impacted by the treatment with menadione than the tropical strain (F1) suggesting that the variations observed are related to their geographic origin

Differential antioxidant response between two Symbiodinium species from contrasting environments

peer reviewedHigh sea surface temperature accompanied by high levels of solar irradiance is responsible for the disruption of the symbiosis between cnidarians and their symbiotic dinoflagellates from the genus Symbiodinium. This phenomenon, known as coral bleaching, is one of the major threats affecting coral reefs around the world. Because an important molecular trigger to bleaching appears related to the production of reactive oxygen species (ROS), it is critical to understand the function of the antioxidant network of Symbiodinium species. In this study we investigated the response of two Symbiodinium species, from contrasting environments, to a chemically induced oxidative stress. ROS produced during this oxidative burst reduced photosynthesis by 30 to 50% and significantly decreased the activity of superoxide dismutase. Lipid peroxidation levels and carotenoid concentrations, especially diatoxanthin, confirm that these molecules act as antioxidants and contribute to the stabilization of membrane lipids. The comparative analysis between the two Symbiodinium species allowed us to highlight that Symbiodinium sp. clade A temperate was more tolerant to oxidative stress than the tropical S. kawagutii clade F. These differences are very likely a consequence of adaptation to their natural environment, with the temperate species experiencing conditions of temperature and irradiance much more variable and extreme