Global Shifts in Gene Expression Profiles Accompanied with Environmental Changes in Cnidarian-Dinoflagellate Endosymbiosis

Unlocking the Complex Cell Biology of Coral-Dinoflagellate Symbiosis: A Model Systems Approach

Symbiotic interactions occur in all domains of life, providing organisms with resources to adapt to new habitats. A prime example is the endosymbiosis between corals and photosynthetic dinoflagellates. Eukaryotic dinoflagellate symbionts reside inside coral cells and transfer essential nutrients to their hosts, driving the productivity of the most biodiverse marine ecosystem. Recent advances in molecular and genomic characterization have revealed symbiosis-specific genes and mechanisms shared among symbiotic cnidarians. In this review, we focus on the cellular and molecular processes that underpin the interaction between symbiont and host. We discuss symbiont acquisition via phagocytosis, modulation of host innate immunity, symbiont integration into host cell metabolism, and nutrient exchange as a fundamental aspect of stable symbiotic associations. We emphasize the importance of using model systems to dissect the cellular complexity of endosymbiosis, which ultimately serves as the basis for understanding its ecology and capacity to adapt in the face of climate change.

Environmental pH signals the release of monosaccharides from cell wall in coral symbiotic alga

Reef-building corals thrive in oligotrophic environments due to their possession of endosymbiotic algae. Confined to the low pH interior of the symbiosome within the cell, the algal symbiont provides the coral host with photosynthetically fixed carbon. However, it remains unknown how carbon is released from the algal symbiont for uptake by the host. Here we show, using cultured symbiotic dinoflagellate, Breviolum sp., that decreases in pH directly accelerates the release of monosaccharides, that is, glucose and galactose, into the ambient environment. Under low pH conditions, the cell surface structures were deformed and genes related to cellulase were significantly upregulated in Breviolum. Importantly, the release of monosaccharides was suppressed by the cellulase inhibitor, glucopyranoside, linking the release of carbon to degradation of the agal cell wall. Our results suggest that the low pH signals the cellulase-mediated release of monosaccharides from the algal cell wall as an environmental response in coral reef ecosystems.

Establishment of a New Model Sea Anemone for Comparative Studies on Cnidarian-Algal Symbiosis

Frequent coral bleaching has drawn attention to the mechanisms of coral dinoflagellate endosymbiosis. Owing to the difficulty of rearing corals in the laboratory, model symbiosis systems are desired. The sea anemone Exaiptasia diaphana, hosting clade B1 of the genus Breviolum, has long been studied as a model system; however, a single species is insufficient for comparative studies and thus provides only limited resources for symbiosis research, especially regarding the specificity of host-symbiont associations. We established a clonal strain of the sea anemone Anthopleura atodai, whose symbiont was identified as a novel subclade of Symbiodinium (clade A) using a novel feeding method. We also developed a method to efficiently bleach various sea anemone species using a quinoclamine-based herbicide. Bleached A. atodai polyps were vital and able to reproduce asexually, exhibiting no signs of harmful effects of the drug treatment. Pilot studies have suggested that host-symbiont specificity is influenced by multiple steps differently in A. atodai and E. diaphana. RNAseq analyses of A. atodai showed that multiple NPC2 genes were expressed in the symbiotic state, which have been suggested to function in the transport of sterols from symbionts to host cells. These results reveal the usefulness of A. atodai in comparative studies of cnidarian-algal symbiosis.

A Rhesus channel in the coral symbiosome membrane suggests a novel mechanism to regulate NH3 and CO2 delivery to algal symbionts

Reef-building corals maintain an intracellular photosymbiotic association with dinoflagellate algae. As the algae are hosted inside the symbiosome, all metabolic exchanges must take place across the symbiosome membrane. Using functional studies in Xenopus oocytes, immunolocalization, and confocal Airyscan microscopy, we established that Acropora yongei Rh (ayRhp1) facilitates transmembrane NH₃ and CO₂ diffusion and that it is present in the symbiosome membrane. Furthermore, ayRhp1 abundance in the symbiosome membrane was highest around midday and lowest around midnight. We conclude that ayRhp1 mediates a symbiosomal NH₄⁺-trapping mechanism that promotes nitrogen delivery to algae during the day-necessary to sustain photosynthesis-and restricts nitrogen delivery at night-to keep algae under nitrogen limitation. The role of ayRhp1-facilitated CO₂ diffusion is less clear, but it may have implications for metabolic dysregulation between symbiotic partners and bleaching. This previously unknown mechanism expands our understanding of symbioses at the immediate animal-microbe interface, the symbiosome.

Establishment of Host-Algal Endosymbioses: Genetic Response to Symbiont Versus Prey in a Sponge Host

The freshwater sponge Ephydatia muelleri and its Chlorella-like algal partner is an emerging model for studying animal: algal endosymbiosis. The sponge host is a tractable laboratory organism, and the symbiotic algae are easily cultured. We took advantage of these traits to interrogate questions about mechanisms that govern the establishment of durable intracellular partnerships between hosts and symbionts in facultative symbioses. We modified a classical experimental approach to discern the phagocytotic mechanisms that might be co-opted to permit persistent infections, and identified genes differentially expressed in sponges early in the establishment of endosymbiosis. We exposed algal-free E. muelleri to live native algal symbionts and potential food items (bacteria and native heat-killed algae), and performed RNA-Seq to compare patterns of gene expression among treatments. We found a relatively small but interesting suite of genes that are differentially expressed in the host exposed to live algal symbionts, and a larger number of genes triggered by host exposure to heat-killed algae. The upregulated genes in sponges exposed to live algal symbionts were mostly involved in endocytosis, ion transport, metabolic processes, vesicle-mediated transport, and oxidation–reduction. One of the host genes, an ATP-Binding Cassette transporter that is downregulated in response to live algal symbionts, was further evaluated for its possible role in the establishment of the symbiosis. We discuss the gene expression profiles associated with host responses to living algal cells in the context of conditions necessary for long-term residency within host cells by phototrophic symbionts as well as the genetic responses to sponge phagocytosis and immune-driven pathways.

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.

Reduced thermal tolerance in a coral carrying CRISPR-induced mutations in the gene for a heat-shock transcription factor

Reef-building corals are keystone species that are threatened by anthropogenic stresses including climate change. To investigate corals' responses to stress and other aspects of their biology, numerous genomic and transcriptomic studies have been performed, generating many hypotheses about the roles of particular genes and molecular pathways. However, it has not generally been possible to test these hypotheses rigorously because of the lack of genetic tools for corals or closely related cnidarians. CRISPR technology seems likely to alleviate this problem. Indeed, we show here that microinjection of single-guide RNA/Cas9 ribonucleoprotein complexes into fertilized eggs of the coral Acropora millepora can produce a sufficiently high frequency of mutations to detect a clear phenotype in the injected generation. Based in part on experiments in a sea-anemone model system, we targeted the gene encoding Heat Shock Transcription Factor 1 (HSF1) and obtained larvae in which >90% of the gene copies were mutant. The mutant larvae survived well at 27 °C but died rapidly at 34 °C, a temperature that did not produce detectable mortality over the duration of the experiment in wild-type (WT) larvae or larvae injected with Cas9 alone. We conclude that HSF1 function (presumably its induction of genes in response to heat stress) plays an important protective role in corals. More broadly, we conclude that CRISPR mutagenesis in corals should allow wide-ranging and rigorous tests of gene function in both larval and adult corals.