CORDIS - EU research results

Molecular Basis of Coral Symbiosis

Final Report Summary - ESYMBIOSIS (Molecular Basis of Coral Symbiosis)

Symbioses, the living together of two distinct organisms, are key to many evolutionary and ecological novelties. For example, eukaryotic cells evolved through the acquisition of bacteria, and coral reef ecosystems depend on the intracellular symbiosis between corals and photosynthetic single-celled algae (dinoflagellates). Most corals produce non-symbiotic larvae that take up symbionts anew each generation via phagocytosis into host endodermal cells. We have only a limited understanding of symbiosis establishment, mostly because corals are not suitable as laboratory systems and only reproduce sexually giving rise to larvae for experimentation once annually. To overcome these challenges, the lab has established the larvae of Aiptasia, a marine sea anemone and emerging model for coral symbiosis, as an experimental platform for investigating the molecular mechanisms of host-symbiont interaction. We developed a robust protocol to induce Aiptasia spawning in the lab that allows us to carry out experiments every day of the week. To set a baseline, we analyzed embryonic and larval development in relation to symbiont phagocytosis, and completed the first comparison of symbiosis specificity between Aiptasia and corals – an important step to manifest Aiptasia as a proxy for corals. Together with collaborators, we sequenced the Aiptasia genome and using RNA-Seq we identified a first set of candidate genes involved in symbiosis establishment setting the stage for a molecular analysis. To characterize and mechanistically analyse key-players, we have developed tools such as in situ hybridization, confocal microscopy, immunofluorescence, live-imaging approaches, protein extraction, immunoprecipitation, metabolomics and lipidomics assays and single-cell transcriptomics. We developed a robust microinjection protocol to deliver molecules (e.g. RNA, DNA constructs) to fertilized eggs, and currently, we are establishing functional approaches including genome engineering via CRISPR to further extend our toolbox.

Taking advantage of our technical advances, the lab is now launching a novel research program to uncover mechanisms of intracellular coral-algal symbiosis using Aiptasia larvae as a model. Fundamental questions that we are currently addressing include: Are symbionts acquired by specialized cells and if so, is this a selective process? What are the molecular key players involved in symbiont phagocytosis, and what are the dynamics? What is the nature of the symbiosome, and how do symbionts escape phagolysosomal digestion by the host cell? How are key nutrients transferred? How do they affect host physiology? It is envisioned that the lab will make important contributions in unraveling fundamental aspects of symbiosis, a phenomenon that has shaped many evolutionary innovations and continues to allow adaptation to ecological niches. Ultimately, the work will contribute in providing the basis to better understand coral bleaching, a phenomenon in which coral lose their symbionts upon environmental stress including elevated sea water temperature and which is severely threating coral reefs worldwide. Thus, we expect that in the future our research may impact decisions by policy makers and nature conservation agencies.

The career integration of the researcher has also been successful as indicated by growth of the research group, acquisition of prestigious national and international funding including an ERC Consolidator grant proving a more stable future perspective. The work has been disseminated and made accessible the public by publications, interviews for science magazines and the radio as well as through outreach activities for school children. For more information visit: