Annika Guse, Heidelberg University

AIPTASIA – A MODEL FOR CORAL SYMBIOSIS

The marine sea anemone Aiptasia allows to experimentally dissect symbiosis establishment, maintenance and coral bleaching

The Question(s)

Symbiosis, the living together of two distinct organisms, is widespread throughout the tree of life. One striking example is the symbiosis between corals and photosynthetic algae (dinoflagellates) which live inside the coral´s cells. The intracellular symbionts transfer essential nutrients to their host which is the basis for the coral´s survival in otherwise very nutrient-poor environments and the formation of the amazingly biodiverse coral reef ecosystems.

How do host and symbiont coordinate their cellular functions and nutrient exchange? How can the symbiont persist in the animal host cell without being considered an invader destined for elimination through digestion? How do coral larvae, which are born without symbionts, find and acquire symbionts from the environment? When phagocytosing symbionts, are coral larvae selective or promiscuous? How do distinct symbionts affect the coral´s ability to adapt to environmental change? What are the cellular mechanisms of coral bleaching, the loss of symbionts from the host tissue upon environmental stress? These and many other fascinating cell biological questions with important ecological ramifications remain unanswered to this day. Investigation of intracellular symbiosis is also interesting from an evolutionary perspective and may for example allow to better understand the evolution of phagocytosis and innate immunity, as well as much more general principles on how two distinct organisms form a functional symbiosis that allows both to expand their ecological habitats and persist through evolutionary times.

The Model

Aiptasia is a tropical marine sea anemone that lives in symbiosis with the same types of symbionts as corals do. Aiptasia produces non-symbiotic larvae that acquire symbionts like their coral relatives from the environment. However, unlike corals Aiptasia is not endangered, grows fast under laboratory conditions and lack the calcareous skeleton that otherwise impairs experimental manipulation and microscopy. Importantly, sexual reproduction of Aiptasia can be, by mimicking a lunar cycle with blue-light LEDs, induced in the lab. Thus, we have unlimited access to fertilized eggs and non-symbiotic larvae to investigate symbiosis establishment.

The genome sequence and transcriptomic resources are available for both host and symbiont, and many important tools are on hand. Methods include in situ hybridization, immunofluorescence and confocal microscopy, western blotting, metabolomic assays, microinjection and basic gain- and loss-of-function approaches. Thus, we are now well-positioned to combine state-of-the-art cell biology, biochemistry, quantitative microscopy and RNA-Seq with comparative fieldwork to better understand many facets of coral symbiosis at a mechanistic level and thus coral reef health. More broadly, Aiptasia serves as a paradigm for how two very distinct organisms form mutualistic symbioses that are key to many ecological and evolutionary novelties.