Symbiosis specificity and innate immunity in Aiptasia, a model system for cnidarian-dinoflagellate symbiosis [electronic resource]
Keywords: Biology.

The future of coral reefs depends upon the endosymbiosis between corals and the dinoflagellate Symbiodinium. Astonishingly complex patterns of specificity between host and symbiont continue to be described, yet we understand little of the cellular and molecular mechanisms underpinning these processes. To investigate these mechanisms, we use a budding model system based on the small sea anemone Aiptasia pallida, which houses the same Symbiodinium as corals but is far more tractable in the laboratory. To create tools necessary for laboratory analysis of this interaction, we generated cultures of four clonal, axenic Symbiodinium strains and showed the strains to be contaminant-free through a novel combination of microscopy, growth on rich media, and PCR-based assays. We used a ribosomal DNA marker (cp23S) to place these strains in phylogenetic context, and we determined each strain's ability to grow autotrophically, hetertrophically, or mixotrophically in a variety of liquid media. We next analyzed the patterns of specificity between these Symbiodinium strains and Aiptasia adults and larvae, addressing outstanding questions of whether and how specificity changes throughout host ontogeny. We showed that two of the strains are compatible with Aiptasia, reproducibly establishing a symbiotic relationship in adults and larvae and proliferating within the animals over time, whereas two of the strains are incompatible. We used microscopy to show that compatible algae become intracellular early during infection, whereas incompatible algae in the gastric cavity are not found to be intracellular, suggesting that a selection step occurs post-ingestion but pre-phagocytosis. I then sought to define a mechanism for symbiont recognition during the onset of symbiosis by testing whether these Symbiodinium cultures interact with the complement immune system in Aiptasia. After determining the sequence of the central complement component C3 in Aiptasia (ApC3), I generated and purified ApC3-specific antibodies. I show through in situ hybridization of ApC3 mRNA and immunohistochemistry that ApC3 is localized at the apical epiderm of the mouth and at the basal endoderm, at the interface with the mesoglea. This localization is potentially well suited for Aiptasia to test incoming particles from the environment, yet I find no co-localization between ApC3 and compatible or incompatible symbionts. Furthermore, the ApC3 localization at the mouth appears to be novel to the cnidarians and is either an invention in the phylum or a vestige of ancient complement function, both of which carry interesting evolutionary implications. The tools and analyses I present herein contribute to our understanding of symbiosis specificity during host ontogeny and, in an attempt to link immunity to symbiont recognition during symbiosis establishment, an innate immune system with deep conservation yet surprising localization. These studies lay the groundwork for future cellular and molecular investigations as we continue to unveil the underpinnings of this ecologically critical symbiosis.