ELUCIDATING THE MECHANISMS OF INFECTION AND PROLIFERATION IN CNIDARIAN-SYMBIODINIACEAE SYMBIOSIS

Coral reef ecosystems are supported by diverse mutualisms formed between cnidarians such as corals, sea anemones, and jellyfish and dinoflagellate algae in the family Symbiodiniaceae. These dynamic symbiotic relationships rely on the successful establishment of algal endosymbionts, often from the surrounding seawater, within cnidarian host tissues. Due to the current limitations in cellular and molecular tools in the field of cnidarian-Symbiodiniaceae symbiosis, the mechanisms of symbiosis establishment including infection, proliferation, and maintenance are poorly understood. The aim of this thesis is to uncover the cellular processes essential to cnidarian-algal symbiosis by developing in vitro and in hospite assays across Symbiodiniaceae and cnidarian species. In chapter two, the trophic flexibility of Symbiodiniaceae was explored. Symbiodiniaceae Breviolum minutum grown in vitro with organic nutrients showed stable growth and photosynthetic function when compared to limited nutrient conditions; this suggests that the oligotrophic waters of coral reef ecosystems may drive free-living Symbiodiniaceae into symbiosis with cnidarians. Next in chapter three, a new protocol for single-cell dissociation of cnidarian hosts is introduced and used to determine the localization of the first Symbiodiniaceae photosynthesis mutant ora1. Here, ora1 was found to retain its ability to form symbiosis in cnidarians, indicating that photosynthesis is not required for symbiosis establishment. Finally, the newly generated Symbiodiniaceae green mutant, gr02, is introduced and co-inoculated with the brown wild type B. minutum in the sea anemone Aiptasia to uncover the cellular events contributing to symbiont proliferation. For the first time, two algal genotypes (gr02 and wild type B. minutum) were observed co-localized in a single host cell via dissociation and microscopy but were rare in frequency. These results suggest that algal cell division and primary infections drive the proliferation of symbionts in hospite. Furthermore, the co-inoculation of gr02 with other species of Symbiodiniaceae in three cnidarian hosts (coral, sea anemone, and jellyfish) reveals intracellular localization and possible interaction between symbionts in host tissues. Together, this work lays the foundation for future cellular biological research using Symbiodiniaceae mutants to answer pressing questions surrounding cnidarian-Symbiodiniaceae symbiosis.