"Coral reefs are the world's most diverse marine ecosystems and their existence depends upon a functional intracellular symbiosis between dinoflagellates (single-celled algae) and their coral host. Stresses, such as elevated seawater temperature, cause the obligate symbiosis to break down, a phenomenon known as coral “bleaching”. Incidences of coral bleaching worldwide are on the rise, linked to global climate change. Most corals (phylum Cnidaria) first initiate symbiosis with dinoflagellates (genus Symbiodinium) during larval stages. The molecular basis of coral symbiosis is not well understood, mainly because corals are difficult to work with and have slow generation times. I propose to use an emerging model system based on the small, experimentally tractable sea anemone Aiptasia (phylum Cnidaria) to analyze symbiosis establishment at the cellular and molecular level, including the ways in which symbiosis establishment is affected by environmental changes. First, I will establish a thorough cellular description of symbiosis establishment in Aiptasia larvae. Next, I will identify conserved genes involved in larval symbiosis in cnidarians and use molecular techniques to dissect their functions. Finally, I will establish gain-of function techniques (transgenesis) for Aiptasia using microinjection into early embryos to visualize membranes, nuclei and the cytoskeleton during symbiosis establishment using life imaging. Analysis of coral symbiosis at the cellular and molecular level will provide the missing link between existing genomic approaches and field population-level approaches and will help identify measures to protect coral symbiosis in the face of environmental change."