This study will examine the cellular and molecular interactions between cnidarians, such as reef-building corals, and their single-celled symbiotic algae that reside within their tissues. Weis' group will identify and characterize genes in host corals and closely related anemones that are turned on specifically due to their symbiosis with algae. The time-course for the expression of these so-called symbiosis genes during the initial establishment of the cnidarian host and symbiotic algae partnership will be explored in microscopic larval corals that acquire their algae from their environment, by ingesting them with food. New state-of-the-art genetic techniques will be used to block the correct functioning of the symbiosis gene products. This approach will help determine the role that symbiosis genes play in the regulation and maintenance of coral/algal symbioses. Symbiotic associations between two or more unrelated organisms are found throughout every ecosystem. These relationships encompass a spectrum of lifestyles, from mutualistic to parasitic, and from extracellular to intracellular. Among the most significant marine mutualisms are those between corals and their algal symbionts, which together form both the trophic and structural foundation of the entire coral reef ecosystem. A great deal is known about corals on a macroscale level, such as their global distribution and ecology. However, microscale processes of inter-partner communication and regulation that occur during the initiation, establishment, and maintenance of these symbioses remain largely unexplored. A lack of basic knowledge of these processes has hampered efforts to understand the global phenomenon of coral bleaching, which is the result of the breakdown of the symbiosis. Widespread bleaching, which leads to coral death and reef degradation, is considered a serious environmental threat. This study will take a multileveled approach to the examination of the events surrounding the initiation, establishment and maintenance of symbioses between corals and their symbiotic algae.