Endosymbiosis is a key phenomenon that has played a critical role in shaping biological diversity, driving gene transfer and generating cellular complexity. During the process of endosymbiosis, one cell is integrated within another to become a critical component of the recipient, changing its characteristics and allowing it to chart a distinct evolutionary trajectory. Endosymbiosis was fundamentally important to the origin and evolution of eukaryotic cellular complexity, because an endosymbiotic event roots the diversification of all known eukaryotes and endosymbiosis has continually driven the diversification of huge sections of the eukaryotic tree of life. Little is known about how nascent endosymbioses are established or how they go on to form novel cellular compartments known as endosymbiotic organelles. Paramecium bursaria is a single celled protist that harbours multiple green algae within to form a phototrophic endosymbiosis. This relationship is nascent as the partners can be separated, grown separately, and the endosymbiosis reinitiated. This project will identify, for the first time, the gene functions that enable one cell to incubate another within to form a stable endosymbiotic interaction. To identify and explore which host genes control endosymbiosis in P. bursaria we have developed RNAi silencing technology. In the proposed project we will conduct genome sequencing, followed by a large-scale RNAi knockdown screening experiment, to identify host genes that when silenced perturb the endosymbiont population. Having identified candidate genes, we will investigate the localisation and function of the host encoded proteins. This project will significantly change our current understanding of the evolutionary phenomenon of endosymbiosis by identifying the cellular adaptations that drive these interactions, advancing our understanding of how these important moments in evolution occur and how core cellular systems can diversify in function.