The cell biology and genome of , a giant cell that embeds symbiotic algae in a microtubule meshwork.

Endosymbiotic events in which an endosymbiont is retained within a cell that remains capable of phagocytosis, a situation known as mixotrophy, provide potentially important clues about the eukaryotic evolution. Here we describe the cell biology and genome of the giant mixotrophic ciliate . We show that contains as an endosymbiont that retains the ability to live outside the host. Within the host, the cells surrounded by microtubule "baskets" near the cell surface. Photosynthetic efficiency of the is reduced inside the cell compared with outside the host, due to increased nonphotochemical quenching. displays positive phototaxis via directed swimming that requires the presence of the , implying a potential flow of information from the symbiont to direct the orientation and swimming of the host cell. We sequenced the genome and found that it employs a standard genetic code, similar to other species but different from most other ciliates. We propose that will serve as a useful model system for studying endosymbiosis, with unique advantages in terms of size and regenerative ability as well as distinct cellular and genomic features compared with other mixotrophic ciliate models.