The cell biology and genome of <i>Stentor pyriformis</i>, 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 <i>Stentor pyriformis</i>. We show that <i>S. pyriformis</i> contains <i>Chlorella variabilis</i> as an endosymbiont that retains the ability to live outside the host. Within the host, the <i>Chlorella</i> cells surrounded by microtubule "baskets" near the cell surface. Photosynthetic efficiency of the <i>Chlorella</i> is reduced inside the <i>Stentor</i> cell compared with outside the host, due to increased nonphotochemical quenching. <i>S. pyriformis</i> displays positive phototaxis via directed swimming that requires the presence of the <i>Chlorella</i>, implying a potential flow of information from the symbiont to direct the orientation and swimming of the host cell. We sequenced the <i>S. pyriformis</i> genome and found that it employs a standard genetic code, similar to other <i>Stentor</i> species but different from most other ciliates. We propose that <i>S. pyriformis</i> 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.