The cell biology and genome of Stentor pyriformis, 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 evolutionary origins of eukaryotes, particularly regarding the relative evolutionary sequence of phagocytosis and endosymbiosis. Mixotrophy in ciliates is commonplace, but has been investigated at a cellular and molecular level almost entirely in one organism, Paramecium bursaria. Reliance on just one model system makes it difficult to know which cell biological aspects of this system represent general features of ciliate mixotrophy versus accidental features of the specific organism. Here we describe the cell biology and genome of the giant heterotrichous ciliate Stentor pyriformis. We show that this giant unicellular organism contains Chlorella variabilis as its endosymbiont, that the Chlorella can live freely outside the host, that within the host the Chlorella cells are docked near the cell surface, surrounded by microtubule "baskets", and that the photosynthetic efficiency of the Chlorella is reduced inside the Stentor cell compared to when it is free-living outside the host, with photon energy instead being shunted to non-photochemical quenching. Compared to the non-mixotrophic Stentor coeruleus, S. pyriformis has several distinct cellular features that may be related to endosymbiosis: the presence of microtubule baskets, which are absent in S. coeruleus; positive rather than negative phototaxis, which is likely an adaptation to allow the photosynthetic symbionts access to sufficient light; and a lack of pigment in the host cell, which may be an adaptation to tolerate high light levels. Compared to P. bursaria, S. pyriformis has several similar cellular features: in both organisms, the symbiont is a strain of Chlorella variabilis; the Chlorella endosymbiont retains the ability to live freely when separated from the host; and the algal symbionts contained in perialgal vesicles are docked at the cell surface. One potentially informative difference between P. bursaria and S. pyriformis is that S. pyriformis employs a standard genetic code, similar to other Stentor species but different from most other ciliates, including P. bursaria, which use a non-standard code in which one or more stop-codons are respecified to encode amino acids. This difference in genetic code could serve as a barrier to impede gene transfer from symbiont to host in other ciliates, but this would not be a factor in S. pyriformis. A second cell biological difference is that whereas P. bursaria performs phototaxis by a kinetic accumulation mechanism, in which swimming is non-directional but cells slow down in regions of higher light intensity, S. pyriformis performs directed swimming towards the direction of high light intensity. However, as in P. bursaria, phototaxis in S. pyriformis requires the presence of the Chlorella, implying a potential flow of information from the symbiont to direct the orientation and swimming of the host cell. We propose that S. pyriformis will serve as a useful model system for studying the evolution of mixotrophy and 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.