A role for contractile vacuole and papilla in transition from cyst to telotroch in Vorticella microstoma

Unicellular ciliates survive adverse environments by forming resting cysts. Using video microscopy and quantitative imaging, we established that differentiation of swimming telotrochs and their release from a cyst followed a strict temporal and spatial pattern in Vorticella microstoma. The temporal pattern was initially marked by formation of a contractile vacuole, which discharged its fluid contents into a newly formed cytopharynx and a membrane invagination. With the pulsatile rhythm of the vacuole, the membrane invagination differentiates into an oral cavity that exhibits polarized cilia. The spatial pattern was delineated by two poles: (i) an apical pole, defined by the position of the pre-existing papilla at the cyst wall, determined the site of oral cavity formation; (ii) a basal pole at the point of rupture of the cyst wall allowed telotroch release. Both temporal and spatial patterns were impaired by concanamycin A, an inhibitor of vacuolar-type H+-ATPases that alters contractile vacuole functions. Our findings indicate that the transition from cyst to telotroch is predetermined by the papilla location at the cyst wall and is controlled by the contractile vacuole. The Vorticella cyst provides a simple, single-cell model for investigating basic principles of integrated organelle neogenesis in protozoa and apical–basal cell polarity in eukaryotes.