Contractile Vacuole and Papilla drive Cyst/Telotroch transition in Vorticella microstoma

The cyst of the protist ciliate Vorticella efficiently differentiates into teletroch, the free-swimming stage. Here, using video microscopy and quantitative imaging we establish that the cyst differentiation follows two strict temporal and spatial patterns. The temporal pattern is initially marked by the functional formation of the contractile vacuole, which then discharges its fluid into the neoformed cytopharynx and, in a third stage, into a membrane invagination which, to the rhythm of the vacuole, differentiates into an oral cavity exhibiting a polarized cilia array revealed by α-tubulin immunostaining. Two poles delineate the spatial pattern. The apical pole is defined by the position of the pre-existing papilla, which determines the site of oral cavity formation directly below it. The second basal pole is placed at the rupture point of the cyst wall. It is characterized by β-actin accumulation and allowed release of the telotroph according to its basal-apical polarity. Both the temporal and spatial patterns are impaired by concanamycin A treatment, a specific inhibitor of vacuolar type H ⁺ -ATPases altering functions of contractile vacuoles. The findings indicate that cyst to telotroch transition is predetermined by the location of the papilla on the cyst wall and under the control of a functional contractile vacuole. We propose that Vorticella cyst is a simple single-cell model for investigating basic principles of integrative neogenesis of organelles in Protozoa, and of apical-basal cell polarity in Eukaryotes.