Signaling in temperature-induced resting cyst formation in the ciliated protozoan Colpoda cucullus

The terrestrial ciliated protozoan Colpoda cucullus inhabits soil. When the habitat conditions become unfavorable, the vegetative cells of C. cucullus quickly transform into resting cysts. C. cucullus culture is established in our laboratory, and encystment is routinely induced by the addition of Ca²⁺ to overpopulated vegetative cells. However, an increase in Ca²⁺ concentration and overpopulation of vegetative cells do not always occur in natural. We investigated the effect of temperature and found that cyst formation was induced by a rapid increase of 5 °C within 2 min but not by a decrease. Moreover, an increase in intracellular Ca²⁺ concentrations is essential, but Ca²⁺ inflow does not necessarily occur during encystment. Ca²⁺ image analysis showed that Ca²⁺ is stored in vesicular structures and released into the cytoplasm within 60 s after temperature stimulation. Multiple signaling pathways are activated after the release of Ca²⁺ from vesicles, and cAMP is a candidate second messenger with a crucial role in the process of temperature-induced encystment. Further studies are needed to clarify the mechanism underlying the sensing of temperature and release of Ca²⁺ from vesicles.