Behavior and Ca²⁺-induced cell signaling for encystment of Colpoda cucullus

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Encystment of Colpoda cucullus is induced by an increase in external Ca²⁺ concentration. When the Colpoda vegetative cells were induced to encyst by being suspended in Ca²⁺-containing medium, the cells that had swum linearly began to turn round which may have modified their ciliary movement, and then stopped prior to the formation of a cyst wall. Such a behavioral modification may be responsible for an elevation of intracellular Ca²⁺ concentration. Slowly and spontaneously induced encystment in Ca²⁺-free medium was suppressed by the addition of EGTA, suggesting that Ca²⁺ contaminating the medium may be partially responsible for the induction of such spontaneous encystment. A cAMP enzyme immunoassay (EIA) revealed that the cAMP level was raised when the vegetative cells were transferred into Ca²⁺-containing medium for encystment induction. This result suggests that Ca²⁺ may activate a signaling pathway leading to encystment events including elevation of the cAMP concentration.