Asexual reproduction in Peridiniumis by binary fission; the parental thecal plates are shed, and each progeny cell synthesizes new plates. Sexual reproduction takes place by the fusion of isogametes. The isogametes form by simple division of a vegetative cell; in some freshwater Peridinium species, this event takes place in the laboratory in media deficient in cell-accessible nitrogen. Gamete fusion results in a planozygote that swims for a few days, and then encysts. The encysted hypnozygote undergoes a short (days or weeks) period of dormancy. Meiosis apparently takes place in the cyst; hence, all swimming cells are haploid.
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