Abstract. This is the first detailed study on the morphology of the resting cysts of an oxytrichid ciliate, Rigidohymena quadrinucleata. Resting cysts were investigated using light microscopy, SEM and TEM. The cyst wall is composed of four distinct layers (from inside to outside), namely the metacyst, the endocyst, the mesocyst and the ectocyst with spine-like protuberances. The cysts of R. quadrinucleata belong to the kinetosome-resorbing (KR) type, which is typical for oxytrichids. The processes of encystation and excystation were observed only in the light microscopy. During the encystation process, the trophic cell changes in shape and volume due to dehydration, four macronuclear nodules fuse into a compact mass, the ciliature is resorbed and cyst wall is formed. The most significant feature is surface ornamentation and yellowish color of resting cysts. We also focuse for the first time on excystation process of R. quadrinucleata. We identified two excystation modes: (i) standard and (ii) rare mode. The beginning of both excystation is characterised by the formation of excystation vacuole which helps the excysting cell to break the cyst wall. The specimen regenerates within a thin, flexible membrane. During the standard mode, the cell leaves the resting cyst in the membrane that is resorbed in the environment. During the rare mode, the excystation vacuole and the pressure of the regenerating cell break the transparent membrane that remains in the resting cyst. The results suggest that not only ciliate resting cysts, but also the excystation process is much more variable than what literature data indicate.
The scope of our study presents a light microscopy study on the encystation, excystation and morphology of the resting cysts of the spirotrich ciliate, Phacodinium metchnikoffi. During the encystation process, the trophic cell changes in shape, size and volume, the horseshoe-shaped macronuclear nodule transforms into a compact rounded mass, the ciliature is resorbed and the cyst wall is formed. A characteristic accumulation of dark substances in the cell cytoplasm was observed. The most significant feature is the surface. Ornamentation in the form of protuberances in regular rows is located on the entire surface of the cysts. We also focused on the excystation process for the first time and uncovered several specifics of P. metchnikoffi excystation. The excystation is characterised by the formation of the excystation vacuole. An escape apparatus is also present. The coexistence of the excystation vacuole and apparatus during the excystation process is an unusual type of escaping and has not yet been described. The results suggest that not only the resting cysts surface, but also the excystation and encystation processes are much more varied than literary data indicate.
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