A Study on Resting Cysts of an Oxytrichid Soil Ciliate, Rigidohymena quadrinucleata (Dragesco and Njine, 1971) Berger, 2011 (Ciliophora, Hypotrichia), Including Notes on its Encystation and Excystation Process

Abstract

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.

How Ciliated Protists Survive by Cysts: Some Key Points During Encystment and Excystment

“…In most cases, the volume of cysts is much smaller than vegetative cells ( Foissner et al, 2006 ) with the exception of Strombidium oculatum ( Jonsson, 1994 ). Despite a few species that form colored cysts resulting from food or pigment granules, cysts of most species are colorless ( Repak, 1968 ; Foissner et al, 2007 ; Benčat’ová et al, 2016 ; Benčat’ová and Tirjaková, 2017 ; Cavaleiro et al, 2018 ).…”

“…(C) Bursaria truncatella : bridge, between the inner and outer cyst walls; opercular, upon emergence pore; modified after Beers (1948) and Krause and Braucker (2009) . (D) Rigidohymena quadrinucleata , modified after Benčat’ová and Tirjaková (2017) and Wang J. et al (2017) . (E) Pleurotricha lanceolata : storage bodies, discontinuous ring of hyaline “lakes” in protoplast; modified after Manwell (1928) , Jeffries (1956) , Jeffries and Mellott (1968) , and Dragesco (1972) .…”

“…Although there are fewer observations on the impact of encystment on other intracellular organelles, for some taxa their mitochondria may cluster or remain scattered in the mature cyst ( Walker and Maugel, 1980 ; Verni et al, 1984 ). For some species, contractile vacuoles disappear, whereas autophagic vacuoles are found intact in resting cysts of many ciliates ( Benčat’ová and Tirjaková, 2017 ; Figures 2 , 4 ).…”

“…For the legend: macronuclei are not drawn in all stages for certain species; in Tetrahymena rostrata , the old macronucleus is degraded, while the new macronuclei are produced by the micronuclei; curved arrows indicate rotation and degree; mucocyst is secreted to form the cyst wall; degrading or regenerating ciliature is drawn as broken lines; vacuoles in Parentocirrus hortualis empty into the so-called contractile vacuole; pockets are disk-like structures consisting of unknown material. Tetrahymena rostrata ( McArdle et al, 1980 ; Segade et al, 2016 ); Holophrya teres ( Benčat’ová et al, 2020 ); Bursaria truncatella ( Beers, 1948 ); Rigidohymena quadrinucleata ( Benčat’ová and Tirjaková, 2017 ); Pleurotricha lanceolata ( Jeffries, 1956 ); Sterkiella histriomuscorum ( Adl and Berger, 1997 ); Stylonychia bifaria ( Verni and Rosati, 2011 ); Stylonychia mytilus ( Zhang and Pang, 1981 ); Halteria grandinella ( Foissner et al, 2007 ); Meseres corlissi ( Foissner et al, 2006 ; Müller, 2007 ); Parentocirrus hortualis ( Benčat’ová et al, 2016 ); Euplotes encysticus ( Rawlinson and Gates, 1985 ; Gu and Zhang, 1992 ; Wang B. et al, 2017 ); Phacodinium metchnikoffi ( Benčat’ová and Tirjaková, 2018 ); Blepharisma stoltei ( Repak, 1968 ).…”

Cyst detection and viability assessment of Balantioides coli in environmental samples: Current status and future needs

“…The problem is compounded when considering identification of the parasite in an environmental sample, as trophozoites and resting cysts of other ciliates could be concomitantly present. Despite the increasing number of publications on the morphology and physiology of ciliate cysts ( Benčaťová and Tirjaková, 2017 ; Gutiérrez et al, 2001 ; Kaur et al, 2019 ; Verni and Rosati, 2011 ), the available data are comparatively insignificant respect to the estimated number of ciliate species ( Chao et al, 2006 ) and the cyst morphology of most of them remains unknown ( Kaur et al, 2019 ). The resting cysts of some free-living ciliates might resemble those of B. coli/Buxtonella sp.…”