Encystment/excystment response and serotypic variation in the gastropod parasite Tetrahymena rostrata (Ciliophora, Tetrahymenidae)

Abstract

Tetrahymena rostrata, which is characterized by a particular encystment-excystment cycle involving autogamy, has been recently found infecting the kidney of edible Helix aspersa snails under farming conditions. In this work, the effects of several factors on its encystment/excystment behaviour and the occurrence of different serotypes were investigated. The encystment/excystment response under starvation conditions was seriously affected by temperature. While a peak of encystment at 48 h followed by a progressive spontaneous excystment was observed at 18 and 25 °C, the encystment response was practically inhibited at 5 °C and clearly slowed down at 10 °C. At 30 °C, most of surviving ciliates remained encysted throughout the experiment, with spontaneous excystment being detected only after switching the temperature to 18 °C. Soil components also affected the encystment/excystment behaviour at 18 °C, with spontaneous excystment occurring in the presence of a sterile-filtered soil extract or mineral water but being strongly minimized with a non-filtered soil extract. Resting cysts formed in the latter extract exhibited a 3–4 times thicker and ultrastructurally more complex wall than that formed in mineral water and retained the excystment ability for about 4 weeks. Incomplete desiccation did not affect significantly the encystment response, while the mucus and kidney extracts from snails as well as a ciliate extract strongly stimulated a rapid excystment. Finally, two different serotypes infecting H. aspersa in heliciculture farms of Galicia (NW Spain) were identified, but no differences were observed between the encystment/excystment responses exhibited by two isolates belonging to each serotype.

Infection of Slugs with Theronts of the Ciliate Protozoan, Tetrahymena rostrata

“…The triggers for ciliate excystment are highly variable [ 21 ]. For T. rostrata , triggers of excystment include media containing peptone and tryptone or milk, kidney extract, mollusk mucous and changes in temperature or salinity [ 3 , 4 , 22 ]. Spontaneous excystment of reproductive cysts is frequent, indicating that metabolic triggers also exist [ 4 , 21 , 22 ].…”

“…For T. rostrata , triggers of excystment include media containing peptone and tryptone or milk, kidney extract, mollusk mucous and changes in temperature or salinity [ 3 , 4 , 22 ]. Spontaneous excystment of reproductive cysts is frequent, indicating that metabolic triggers also exist [ 4 , 21 , 22 ]. Our method of preparation of cysts results in highly efficient, synchronized encystment with almost 100% of cells encysting.…”

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

“…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 ).…”

Intraspecies Variation in Tetrahymena rostrata

“…However, the life cycle of T. rostrata is different from ciliates which have genetic exchange via conjugation. During encystment-induced autogamy, the pronuclei from the same cell fuse and the new macronuclei derived from them are expected to be homozygous [6,26].…”