The differentiation of cellular structure during encystment in the soil hypotrichous ciliateAustralocirruscf.australis(Protista, Ciliophora)

Abstract

Ciliates are able to form resting cysts as a survival strategy in response to stressful environmental factors. Studies on the characteristics of cellular structure during encystment may provide useful information for further understanding of the regulatory mechanism of cellular patterns and supply new clues regarding the phylogeny of ciliates. Scanning and transmission electron microscopies were used to observe the ultrastructure of cells during encystment of the soil ciliate Australocirrus cf. australis. The dedifferentiation of ciliature was revealed for the first time. Ciliary shafts first shortened, and the remaining ciliature, including basal bodies and the fibrillar cirral basket, retracted into the cytoplasm and was surrounded by the autophagic vacuoles and then gradually digested. A large number of autophagic vacuoles were observed in mature resting cysts. Autophagy might not only be necessary for the differentiation of cellular structures during encystment but might also be important to sustain the basic life activities in the resting stage. Australocirrus cf. australis formed a kinetosome-resorbing cyst and contained four layers in the cyst wall: the ectocyst, mesocyst, endocyst and granular layer. The ciliature resorbing state and the number of layers in the cyst wall were consistent with those found in other oxytrichous ciliates. However, the phenomenon wherein the two macronuclear nodules are not fused during encystment is not commonly observed among oxytrichids. Additionally, the octahedral granules in the mesocyst of this species exhibit different morphology from the congeners. ARTICLE HISTORY

Ultrastructure of vegetative cells and resting cysts, and live observations of the encystation and excystation processes in Diophrys oligothrix Borror, 1965 (Protista, Ciliophora)

“…Similarly, unlike most other oxytrichids (Berger, ; Blatterer & Foissner, ; Kumar & Foissner, ), the two macronuclear nodules of Australocirrus cf. australis and Australocirrus shii did not fuse during encystment either (Kumar & Foissner, ; Li et al, ). These exceptions indicated that the behavior of macronuclear nodules is not concordant among the NKR‐ and KR‐ type of cysts.…”

“…When environmental conditions are suitable again, excystment occurs, and they return to an active state (Delgado, Calvo, & Torres, ; Grisvard, Lemullois, Morin, & Baroin‐Tourancheau, ; Pomajbíková et al, ; Rios, Sarmiento, Torres, & Fedriani, ; Tadao & Fumikazu, ). During encystment, somatic ciliature and microtubular organelles dedifferentiate; tubulin gene expression may also be decreased due to the decline in demand; the cyst walls gradually form; the shape of the macronucleus is often changed, along with chromatin condensation; and autophagic activity occurs in cytoplasmic organelles, namely, lysosomes and autophagic vesicles (Aslan, Küçükoğlu, & Arslanyolu, ; Cavaleiro, Fernandes, Silva‐Neto, & Soares, ; Li et al, ; Slabodnick et al, ; Wang et al, ; Wang, Wu, Fan, & Gu, ). During excystment, the ciliature is regenerated or reactivated, often with the activation of an excystment vacuole; certain genes and proteins related to initiating the excystment process and reconstructing the vegetative cell structure are upregulated (Funadani, Suetomo, & Matsuoka, ; Grisvard et al, ; Müller, ; Sogame et al, ).…”

“…Details of the morphology and physiology of cysts have been studied for many species (Verni & Rosati, ). Resting cysts differ in resistance and also have a great morphological and morphogenetic diversity (Kumar & Foissner, ; Li et al, ; Sun, Yun, Gu, & Fan, ). Mechanisms of encysting and excysting, as well as structural changes in the ciliature and nuclear apparatus during these two processes, depend on the species (Funadani et al, ; Müller, ).…”

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

“…According to new study by Li et al (2017), the ectocyst morphology of oxytrichids exhibits two different types of cyst surface, that is a spine-like protuberances (e.g. Rigidohymena tetracirrata, Laurentiella acuminata, Steinia sphagnicola, Oxytricha bifaria and others) or a slightly wrinkled surface of resting cyst (Gastrostyla steinii, Australocirrus cf.…”

“…by Berger (1999), Australocirrus cf. australis by Li et al (2017), Histriculus similis [now accepted as Sterkiella quadrinucleatus (Sick, 1933) Berger, 1999 by Calvo et al (1983), Calvo et al (1986) and others. In comparison with different taxonomic group of ciliates, e.g.…”