Abstract
ABSTRACTCiliates 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.
Highlights
Ciliated protists are eukaryotic unicellular organisms with wide distributions in various habitats (Gu 1991; Berger 1999, 2006, 2011)
Based on the extent of resorption of the ciliature after encystment, resting cysts of hypotrichids are classified into three typical groups: (1) non-kinetosome-resorbing cysts of euplotids; (2) partial-kinetosome-resorbing cysts of urostylids; and (3) kinetosome-resorbing cysts of oxytrichids (Grimes 1973; Rios et al 1985; Berger 2006; Zhao et al 2009)
Matsusaka et al (1984) revealed that the axonemes and basal bodies disintegrated after they retracted into the cytoplasm in Histriculus muscorum
Summary
Ciliated protists are eukaryotic unicellular organisms with wide distributions in various habitats (Gu 1991; Berger 1999, 2006, 2011). Ciliates in many groups form resting cysts when cells are subjected to stressful environmental conditions, such as starvation, high population density, and salinity variation; whereas excystment occurs when environmental conditions become favorable. This process is typically regarded as a survival strategy of ciliates responding to their living conditions (Gu et al 1996; Li et al 2012). During such encystment and excystment, cellular structures undergo dedifferentiation and redifferentiation (Grimes 1973; Walker et al 1975, 1980; Walker & Maugel 1980). Ultrastructural characteristics may provide further insight to the taxonomy and phylogeny of hypotrichous ciliates (Berger 1999, 2006, 2011)
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