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

Ciliated protists can form cysts to resist unfavorable environmental conditions and then excyst when environmental conditions become favorable. This study used electron and light microscopy to investigate the structure of vegetative cells and resting cysts, as well as the encysting and excysting processes of Diophrys oligothrix. For the first time, ampules were revealed beneath the pellicle in the genus Diophrys, and their extrusome types differed between Diophrys species. Membrane‐packed discs of diverse shapes were found in the cytoplasm just beneath the pellicle around the cytopharynx and were separated by rows of microtubule units. Beneath the discs, some double‐layer microtubule structures were detected as well. During encystment, the ventral ciliature was folded in a ventral cavity of the cell, and the caudal cirri were retracted directly into the cyst in a separate cavity on the dorsal side. In the resting cysts, high autophagic activity occurred, possibly including digestion of membrane‐packed discs and ampules. Two macronuclear nodules kept their basic shape, although the chromatin aggregation and fusion region were observed in ultrathin sections. The cyst wall contained two layers, namely, the ectocyst, and endocyst. In mature cysts, basal bodies and ciliary shafts were observed, demonstrating that D. oligothrix forms non‐kinetosome‐resorbing cysts. The process of excystment occurred in two modes, either with or without participation of a contractile vacuole. Ciliated protozoa form resting cysts in responding to stressful environmental conditions. During encystation, Diophrys oligothrix form non‐kinetosome‐resorbing cysts with two cyst wall layers and two non‐fused macronuclear nodules. The autophagy happens and involves several organelles including membrane‐packed discs and ampules. There are two ways of excystation according to whether the contractile vacuole participates.