Effects of cyst age on excystment of the ciliate Histriculus cavicola (Kahl, 1935) (Ciliophora: stichotrichia)

Emergence of the terrestrial ciliate Colpoda cucullus from a resting cyst: rupture of the cyst wall by active expansion of an excystment vacuole

The first sign of excysting Colpoda cucullus cells is the initiation of the pulsation of a contractile vacuole, which is then replaced by a non-pulsating vacuole (excystment vacuole) that continues to expand and finally ruptures the outermost cyst wall (ectocyst) due to inner pressure. A ciliate surrounded by flexible membranes (endocyst) thus emerges. The osmolarity of the excysting cells is estimated to be 140 mOsm L⁻¹ from the relationship between the frequency of contractile vacuole pulsation and the external sucrose concentration. Both the expansion of the excystment vacuole and the emergence of ciliates occurred even when the cysts were immersed in hypertonic medium. In hypotonic medium containing sodium azide (NaN₃, a cytochrome c oxidase inhibitor), the contractile vacuole of vegetative cells stopped pulsating and gradually expanded, causing cells to burst. When C. cucullus was induced to encyst in a hypotonic medium containing NaN₃, the expansion of the excystment vacuoles was inhibited. These results suggest that the active uptake of water may be responsible for the expansion of the excystment vacuole required for the ectocyst to rupture.

Live observation of excystment in the spirotrich ciliate Meseres corlissi

Excystment of Meseres corlissi Petz and Foissner 1992 was observed by light microscopy. Excysting cells were first recognized by the appearance of a non-pulsating vacuole. With increasing size of this vacuole, the outer cyst wall ruptured, creating a narrow slit. The emerging protoplast was enclosed by an inner flexible membrane. Oral membranelles were faintly visible, but inactive at this stage. Within the next hour, oral membranelles and a pulsating vacuole became active, and the ciliate started to rotate. During a phase of rapid movement, the flexible membrane became wider and thinner. Finally, it burst open and released the now fully differentiated ciliate. The emergence of M. corlissi from the cyst is similar to descriptions in the literature for Didinium nasutum, Nassula ornata, Tillina magna and several stichotrich ciliates.

Timing of life cycle morphogenesis in synchronous samples of Sterkiella histriomuscorum. II. The sexual pathway

Isolates of Sterkiella (Oxytrichidae, Stichotrichia, Ciliata) are commonly used to study macronuclear development. These organisms respond to changes in food abundance variably by encystment-excystment, conjugation, cannibalism or rescaling cell size. An isolate of Sterkiella histriomuscorum (previously Oxytricha fallax and O. trifallax) is used because two complementary mating types are available. We provide observations on conjugation in cultures of this isolate. Using synchronous samples of conjugants, the timing of stages of nuclear divisions during conjugation was determined. Following ex-conjugant cultures over time, the onset of clonal aging and senescence is described. Cells become sexually mature after a brief period of "adolescence", during which time selfing is possible. Senescent cultures are less vigorous, unable to conjugate and encyst more readily. Excystment survival decreases with clonal age. These results can serve as reference for long-term cultures of this species and for analysing particular stages of developmental processes during conjugation.

Temporal suspension of excystment by salt solution in the ciliate, Histriculus cavicola (Kahl, 1935) (Ciliophora: Stichotrichia)

Cysts were incubated in excystment medium (EM) for varying periods and then transferred to salt solutions or to 5 μg/ml cycloheximide (CH). When the transfer was to salt solutions, almost no cell excysted within 24 h after initiation of incubation in EM if the incubation was shorter than 30 min, but the proportion of excysted cells increased with increasing incubation time longer than 40 min. When the transfer was to CH, no cell attained stage 2 of excystment after incubation in EM shorter than 30 min but normal excystment ensued after an incubation longer than 70 min. If incubation in EM was between the above mentioned periods, some cells attained stage 3 but never excysted. Kinetosome formation occurred after incubating cysts for 30 min in EM, regardless of the presence or absence of CH. Formed kinetosomes, however, disappeared if cysts were maintained in salt solutions for 210 min or in CH for 240 min. These results indicate that: 1) excystment requires protein synthesis and incubation of cysts in EM longer than 30 min, 2) kinetosome formation does not need protein synthesis and does not necessarily direct cysts to complete excystment, and 3) maintenance of new kinetosomes requires continuation of the excystment process.

Reversibility of encystment in the ciliate, Histriculus cavicola (kahl, 1935) (ciliophora: stichotrichia)

An early phase of encystment of the ciliate, Histriculus cavicola is subdivided into 5 stages, stages A to E. Resorption of feeding organelles, undulating membranes (UM) and the adorai zone of membranelles (AZM) start from stage B. In stage E cells, resorption of UM is completed and that of the AZM reaches to the level of anterior end of left marginal cirral row from its posterior end. When stage A cells are transferred in culture medium with or without food organisms, or in normal or 10 times concentrated Osterhout's solutions with food, most cells do not encyst. More than half of stage C cells do not encyst at 20°C after transfer in culture medium. All stage E cells encyst, irrespective of the medium or of the presence or -the absence of food into which they are transferred. About half of stage A cells and all stage B to E cells contain ectocyst precursors. Our results indicate that initiation of ectocyst precursor formation does not necessarily direct the cells to encyst but cells in stage E are determined to encyst.