EFFECT OF CYCLOHEXIMIDE ON THE ENCYSTMENT AND ULTRASTRUCTURE OF THE CILIATE, HISTRICULUS

The effect of the protein synthesis inhibitor cycloheximide on the experimentally induced synchronous encystment of a hypotrich ciliate, Histriculus, was examined and the cytoplasmic ultrastructures of the control and the inhibitor-treated organisms were compared. Encystment was divided into the following 5 stages based on observations of living cells: stage 1, cells with brown cytoplasm; stage 2, cells with smooth, transparent cytoplasm; stage 3, spindle-shaped cells; stage 4, spherical cells without cyst walls; and stage 5, young cysts with cyst walls. When cycloheximide treatment was preceded by stage 2, encystment and transformation into the next stages were totally blocked, whereas even in the presence of the inhibitor, stage 4 and stage 5 cells encysted normally, and stage 3 cells transformed into stage 4 although they did not form cyst walls. On the basis of ultrastructural studies it is suggested that the formation and excretion of ectocyst precursors are severely inhibited by cycloheximide and that polysome formation, active in stages 1 and 2, may be almost finished by stage 3. The role of lysosomal enzymes in the early stages of encystment is discussed.