The role of protein synthesis in prefission morphogenesis of Paramecium aurelia

The effect of cycloheximide on formation of contractile vesicles, development of buccal anlage, and fission in Paramecium aurelia was examined. Paramecium exhibits remarkably strong resistance to this inhibitor of protein synthesis. With continuous exposure to lower concentrations (0.14 to 0.94 mg/ml), the daily fission rate is reduced in proportion to the concentration of cycloheximide and is maintained at a constant level. Exposure to higher concentrations (1.83 mg/ml) starting at various specific times in the interfission cycle reveals transition and stabilization points for each morphogenetic event; exposure prior to the transition point completely blocks development of a given event, but after the stabilization point exposure does not affect morphogenesis, and between these points blockage may or may not take place. The formation of the contractile vesicles is a morphogenetic process distinct from other events related to cell division. It occurs earlier than oral morphogenesis and fission and occurs in a step‐wise fashion with the anterior vesicle appearing first and then the posterior one. For fission and buccal development the transition and stabilization points are very similar to those found previously for actinomycin D treatments, suggesting that translation follows immediately on transcription. Resorption of new vesicles was observed; buccal anlage were never resorbed.