Studies on the Maintenance of Development in Tetrahymena pyriformis GL‐C. I. An analysis of the mechanism of resorption of developing oral structures

Resorption of differentiating oral structures of Tetrahymena pyriformis GL‐C can be brought about by exposures to high and to low temperatures, to antagonists of RNA and of protein synthesis, and to inhibitors of energy metabolism. The resorption process is accompanied by a blockage (or reversal) of cell division furrowing. At a late developmental stage, oral primordium development and the cell division process become simultaneously stabilized, so that both processes can be completed (although often at a reduced rate) even in the presence of the agent. The stage at which stabilization takes place is independent of the agent employed. The sequence of events observed during the resorption process was generally the same regardless of the agent which brought the process about. However, the rate of resorption depended on the agent and also on the concentration at which it was employed. At a sublethal high temperature (34°C) and in the presence of antagonists of RNA and protein synthesis, resorption was rapid, being completed in an hour or less. On the other hand, in the presence of 2,4‐dinitrophenol (at a concentration just above the threshold for blocking cell division) resorption was extremely slow and incomplete. At low temperatures, resorption was also slow, the rate depending on the temperature; a linear relationship was obtained if the logarithm of the resorption rate was plotted against the reciprocal of the absolute temperature (Arrhenius plot). Following the addition of dinitrophenol or the onset of low temperature treatment, there was an initial interval during which oral development was blocked or considerably retarded, but oral primordium resorption had not yet commenced. If cells were returned to optimal growth conditions (by washing out the DNP or returning samples from the cold to 28°C) late in this interval, they underwent immediate and rapid resorption of oral primordia in which membranelle differentiation had begun. This resorption process always went to completion. Resorption is thus an irreversible process, which is in some way pre‐determined or triggered by dinitrophenol, low temperature, and probably the other agents also. The time required to bring about the triggering of resorption was ascertained by exposing synchronized cultures to DNP or low temperature treatments of differing durations. Resorption was triggered quite rapidly by DNP, and more slowly by low temperature. Furthermore, the time required to trigger resorption at 1°C was much longer than the time required at 9°C. Hence the triggering of resorption probably involves some metabolic activity, but not a substantial energy requirement. It is suggested, in the discussion, that the resorption process involves the localized enzymatic degradation of the developing oral structures and that the triggering of this process probably corresponds to the activation and/or mobilization of the degradative enzymes. Alternative hypotheses of the nature of this triggering mechanism are presented and discussed.