Temperature adaptation of the growth and division process of Tetrahymena pyriformis: II. Relationship between cell growth and cell replication

Abstract

Replication rates and volume distributions of Tetrahymena pyriformis GL determined with a commercially available electronic cell counter are reported for several temperatures. Tetrahymena pyriformis GL was cultured in proteose peptone yeast extract medium at 28.0, 30.8, 32.5, and 33.9 °C. During the period of exponential cell replication at all temperatures, the volume distribution was constant, lognormal and the standard deviation from the median volume was temperature independent. This indicates a steady state of balanced growth at 28.0, 30.8, and 32.5 °C during exponential cell replication. It is demonstrated that the knowledge of volume distribution and generation time is insufficient for calculating rates of cell growth between divisions unless the parameters for the momentary volume distribution can be measured independently. Statistical evaluation of data from the literature confirms previously reported findings which indicate that growth of Tetrahymena over extended periods of the division cycle is approximately exponential and not linear. The previous proposition that control mechanisms for growth of Tetrahymena and Paramecium are different, is thus unnecessary. A new model, which describes the control of cell growth and cell replication is suggested. The model accounts for the lognormal distributions of generation times and volumes as well as the nearly exponential rate of growth between divisions. The mean rate of cell replication which decreases with increasing temperatures above the optimum rather than all temperatures shows a negative correlation with the median log volume of the culture. This is in accordance with the predictions of the proposed growth model and furthermore indicates that the growth rate is to a large extent independent of cell replication.