Novel features of computer-simulated clonal life of Paramecium caudatum.

The clone of the ciliated protozoan Paramecium caudatum has the immaturity period of about 60 fissions and the lifespan of about 600 fissions. These life cycle figures have been depicted through laboratory experiments that allow continuous cell divisions for hundreds, which never occur in nature. We here constructed the nature-mimicking model culture that alternated the log- and stationary phases to allow conjugation, and computer-simulated the age structure modifying parameters such as cell distributions to start the culture, fission rates, death rates, immaturity periods, probabilities of conjugation, proportions of transplantation and so on. The average and maximum ages in the culture after thousands of alternations were converged to 43+/-2 and 140+/-5 fissions, respectively, when parameters for the immaturity period and the maximum clonal lifespan were set at 60 and 600 fissions. This result explains why cells collected in nature are usually young and vigorous. The average and maximum ages proportionally prolonged as the immaturity period was prolonged, as reported true for species of the ciliate. These results indicate the validity of our simulation. The average and maximum ages remained unchanged when the initial condition for starting the culture was changed from two complementary mating-type cells to a population with a quadratic-function distribution, and when the fission rate at the log-phase and the death rate at the stationary phase were modified for older ages. The average and maximum ages changed slightly when either the conjugation rate or the proportion of transplantation was somewhat lowered. Although they changed considerably when such parameters as the immaturity period, conjugation rate and death rate were extremely modified, no clones with the age over 230 fissions appeared in any simulations. These results indicate the robustness of the model, which provides us with fresh insight into the structural system of the clonal lifespan of P. caudatum in nature.