Regulation of cell cycle duration, cell size and life history in the ciliate sterkiella histriomuscorum

As eukaryotic cells grow and divide, their size remains within a limited range despite changes in resources available. Understanding cell cycle regulation can help understand how size constancy is achieved and provide the molecular and physiological clues to size regulation. However, it is not always obvious how to reconcile the physiological behaviour of cells with this molecular information. We chose the ciliate Sterkiella histriomuscorum (Oxytrichidae) to obtain new insights into size regulation. This organism re-adjusts its cell dimensions as resources diminish. It exits the cell cycle for conjugation or encystment when resources are scarce. A general model is proposed for ciliate cell cycle regulation and the cell cycle of Sterkiella was described within this context. Conditions for inducing conjugation, encystment and excystment were determined to obtain synchronous cultures, and to describe the timing of each process. Observations on the effect of clonal ageing on each process is described. These allow one to manipulate the organism predictably in the laboratory. The consequences of changes in food particle size, prey species, prey abundance and temperature on the size and cell cycle duration of Sterkiella were explained using Michaelis- Menten kinetics. Two parameters (Pmin and Pdiv) were then described and calculated, that were useful in predicting tendencies of cultures with one or two prey species. Two approaches were taken to understand this regulation of cell size physiologically. First, an attempt was made to find out if cAMP is an important regulator of cell cycle progression and duration of growth. Some ciliate "fat" mutants were rescued by drug treatments with inhibitors of kinases. Second, the c d mutant was rescued by transformation with a partial DNA library from wild type Paramecium. However, the rescuing element was not the c d gene. Finally, cell size regulation in several protists is provided with a unified jargon, from a physiological and molecular perspective. This allowed for the first time, a comparison of regulatory processes between species and the development of several ideas regarding cell size regulation. Several principles are proposed as generalities for testing.Science, Faculty ofZoology, Department ofGraduat