The Cell Cycle of Euplotes aediculatus (Ciliophora) at Various Population Densities

Abstract

The population density in cultures of the ciliate Euplotes aediculatus directly influences the rate of asexual population growth. At continuous exchange of medium and continuous supply of food the cultures grow to a density of 2100 cells/ml ± 100 (standard deviation) that is retained at further cultivation. The mortality rate is not substantially increased at this steady population density, instead the cell cycle time is lengthened from 37 hours to about 500 hours. Changes of the duration of individual cell cycle phases with changing reproduction rates were studied by light microscopy of the morphology of the cells and their macronuclei. The M phase had a duration of 1.9 ± 0.1 hours, independent of the population density. The G 1 phase of the macronucleus increased in relative proportion of the cell cycle from 34% at a low population density (below 100 cells/ml) to approximately 70% at a high population density (above 1500 cells/ml). It increased in length from 13 hours to approximately 350 hours. The S-phase of the macronucleus was shortened in relative proportion from 61% at a low population density to approximately 30% at a high population density. Thus the composition of the cell cycle changed with increasing population density to a prolonged G 1 phase. The absolute duration of the macronucleus S phase increased significantly (from 23 to approximately 150 hours) with increasing population density in contrast to its shortened relative proportion of the cell cycle. Accordingly the S-phase is regulated after its start in E. aediculatus just as the G 1 phase. Apparently at high population density most of the cells of the populations reside in G 1 , possibly they enter a G 0 state. The observations imply a direct regulation of the cell cycle by the population density in this unicellular organism.