Persistent intraclonal variations in cell dry mass and DNA content in Paramecium aurelia

Abstract

1. 1. Different cell lineages in genetically homogeneous stocks of Paramecium aurelia differ from one another in the mass of the cell at division and in its macronuclear DNA content. 2. 2. These differences persist for varying periods of time during vegetative reproduction, but the descendant cells become randomized in respect to the ancestral cells in these properties after a number of days and many cell generations. 3. 3. There is a highly significant correlation between cell mass and DNA content. There is at most a small positive correlation, perhaps no correlation at all, between mass and division rate. Therefore, the differences in mass at division result from differences in cell growth rate, not from differences in division rate. 4. 4. A stock homozygous for a gene ( am) that causes unequal macronuclear division to be more frequent than usual shows a greater variability than the normal stock in the dry mass and in the DNA content of predivision cells. Again the two quantities are closely correlated. 5. 5. These data are interpreted to mean that the initial variable is the amount of DNA, and that this in turn affects the growth rate and consequently the cell size. 6. 6. No direct evidence for regulatory mechanisms for restoring a “normal” cell size has been obtained, but such regulatory mechanisms exist since the coefficients of variation do not continue to increase indefinitely. Regulation must be slow, however, in respect to the cell generation time. 7. 7. A clone of P. aurelia can be looked upon as consisting of a number of cell lineages undergoing irregular cyclic variations in cell size and DNA content as a result of repeated unequal division and opposing regulation towards normal. These regulatory cycles appear to last for several days or perhaps even longer and for many cell generations. 8. 8. Comparison is made with somewhat similar information for mammalian cells and for another ciliate, Tetrahymena pyriformis.