Life cycle variation and regulation of macronuclear DNA content in Tetrahymena thermophila

The mean DNA content of G2 macronuclei varies during the life cycle of the ciliate Tetrahymena thermophila. Early in the life cycle the mean is about 130 C; later it is about 94 C. In hybrids between strains A and B the decrease from 130 C to 94 C usually began after 60 fissions after conjugation. In B X B clones the decrease was complete by 50 fissions. The data suggest that there may be a genetic difference between strains A and B with respect to the onset of the decrease in DNA content. The downward regulation of the mean DNA content appears to be related to the mechanism which removes the variance in macronuclear DNA content which is added to macronuclei by unequal macronuclear division. Unequal macronuclear division regularly occurs at all stages of the life cycle, with larger macronuclei tending to divide more unequally. In the absence of regulation, unequal macronuclear division would constantly add variance to G1 macronuclei and their range would continue to increase. Analysis of the variances of G1 and G2 macronuclei suggests that at all stages of the life cycle the added variance is removed by acting upon nuclei which become too small or too large. According to this model, macronuclei with smaller amounts of DNA are regulated upward by an extra macronuclear S phase, while larger amounts are regulated downward by chromatin extrusion and the skipping of macronuclear S. The mean DNA content appears to change during the life cycle because the thresholds at which macronuclei become too small or too large are readjusted. It is postulated that these thresholds are a function of gene dosage.