Macronuclear persistence of sequences normally eliminated during development in Tetrahymena thermophila

During conjugation in the ciliated protozoan, Tetrahymena thermophila , a somatic MAC‐ronucleus develops from the germinal MICronucleus. Ten to 20 percent of the MIC genome is eliminated during this process. Three repetitive families have been identified which have different levels of repetition in the MIC and are eliminated to different degrees in the MAC. Some members of two of these families persist in the MAC. In this study, we have looked at these persistent sequences in the MAC of cell lines from a variety of sources including several inbed strains, two sets of caryonides, caryonidal subclones, and vegetatively aged cell clones. The results suggest that the sequences that remain in the MAC have a genetic predisposition to persist. However, epigenetic variations occur as the MAC develops so that only some of the persistent sequences are actually observed in a particular MAC. Polymorphisms may be generated if alternative processing of a single MIC segment occurs. These polymorphisms can later be resolved by phenotypic assortment during vegetative growth. These facultatively persistent sequences appear to differ from sequences previously described in this organism.