Abstract
Chloramphenicol-resistant mutants of Tetrahymena pyriformis were obtained after mutagenesis with nitrosoguanidine at an estimated frequency of 10(-3) mutants per mutagenized cell. The mutants are still partially sensitive to chloramphenicol and have a lowered growth rate, compared to the wild type strain, in rich medium without chloramphenicol. The genetic analysis described here indicates that chloramphenicol resistance is inherited as a cytoplasmic determinant that is not exchanged during conjugation. This represents the first simple cytoplasmic genetic determinant described in this species. A number of arguments favor a mitochondrial localization for this genetic determinant. In addition to the possible utility of such mutants for studies of mitochondrial structure and function in Tetrahymena, analogous mutations might serve as cytoplasmic "tags" in other ciliate species where the results of selfing need to be distinguished from those of outcrossing.