Abstract
Somatic cycloheximide-resistant mutants of syngen 1 of Tetrahymena pyriformis were isolated and genetically characterized. Two properties of the mutants were independently examined: (a) The transmission of the mutant phenotype during conjugation and (b) the kinetics of phenotypic assortment during vegetative propagation. The results of both studies strongly support the idea that these somatic mutations have a macronuclear location. The kinetics of assortment are consistent with the idea that the syngen 1 macronucleus contains about 45 assorting genetic units. The sib-selection method employed here, used in conjunction with the analysis of assortment kinetics and a previously described test for randomness of distribution, provides a probe of macronuclear genetics applicable to many ciliates, including those in which conjugation is not known to occur or is not under experimental control.