USE OF GENOMIC EXCLUSION TO ISOLATE HEAT-SENSITIVE MUTANTS IN TETRAHYMENA

Eduardo Orias ¹ and Miriam Flacks ²

¹ Section of Biochemistry, University of California at Santa Barbara, Santa Barbara, California 93106
² Section of Molecular Biology, University of California at Santa Barbara, Santa Barbara, California 93106

We have used the abnormal form of conjugation known as "genomic exclusion" to isolate a collection of heat-sensitive mutants of Tetrahymena pyriformis, syngen 1. Growth at room temperature in bacterized medium and no growth at 40°C in the same medium was the criterion used for the isolation. The mutant strains were tested for growth in pure (axenic) culture in proteose peptone medium; of the 31 strains which grew normally at room temperature and not at 40°C in that medium, 21 also failed to grow at 37°C. Preliminary results of complementation tests suggest that most, if not all, the mutations are recessive and that a variety of genes was affected. A detailed genetic analysis was performed on one mutant (H9). The results are all consistent with the idea that the heat-sensitive phenotype of this mutant is determined by a single recessive mutation, designated ts-2. Heterozygotes ts-2/+ yield heat-sensitive segregants during vegetative growth; we interpret this finding as another example of allelic exclusion, a phenomenon universally encountered among heterozygotes in syngen 1 of T. pyriformis. Our results are discussed in the context of some questions of current interest in Tetrahymena genetics.