An amicronucleate mutant of Tetrahymena thermophila.

A stable amicronucleate strain of Tetrahymena thermophila was isolated following nitrosoguanidine mutagenesis. The mutant has the same growth rate and viability as the micronucleate parent strain, and has no micronucleus detectable by chromatin-specific staining in vegetative growth or during conjugation. The mutant pairs with normal efficiency with cells of complementary mating type. Matings of the mutant with aneuploid strains which lose their micronucleus during meiosis produced cell pairs yielding one viable and one inviable cell. The mutant receives a micronucleus from a normal mating partner, but this micronucleus is lost by the mutant cells within two hundred generations.