Arrest of micronuclear DNA replication during genomic exclusion in Tetrahymena produces haploid strains.

Diploid cells of Tetrahymena thermophila were crossed to strain AV, whose micronucleus is defective, to induce the unilateral transfer of gametic nuclei from the diploid cells to the AV cells (round I of genomic exclusion). These haploid nuclei presumably undergo one endomitotic cycle and then become diploid with a G1 (2C) DNA content. However, further DNA replication from 2C to 4C was transiently arrested until the pairs separated. When endomitosis was blocked by treatment with cycloheximide during 6-8 hours of conjugation, the exconjugants of round I of genomic exclusion remained haploid. Competence for diploidization is apparently limited to some period of time after nuclear transfer. Blocking of diploidization during round I of genomic exclusion can be used as an efficient way to induce haploid strains in Tetrahymena.