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

Diploid cells of Tetrahymena thermophila were crossed to strain A*V, whose micronucleus is defective, to induce the unilateral transfer of gametic nuclei from the diploid cells to the A*V 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.