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Genomic exclusion (GE) is an abnormal form of conjugation which has previously been described in detail for three hypodiploid strains of Tetrahymena thermophila. These strains cannot form gametic nuclei and by failing to participate in normal reciprocal fertilization their genes are excluded from exconjugants. To determine whether GE is a general property of infertile strains, we surveyed genetically and cytogenetically 19 additional strains of T. thermophila to determine why they failed to contribute genes to sexual progeny. Crosses to genetically marked tester strains showed that seventeen of these strains undergo GE. In each case GE appears to be due to the failure of the defective partner to form functional gametic nuclei. The normal conjugant, however, contributes to its defective partner a haploid nucleus identical to its own, and following diploidization of the unfertilized nuclei, the conjugants separate retaining the old macronuclei. Cytofluorimetric measurement of micronuclear DNA content in 18 strains suggests that aneuploidy is the proximate cause of GE; eleven strains were hypodiploid, five were diploid and three were hyperdiploid. Many irregular cytogenetic events were observed in conjugants presumably not undergoing GE, including, in some instances, abnormal meiosis in the normal partner. Since genomic exclusion was found in both wildtype and mutant clones, the results suggest that it should be possible by appropriate crosses to identify genomic exclusion strains of any genotype.

Genomic exclusion and other micronuclear anomalies are common in genetically defective clones of tetrahymena thermophila.