Resistance of germinal nucleus to aging in Paramecium: evidence obtained by micronuclear transplantation

A diversity of time when increase in mortality after conjugation occurs during the lifespan was found in subclones of three stocks of Paramecium caudatum. A possible micronuclear contribution to the increase in sterility has been investigated by micronuclear transplantation. We found two classes of micronuclei in aged clones: those that can function normally if the cytoplasmic environment is young, and those that cannot, even in a young cytoplasmic environment. The results indicate that in the former, the age-dependent increase in sterility is due to a deleterious macronucleus and/or cytoplasm, and in the latter it is due to micronuclear damage. The micronuclear damage in aged clones is probably induced by a deleterious cytoplasmic environment because aged clones with transplanted young micronuclei showed an abrupt decrease in progeny survival between 14 and 42 cell divisions after transplantation. Overall, the micronucleus seems not to be a source of age-related damage but rather is subjected to damage from macronuclear and/or cytoplasmic sources.