NON-MENDELIAN INHERITANCE OF MACRONUCLEAR MUTATIONS IS GENE-SPECIFIC IN PARAMECIUM-TETRAURELIA

Paramecium tetraurelia contains two types of nuclei, a diploid germina l micronucleus and a large transcriptionally active macronucleus. The macronuclear genome is formed from the micronuclear DNA during sexual reproduction. Previous studies have shown that the processing of the A -type variable surface protein gene during formation of a new macronuc leus is dependent on the presence of the A gene in the old macronucleu s. It is not clear if this is a general feature that controls the form ation of the Paramecium macronuclear genome or a unique feature of the A locus. Using micronuclear transplantation, we have constructed a st rain that has a wild-type micronucleus but has macronuclear deletions of the A- and B-type surface protein genes. Neither the A nor the B ge ne is incorporated into the new macronucleus after sexual reproduction . Macronuclear transformation of this strain with the B gene rescues t he B-gene deletion after formation of the next macronucleus but has no effect on the A deletion. Similarly, transformation with the A gene s hows gene-specific rescue for A but not B. The effect of the old macro nucleus on the processing of the new macronucleus results in a pattern of non-Mendelian inheritance of both macronuclear deletions. Progeny from the wild-type exconjugant are all wild type, and progeny from the A- B- exconjugant are mutant. The features of this A- B- non-Mendelia n mutant demonstrate that the regulation of macronuclear DNA processin g is gene specific, and our results open the possibility that this typ e of regulation affects many regions of the Paramecium genome.