Non-Mendelian inheritance of macronuclear mutations is gene specific in Paramecium tetraurelia.

Abstract

The ciliated protozoan Paramecium tetraurelia possesses two kinds of nuclei within a single cell. The highly polyploid macronucleus is transcriptionally active and therefore determines the phenotype of the cell. In contrast, the two diploid micronuclei are transcriptionally silent but participate in the nuclear reorganization events of autogamy (self-fertilization) and conjugation (reciprocal exchange of nuclei with cell of the opposite mating type). During either of these events, a new macronuclear genome is produced from the micronuclear genome through a process which involves extensive amplification and rearrangement of micronuclear DNA. This process of macronuclear development results in differences in the genetic content of the two types of nuclei.The previously isolated mutant d48 provides an example of how the macronuclear content in the Paramecium cell can differ from the micronuclear content.

Sequence-Specific Epigenetic Effects of the Maternal Somatic Genome on Developmental Rearrangements of the Zygotic Genome inParamecium primaurelia

“…A mechanism involving pairing has also been proposed to account for the ability of several nonoverlapping DNA segments from the A gene, after injection into the d48 maternal macronucleus, to prevent A gene terminal deletion of the developing macronucleus, thus permanently rescuing the d48 macronuclear mutation in sexual progeny (21,48). The experiment has been repeated with a macronuclear deletion of the B gene, a related surface-antigen gene; despite the fact that the rescuing ability is not confined to a particular sequence within the A gene, a study of cell lines carrying both A gene and B gene macronuclear deletions has shown that the rescue mechanism is gene specific (41), which supports the pairing hypothesis. Finally, a formally similar trans-nuclear effect, the inhibition of excision of some IESs by homologous sequences present in the maternal macronucleus, may also involve pairing (13a, 14).…”

“…Which rearrangement takes place in the developing macronucleus is determined by the old macronucleus through unknown trans-acting factors that can be detected in the cytoplasm at the time of nuclear reorganization (25). A similar maternal inheritance has also been observed for variant rearrangement patterns in the B gene region (41) and for the excision of some IESs, which can be excised in the developing macronucleus only if they are absent from the old macronuclear genome (13a, 14).…”

RNA-Mediated Programming of Developmental Genome Rearrangements in Paramecium tetraurelia

“…Like terminal deletions of the A gene and other subtelomeric genes (26,36), we found that dsRNA-induced internal deletions of the ND7 gene are spontaneously reproduced in subsequent sexual generations. Epigenetic inheritance of experimentally induced macronuclear deletions may thus be quite general.…”

“…Indeed, introduction of the missing A gene into the d48 macronucleus can rescue the developmental amplification of the germ line A gene in sexual progeny (18,21,42). Epigenetic inheritance of macronuclear deletions has also been observed with other genes (26,36). By using a cell line carrying macronuclear deletions for two different genes, the rescue effect was shown to be gene specific (36).…”

“…Epigenetic inheritance of macronuclear deletions has also been observed with other genes (26,36). By using a cell line carrying macronuclear deletions for two different genes, the rescue effect was shown to be gene specific (36). Since maternal and zygotic nuclei do not fuse, a gene-specific signal must be transmitted through the cytoplasm.…”