Abstract EN:

The germline genome of ciliates is extensively rearranged during the development of a new somatic macronucleus from the germline micronucleus, after sexual events. In Paramecium tetraurelia, single-copy internal Eliminated Sequences (lESs) are precisely excised from coding sequences and intergenic regions. For a subset of IESs, introduction of the IES sequence into the maternal macronucleus specifically inhibits excision of the hornologous IES in the developing zygotic macronucleus, suggesting that epigenetic regulation of excision involves a global comparison of germline and somatic genomes. ScanRNAs (scnRNAs) produced during micronuclear meiosis by a developmentally regulated RNA interference pathway have been proposed to mediate this trans-nuclear crosstalk. In this study, microinjection experiments provide direct evidence that 25-nt micronuclear scnRNAs indeed promote IES excision. We further show that non-coding RNAs are produced from the somatic maternal genome, both during vegetative growth and during sexual events. Maternal inhibition of IES excision is abolished when maternal somatic transcripts containing an IES are targeted for degradation by a distinct RNA interference ] pathway involving 23-nt siRNAs. These data support a scnRNA/macronuclear RNA scanning mode! in which a natural genomic subtraction, occurring during meiosis -between deletion-inducing scnRNAs and antagonistic transcripts from the maternal macronucleus, regulates rearrangements of the zygotic genome