Résumé
The ciliated protozoan Tetrahymena undergoes programmed DNA elimination during sexual reproduction, a process determined by transgenerational communication between the germline and soma. This study by Mochizuki and colleagues provides long-awaited deep-sequencing analysis of the scnRNAs that mediate this epigenetic inheritance. The pattern of DNA elimination is found to be influenced by two factors: biased transcription of the germline genome, and selective degradation of the small RNAs complemetary to the somatic genome. The ciliated protozoan Tetrahymena undergoes extensive programmed DNA elimination when the germline micronucleus produces the new macronucleus during sexual reproduction. DNA elimination is epigenetically controlled by DNA sequences of the parental macronuclear genome, and this epigenetic regulation is mediated by small RNAs (scan RNAs [scnRNAs]) of ∼28–30 nucleotides that are produced and function by an RNAi-related mechanism. Here, we examine scnRNA production and turnover by deep sequencing. scnRNAs are produced exclusively from the micronucleus and nonhomogeneously from a variety of chromosomal locations. scnRNAs are preferentially derived from the eliminated sequences, and this preference is mainly determined at the level of transcription. Despite this bias, a significant fraction of scnRNAs is also derived from the macronuclear-destined sequences, and these scnRNAs are degraded during the course of sexual reproduction. These results indicate that the pattern of DNA elimination in the new macronucleus is shaped by the biased transcription in the micronucleus and the selective degradation of scnRNAs in the parental macronucleus.