Small RNA-Mediated trans-Nuclear and trans-Element Communications in Tetrahymena DNA Elimination

Epigenetic inheritance of acquired traits is widespread among eukaryotes, but how and to what extent such information is transgenerationally inherited is still unclear. The patterns of programmed DNA elimination in ciliates are epigenetically and transgenerationally inherited, and it has been proposed that small RNAs, which shuttle between the germline and the soma, regulate this epigenetic inheritance. In this study, we test the existence and role of such small-RNA-mediated communication by epigenetically disturbing the pattern of DNA elimination in Tetrahymena. We show that the pattern of DNA elimination is, indeed, determined by the selective turnover of small RNAs, which is induced by the interaction between germline-derived small RNAs and the somatic genome. In addition, we show that DNA elimination of an element is regulated by small-RNA-mediated communication with other eliminated elements. By contrast, no evidence obtained thus far supports the notion that transfer of epigenetic information from the soma to the germline, if any, regulates DNA elimination. Our results indicate that small-RNA-mediated trans-nuclear and trans-element communication, in addition to unknown information in the germline genome, contributes to determining the pattern of DNA elimination.