Non-coding RNAs mediate the rearrangements of genomic DNA in ciliates

Most eukaryotes employ a variety of mechanisms to defend the integrity of their genome by recognizing and silencing parasitic mobile nucleic acids. However, recent studies have shown that genomic DNA undergoes extensive rearrangements, including DNA elimination, fragmentation, and unscrambling, during the sexual reproduction of ciliated protozoa. Non-coding RNAs have been identified to program and regulate genome rearrangement events. In Paramecium and Tetrahymena, scan RNAs （scnRNAs） are produced from micronuclei and transported to vegetative macronuclei, in which scnRNA elicits the elimination of cognate genomic DNA. In contrast, Piwi-interacting RNAs （piRNAs） in Oxytricha enable the retention of genomic DNA that exhibits sequence complementarity in macronuclei. An RNA interference （RNAi）-like mechanism has been found to direct these genomic rearrangements. Furthermore, in Oxytricha, maternal RNA templates can guide the unscrambling process of genomic DNA. The non-coding RNA-directed genome rearrangements may have profound evolutionary implications, for ex- ample, eliciting the multigenerational inheritance of acquired adaptive traits.