Impact of nuclear Piwi elimination on chromatin state in Drosophila melanogaster ovaries

The Piwi-interacting RNA (piRNA)-interacting Piwi protein is involved in transcriptional silencing of transposable elements in ovaries of Drosophila melanogaster. Here we characterized the genome-wide effect of nuclear Piwi elimination on the pres-ence of the heterochromatic H3K9me3 mark and HP1a, as well as on the transcription-associated mark H3K4me2. Our results demonstrate that a sig-nificant increase in the H3K4me2 level upon nu-clear Piwi loss is not accompanied by the alterations in H3K9me3 and HP1a levels for several germline-expressed transposons, suggesting that in this case Piwi prevents transcription by a mechanism distinct from H3K9 methylation. We found that the targets of Piwi-dependent chromatin repression are mainly related to the elements that display a higher level of H3K4me2 modification in the absence of silenc-ing, i.e. most actively transcribed elements. We also show that Piwi-guided silencing does not signifi-cantly influence the chromatin state of dual-strand piRNA-producing clusters. In addition, host protein-coding gene expression is essentially not affected due to the nuclear Piwi elimination, but we noted an increase in small nuclear spliceosomal RNAs abun-dance and propose Piwi involvement in their post-transcriptional regulation. Our work reveals new as-pects of transposon silencing in Drosophila, indicat-ing that transcription of transposons can underpin their Piwi dependent silencing, while canonical het-erochromatin marks are not obligatory for their re-pression