Maternal Depletion of Piwi, a Component of the RNAi System, Impacts Heterochromatin Formation in Drosophila (original) (raw)
Figure 6
Model for Piwi's role in the heterochromatin formation and PEV reporter silencing.
In wild type embryos, Piwi guides HP1a deposition to a subset of TEs in heterochromatin, maintaining normal levels of silent marks. PEV reporter silencing is dependent on the spreading of heterochromatin components from the heterochromatin mass. During development, the silent state is maintained during mitosis independent of Piwi. When Piwi is depleted at the critical stage of heterochromatin establishment in the embryos, the HP1a level in heterochromatin in this region is lower; the reduced HP1a enrichment results in loss of silencing of the sensitive PEV reporter, a state that is propagated to adult animals. Depletion of HP1a in the early embryo has a similar effect, even when HP1a is present at normal levels during mitotic cell divisions. Piwi may act at the border regions between euchromatin and heterochromatin in particular, to prevent aberrant transcription, making PEV reporters particularly sensitive to the depletion of Piwi.