Aub, Vasa and Armi concentration in phase separated nuage is dispensable for piRNA biogenesis and transposon silencing

Abstract

The piRNA biogenesis machinery localizes to phase separated nuage granules, but nuage function is not well understood. We therefore assayed nuage composition, piRNA expression and transposon silencing in Drosophila mutants that disrupt piRNA precursor production and nuclear export, ping-pong amplification and phased piRNA biogenesis. These mutations destabilize the genome and activate Chk2 signaling and chk2/mnk double mutants were therefore analyzed in parallel. Aub and Vasa are required for ping-pong amplification and Armi promotes phased piRNA processing. We show that Chk2 activation releases Aub and Vasa from nuage and that piRNA precursors are required for nuage localization of the ping-pong and phased biogenesis machinery. However, this analysis also indicates that Vasa, Aub, and Armi concentration in nuage is dispensable for piRNA production and transposon silencing, indicating that dispersed cytoplasmic proteins can drive these processes. We speculate that nuage sequesters silencing effectors, which are released by Chk2 in response to transposon mobilization.