Figure 4.

The germinal piRNA pathway within and downstream of the Pilp. (A) Schematic structure of a germarium with an egg chamber. The germline stem cells (pink) give rise to the cystoblast (black) which divides four times (light grey) to form a cyst of sixteen cells (dark grey), one of which differentiates into the oocyte (red). The germinal cells are surrounded by somatic follicular cells (green). (B) In the Pilp, Piwi is downregulated leading to an increase of TE transcription (blue box). TE transcripts may have two fates: they are translated and engage the TE in a new replication cycle; they are processed in piRNAs and boost the ping-pong cycle through their complementarity to transcripts produced from piRNAs cluster (red box). (C) In germ cells downstream of the Pilp, Piwi is present and mediates TE repression through Transcriptional Gene Silencing (TGS). TE transcription is then reduced and only a few secondary piRNAs are produced from the ping-pong cycle.