12/ This work is part of my PhD project at the @viennabiocenter.bsky.social‬. A big Thank You to @juliusbrennecke.bsky.social and Kirsten Senti for their supervision! To my co-authors Liudmila and @86dominik.bsky.social‬, and to the rest of the Brennecke lab!

11/ All in all, our work strongly suggests that the decision to process a transcript into piRNAs is not a feature of the genomic source locus but rather a decision that is taken in the cytoplasm after export of a TE antisense-containing transcript.

10/ And @astridhaase.bsky.social‬’s group uncovered compelling evidence that in mammals, including humans, antisense insertions of endogenous retroviruses downstream of genes also correlate with piRNA production suggesting that this mechanism is conserved. www.cell.com/cell-reports...

9/ Importantly, this is not restricted to flies, Bill Theurkauf and colleagues, showed that silencing of the invading KoRV-A retrovirus in wild koala populations strongly correlates with the presence of a single antisense insertion in a host gene 3′ UTR. www.sciencedirect.com/science/arti...

8/ Altogether, our data show that antisense insertions of TEs in host gene exons are sufficient to elicit a piRNA response, which expands the repertoire of piRNA source loci to host genes expressed in gonads in addition to “classical” piRNA clusters.

7/ To test this idea, we generated a synthetic construct in which we placed antisense tirant sequence into the 3′ UTR of a UAS-GFP transgene. When expressed in follicle cells, this transgene silenced tirant expression, confirming our model.

6/ This led us to propose a model in which antisense piRNAs can only be generated if a TE inserts itself into a host transcription unit (gene or piRNA cluster). This generates a cytoplasmic antisense RNA that is processed into piRNAs.

5/ Surprisingly, we found that a single antisense insertion of tirant in the 3′ UTR of a host gene, Fs(2)Ket, is sufficient to trigger piRNA production and silence tirant. piRNA production from this insertion requires host gene transcription and a chimeric mRNA-TE transcript.

4/ Some strains lack tirant insertions in flamenco or in any other known piRNAs cluster. However, they are still able to produce piRNAs and silence tirant. So where are these piRNAs coming from?

3/ We first show that tirant is exclusively expressed in somatic cells of the ovary. Here, most strains also produce tirant piRNAs from tirant insertions in flamenco, the major piRNAs cluster active in that tissue. But this is not true in all strains …

2/ Over the past century, several TEs invaded Drosophila melanogaster (thanks to the work of @rokofler.bsky.social‬). We traced how natural strains acquired piRNA immunity against one of these TEs, tirant, an LTR retrotransposon.