Retrovirus insertions in host transcripts trigger de novo piRNA immunity.

How host organisms adapt their defense systems to newly invading transposable elements remains poorly understood. Here, we show how Drosophila melanogaster acquired PIWI-interacting RNA (piRNA)-mediated immunity against the endogenous retrovirus Tirant. We uncover two distinct modes of de novo piRNA biogenesis by combining genetics, small RNA profiling, and population genomics. The primary route involves antisense insertions into the flamenco cluster, a master locus for transposon control. Unexpectedly, a second, equally potent mechanism arises from antisense Tirant insertions within host gene 3' UTRs. This process requires host gene transcription but is independent of host gene identity. Our findings challenge prevailing models that tie piRNA precursor specification to genomic origin or nuclear RNA processing context. Instead, they reveal a flexible mechanism that turns a critical vulnerability of transposons into an advantage for the host. When transposition occurs into host gene exons, chimeric antisense transcripts are exported to the cytoplasm, inadvertently initiating piRNA production and enabling rapid, adaptive genome defense against new invaders.