Drosophila melanogaster evolved piRNA-mediated immunity against the invading tirant retrovirus through two distinct mechanisms: antisense insertions into the flamenco piRNA cluster, and a novel pathway in which antisense insertions into host gene 3' UTRs trigger robust piRNA production through host gene transcription, demonstrating that cytoplasmic transcripts containing antisense transposon sequences are sufficient to initiate piRNA biogenesis independent of canonical piRNA cluster architecture or chromatin context.

Transposable elements pose a persistent threat to genome integrity, yet how host defense systems adapt to newly invading elements remains poorly understood. Here, we reveal how Drosophila melanogaster acquired PIWI-interacting RNA (piRNA)-mediated immunity against the recently invading endogenous retrovirus tirant. By integrating genetics, small RNA profiling, and population genomics, we identify two distinct modes of de novo piRNA biogenesis. The primary mechanism involves antisense insertions into the flamenco cluster, a well-established master locus for transposon control. Strikingly, we also find that antisense tirant insertions into 3′ UTRs of host genes robustly trigger piRNA production, a process driven by host gene transcription but independent of gene identity. These findings challenge prevailing models that link piRNA precursor specification to genomic origin or nuclear processing context. Instead, they uncover a flexible, general mechanism in which transposition into host gene exons represents a critical vulnerability for transposons: by generating chimeric antisense transcripts that are exported to the cytoplasm, transposons inadvertently initiate their own silencing, enabling rapid and adaptive genome defense against new invaders.