Wolbachia infection causes male-specific death in Ostrinia furnacalis, but its removal from infected strains results in female-specific death instead of restoring 1:1 sex ratio, suggesting that cytoplasmic Wolbachia, not the host genome, primarily determines femaleness in infected strains. This phenomenon is a striking example of the evolutionary outcome of cytoplasmic sex determination, potentially arising from prolonged host-symbiont co-evolution. Although we recently identified Oscar, the Wolbachia-encoded male-killing effector targeting the host masculinizing factor OfMasc in Ostrinia moths, inactivation or loss of the host’s endogenous feminizer remains unknown. Here we identify a W-linked primary feminizer, OfFem piRNA, which targets an mRNA encoding an OfMasc-interacting protein Ofznf-2. We demonstrate that Ofznf-2 is essential for both masculinization and dosage compensation. We also show that OfFem piRNA is entirely absent in the Wolbachia-infected lineage, providing molecular evidence that a male-killing Wolbachia hijacks the host feminizing piRNA function by acquiring the Oscar protein during prolonged endosymbiosis.

Competing interests: The authors declare no competing interests.