Parasitic castration by a viral protein tyrosine phosphatase targeting the host cell cycle checkpoint protein Rad9A

Parasitic castration is a widespread strategy where parasites hijack host reproductive resources, yet the key molecular mechanisms driving this phenomenon remain poorly understood. Here, we reported that parasitization by the parasitic wasp <i>Cotesia vestalis</i> triggers apoptosis-mediated castration in the larval testes of its lepidopteran host, <i>Plutella xylotella</i>. Such a phenomenon was mediated by <i>CvBV_22-9</i>, a testis-enriched <i>protein tyrosine phosphatase</i> (PTP) encoded by Cotesia vestalis bracovirus (CvBV), a domesticated virus endogenized in the wasp. Similarly, a homolog of CvBV_22-9, encoded by the Microplitis manilae bracovirus, is involved in testis castration by inducing apoptosis in parasitized fall armyworm, <i>Spodoptera frugiperda</i>. Mechanistically, CvBV_22-9 binds to a cell cycle checkpoint protein, Rad9A, but does not alter its tyrosine phosphorylation level. Crucially, CRISPR-Cas9 knockout of <i>Rad9A</i> causes embryonic lethality and severe testis defects. Validation in <i>Drosophila melanogaster</i> shows that testis-specific expression of <i>CvBV_22-9</i> or <i>Rad9A</i> knockdown induces apoptosis, while combined targeting synergistically enhances this effect, suggesting a conserved function of both proteins in insects. Our study uncovers a regulatory mechanism where a parasitoid wasp deploys a domesticated viral PTP that functions as a pseudophosphatase to induce Rad9A-mediated apoptosis and disrupt host testis development and spermatogenesis. This mechanism highlights a sophisticated strategy of host exploitation by parasitoid wasps, providing insights for the biocontrol of lepidopteran pests.