A fungal pathogen deploys a small silencing RNA that attenuates mosquito immunity and facilitates infection

Insecticidal fungi represent a promising alternative to chemical pesticides for disease vector control. Here, we show that the pathogenic fungus Beauveria bassiana exports a microRNA-like RNA (bba-milR1) that hijacks the host RNA-interference machinery in mosquito cells by binding to Argonaute 1 (AGO1). bba-milR1 is highly expressed during fungal penetration of the mosquito integument, and suppresses host immunity by silencing expression of the mosquito Toll receptor ligand Spätzle 4 (Spz4). Later, upon entering the hemocoel, bba-milR1 expression is decreased, which avoids induction of the host proteinase CLIPB9 that activates the melanization response. Thus, our results indicate that the pathogen deploys a cross-kingdom small-RNA effector that attenuates host immunity and facilitates infection. Fungi that infect insects can potentially be exploited for disease vector control. Here the authors show that the fungus Beauveria bassiana exports a microRNA-like RNA into mosquito cells that modulates host immunity by suppressing expression of Toll receptor ligand Spätzle 4.