The Masc-PSI complex directly induces male-type doublesex splicing in silkworms.

The WZ sex determination system is found in a diverse range of animals, including lepidopteran insects. In the silkworm Bombyx mori, the masculinizing protein Masculinizer (BmMasc) induces the production of the male-type Bombyx mori doublesex (Bmdsx(M)), which is the master genetic switch of B. mori sex differentiation. However, the molecular mechanism through which BmMasc transduces the masculinizing signal to the Bmdsx(M) production remains unknown. Here, we show that BmMasc physically interacts with Bombyx mori P-element somatic inhibitor (BmPSI), a RNA binding protein required for Bmdsx(M) expression. Knockdown experiments indicate that BmPSI is essential for the masculinizing activity of BmMasc. RNA immunoprecipitation experiments also reveal that BmMasc-containing protein complex is associated with female-specific regions of Bmdsx pre-mRNA. Taken together, our findings show that the BmMasc-BmPSI protein complex binds to female-specific Bmdsx regions, inducing exon skipping, and thereby promoting Bmdsx(M) expression in B. mori males.