A feminizing switch in a hemimetabolous insect.

The mechanism of sex determination remains poorly understood in hemimetabolous insects. Here, in the brown planthopper (BPH), , a hemipteran rice pest, we identified a feminizing switch or a () that encodes a serine/arginine-rich protein. Knockdown of in female nymphs resulted in masculinization of both the somatic morphology and splicing. The female-specific isoform of , , is maternally deposited and zygotically transcribed. Depletion of by maternal RNAi or CRISPR-Cas9 resulted in female-specific embryonic lethality. Knockdown of an hnRNP40 family gene named () also conferred masculinization. In vitro experiments showed that an isoform, NlFMD2, bound the RAAGAA repeat motif in the pre-mRNA and formed a protein complex with NlFMD-F to modulate splicing, suggesting that NlFMD2 may function as an RNA binding partner of the feminizing switch NlFMD. Our results provide novel insights into the diverse mechanisms of insect sex determination.