Xio is a component of the Drosophila sex determination pathway and RNA N6-methyladenosine methyltransferase complex.

N6-methyladenosine (m6A), the most abundant chemical modification in eukaryotic mRNA, has been implicated in Drosophila sex determination by modifying Sex-lethal (Sxl) pre-mRNA and facilitating its alternative splicing. Here, we identify a sex determination gene, CG7358, and rename it xio according to its loss-of-function female-to-male transformation phenotype. xio encodes a conserved ubiquitous nuclear protein of unknown function. We show that Xio colocalizes and interacts with all previously known m6A writer complex subunits (METTL3, METTL14, Fl(2)d/WTAP, Vir/KIAA1429, and Nito/Rbm15) and that loss of xio is associated with phenotypes that resemble other m6A factors, such as sexual transformations, Sxl splicing defect, held-out wings, flightless flies, and reduction of m6A levels. Thus, Xio encodes a member of the m6A methyltransferase complex involved in mRNA modification. Since its ortholog ZC3H13 (or KIAA0853) also associates with several m6A writer factors, the function of Xio in the m6A pathway is likely evolutionarily conserved.