Xio is a component of the Drosophila sex determination pathway and RNA N 6 -methyladenosine methyltransferase complex

N 6 -methyladenosine (m 6 A), 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 m 6 A 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 m 6 A factors, such as sexual transformations, Sxl splicing defect, held-out wings, flightless flies, and reduction of m 6 A levels. Thus, Xio encodes a member of the m 6 A methyltransferase complex involved in mRNA modification. Since its ortholog ZC3H13 (or KIAA0853) also associates with several m 6 A writer factors, the function of Xio in the m 6 A pathway is likely evolutionarily conserved.