The master male sex determinant Gdf6Y of the turquoise killifish arose through allelic neofunctionalization.

Although sex determination is a fundamental process in vertebrate development, it is very plastic. Diverse genes became major sex determinants in teleost fishes. Deciphering how individual sex-determining genes orchestrate sex determination can reveal new actors in sexual development. Here, we demonstrate that the Y-chromosomal copy of the TGF-β family member gdf6 (gdf6Y) in Nothobranchius furzeri, an emerging model organism in aging research, gained the function of the male sex determinant through allelic diversification while retaining the skeletal developmental function shared with the X-chromosomal gdf6 allele (gdf6X). Concerning sex determination, gdf6Y is expressed by somatic supporting cells of the developing testes. There it induces the male sex in a germ cell-independent manner in contrast to sex determination in zebrafish and the medaka. Looking for downstream effectors of Gdf6Y, we identified besides TGF-β signaling modulators, especially the inhibitor of DNA binding genes id1/2/3, the mRNA decay activator zfp36l2 as a new GDF6 signaling target.