Gene variant W of Barrier to autointegration factor 2 (Banf2w) is concordant with female determination in cichlids. Banf2w is concordant with female determination in cichlids

Oreochromis fishes exhibit variability of genetic sex-determination (SD) mecha-nisms and therefore contribute to understanding of the sex differentiation network. Oreochromis SD is a key factor for production of commercial all-male progeny. However, only one master key regulator (MKR) of SD has been dis-covered for these species, i.e., an XX/XY SD-system driven by amh on linkage group 23 (LG23) for O. niloticus (On). The LG3 WZ/ZZ SD-system that controls sex in O. aureus (Oa) has been recently delimited within a critical genomic region (9.2 Mbp) with an embedded interval of high female specific variation, harboring two paics genes and banf2. Developing genetic markers in this interval, we narrowed the SD-region to 235 Kbp on the orthologous On physical map showing association with a WZ/ZZ SD system in an Oa stock (n=96; p<1.5×10-26) for which hybridization broke LG3's non‐recombining block. In males, DNA-seq assembly and peak-proportion analysis of banf2 Sanger chromatograms showed three exons capable of encoding 90-aminoacid polypeptides. Yet, in females, we found an extra copy with an 89-aminoacid polypeptide having three non-conserved aminoacid substitutions. We named this gene copy banf2w. Disrupting Hardy-Weinberg equilibrium (p<4.2×10-3), banf2w was concordant with female determination in four cichlid fishes with LG3 WZ/ZZ SD-systems. Coding variation and exclusive RNA-seq expression during differentiation stages of Oa females highlighted banf2w as the long-sought SD MKR of LG3. As banf genes are known to mediate nuclear assembly, chromatin organization, gene expression and gonad development, banf2w may play fundamental roles in inducing female nucleus formation that is essential for SD.