ABSTRACT Many salmonids have a male heterogametic (XX/XY) sex determination system, and they are supposed to have a conserved master sex determining gene ( sdY ), that interacts at the protein level with Foxl2 leading to the blockage of the synergistic induction of Foxl2 and Nr5a1 of the cyp19a1a promoter. However, this hypothesis of a conserved master sex determining role of sdY in salmonids is still challenged by a few exceptions, one of them being the presence of some naturally occurring “apparent” XY Chinook salmon females. Here we show that XY Chinook salmon females have a sdY gene ( sdY-N183 ), which has one missense mutation leading to a substitution of a conserved isoleucine to an asparagine (SdY I183N). In contrast, Chinook salmon males have both a non-mutated sdY-I183 gene and the missense mutation sdY-N183 gene. The 3D model of SdY-N183 predicts that the I183N hydrophobic to hydrophilic amino acid change leads to a local modification of the β-sandwich structure of SdY. Using in vitro cell transfection assays we found that SdY-N183, like SdY-I183, is preferentially localized in the cytoplasm. However, compared to SdY-I183, SdY-N183 is more prone to degradation, its nuclear translocation by Foxl2 is reduced and SdY-N183 is unable to significantly repress the synergistic Foxl2/Nr5a1 induction of the cyp19a1a promoter. Altogether our results suggest that the sdY-N183 gene of XY Chinook females is a non-functional gene and that SdY-N183 is no longer able to promote testicular differentiation by impairing the synthesis of estrogens in the early differentiating gonads of wild Chinook salmon XY females.

A non-functional copy of the salmonid sex determining gene (<i>sdY</i>) is responsible for the “apparent” XY females in Chinook salmon, <i>Oncorhynchus tshawytscha</i>

A non-functional copy of the salmonid sex determining gene (sdY) is responsible for the “apparent” XY females in Chinook salmon, Oncorhynchus tshawytscha