Rainbow trout has a strict genetic XX/XY sex determination system. However spontaneous masculinization has been observed in genetic females (XX) obtained through endomitotic gynogenesis. The masculinized phenotype does not affect all adult animals and would be the consequence of an uncharacterized mutation named " mal ". In order to identify the causal gene harboring this mutation, a better characterization of this phenotype is needed. For this, we characterized the gonadal phenotype using both histological and molecular approaches. All XXmal animals have a highly disturbed sex differentiation based on results from histology and gene expression profiles. Among these perturbations the inhibition of the expression of ovarian form of aromatase gene (cyp19a1a) encoding the enzyme needed for estrogen synthesis, could be an explanation for this masculinization. Moreover high rearing temperatures (18°C) increase the masculinization of these XXmal animals. And surprisingly, the growth of these XXmal animals was negatively affected especially for those having an adult masculinized phenotype. These results suggest that the XXmal masculinization is the result of a deep perturbation upstream from the transcription of cyp19a1a gene, with as consequence a precocious unstable ovarian differentiation followed by an active masculinization of only some of these XXmal animals. This precocious unstable ovarian differentiation would be amplified by high temperatures that would affect the development of the XXmal animals and the transcription of the cyp19a1a gene. Our results also suggest that the "mal" phenotype is more complex than a simple "masculinized female" phenotype.
