Ovary-predominant wnt4 expression during gonadal differentiation is not conserved in the rainbow trout (Oncorhynchus mykiss)
Résumé
The Wnt/β-catenin pathway is crucial for ovarian differentiation in mammals, and WNT4 is an important protein that regulates this process. While the role of Wnt4 in gonadal differentiation is relatively well characterized in mammals, little is known regarding its role in teleost fish. Therefore, we investigated the potential activity of wnt4 in gonadal differentiation in rainbow trout (Oncorhynchus mykiss), focusing on the teleost and salmonid gene duplications. Phylogenetic and synteny analyses demonstrated that teleost fish possess two wnt4 genes, wnt4a and wnt4b, as a consequence of the teleost-specific whole-genome duplication (3R). In rainbow trout, we also identified an additional wnt4 gene, which is a wnt4a paralog that likely resulted from the salmonid-specific whole-genome duplication (4R). These two Wnt4a proteins (Wnt4a1 and Wnt4a2) share a high identity (>80%) with other vertebrate Wnt4 proteins, whereas Wnt4b is clearly more divergent (60% identity). During embryogenesis and adulthood, the wnt4a1/2 transcripts were expressed in various tissues, including the ovaries and testes. In contrast, wnt4b expression was restricted to the nervous system, suggesting a sub- or a neo-functionalization of this divergent paralog. During early gonadal differentiation in both males and females, the wnt4a1/2 transcripts were detected in the somatic cells surrounding the germ cells, with a slight sexual dimorphism in favor of males. These results demonstrate that, unlike mammals, rainbow trout do not display an ovary-predominant wnt4 expression profile during early gonadal differentiation.