Wnt4a is indispensable for genital duct elongation but not for gonadal sex differentiation in the medaka Oryzias latipes

In most vertebrates, oviducts are derived from Mullerian ducts and sperm ducts from Wolffian ducts. In teleosts, however, Mullerian ducts are absent. Wolffian ducts function as nephric ducts in both sexes, and genital ducts are formed by posterior extension of either ovaries or testes. Whether genital ducts of teleosts are newly evolved organs or are a variant of the Mullerian ducts is an important question for evolutionary mechanisms of morphogenesis. One of the genes essential for Mullerian duct formation in mice, wnt4, is expressed in the mesenchyme and induces invagination of the coelomic epithelium and its posterior elongation. Here, we approached to the above question by examining genital duct development in mutants of two Wnt4 genes in medaka (wnt4a is orthologous to mouse wnt4 and wnt4b paralogous). The wnt4b mutants had short body but were fertile with normal genital ducts. In contrast, both male and female wnt4a mutants had posterior elongation of the gonads stopped within or just outside the coelom, depending on the mutated alleles. Genetic females of the scl mutants (unable to synthesize testosterone or estrogens) have gonads containing both previtellogenic oocytes and spermatogenic cysts. Their gonads do not have ovarian cavities or sperm duct primordia and are lacking genital ducts completely. The results suggest Wnt4a target organs are posterior parts of the ovarian cavities or the sperm duct primordia. Medaka wnt4a was expressed in the mesenchyme ventral to the genital ducts in both sexes. Thus, the aborted elongation of genital ducts in the wnt4a mutants, the ortholog of mouse wnt4, suggests strongly that mouse Mullerian ducts and teleost genital ducts share homologous developmental processes. To further demonstrate this possible homology, mechanisms of genital duct formation and possible roles of Wnt4 should be compared before and after the appearance of Mullerian ducts in vertebrate phylogeny, namely jawless fish and cartilaginous fish. Additionally, wnt4a and wnt4b single mutants or double mutants did not show sex-reversal, suggesting both genes are dispensable for gonadal sex differentiation in medaka. This is in contrast to indispensable function of Wnt4 in mammalian ovarian differentiation.