Figure 2. Diverse roles of DMRT1 orthologues in vertebrate sex determination.

Blue shading indicates masculinizing function and pink indicates feminizing function. a | In mammals, expression of the X-chromosome-linked sex-determining region of chromosome Y (SRY) gene in the fetal gonad triggers male development by activating SOX9 transcription during a crucial period and, in the absence of SOX9 activation, ovarian development ensues. DMRT1 is expressed in the fetal gonad in mammals before sex determination but is required only after gonadal sex is determined. b | In medaka, a Y-chromosome-linked, recently duplicated Dmrt1 gene, DM domain on Y (Dmy; also known as Dmrt1bY), has a role that is analogous to that of SRY, triggering testis development in XY fish. c | In birds, Dmrt1 is Z-chromosome-linked and thus genetic males have two copies, whereas females have one. Dmrt1 expression is much higher in the genital ridge of ZZ birds, and Dmrt1 is required for testis determination, suggesting that testis fate requires a critical threshold of Dmrt1 activity. d | In the frog Xenopus laevis, a duplicated and truncated Dmrt1 gene (dmw) is found on the female-specific W chromosome and appears to block the ability of the autosomal Dmrt1 gene to determine testis fate. In medaka, birds and X. laevis, the autosomal Dmrt1 genes also are believed to function in testicular differentiation after gonadal sex is determined, similar to the situation for their mammalian counterparts.