Masculinization-Related Genes and Cell-Mass Structures During Early Gonadal Differentiation in the African Clawed Frog Xenopus laevis

The African clawed frog Xenopus laevis has a female heterogametic ZZ/ZW-type sex-determining system. We previously discovered a W-linked female sex-determining gene dm-W that is involved in ovary formation, probably through the up-regulation of the estrogen synthesis genes cyp19a1 and foxl2. We also reported that a unique “mass-in-line structure”, which disappears from ZZ gonads during early testicular development, might serve as the basis for ovary differentiation in ZW gonads. However, the molecular mechanisms underlying early masculinization are poorly understood. To elucidate the development of bipotential gonads into testes after sex determination in this species, we focused on the orthologs of five mammalian sex-related genes: three nuclear factor genes, dax1, sf1 (also known as ad4bp), and sox9, and two genes encoding members of the tumor growth factor-β (TGF-β) family, anti-Mullerian hormone (amh) and inhibin βb (inhbb). Quantitative RT-PCR analysis revealed that the expression of dax1, sox9, amh,...