Genetic sex-determining systems in vertebrates include two basic types of heterogamety; XX (female)/XY (male) and ZZ (male)/ZW (female) types. The African clawed frog Xenopus laevis has a ZZ/ZW-type sex-determining system. In this species, we previously identified a W-specific sex (female)-determining gene dmw, and specified W and Z chromosomes, which could be morphologically indistinguishable (homomorphic). In addition to dmw, we most recently discovered two genes, named scanw and ccdc69w, and one gene, named capn5z in the W- and Z-specific regions, respectively. In this study, we revealed the detail structures of the W/Z-specific loci and genes. Sequence analysis indicated that there is almost no sequence similarity between 278kb W-specific and 83kb Z-specific sequences on chromosome 2Lq32-33, where both the transposable elements are abundant. Synteny and phylogenic analyses indicated that all the W/Z-specific genes might have emerged independently. Expression analysis demonstrated that scanw and ccdc69w or capn5z are expressed in early differentiating ZW gonads or testes, thereby suggesting possible roles in female or male development, respectively. Importantly, the sex-determining gene (SDG) dmw might have been generated after allotetraploidization, thereby indicating the construction of the new sex-determining system by dmw after species hybridization. Furthermore, by direct genotyping, we confirmed that diploid WW embryos developed into normal female frogs, which indicate that the Z-specific region is not essential for female development. Overall, these findings indicate that sex chromosome differentiation has started, although no heteromorphic sex chromosomes are evident yet, in X. laevis. Homologous recombination suppression might have promoted the accumulation of mutations and transposable elements, and enlarged the W/Z-specific regions, thereby resulting in differentiation of the W/Z chromosomes.

In many animals, primordial germ cells ( PGC s) migrate into developing gonads. There, they proliferate and differentiate into female and male germ stem cells ( GSC s), oogonia and spermatogonia, respectively. Few studies have focused on the molecular mechanisms underlying the development of GSC sex determination. Here, we investigated the expression of the transcription factor Dmrt1 and a phosphorylated form of the histone variant H2 AX (γH2 AX ) during gonadal development in Xenopus laevis . During early sexual differentiation, Dmrt1 was expressed in the GSC s of the ZW (female) and ZZ (male) gonads as well as somatic cells of the ZZ gonads. Notably, the PGC s and primary GSC s contained large, unstructured nuclei, whereas condensed, rounder nuclei appeared only in primary oogonia during tadpole development. After metamorphosis, Dmrt1 showed its expression in secondary spermatogonia, but not in secondary oogonia. Like Dmrt1, γH2 AX was expressed in the nuclei of primary GSC s in early developing gonads. However, after metamorphosis, γH2 AX expression continued in primary and secondary spermatogonia, but was barely detected in the condensed nuclei of primary oogonia. Taken together, these observations indicate that spermatogonia tend to retain PGC characteristics, compared to oogonia, which undergo substantial changes during gonadal differentiation in X. laevis . Our findings suggest that Dmrt1 and γH2 AX may contribute to the maintenance of stem cell identity by controlling gene expression and epigenetic changes, respectively.

The transcription factor DMRT1 has important functions in two distinct processes, somatic-cell masculinization and germ-cell development in mammals. However, it is unknown whether the functions are conserved during evolution, and what mechanism underlies its expression in the two cell lineages. Our analysis of the Xenopus laevis and Silurana tropicalis dmrt1 genes indicated the presence of two distinct promoters: one upstream of the noncoding first exon (ncEx1), and one within the first intron. In contrast, only the ncEx1-upstream promoter was detected in the dmrt1 gene of the agnathan sand lamprey, which expressed dmrt1 exclusively in the germ cells. In X. laevis, the ncEx1- and exon 2-upstream promoters were predominantly used for germ-cell and somatic-cell transcription, respectively. Importantly, knockdown of the ncEx1-containing transcript led to reduced germ-cell numbers in X. laevis gonads. Intriguingly, two genetically female individuals carrying the knockdown construct developed testicles. Analysis of the reptilian leopard gecko dmrt1 revealed the absence of ncEx1. We propose that dmrt1 regulated germ-cell development in the vertebrate ancestor, then acquired another promoter in its first intron to regulate somatic-cell masculinization during gnathostome evolution. In the common ancestor of reptiles and mammals, only one promoter got function for both the two cell lineages, accompanied with the loss of ncEx1. In addition, we found a conserved noncoding sequence (CNS) in the dmrt1 5'-flanking regions only among amniote species, and two CNSs in the introns among most vertebrates except for agnathans. Finally, we discuss relationships between these CNSs and the promoters of dmrt1 during vertebrate evolution.

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-Müllerian hormone (amh) and inhibin βb (inhbb). Quantitative RT-PCR analysis revealed that the expression of dax1, sox9, amh, and inhbb or sf1 was greatly or slightly higher in ZZ than in ZW gonads during early sex development. In situ hybridization analysis revealed that amh and inhbb mRNAs were expressed in somatic cells on the inner and outer sides of cell masses in the mass-in-line structure, respectively, in the developing ZZ gonads. Interestingly, estrogen exposure prevented the disappearance of the mass-in-line structure in early developing ZZ tadpoles. These findings suggest that TGF-β signaling is involved in the destruction of the mass-in-line structure, which may be maintained by estrogen.