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A gamete is derived initially from a presumptive primordial germ cell (pPGC) and transmits genetic potential to the next generation. Xenopus tropicalis, which is a close relative of Xenopus laevis, has a diploid genome and advantages for genetic and genomic research; however, little is known about the developmental mechanism of its germinal lineage. Here, we identified the Xenopus tropicalis DAZ-like gene (Xtdazl), which encodes RNA-binding proteins homologous to Xdazl in Xenopus laevis and examined the expression patterns of Xtdazl transcripts during embryogenesis. In this work, we showed that Xtdazl mRNA was localized in the germ plasm and was expressed from the previtellogenic oocyte to early tadpole, in testis and ovary. The same localization patterns have been reported in Xenopus laevis germ plasm and pPGCs. These results indicate that Xtdazl mRNA is the first specific marker of germ plasm and pPGCs in Xenopus tropicalis and is very useful to trace Xenopus tropicalis pPGCs, including germ plasm until the early tadpole stage.
Figure 3. Spatial expression patterns of Xenopus tropicalis DAZ-like gene (Xtdazl) mRNA until neurula stage by whole-mount in situ hybridization. A,B: Section of Xtdazl-stained oocyte showed that Xtdazl mRNA was expressed in the perinuclear region in a stage (St.) I oocyte (A) and (B) in the vegetal pole of a stage III oocyte. C: Section of St. 4 embryo stained with Azan. Arrows indicate vegetal pole cytoplasm. D-I: Spatial distribution of Xtdazl mRNA in a fertilized egg (St.1, D), eight-cell stage embryo (St. 4, E), late blastula (St. 9, F), gastrula (St. 10.5, G), early neurula (St.13, H), and late neurula (St. 18. I). J-L: Section of St.1 (J), St.4 (K), and St. 9 (L) embryo with staining for Xtdazl. Animal side is up. M-O: Sagittal section of Xtdazl-stained embryo at the gastrula stage (St. 10.5, M), early neurula stage (St.13, N), and late neurula stage (St. 18, O). Scale bars = 100 mu m in A,B, 200 mu m in C,J-O, 500 mu m in D-I.
Figure 4. Whole-mount in situ hybridization for Xenopus tropicalis DAZ-like gene (Xtdazl) mRNA (A-E) and transverse section of Xtdazl-stained embryo (F-I). A,F: Xtdazl mRNA-positive cells are found in the posterior and lateral regions of the endoderm at St. 23. B,G: Some cells expressing Xtdazl mRNA gradually migrated to more anterior and dorsal endodermal regions at St. 28. C,H: Xtdazl mRNA-positive cells indicated by the arrow in H are localized beneath the dorsal mesentery at St. 35. D,I: Xtdazl mRNA-positive cells indicated by the arrow in I are confined to the dorsal crest at St. 40. E: The cells indicated by the arrowheads are not spread laterally. Scale bars = 500 mu m in A-E, 200 mu m in F-I.
Figure 5. Spatial expression patterns of Xtcat-2 mRNA analyzed by whole-mount in situ hybridization. A: Fertilized egg (St.1). B: Eight-cell stage embryo (St. 4). C: Mid-gastrula (St. 11). D: Late neurula (St.18). E: Early tail bud (St. 26). F: Late tail bud (St. 30). G: No cells expressing Xtcat-2 mRNA are observed at St. 40. H,I: Section of Xtcat-2 stained fertilized (H) or eight-cell stage (I) embryo revealed the localized expression of Xcat-2 mRNA. J,K: Sagittal section of Xtcat-2-stained St. 11 (J) or St. 18 (K) embryo. L,M: Transverse section of tail bud stained with Xtcat-2 at the same stage of E and F, respectively. Scale bars = 500 mu m in A-G, 200 mu m in H-M.
