Cell-Mass Structures Expressing the Aromatase Gene Cyp19a1 Lead to Ovarian Cavities in Xenopus laevis

Abstract

The African clawed frog, Xenopus laevis, has a ZZ/ZW-type sex-determination system. We previously reported that a W-linked gene, Dm-W, can determine development as a female. However, the mechanisms of early sex differentiation remain unclear. We used microarrays to screen for genes with sexually dimorphic expression in ZZ and ZW gonads during early sex differentiation in X laevis and found several steroidogenic genes. Importantly, the steroid 17α-hydroxylase gene Cyp17a1 and the aromatase gene Cyp19a1 were highly expressed in ZZ and ZW gonads, respectively, just after sex determination. At this stage, we found that Cyp17a1, Cyp19a1, or both were expressed in the ZZ and ZW gonads in a unique mass-in-line structure, in which several masses of cells, each surrounded by a basement membrane, were aligned along the anteroposterior axis. In fact, during sex differentiation, ovarian cavities formed inside each mass of Cyp17a1- and Cyp19a1-positive cells in the ZW gonads. However, the mass-in-line structure disappeared during testicular development in the ZZ testes. These results suggested that the mass-in-line structure found in both ZZ and ZW gonads just after sex determination might be formed in advance to produce ovarian cavities and then oocytes. Consequently, we propose a view that the default sex may be female in the morphological aspect of gonads in X laevis.

Sex Reversal in Amphibians

“…Such a dimorphic expression of aromatase has also been reported in several anuran species, including G. rugosa, X. laevis, and P. nigromaculatus [Kato et al, 2004;Sakata et al, 2006;Maruo et al, 2008;Mawaribuchi et al, 2014;Xu et al, 2015], and was used to predict the phenotypic sex in L. sylvaticus and S. tropicalis [NavarroMartín et al, 2012]. In X. laevis , the dimorphic expression of cyp19A1 occurred just after sex determination in several masses of cells, each surrounded by a basement membrane, that were aligned along the anteroposterior axis [Mawaribuchi et al, 2014]. During sex differentiation, ovarian cavities formed inside each mass of cyp19a1 -positive cells in the ZW gonads, whereas the mass-in-line structure disappeared during testicular development in the ZZ gonads.…”

“…Similarly, a dimorphic expression of cyp19 was observed in H. retardatus in which it occurred 15 days after hatching, i.e., prior to the morphological differentiation of the gonads [Sakata et al, 2005[Sakata et al, , 2006. Such a dimorphic expression of aromatase has also been reported in several anuran species, including G. rugosa, X. laevis, and P. nigromaculatus [Kato et al, 2004;Sakata et al, 2006;Maruo et al, 2008;Mawaribuchi et al, 2014;Xu et al, 2015], and was used to predict the phenotypic sex in L. sylvaticus and S. tropicalis [NavarroMartín et al, 2012]. In X. laevis , the dimorphic expression of cyp19A1 occurred just after sex determination in several masses of cells, each surrounded by a basement membrane, that were aligned along the anteroposterior axis [Mawaribuchi et al, 2014].…”

“…The Cyp17 protein is present in somatic cells of indifferent gonads, and the conversion of [ 3 H]progesterone into [ 3 H]androstenedione is higher in males than in females [Sakurai et al, 2008]. This has also been described in X. laevis where cyp17A1 expression is 5-40-fold higher in ZZ than in ZW gonads during early sex differentiation (stage 50-62) [Mawaribuchi et al, 2014].…”

Determining the optimal developmental stages of Xenopus laevis for initiating exposures to chemicals for sensitively detecting their feminizing effects on gonadal differentiation

“…In Ito's reports, gonadal sex differentiation in X. laevis was believed to occur during stages 48-50 (Yoshimoto et al, 2008;Yoshimoto and Ito 2011;Mawaribuchi et al, 2014). However, in present study 1 nM EE2 resulted in high proportions of complete male-to-female reversals (81%) when exposure began at stages 45/46, whereas 1 nM EE2 caused low proportions (17%) of phenotypic female in genetic males when exposure begin at stage 48 (Fig.…”

Extraordinary Diversity in the Origins of Sex Chromosomes in Anurans Inferred from Comparative Gene Mapping

“…buergeri CYP17A1 (cytochrome P450 family 17 subfamily A polypeptide 1), which was localized to the inverted region of the W chromosome in B. buergeri , is known to be a sexual differentiation gene in vertebrates [Shimada, 1998;Zhou et al, 2007;Iwade et al, 2008;Houmard et al, 2009;Mawaribuchi et al, 2014]. To identify candidate sex-determining genes, we attempted molecular cloning of the W form of CYP17A1 from the ovaries.…”