Abstract
Xenopus laevis is an amphibian (frog) species widely used in developmental biology and genetics. To unravel the molecular machinery regulating sex differentiation of Xenopus gonads, we analyzed for the first time the transcriptome of developing amphibian gonads covering sex determination period. We applied microarray at four developmental stages: (i) NF50 (undifferentiated gonad during sex determination), (ii) NF53 (the onset of sexual differentiation of the gonads), (iii) NF56 (sexual differentiation of the gonads), and (iv) NF62 (developmental progression of differentiated gonads). Our analysis showed that during the NF50, the genetic female (ZW) gonads expressed more sex-specific genes than genetic male (ZZ) gonads, which suggests that a robust genetic program is realized during female sex determination in Xenopus. However, a contrasting expression pattern was observed at later stages (NF56 and NF62), when the ZW gonads expressed less sex-specific genes than ZZ gonads, i.e., more genes may be involved in further development of the male gonads (ZZ). We identified sexual dimorphism in the expression of several functional groups of genes, including signaling factors, proteases, protease inhibitors, transcription factors, extracellular matrix components, extracellular matrix enzymes, cell adhesion molecules, and epithelium-specific intermediate filaments. In addition, our analysis detected a sexually dimorphic expression of many uncharacterized genes of unknown function, which should be studied further to reveal their identity and if/how they regulate gonad development, sex determination, and sexual differentiation. Comparison between genes sex-specifically expressed in developing gonads of Xenopus and available transcriptome data from zebrafish, two reptile species, chicken, and mouse revealed significant differences in the genetic control of sex determination and gonad development. This shows that the genetic control of gonad development is evolutionarily malleable.
Highlights
Xenopus laevis is a good model to study molecular mechanisms of gonad development because the structural changes in developing gonads and the master gene determining sex, the Wlinked DM-domain gene, are well known
In developing Xenopus laevis gonads, we detected the expression of 63,084 transcripts in total
We found that while the expression level of the majority of genes was similar between stages and between male and female gonads, a subpopulation of genes showed distinct changes in the expression level between stages and sexes, which suggested that they may play a role in sex determination and/or sexual differentiation (Figs. 2A, B and 3, Tables 1 and 2)
Summary
Xenopus laevis is a good model to study molecular mechanisms of gonad development because the structural changes in developing gonads and the master gene determining sex, the Wlinked DM-domain gene (dm-w), are well known. The medulla is sterile and contains medullar cells only (Piprek et al 2016, 2017) At this stage, the sex-determining genes (dm-w and dmrt1) are expressed in the somatic cells of the gonads. In differentiating ovaries, which express dm-w, the germ cells remain in the cortical position, and at stage NF56, the ovarian cavity forms inside the gonad. Around NF62, the ovaries are fully differentiated, with the oocytes located in the cortex (Piprek et al 2017; Yoshimoto et al 2008). This divergent development of the female and male gonads has to be controlled by differential gene expression. A global analysis of Xenopus gonad transcriptome, which we performed in this study, is the step in obtaining a broad database of gene expression pattern in developing male and female Xenopus gonads
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