Abstract
Germ cells give rise to eggs or sperm. However, recent analyses in medaka (Oryzias latipes) showed that germ cells are also important for feminization of gonads, although this novel role of germ cells has not been characterized in detail. Here, we show that the feminizing effect is inherent to germ cells and is not affected by gametogenic stages or the sexual fate of germ cells. Three medaka mutants were generated to demonstrate this effect: figlα mutants, in which follicle formation is disrupted; meioC mutants, in which germ cells are unable to commit to gametogenesis and meiosis; and dazl mutants, in which germ cells do not develop into gonocytes. All these different stages of germ cells in XX mutants have an ability to feminize the gonads, resulting in the formation of gonads with ovarian structures. In addition to normal ovarian development, we also suggest that the increased number of gonocytes is sufficient for male to female sex reversal in XY medaka. These results may genetically demonstrate that the mechanism underlying the feminizing effect of germ cells is activated before the sexual fate decision of germ cells and meiosis, probably by the time of gonocyte formation in medaka.Author summaryGerm cells are the only cells that can transfer genetic materials to the next generation via the sperm or egg. However, recent analyses in teleosts revealed another essential role of germ cells: feminizing the gonads. In our study, medaka mutants in which gametogenesis was blocked at specific stages provides the novel view that the feminizing effect of germ cells occurs in parallel with other reproductive elements, such as meiosis, the sexual fate decision of germ cells, and gametogenesis. Germ cells in medaka may have a potential to feminize gonads at the moment they have developed.
Highlights
The sex of organisms is determined either by genetic factors and/or by environmental factors
Medaka mutants in which gametogenesis was blocked at specific stages provides the novel view that the feminizing effect of germ cells occurs in parallel with other reproductive elements, such as meiosis, the sexual fate decision of germ cells, and gametogenesis
Ovarian follicles are not required for ovary fate and female development in medaka After cystic division and the start of meiosis, each oocyte at diplotene stage is surrounded by somatic cells to form follicles in female gonads
Summary
The sex of organisms is determined either by genetic factors and/or by environmental factors. Many organisms with genetic sex determination systems have sex determination genes, but these genes differ from species to species [1]. The environmental factors that determine sex include temperature, density, pH and social status [2]. Initial triggers for sex determination vary, there are some common features in the subsequent sex differentiation process in gonads. After primordial germ cells (PGCs) reach the gonads, PGCs develop to become gametogenesis-competent germ cells [3]. Whereas the germ cells in females begin to differentiate to undergo oogenesis, the germ cells in males are in a mitotically quiescent state for a while before the initiation of spermatogenesis [4, 5]
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