Germ cells in the teleost fish medaka have an inherent feminizing effect

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.

Zebrafishdazlregulates cystogenesis upstream of the meiotic transition and germline stem cell specification and independent of meiotic checkpoints

“…This process begins with congregation of individual migratory GCs that then undergo synchronous division to generate premeiotic cysts. Notably, as we observe in zebrafish dazl mutants, dazl -/-GCs of mouse (Chen et al, 2014;Gill et al, 2011;Hu et al, 2015a;Lin et al, 2008) and medaka (Nishimura et al, 2018) remain as individual cells, which were described as PGC-like in mice and medaka (Chen et al, 2014;Gill et al, 2011;Nishimura et al, 2018). Here we show that PGCs lacking dazl begin the cystogenic process, including formation of ring canals.…”

“…Similarly, in mouse dazl is required for "licensing" or acquisition of meiotic competence and to generate gametes; dazl mutant GCs remain PGC like and fail to develop as male or female in response to masculinizing or feminizing signals (Gill et al, 2011;Hu et al, 2015b). In medaka, dazl is also required for gametogenesis and GC development (Li et al, 2016;Xu et al, 2007) but is not required for sexually dimorphic gonad development in response to somatic cues (Nishimura et al, 2018). Here we show that zygotic dazl is required for gametogenesis and meiotic progression in zebrafish.…”

“…Here we show that zygotic dazl is required for gametogenesis and meiotic progression in zebrafish. However, in contrast to mouse dazl mutant gonads which fail to sexually differentiate, and medaka dazl mutants, which develop as either sex (Nishimura et al, 2018), zebrafish dazl mutants develop exclusively as males. In medaka, recovery of male or female dazl mutants with only early PGCs provided evidence that PGC-like cells could support development of both sexes (Nishimura et al, 2018).…”

Starvation causes female-to-male sex reversal through lipid metabolism in the teleost fish, medaka (Olyzias latipes)

“…Fish were maintained on a 14 h light/10 h dark cycle at 25-28°C. Phenotypic sex was determined by observing the anal fin, dorsal fin and gonads under a stereomicroscope (Yamamoto, 1958;Nishimura et al, 2015Nishimura et al, , 2018. For starvation and pantothenate treatment, 50 larvae were treated with complete starvation or pantothenate (20, 200 and 2000 μM; 033-14165, Wako) in 1 l of water from 0 dph to 5 dph.…”

“…The TALEN Targeter program (https://tale-nt.cac.cornell.edu/node/add/talen) was used to search the TALEN target site for dmrt1. TALEN assembly was performed as previously described (Nishimura et al, 2018). The left and right TALEN arms (250 ng/µl each) were injected into one-or two-cell-stage embryos from OK-Cab wild-type fish.…”