foxl3 is a germ cell–intrinsic factor involved in sperm-egg fate decision in medaka

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Science  17 Jul 2015:
Vol. 349, Issue 6245, pp. 328-331
DOI: 10.1126/science.aaa2657

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How germ cells become sperm or egg

During vertebrate development, germ cells switch from a sexually indifferent to a committed state for either egg or sperm. Signals from somatic gonadal cells are generally thought to influence the sexual differentiation of germ cells. However, Nishimura et al. demonstrate that germ cell–intrinsic sex determination cues are at play in the teleost fish medaka. The forkhead box transcriptional factor foxl3 represses the initiation of spermatogenesis. In the absence of foxl3 function, females develop ovaries filled with functional sperm. Thus, the male gonad environment is not required for spermatogenesis.

Science, this issue p. 328


Sex determination is an essential step in the commitment of a germ cell to a sperm or egg. However, the intrinsic factors that determine the sexual fate of vertebrate germ cells are unknown. Here, we show that foxl3, which is expressed in germ cells but not somatic cells in the gonad, is involved in sperm-egg fate decision in medaka fish. Adult XX medaka with disrupted foxl3 developed functional sperm in the expanded germinal epithelium of a histologically functional ovary. In chimeric medaka, mutant germ cells initiated spermatogenesis in female wild-type gonad. These results indicate that a germ cell–intrinsic cue for the sperm-egg fate decision is present in medaka and that spermatogenesis can proceed in a female gonadal environment.

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