“This is exciting because it is absolutely unexpected,” said Manfred Schartl, who studies gonadal development at the University of Würzburg in Germany and was not involved in the current study. “That these germ cells have genetically determined sexual fate is new.”
The gene, foxl3, is an ancient lineage duplicate of the foxl2 gene, which is known to be involved in mammalian ovary formation. But the function of foxl3 was previously unclear. “In mammals, foxl2 is well known to antagonize male somatic cell fate and now foxl3, at least in medaka, seems to antagonize male germ cell fate,” said Josephine Bowles, a senior research fellow at the University of Queensland in Australia who studies sex determination in mammals and was not involved in the study.
The team created two medaka mutants that resulted in no detectable expression of the FOXL3 protein. In female homozygous mutant fish, spermatogenesis within the ovary was detectable one week after the fish hatched—three weeks earlier than the typical timing of male puberty, when sperm production starts in these fish. While spermatogenesis within the ovary was somewhat irregular, the resulting sperm were viable. Extracted ovary-produced sperm fertilize half of the eggs of wild-type female fish, and the majority of these fertilized eggs hatched. Other than producing sperm, the mutant fish were overtly female, and the absence of foxl3 did not affect the development of their surrounding somatic gonadal cells. The researchers detected some oocytes within the sperm-producing ovaries of some mutant fish.
Because the sperm could form in the context of female signaling and hormones, the results suggest that male germ cells may not register regulatory signals from female somatic cells, Tanaka told The Scientist in an e-mail.
“In the environment of a seemingly perfect ovary, the germ cells are largely undergoing the spermatogenesis pathway, suggesting that the only factor normally stopping germ cells in the medaka ovary from adopting the spermatogenesis pathway is foxl3,” said Bowles. “This shows how plastic the sexual fate of these germ cells is in this species.”
According to Schartl, foxl3 appears to allow female germ cells to react to environmental signals while the mutant is shielded from these gonadal somatic cell cues. That the default development track appears to be male is unexpected, added Scharl, as “all of the data from higher vertebrates shows that the default development [track] is female.”
Sex determination mechanisms vary greatly among different fish species. Unlike many other fish, in which temperature-dependent mechanisms determine sex, sex determination in medaka is chromosomally determined, noted Bowles.
It’s unclear whether the results in medaka will have any impact on the study of sex determination in mammals, said Allan Spradling, a developmental biologist at the Carnegie Institution for Science in Baltimore, Maryland, who was not involved in the work.
The team is now studying how foxl3 is regulated, what it regulates, and how this gene is linked to the factors that determine sex in the somatic cells of the gonads in medaka, Tanaka said.
“It’s a surprising study and, technically, wonderful,” said Schartl. “It gives a lot for us [developmental biologists] to think about.”
T. Nishimura et al., “foxl3 is a germ cell-intrinsic factor involved in sperm-egg fate decision in medaka,” Science, doi:10.1126/science.aaa2657, 2015.