Scientists from the National Institute for Basic Biology in Japan published experiments involving a small fish called the medaka, or Japanese rice fish, showing that a gene called foxl3 controls the fate of germ cells.
When the researchers created homozygous mutants with no foxl3 genes, fully functional sperm formed in the ovary. They were then able to use the sperm to fertilise eggs from wild-type (normal) fish, many of which went on to hatch.
The mutant fish appeared female in all other regards, and some still had detectable oocyte production in their ovaries.
Most previous research on germ cells indicated that they need the environment inside the ovary or testis to properly form eggs or sperm.
'In spite of the environment surrounding the germ cells being female, the fact that functional sperm has been made surprised me greatly. That this sexual switch present in the germ cells is independent of the body's sex is an entirely new finding,' co-author Toshiya Nishimura told Reuters.
The team's results, published in the journal Science, suggest that in medaka fish a single gene suppresses 'maleness' within developing reproductive cells.
Sexual development and reproduction are very different in medaka fish than in mammals, so this gene may not have the same role in mammalian sexual and reproductive development. The function of foxl3 in humans is not known, but the related foxl2 gene is expressed in mammalian gonads and appears to be involved in the development of ovaries and testes.
'It's unclear whether the results in medaka will have any impact on the study of sex determination in mammals,' Allan Spradling, from the Carnegie Institution for Science in Baltimore, Maryland, and who was not involved in this research, told The Scientist.