Knocking out just a single gene in Japanese rice fish makes otherwise female fish produce sperm. Not only that, but the sperm is produced in the ovaries, and more impressive still, the sperm is perfectly healthy and can go on to fertilize eggs that then hatch.

The researchers, led by the National Institute for Basic Biology in Japan, identified the specific gene foxl3 in the fish, which are locally known as ‘medaka’. They discovered that this gene works as a genetic switch, and determines if germ cells, the early cells that have the potential to become either eggs or sperm, develop into one or the other.

“While germ cells can become either sperm or eggs, nobody knew that in vertebrates the germ cells have a switch mechanism to decide their own sperm or egg fate,” explained Professor Minoru Tanaka, co-author of the study published in Science. “Our result indicates that once the decision is made the germ cells have the ability to go all the way to the end. I believe it is of very large significance that this mechanism has been found.”

The scientists produced mutant female medaka embryos that lack the foxl3 gene, and found that they started producing sperm only a week after hatching. As if female fish making sperm wasn’t weird enough, the mutant female fish were producing it three weeks earlier than is typical in normal, wild-type males. The sperm production was irregular, but the cells were still able to go on and fertilize normal fish eggs, the majority of which did hatch.

The mutant medaka by all other accounts looked normal, showing a female body type and developed ovaries. The researchers also found that amongst the sperm, there were even some normal egg cells. Most surprising to the scientists, the working sperm cells were formed despite being surrounded by a female environment and receiving female cues and hormones.

“In spite of the environment surrounding the germ cells being female, the fact that functional sperm has been made surprised me greatly,” says first author Dr. Toshiya Nishimura. “That this sexual switch present in the Germ Cells is independent of the body's sex is an entirely new finding.”

Whether this new discovery will have further implications is unknown. A similar gene, called foxl2, is involved in ovary development in mammals, but sex determination even just amongst fish is highly varied. So, whether foxl3 has any role in germ cell determination in mammals is still up in the air.