Germ cells can become either eggs or sperm in the human body, and now, researchers believe they have found a genetic switch that decides the fate of the biological unit of life. This finding could pave the way to discovering how the same process works in human beings.

Medaka (Oryzias latipes), also known as the Japanese rice fish, were examined, and researchers discovered a gene called foxl3 was necessary for proper functioning of reproductive systems. Female fish who had this gene removed by researchers developed sperm cells in their ovaries instead of eggs. These reproductive cells were found to be perfectly normal sperm, capable of producing offspring.

"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. 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," Minoru Tanaka of the National Institutes of Natural Sciences in Japan said.

Germ cells are present in both males and females but normally develop into either eggs or cells within an individual. However, the process by which this transformation took place remained elusive until this recent discovery. Until this study, biologists believed germ cells were passive, being regulated strictly by other cells in the body.

"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," said Toshiya Nishimura, a reproductive biologist on the study.

Together with the large number of sperm cells produced in fish lacking the foxl3 gene, the animals also produced a number of eggs as well. Despite the strange production of sperm by females, the animals appear normal on their exterior, including their female genitalia.

These sperm-producing female fish actually produce the male reproductive cells in less time than it takes wild-born males. Commercial fisheries could, one day, raise females to produce sperm for reproduction.

Humans do not possess foxl3 genes, but researchers expect that our species may have a similar segment of code within our own genetic structure, carrying out the same process.

Investigation of the role of foxl3 in the development of reproductive cells in medaka fish was published in Science Express, the advance online version of the journal Science.