WASHINGTON, June 11 -- Japanese researchers said Thursday they have found, for the first time, in vertebrates a genetic switch that determines whether germ cells become sperm or eggs.

The gene is named foxl3, and has been identified using a small fish called medaka, or Japanese rice fish, they reported in the U. S. journal Science.

In medaka lacking functional foxl3 genes, a large number of sperm were found to form in the ovaries of females, and they were functional and could produce normal offspring. Only a small number of eggs are formed at the same time.

First author Toshiya Nishimura of the National Institute for Basic Biology explained that foxl3 gene works in a way that " suppresses differentiation into sperm" in the germ cells of females.

"In spite of the environment surrounding the germ cells being female, the fact that functional sperm has been made surprised me greatly," Nishimura said in a statement.

"That this sexual switch present in the germ cells is independent of the body's sex is an entirely new finding."

The study also showed that in these females lacking the function of foxl3, functional sperm can be obtained in a shorter period of time than in normal wild-type males.

Applied research into uses for aquaculture is already underway, they said.

The results were believed to help researchers learn more about how the sexual fate of germ cells is determined during vertebrate development.

"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," said Associate Professor Minoru Tanaka of the National Institute for Basic Biology, who led the study.

"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."