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Germ Cell Gene Switch In Fish Decides Sperm Or Egg Creation

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Researchers studying a small fish popular with aquarium enthusiasts say they've identified a gene that determines whether reproductive germ cells in vertebrates will eventually become sperm or eggs.

Medaka, also known as the Japanese rice fish, are commonly found in rice paddies, ponds and marshes.

The researchers in Japan claim to have found the foxl3 gene, which acts as a genetic switch on precursor germ cells known to be present in both sexes of vertebrates.

"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 Minoru Tanaka of Japan's National Institute for Basic Biology.

The gene is primarily active in the germ cells of female medaka fish to prevent them from becoming sperm cells.

However, when the gene was inactivated in females, their germ cells began to turn into sperm in their ovaries, the researchers reported in the journal Science.

"In spite of the environment surrounding the germ cells being female, the fact that functional sperm has been made surprised me greatly," said study co-author Toshiya Nishimura.  "That this sexual switch present in the germ cells is independent of the body's sex is an entirely new finding."

The scientists say they were able to take the sperm cells produced in the ovaries of the female fish and successfully fertilize egg cells from other females to produce normal offspring.

It came as a surprise that vertebrate germ cells contain their own switch to decide their individual egg-or-sperm outcome.

"The germ cells were regarded as passive cells that are regulated by other cells," Tanaka noted.

"Our result indicates that once the decision is made the germ cells have the ability to go all the way to the end," he said. "I believe it is of very large significance that this mechanism has been found."

Prior to this study, the molecular process that drives germ cells toward becoming sperm of eggs had been poorly understood. While humans do not possess an identical foxl3 gene, the researchers suggest it is likely a similar mechanism of some form of genetic switch may operate in people.

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