The diminutive medaka (Japanese rice fish) have been kept as pets since the Edo Period (1603-1868). They are hardy animals, an important quality for a pet, and they naturally occur in a variety of colors, including gold. They have distinctive, some say attractive, eyes (for a fish) — indeed, medaka in Japanese means “with high eyes.”

These characteristics made them popular among all classes of feudal Japanese in the past. The natural historian Baien Mouri included a number of color varieties in his 1843 encyclopedia.

Three hundred years ago, however, no child taking delight in the glittering little fish in his or her rice field could have possibly imagined the incredible adventures this species would go on, nor the contribution to our understanding of the world it would make. For all the cultural and culinary impact of carp or puffer fish, medaka — scientific name Oryzias latipes — are arguably more important.

In the early 1900s, they contributed to genetics. Medaka were the first backboned animal that were found to follow Mendel’s law of inheritance, which explains the way different traits are passed on down the generations.

In 1994, medaka became the first vertebrate to have sex and reproduce in space. Male and female fish traveled to the Earth orbit on the space shuttle Columbia, mated in orbit and produced normal babies, all in a weightless environment. Sure, there are rumors, denied by NASA, that humans have had sex in space, too, but the first vertebrates to do it were medaka.

Right now, there is a special chamber for them on board the Kibo module of the International Space Station. In the Aquatic Habitat built by the Japanese Space Agency, medaka are being studied to tell us about the impact of micro-gravity in life — not so much for telling us about what will happen to us when we live far from the planet, but for improving life on it.

“Studies on bone degradation mechanisms and muscle atrophy mechanisms are applicable to human health problems, especially for the aging society,” says Nobuyoshi Fujimoto of the agency’s Space Environment Unitization Center.

The fish, which had its genome sequenced in 2007, will also be bred for three generations on board the space station. So we know a huge amount about medaka, but what we learned last week was still a massive surprise, and could have illuminating implications for humans, too.

Scientists discovered last week that changing only one gene in female fish makes them produce sperm in their ovaries, instead of eggs.

In every other way the fish are normal female fish. The sperm produced by them are normal sperm, which fertilize eggs normally and produce normal offspring.

Scientists at the National Institute for Basic Biology in Okazaki achieved the feat by breeding female fish without a key gene, called foxl3.

“In spite of the environment surrounding the germ cells being female, the fact that functional sperm has been made surprised me greatly,” geneticist Toshiya Nishimura says.

It’s an entirely new finding, he says, that the sex of the body is independent of the germ cells (the sex cells that produce sperm or eggs) made by the body.

“Nobody knew that the germ cells in vertebrates have a switch mechanism to decide their own sperm or egg fate,” says Minoru Tanaka, a colleague of Nishimura. “Our result suggests that once a 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.”

It’s tempting to consider what this finding means for our understanding of human biology. That would be premature: Of course, fish are very different to mammals. Mammals have the foxl3 gene but what it does in humans is as yet unknown — perhaps it has something to do with stopping basic sex cells becoming male? Finding out what it does in us will be trickier than it has been in fish.

(It’s worth noting that some 60 percent of medaka genes have equivalent forms in humans, making it a good “model” organism for scientists to study.)

However, look again at what Nishimura says and it’s hard not to draw analogies with humans. “That this sexual switch present in the germ cells is independent of the body’s sex is an entirely new finding,” he says. In other words, the body can look like a female, but the germ cells it produces — from ovaries that look like normal ovaries — are sperm.

The analogy is comparable with the way we consider gender in humans. We now know that gender is not a binary male or female thing, but a spectrum. Some people who look like they are male or female on the outside, may feel different on the inside.

Another possibility is that scientists could potentially flick the same switch in human females and get them to produce sperm. The implications for lesbian couples wanting to have children are profound — though, of course, no one is even talking about this as a possibility yet.

I’ve only scratched the surface of what scientists know about medaka. As well as being pets for hundreds of years in Japan, they’ve also been studied in-depth for many, many years.

Cute factoid: rather than simply spawning their eggs into the water, female medaka carry their eggs like a bunch of grapes between their fins. Once the eggs are fertilized, female medaka don’t just dump them into the water, they deposit them carefully, one by one, on plants.

Fish owners love them, and genetically modified fish are available that glow red, green or yellow, the result of having jellyfish genes inserted to the genome.

My favorite is the moonlight medaka — genetically modified so it glows in ultra-violet light. Someone should write a haiku about them: they glow in the dark, they’ve had sex in space and some of the females can make sperm. There’s plenty of material to work with there.

Rowan Hooper is the news editor of New Scientist magazine. The second volume of Natural Selections columns translated into Japanese is published by Shinchosha. The title is “Hito wa Ima mo Shinka Shiteru” (“The Evolving Human”). Follow Rowan on Twitter @rowhoop.

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