By Richard Black
BBC science correspondent
Japanese scientists have developed a new kind of assisted reproduction for animals, in which one species can create another.
Trout, like this five-month old juvenile, were created using surrogate sperm from salmon
They implanted tissue from trout embryos into salmon embryos; and when the salmon became adults and mated, they produced trout.
The researchers suggest this could be a way to improve the chances of endangered species, they say in Nature.
It could also help secure supplies of bluefin tuna for sushi restaurants.
The radically new technique involves both surrogate mothers and surrogate fathers.
Scientists at Tokyo University of Marine Science and Technology first extracted primordial germ cells from rainbow trout embryos.
These are cells which will, as the embryo develops, become testes or ovaries.
Then they put these cells into the peritoneal cavities of 60 developing masu salmon embryos.
When the salmon had grown to maturity and mated with each other, most of the offspring were trout-salmon hybrids which died quickly.
However, some were pure trout - an identity confirmed by genetic analysis.
The lead researcher, Professor Goro Yoshizaki, now aims to apply the technique to tuna. Japanese fish farmers, or marine ranchers as they're known, rear tuna in sea pens - but they're difficult to manage and take years to reach reproductive age.
"We consume a lot of tuna as ingredient for sushi. And the biggest problem for bluefin tuna marine ranching is rearing the big parent fish, because bluefin tuna reach to two hundred, three hundred kilo - it's a very huge fish," Professor Yoshizaki told the BBC.
"So my idea is, if we can make a small mackerel fish that can spawn tuna eggs and sperm, we can make marine ranching of tuna much easier and simpler."
Professor Yoshizaki also believes his technique could improve the chances of endangered species.
For example, masu salmon reproduce faster than rainbow trout, so if the trout were endangered, this approach could perhaps produce new inviduals more quickly than nature could manage.
Professor Norman MacLean from Southampton University, a leading UK authority on fish genetics and genomics, said the technique would be difficult to master.
"First you have to locate what are called the genital ridges in the trout embryo in order to extract the germ cells, then micro-inject the cells into another embryo" he told the BBC.
However Professor MacLean believes there might be commercial applications.
"The obvious one is the sturgeon, which only become reproductively mature around the age of ten," he said, "so perhaps you could have them reared by another species which develops much faster."
The researchers transferred germ cells from a trout into masu salmon
As details of the technique have only just been published, it's difficult to judge how widely it might be applied.
How closely related do the species have to be? Would it work in mammals as well as fish?
Already cross-species surrogate motherhood is used in the conservation world - in 1999 a domestic cat carried and delivered an African wildcat baby.
But Susan Mainka, Head of the Species Programme at the International Union for the Conservation of Nature, says there are some endangered species for which that just does not work.
"With the giant panda, for example, people have tried the bear as a surrogate mother but there are ways in which the two species are very different," she told the BBC.
However she cautioned that high-tech science can only ever be a partial solution to species loss.
"You need to conserve the habitat as well," she said, "in order that you have somewhere to return the animals.
"We need to do a much better job of preventing them from going extinct in the first place."