Salmon gives birth to trout in scientific leap that gives hope to endangered fish

Biology textbooks will never quite be the same again. Scientists have altered the reproductive organs of salmon so that they produce trout offspring.

A "germ" tissue from young trout was put into young salmon so that when the salmon became sexually mature they produced the sperm and eggs of trout. In a study published today in the journal Nature, the researchers report that they have successfully used the technique to breed healthy rainbow trout from salmon parents.

The scientists, led by Yutaka Takeuchi of the Tokyo University of Marine Science and Technology, said that the development could help many of the world's endangered species of fish. Dr Takeuchi said tissue transplants from one endangered species to a related but more common species that is easier to rear in captivity could help to boost the wild populations of threatened or commercially valuable fish.

"The seed production for a species with a large body size and longer generation time could be carried out in surrogate parents with a smaller body size and shorter generation time," Dr Takeuchi said.

The technique relies on transplants of "primordial germ cells", which are the specialised tissues of embryonic fish that eventually develop into the gonads, the sex organs of adults that produce the sperm and eggs. Dr Takeuchi and his colleagues took primordial germ cells from an embryonic North American rainbow trout and transplanted them into the embryos of the masu salmon, which is only found in east Asia. Although the two species are related - they both belong to a group known as the salmonids - they have been separated by at least eight million years of evolutionary history.

"The most striking biological difference between them is that rainbow trout are able to spawn several times during their lives, whereas masu salmon die after their first spawning," the researchers write in Nature.

When the salmon in the experiment grew into adults, some of them produced sex cells - sperm or eggs - from both species. The scientists went on to show that these salmon could produce healthy, viable trout offspring. "It is the first time in animals that the primordial germ cell transplantation experiment has worked on the trans-species level and showed the expected ultimate success, namely the production of live offspring," Dr Takeuchi said.

The findings have important commercial implications, because some of the rarest fish in the world, such as the bluefin tuna, are those that are being hunted to extinction.

"If primordial germ cells of bluefin tuna could be transplanted into mackerel, the surrogate mackerel would produce mature eggs and sperm derived from the donor tuna in a short period and in a small facility. Therefore, our technique may help to feed the world's sushi habit," Dr Takeuchi said.

In addition to helping endangered species and boosting the number of valuable fish such as the bluefin tuna, the research raises the possibility of producing expensive sturgeon caviar in the bodies of related but more common species which could be reared on fish farms.

Simon Davies, a fish biologist at the University of Plymouth, first heard of the study at a scientific conference in Hawaii in March where the Japanese scientists presented their results. "When I saw this research I said, 'My God this is really interesting'. I came away very stunned by the quality of the science. In the right hands it could be very beneficial," Dr Davies said.

Professor Gordon Reid, director of Chester Zoo and a leading fish specialist, was equally impressed by the Tokyo team's ability to transplant viable germ tissue from one species to another. "It does sound interesting because one could imagine in the area of declining populations of fish when one species is threatened you can salvage that species by utilising a more common species," he said.

Freezing the sperm of endangered fish has had limited success and freezing eggs has been even more problematical. Embryonic germ cells are easy to freeze so they could provide an answer to the problem, Professor Reid said.

However, the idea of using tissue transplants between fish species as a way of preserving endangered animals in the wild is a simplistic solution, warned Professor John Sumpter, a fish biologist at Brunel University.

"They are trying to sell this on the grounds of helping to preserve endangered species, but it strikes me as a hi-tech solution," he said. "The problem is not going to be addressed by this solution. The problem is one of habitat loss, over-fishing, and possibly climate change and pollution. These are what need to be addressed."