Salmon produces trout sperm

Scientists have taken assisted reproduction to new heights, using a salmon to produce trout sperm to produce a trout. This amazing feat, revealed in this week's edition of the journal Nature, is the work of a team from the University of Marine Sciences and Technology in Japan. The team has high hopes of a benefit to Japan's lucrative sushi market, which is exhausting the Pacific of bluefin tuna.

The researchers made masu salmon, a native of East Asia, produce the sperm of rainbow trout, a native of North America. They achieved this by transplanting cells from trout embryos into salmon embryos. As the salmon embryos developed, these cells formed germ cells in the gonads of the salmon. Germ cells produce either eggs or sperm, depending on whether they are in the ovaries or testes.

When male salmon that had received the transplant were mature, the researchers harvested sperm and found that it contained a mixture of salmon and trout sperm. They then used this sperm to fertilise trout eggs, producing baby trout that had been fathered by a salmon. A female trout had produced the trout eggs used, but the team are also investigating whether salmon can be used to produce trout eggs.

This is the first time a cross-species transplant of this type has produced live offspring. Dr Yutaka Takeuchi, who led the researchers, said, 'if cells of bluefin tuna could be transplanted into mackerel, the surrogate mackerel would produce mature eggs and sperm from the donor tuna in a short period and in a small facility. Therefore, our technique may help feed the world's sushi habit'.

The research may also be beneficial for conservation: scientists have had difficulty freezing the sperm and eggs of endangered species, but the embryo tissue used is much easier to store. The stored material could then be used to restore the population, with a more common related species acting as surrogate.

Not surprisingly, questions have already been raised about the use of this technology in mammals and even humans. One important factor that will determine whether the technique will work is the relatedness of the two species. The salmon and trout used are closely related: it is thought that they diverged into different species around 8 million years ago. Man and chimpanzees diverged about 5 million years ago.