BioEd Online

Japanese researchers have pioneered a breeding technique that allows salmon to father baby trout. The method could potentially revolutionize fish-farming and even resurrect extinct species, they claim.

The researchers managed to create male salmon that produce sperm of a closely related trout species. When used to fertilize trout eggs, the sperm produced perfectly healthy young trout, report Goro Yoshizaki and his colleagues at the University of Marine Sciences and Technology in Tokyo.

The technique involves cells called primordial germ cells (PGCs), which are found in embryos and can develop into either eggs or sperm. Yoshizaki's team injected PGCs from rainbow trout (Oncorhynchus mykiss) into young male masu salmon (Oncorhynchus masou), and let the salmon mature.

When sperm from the fully-grown salmon were used to fertilize trout eggs, 0.4% of the resulting offspring were healthy trout, showing that the injected PGCs had developed into trout sperm. The rest of the offspring fathered by the salmon died at a young age, showing that they were hybrids created as the trout eggs were fertilized by ordinary salmon sperm, the team reports in this week's Nature¹.

Fishy family

It is the first time that transplanted PGCs from one species have successfully produced offspring through a surrogate parent of another species, says Yoshizaki. The achievement is made even more remarkable by the fact that the salmon and trout are native to east Asia and North America, respectively, and are separated by around 8 million years of evolution.

The researchers are also investigating whether PGCs transplanted into female fish will develop into eggs. They expect to find out next year, as female masu salmon take three years to mature compared with the males' two.

If the technique does work for eggs as well as sperm, then PGCs may offer the chance to preserve endangered species or even raise extinct ones from the dead, Yoshizaki predicts. "We can preserve frozen PGCs forever," he says. "So theoretically, even if a species goes extinct, we can transplant the cells into a closely related species."

PGCs may become a useful conservation tool, says Brendan McAndrew, who studies fish rearing at the University of Stirling, UK. But he adds that the technique has not yet been tested on species with more complex development. "Nearly everything you try in rainbow trout seems to work," he says. "Other species may not be so simple."

If the technique does work in other species, it could be a boon to sushi lovers, Yoshizaki hopes. Bluefin tuna is a key ingredient in many sushi recipes, but because they weigh up to 500 kilograms, adult fish are difficult and expensive to house on fish farms. If the related, but much smaller, mackerel can be made to produce tuna eggs and sperm, Yoshizaki suggests, the young tuna could be reared in huge numbers and then released into the sea to be caught by local fishermen.

It is an interesting way to tackle Japan's insatiable appetite for bluefin tuna, says McAndrew. But he argues that traditional conservation measures are ultimately more reliable than high-tech fixes. "Mackerel are more complex than trout, so it might not work," he warns. "Maybe they're better off not fishing their wild tuna so hard."