TROUT have been created whose surrogate fathers were salmon. The salmon had been engineered so they had the sperm-producing cells of a trout. Although sex cells have been transferred between species before, this is the first time that live offspring have been produced. The technique could eventually be used to breed endangered or valuable species, such as the blue-fin tuna that is highly prized by Japanese diners.
The pioneer of such cross-species transfers was Ralph Brinster and his team at the University of Pennsylvania in Philadelphia. In 1996, they showed that sperm precursor cells from rats would develop into mature sperm in a mouse testis, although no young were created. In the same year, another team injected chickens with primordial germ cells from quails. The cells began to develop into sperm and eggs but were eventually rejected by the chickens鈥� immune systems.
Now Goro Yoshikazi and his colleagues at the Tokyo University of Marine Science and Technology have successfully applied the same technique to fish. They extracted primordial germ cells from rainbow trout embryos, added a marker gene, and then injected them into 60 newly hatched masu salmon. Thirty days later, 10 salmon had trout cells in their ovaries and testes. Those cells were not immediately rejected by the salmons鈥� immune systems and were still present in the gonads of some fish at least a year later (Nature, vol 430, p 629).
Advertisement
The team then used the trout sperm from the salmon to fertilise trout eggs laid by trout. Ten of more than 2000 embryos developed into normal trout, proving that trout sperm from the surrogates were viable.
The achievement is remarkable because although the two species are in the same genus, and salmon sperm can fertilise trout eggs, those embryos always die soon after hatching. What鈥檚 more, the two species diverged at least 8 million years ago and have different reproductive strategies: rainbow trout spawn numerous times during their lifetimes, while salmon die after one spawning.
Yoshikazi believes the technique should also work in mammals and amphibians, and even in species from different genera. 鈥淚f the recipient and donor belong to the same family, we should be able to do it,鈥� he says. He has already done preliminary experiments using trout as the surrogate for primordial germ cells from charr, a more distantly related fish species. There were no signs of immune rejection, but there have been no offspring yet.
The most useful application may be in breeding species that are difficult to keep in captivity. Blue-fin tuna are close to extinction, but captive breeding is expensive because individuals can grow to more than 3 metres long. An alternative would be to grow tuna sperm and eggs in mackerel, breed them, and then release the fry into the ocean to grow to full size.