Fish switch: salmon make baby trout after species, sex swap.

Biologists have implanted male-reproductive tissue from rainbow trout into male and female salmon, which then bred a new generation of baby trout.

In male-salmon recipients, the trout tissue produced sperm, but in female salmon, the same tissue produced eggs, says Goro Yoshizaki of the Tokyo University of Marine Science and Technology. These surrogate parents produced trout that grew up to breed in the usual way, Yoshizaki and his colleagues report in the Sept. 14 Science.

Showing that surrogates can create the young of another species implies that "cryo-preserved germ cells from testes of endangered, or even nearly extinct, species should be kept for the future," comments Rune Male, who studies fish reproduction at the University of Bergen in Norway.

Developing techniques for the switch took almost 5 years, Yoshizaki says. The team learned a few tricks from fish farms, such as dipping fertilized eggs in warm water to create sterile fish. Sterilized masu salmon served as recipients for trout tissue.

The Yoshizaki team showed that newly hatched salmon would not reject implants of foreign tissue. Researchers treated the salmon before they were 46 days old.

The implants came from tissues called spermatogonia, which give rise to germ cells, or sperm, in male trout. The researchers simply injected the trout tissue into the body cavities of the hatchling salmon. "Germ cells have a very special ability to find, and migrate to, gonads," says Yoshizaki.

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The researchers had earlier discovered that spermatogonia tissue would make eggs when tucked into a female body. That's one of the "important tricks" that makes the surrogacy practical, says Yoshizaki.

After the salmon matured, in 3 years, 10 of the 29 male recipients produced sperm and 5 of the 50 females ovulated. In the best batch of fertilized eggs, 90 percent hatched. Genetic tests confirmed that the young fish came from the tissue-donating trout, the researchers say.

For basic biology, says Male, the demonstration "opens the way for new experiments that may help explain the process of sex determination and differentiation." Trout are related to salmon and, as the new experiment suggests, share sex-determining mechanisms.

Male notes that salmon and trout eggs can reach 6 millimeters in diameter, too big to preserve well by freezing. The ability to preserve and later use the tissue that generates germ cells could be a boon in efforts to breed fish for both commerce and conservation.

Yoshizaki's team has found that 45 percent of cryogenically preserved spermatogonia are alive after thawing. Yoshizaki says that he's trying to produce bluefin tuna using mackerel as surrogates. Such surrogate parents, a thousandth the size of their offspring as adults, could allow fish farmers to keep tuna-breeding stock in tanks of manageable size. Yoshizaki is also working to create surrogate parents for the endangered Idaho sockeye salmon.

Populations of salmon, trout, and their relatives are declining worldwide, and in the lower 48 states, wild salmon are down to 4 percent of their historic abundance, says Guido Rahr, president of the Wild Salmon Center in Portland, Ore. He points out, though, that high-tech breeding techniques won't help without habitat preservation. "The best fish hatchery in the world is a healthy river system," he says.