Cross-fertilization.
A salmon embryo gets a dose of immature trout reproductive cells.

Credit: Science

Trout, Your Mama Was a Salmon

By Adam Hinterthuer
ScienceNOW Daily News
13 September 2007

The study clears a major conservation hurdle. Scientists often freeze eggs and sperm of endangered animals to keep viable genetic material around in case of extinction. However, fish eggs cannot be preserved this way because of their large size and high fat content. But undeveloped male sperm cells, called spermatogonia, do fine in a freezer. When thawed and implanted into another fish, they migrate to the gonads and grow into either sperm or eggs, depending on the sex of their host. This technique works within a species, but no one had tried transfers between fish species.

Researchers at the Tokyo University of Marine Science and Technology collected spermatogonia from adult rainbow trout and injected them into sterile salmon embryos. The researchers raised the fish to sexual maturity and found that 10 of the 29 male salmon produced trout sperm, and five of the 50 female salmon produced trout eggs. In comparison, a control group of sterile salmon that did not receive transplants had no mature sex cells at all. As the team reports in the 14 September issue of Science, when it combined eggs and sperm from the recipients, a new generation of healthy trout hatched. Co-author Goro Yoshizaki says his team used sterile recipient fish so that only donor-derived sex cells could be produced. That way, when the researchers mixed eggs from the female salmon with sperm produced by the males, they got "100% trout."

Fish geneticist Gary Thorgaard of Washington State University in Pullman says he is "very excited" by the study because it "opens up new directions both in research with salmon and trout and in conservation approaches." But, he warns, only a small percentage of salmon in the study produced sperm and eggs, and that could create a "genetic bottleneck" where inbreeding occurs. To prevent this, Thorgaard says, researchers will need to get more surrogates reproducing so that enough genetic diversity is available to keep endangered fish safely in the gene pool.

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