Salmon + Salmon = Trout?

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A trout germ cell is transplanted into the body cavity of a newly hatched salmon embryo. This is part of the process that allowed adult salmon to successfully spawn trout offspring.

Image: Science magazine

Have you ever heard of a trout with salmon for parents? Since when has one species given birth to another species? Well, ever since scientists began experimenting with salmon in the hope that they could genetically alter these fish by injecting sex cells from trout so the salmon would produce trout offspring when they bred.

"There are a lot of fish species or populations facing extinction because of environment destruction and overharvesting," says marine bioscientist Goro Yoshizaki, who led the team. "Obviously environmental protection and [halting] fishing is the best solution...[but] we need some backup system to conserve endangered species."

To this end, Yoshizaki and his colleagues at the Tokyo University of Marine Science and Technology refer to their method as "surrogate broodstocking." Basically, they collected undeveloped male sperm cells, known as spermatogonia, from adult rainbow trout, Oncorhynchus mykiss, and injected them into sterile masu salmon embryos, O. masou. Masu salmon are native to Japan.

Interestingly, injecting male trout sex cells into sterile female Masu salmon also worked.

Once inside the salmon's body, the sex cells migrated to the host fish's testes or ovaries, where they developed normally.

"They can walk to their home by themselves," Yoshizaki observed. The process that the team used can be summarized as shown below;

Image: Science magazine.

To mark these trout sex cells, referred to as "primordial germ cells (PGCs)" as being different from those of the surrogate salmon recipients', the researchers included a green fluorescent protein in the trout DNA. Additionally, histology and DNA fingerprinting showed that spermatozoa produced by the salmon originated from the trout donors. These salmon are referred to as "xenogeneic salmon" because of they produce offspring from a different species. (Figure 1, below).

Image: Science magazine [larger size].

As a result of this work, these xenogeneic surrogate salmon were raised to sexual maturity and one-third of the males produced viable trout sperm while one out of five females produced viable trout eggs. Further, when these eggs and sperm were combined, they gave rise to a new generation of trout. How do the researchers know that these offspring are pure trout? First, it is known that hybrids of Masu salmon and rainbow trout do not survive and further, DNA fingerprinting and breeding experiments confirmed that these fish were "100% trout."

"They showed nicely that ... they produced the fish they were shooting for," commented John Waldman, a fisheries biologist at Queens College in New York.

Further, these trout were able to reproduce when they reached maturity.

These results are not specific to salmon and trout: Yoshizaki and his team obtained similar results when they injected trout spermatogonia into brown trout and Japanese char.

This success is of great interest to conservation biologists, who have been unable to save extinct fishes by freezing their eggs because fish eggs have a high fat content and are so large. However, they can successfully freeze fish sperm.

"Future work should look to expand this approach to other fishes in need of conservation, in particular, the sturgeons and paddlefish," Waldman added. "We have a lot of species of fish around the world that are really in danger of becoming extinct."

Fish geneticist Gary Thorgaard of Washington State University in Pullman says he is "very excited" by this work 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 avoid this, Thorgaard says, researchers need to get more surrogates reproducing so that enough genetic diversity is available to keep endangered fish safely in the gene pool.

Yoshizaki is developing this technique specifically for use with bluefin tuna, a commercially important species that is rapidly approaching extinction due to overfishing. Bluefin tuna grow to be the size of a human, so they are difficult to ranch using standard techniques.

To accomplish this goal, Yoshizaki has begun developing techniques to produce baby tuna from mackerel, a fish species that are approximately one thousand times smaller than adult tuna.

"If we can produce surrogate mackerel, whose body size is 500 to 1,000 grams--nearly 1,000 times smaller than tuna -- we can save space, cost and labor to produce [a] tuna baby," Yoshizaki predicts.

This research was published in last Friday's issue of the journal, Science.


T. Okutsu, S. Shikina, M. Kanno, Y. Takeuchi, G. Yoshizaki (2007). Production of Trout Offspring from Triploid Salmon Parents Science, 317 (5844), 1517-1517 DOI: 10.1126/science.1145626.

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