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Trout+Trout = Salmon
After years of research and careful experimentation, Joseph Cloud, professor of biological science, is hoping to get an unexpected product from combining male and female rainbow trout: sockeye salmon. That’s right: a different fish.
“I like to call our project genetic insurance,” Cloud says of his conservation efforts. “But to have a great policy you need eggs. Freezing and preserving sperm is simple, but even a tiny zebrafish egg is too large to freeze. So certainly with salmon it’s technically impossible.”
Sockeye salmon from Redfish Lake are endangered, part of the reason why Cloud is trying to ensure their future and why those salmon eggs are a hot commodity. So, to secure an adequate, fresh egg supply will be available for conservation efforts, Cloud is turning to rainbow trout through a procedure that doesn’t require a single female sockeye salmon.
When a fish is first hatched, it has no sexual identity. Instead, primordial germ cells migrate to structures called genital ridges where they develop into either testes or ovaries, depending on the fish’s DNA. But it turns out these primordial germ cells aren’t the only cells capable of doing the job. The same cells from other fish, when injected before the migration begins, will also make the journey.
In an even bigger twist, scientists recently discovered stem cells in the testes of fish that act like primordial germ cells and will also get the job done. Even though the stem cells come from males, they will become ovaries once implanted into undeveloped females.
“The DNA is actually telling the stem cells what they’re going to major in,” says Cloud. “They’re going to become ovaries.”
The overall idea is that if scientists take these specific stem cells from a male and put them into a sterile female trout just after hatching, then that female trout will produce salmon eggs. This is important because undeveloped male testes can be frozen for at least a year – perhaps decades – possibly giving scientists an enduring bank of genetic material without any ovaries or eggs required.
However, because it takes rainbow trout three years to sexually mature, and the first specimens were injected only a year ago, Cloud is still waiting to see if it worked.
“We tagged the injected stem cells with fluorescent markers,” says Cloud. “So, after 61 days of development, we have cells that light up. That’s how it’s known our cells are still there and that there are eggs present. But are they sockeye salmon eggs? We don’t know. We’re now at one of those points where we just have to wait and see.”