Trout with salmon parents could help to revive endangered fish species

Japanese researchers have developed a way of using one species of fish as a surrogate parent for an endangered one by transplanting the sexual equivalent of stem cells. If enough of these cells can be preserved, an extinct species could be resurrected.

Getting excited when fish produce sperm would usually get you strange looks. But for Tomoyuki Okutsu and colleagues at the Tokyo University of Marine Science and Technology, it’s all part of a day’s work. They are trying to use one species of fish as surrogate parents for another, a technique that could help to preserve species that are headed for extinction.

Baby trout can be born from salmon parents using transplanted sex cells.Okutsu works on salmonids, a group of fish that includes salmon and trout. Many members of this tasty clan have suffered greatly from over-fishing in the last few decades, and their populations are dwindling their way to extinction.

If stocks fall below a critical level, they may need a jump-start. One strategy is to freeze some eggs to be fertilised artificially, in the way that many human eggs are in fertility clinics. But it’s much harder for fish eggs – they are large and have lots of fat, which makes them difficult to freeze effectively.

Okutsu’s group have hit on a more effective solution. They use transplanted sexual stem cells to turn another species of fish into surrogate parents for the endangered ones.

Fishy surrogates

The technique revolves around special cells called spermatogonia. In humans, these eventually become sperm. But in an earlier study, Okutsu found that the spermatogonia of fish are like sex stem cells, with the ability to become either sperm or eggs. When transplanted into a host species, they should develop into both types of sex cell.

But that wasn’t to be. The transplanted cells only produced a very small number of donor sperm cells, which would have needed to be sorted from the host’s far greater supply of its own sperm. The transplanted cells also never produced viable eggs, putting the kibosh on any breeding attempts.Sterile masu salmon can be transformed into surrogate parents for trout.

Undeterred, Okutsu’s group found a solution for this too and it was a remarkably simple one. They used sterile hosts that couldn’t produce their own sex cells – if they made any, it must have been because the transplants had taken.

As hosts, they chose masu salmon that suffered from a condition called triploidy. Rather than pairs of chromosomes, their cells carried triplets, and the extra set rendered them infertile.

Okutsu took spermatogonia from rainbow trout and implanted them in the newly hatched salmon. Normally, the male triploid salmon never develop proper testes and the females never produce viable eggs. But not so for the transplant recipients – most of them developed normal sets of both sex organs and sex cells.

Success

Ten of the 29 males produced milt, the fish equivalent of seminal fluid and five of the 50 females released mature eggs.The group used sperm from the surrogate males to fertilise eggs from the surrogate females and to their relief, the resulting embryos did very well. The vast majority hatched and developed normally.

Overfishing is putting untenable pressure on the world’s fish stocks.When he checked the DNA fingerprints of this new generation, he saw that they were all 100% trout. Even their mitochondria were trout mitrochondria. The team had successfully bred baby trout from surrogate salmon parents. The newborns even grew up to become fertile adults, capable of producing young trout of their own.

Best of all, the team found that 45% of frozen spermatogonia survive the thawing process intact. That means that their technique is not only feasible but practical. Even if rainbow trout went extinct, Okutsu could breed new ones if he had enough of these cells in cold storage.

More on fish and conservation:

Farmed salmon decimate wild populations by exposing them to parasites

Shark-hunting harms animals at bottom of the food chain

Restoring predator numbers by culling their prey

Reference: Okutsu, Shikina, Kanno, Takeuchi & Yoshizaki. 2007. Production of trout offspring from triploid salmon parents. Science 317: 1517.

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