Sperm transplant hope for endangered species

14 September 2007

In a world first, one species of fish has conceived and given birth to a completely different species following a sperm stem-cell transplant. Could the breakthrough offer hope for critically endangered animals? Antenna investigates...

This research was published in the journal Science on 14 September 2007.

These trout were conceived and borne by salmon.

Image: Goro Yoshizaki/Science


Over 1500 species of animal are currently critically endangered, which means they face an extremely high risk of becoming extinct. A further 38 animal species have already died out in the wild, surviving only in captivity.

If only males or females of a species survive, the outlook is bleak... or is it?

Lonesome George, a male Pinta tortoise from the Galapagos Islands, is the last known member of his species. When he dies, his species will die with him.

Image: Dan Cooper/iStockphoto.com

Now, Japanese scientists think they've got a route back from the brink of extinction. They've transplanted sperm stem cells from male trout into newborn sterile salmon. Male salmon went on to produce trout sperm, and - remarkably - female salmon produced trout eggs. When these salmon mated, they produced trout babies.
'It's the first time the technique has produced functional eggs in any animal species,' says Goro Yoshizaki, from Tokyo University of Marine Science and Technology.

Each small dark dot in the bottom half of this image is a trout sperm, growing in the testes of a salmon. The top half shows tissue from the testes of a sterile salmon that didn't receive a stem-cell transplant.

Image: Goro Yoshizaki/Science

How to make eggs from sperm stem cells...
The key to Goro and his team's success was that they used newborn salmon, implanting the stem cells before their ovaries had fully developed. This environment could have triggered the cells to change type. 'We think the sperm stem cells went back to a very immature stage, and then developed into eggs,' says Goro.

The bottom half of this image shows a salmon ovary full of trout eggs. The top half shows an ovary from a sterile salmon that didn't receive a stem-cell transplant.

Image: Goro Yoshizaki/Science


What does this mean for endangered species?


'By freezing sperm stem cells from an endangered species and transplanting them into closely related species, we can obtain eggs, sperm and offspring even if the target species becomes extinct,' says Goro.

Goro Yoshizaki, Tokyo University of Marine Science and Technology.

Image: Tokyo University of Marine Science and Technology.

The technique succeeded with salmon, but the jury's out on whether it would work with other animals, such as mammals. 'At this moment, we don't know,' explains Goro. 'I think there's a possibility it can work.'
Salmon and trout are pretty closely related - the amount of genetic difference between them is roughly the same as that between human and chimp. But it's not just genetics that matter. The two species would need to be physiologically similar too. And the production of eggs in other animals is more complicated than in fish.

The salmon only produced trout babies (pictured) and no salmon babies because their own sperm and egg cells couldn't function properly.

Image: Goro Yoshizaki/Science

For now, Goro and his team have already started using their research to help vulnerable species of fish, freezing sperm stem cells from an endangered species of salmon. But other experts are concerned the approach could distract attention from the real issue - habitat destruction.
'Technically, it's extremely clever research,' says Mark Wright, an expert on endangered species at the WWF. 'But this kind of work allows us to take our eye off the problem. Our efforts should really be going into protecting the environment. We should only be using these technologies as a matter of last resort.'

Mark Wright, WWF.

Image: Mark Wright/WWF

Goro agrees work needs to focus on restoring habitats, but says his research could help provide a safety net. 'By freezing sperm stem cells we can pretty much preserve them forever. Once the environment has improved, we can use them to regenerate an endangered population.'
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