13 September 2007; | doi:10.1038/news070910-11 / http://www.nature.com/news/2007/070910/full/070910-11.html
Salmon parents give birth to trout
Genetic technique creates viable fish sperm and eggs.Nicola Jones


| Trout juveniles at 6 months old, generated from surrogate salmon parents. Science |
|
Researchers
have succeeded in making salmon couples give birth to trout - using a
technique that they argue could help to preserve rare species of fish.
Goro
Yoshizaki and his colleagues at the Tokyo University of Marine Science
and Technology in Japan had previously shown that male salmon could be
injected with cells from closely-related trout to produce viable trout
sperm. When the sperm were introduced to trout eggs, healthy trout
offspring were produced.
Now
the researchers have taken the work a step further, showing that salmon
can be not only the biological fathers but also the mothers of trout
offspring. The new work, published in
Science, shows how two sterile masu salmon (
Oncorhynchus masou) can together produce nothing but healthy rainbow trout (
O. mykiss).
The
technique relies on the injection of trout spermatagonia - the early,
stem-cell stage of sperm - into salmon embryos, so that the growing
salmon produce trout sperm and eggs. The technique could be very useful
for storing back-up genetic material of different fish species that are
today under threat, because spermatagonia can be easily cryopreserved,
says David Penman, a fish geneticist at the University of Stirling, UK.
With
many plants and animals, seeds, sperm and eggs can be cryopreserved to
later resurrect a species that has died out. "The problem with
gene-banking when you come to fish, is that it's almost impossible with
eggs," says Penman. "The eggs are very big, very yolky," he says, which
makes them nearly impossible to freeze.
If
Yoshizaki's technique is broadly applicable to other fish, it will mean
that their eggs don't need to be preserved - they could be made at a
later date, in a surrogate fish.
The sperm that turned

| Injecting trout spermatogonia into a salmon embryo. Science |
|
If
trout spermatagonia are injected into normal male salmon embryos, the
fish will grow up to produce a mix of different types of sperm - some
salmon, and some trout. In the team's previous work, when sperm from
male salmon treated with primordial germ cells (an even earlier stage
of sperm) were used to fertilize trout eggs, just 0.4% of the resulting
offspring were healthy trout. The rest were hybrids that did not
survive.
To
increase this percentage, the team turned to salmon designed to have
three sets of chromosomes instead of the usual two, making them
sterile. When they are injected with spermatagonia, the only viable
sperm the males later produce comes from these injected cells - making
them 100% trout.
The
group also met with success doing the same thing with sterile female
salmon. When the female fish were injected as embryos with
spermatagonia, the eggs they produced were all trout eggs.
The fact that an early form of sperm could be used to
prompt the growth of eggs shows just how flexible these fish are to
this type of treatment, notes Penman. "It's really amazing to see that
they injected spermatagonia and got eggs - the fish are very plastic."
When mated, these salmon parents produced healthy trout offspring, which in turn mated to give a healthy second generation.
Save the fish
Yoshizaki and his colleagues hope the technique could be used to help save endangered fish.
"There
are lots of species of fish that are threatened," says Penman. He says
he thinks that to use this approach properly will mean thinking about
cryopreserving a lot of material, to save all species and ensure that
resulting populations are not too inbred. And some species are likely
to be easier to work with than others, he adds. "If you don't happen to
have a closely related recipient [for a surrogate] then you'd struggle.
There are some groups where there are lots of close relatives, but some
where there are only a few," he says.
"It's
better to look at all the aspects of conservation biology - but this
will be a useful additional technique, as a backup," Penman says.
Reference : Okutsu, T., Shikina, S., Kanno,
M., Takeuchi, Y.& Yoshizaki, G.,Science 317,1517(2007).