Salmon parents, trout offspring

Researchers have developed a practical method of breeding endangered fish species, according to a paper in this week's Science. The authors report that transplanting reproductive cells from rainbow trout into sterile salmon surrogates led to the birth of healthy trout offspring."Here is a new way that you can breed and bring back an endangered species or an extinct species," said Yonathan Zohar of the University of Maryland Biotechnology Institute in Baltimore, who was not involved in the work. "It's absolutely feasible."In previous work, researchers at the Tokyo University of Marine Science and Technology (TUMSAT) in Japan transplanted primordial germ cells from trout into male salmon. The salmon produced trout sperm and offspring, but two problems with the technique remained. First, only a small number of primordial germ cells could be isolated from the donor trout, making the process inefficient. Second, the salmon also produced their own sperm along with trout sperm, suggesting that a genetic marker would be required to differentiate the two types of offspring. In later work, the researchers found a potential solution to the first problem. They discovered that sperm progenitor cells called spermatogonia, which can be isolated in large numbers from adult males, not only develop into donor-derived sperm in males but also into donor-derived eggs in females. "You can make eggs and sperm from the same individual," Zohar told The Scientist."That's pretty amazing."In the new study, the researchers addressed the second problem by using genetically sterile salmon as surrogates. Led by Tomoyuki Okutsu of TUMSAT, they transplanted rainbow trout (Oncorhynchus mykiss) spermatogonia into newly hatched sterile masu salmon (Oncorhynchus masou). They found that all mature gametes that developed in both female and male salmon were genetically identical to rainbow trout. The technique is not entirely robust, however. Out of 29 male salmon the researchers transplanted, just 10 recipients of spermatogonia produced sperm, and 4 out of 8 female recipients produced viable oocytes. Once the eggs were fertilized, the hatching rates varied widely across different females, with some producing hatchlings from just 2 percent of their eggs and others from up to about 90 percent of eggs.The most innovative part of the study was using sterile salmon as parents, Zohar said. "You know that whatever eggs or whatever sperm are being produced in the surrogate are going to be [from] the donor," he told The Scientist. "Then you simply breed them." Indeed, when sperm from surrogate salmon males fertilized eggs of surrogate salmon females, normal trout offspring developed. This first trout generation was also fertile and produced a healthy second generation."It's an extremely elegant study," said Ina Dobrinski of the University of Pennsylvania School of Veterinary Medicine in Kennett Square, who was not involved in the research. The authors also found that about 45 percent of trout spermatogonia survive freezing, which is "very important from a practical point of view," Dobrinski said. Cryopreservation of fish eggs and embryos has been unsuccessful, but the survival of spermatogonia after freezing suggests that scientists could preserve these cells from endangered trout and then breed them later if necessary. "The fish could even go extinct, if you made sure that you cryopreserved a few of those germ cells," Zohar said.Senior author Goro Yoshizaki, of TUMSAT and the Japan Science and Technology Agency in Tokyo, said that his group is currently trying to transplant bluefin tuna spermatogonia into mackerel recipients. It may also be possible to perform similar techniques in other types of animals, including mammals, Yoshizaki said. A 2006 study showed that rat sperm can mature normally in mouse testis, "but there is no method to produce donor-derived eggs in mammals," he said.It appears that a species closely related to the donor is needed as a surrogate, Dobrinski told The Scientist, which may limit the animals that could be bred this way. But "there are some wild strains of trout that are about to go extinct," she said. "This probably would work."Melissa Lee Phillips mail@the-scientist.comLinks within this article:T. Okutsu et al., "Production of trout offspring from triploid salmon parents," Science, September 13, 2007. http://www.sciencemag.orgS. Bunk,"Righting the rainbow," The Scientist, October 14, 2002. http://www.the-scientist.com/article/display/13306/S. Bunk, "What price salmon?" The Scientist, January 22, 2001. http://www.the-scientist.com/article/display/12212/Yonathan Zohar http://www.umbi.umd.edu/~comb/Y. Takeuchi et al., "Biotechnology: surrogate broodstock produces salmonids," Nature, August 5, 2004. http://www.the-scientist.com/pubmed/15295587D. Secko, "How to spot a primordial germ cell," The Scientist, September 16, 2003. http://www.the-scientist.com/article/display/21591/T. Okutsu et al., "Testicular germ cells can colonize sexually undifferentiated embryonic gonad and produce functional eggs in fish," PNAS, February 21, 2006. http://www.the-scientist.com/pubmed/16473947Ina Dobrinski http://w3.vet.upenn.edu/faculty/dobrinskGoro Yoshizaki http://www.soi.wide.ad.jp/class/20040015/profile/yoshizaki.htmlT. Shinohara et al., "Rats produced by interspecies spermatogonial transplantation in mice and in vitro microinsemination," PNAS, September 12, 2006. http://www.the-scientist.com/pubmed/16945902

Salmon parents, trout offspring

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