Sperm transplant could save endangered fish

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Scientists from the Tokyo University of Marine Science and Technology have developed a surrogacy method that shows great potential in conserving endangered (or even extinct) fish species.

The results are published in a report in the latest issue of the journal Science by Tomoyuki Okutsu, Shinya Shikina, Megumi Kanno, Yutaka Takeuchi and Goro Yoshizaki.

The method, dubbed surrogate broodstocking by the authors, involves the transplantation of primordial germ cells or spermatogonia from the target species into a related species for which rearing techniques are well developed.

This causes the recipient species to produce the eggs and sperm of the target species.

As an experiment, the authors introduced spermatogonia and primordial germ cells of adult rainbow trout, Oncorhynchus mykiss, via intraperitoneal microinjection into embryos of triploid sterile masu salmon, O. masou (the hybrids of the two species do not survive).

The testes of the control fish, in which no transplantation took place, were immature and contained only spermatogonia, while those of the transplanted fish were normal and mature.

Of the fish receiving the spermatogonia, 10 of the 29 male recipients produced milt, which when used to fertilise wild-type rainbow trout eggs, produced normal rainbow trout young (confirmed by DNA fingerprinting).

Similarly, 4 of 8 female recipients produced vitellogenic oocytes after 17 months (the ovaries of intact triplod salmon of the same age contained no vitellogenic oocytes).

These eggs, when fertilised with milt from male recipient salmon, also produced normal rainbow trout young at a hatching rate of 89.5%.

A bigger step for surrogate broodstocking is to be taken next month, when scientists from Idaho attempt to produce the sockeye salmon (which is highly endangered in the state) using this method on more plentiful trout.

For more information, see the paper: Okutsu, T, S Shikina, , M Kanno, Y Takeuchi and G Yoshizaki (2007) Production of trout offspring from triploid salmon parents. Science 317, p. 1517.