Transplant reroutes cells from sperm to eggs.

Fish cells destined to become sperm can morph into eggs when transplanted into larvae, a new study shows.

Some fish species can switch from male to female either spontaneously or when treated with steroids. This knowledge led Goro Yoshizaki of the Tokyo University of Marine Science and Technology and his colleagues to wonder whether the gonads of male fish contain cells that might become either eggs or sperm.

From rainbow trout carrying a gene that makes their tissues glow green, Yoshizaki's team removed cells, called spermatogonia, that normally give rise to sperm. The researchers then transplanted these cells into fish larvae so immature that their gender wasn't yet known.

As the larvae matured into adult fish, the scientists found glowing green sperm, produced from the donor fishes' spermatogonia, in males. However, they also found glowing green eggs in females. When Yoshizaki and his colleagues fertilized these eggs with sperm from normal rainbow trout, they developed into healthy, fertile fish, the team reports in the Feb. 21 Proceedings of the National Academy of Sciences.

Yoshizaki says that he and his colleagues don't yet know whether spermatogonia can become eggs in animals other than fish. However, his lab has done spermatogonia transplants between fish species, producing trout eggs and sperm in salmon. He suggests that scientists could use the technique to turn small fish species easily kept in captivity, such as mackerels, into brood fish for large, hard-to-keep species such as overfished bluefin tuna.--C.B.