Bait and switch has a whole new meaning. A team of scientists has coaxed a rainbow trout cell slated to become sperm to develop into a female egg. Researchers say the technique offers a potential conservation backup plan for threatened fish populations worldwide and may also help explain the ability of fish to swap sexes.
Scientists began wondering just how versatile fish could be several years ago. In some species, both sexes possess ovaries and testes, and in one type of tropical fish, females can spontaneously switch their sex when the male leader of an all-female harem is removed from the tank; males of the same species can do the opposite if more than one male leader is around. Researchers wanted to better understand how sexual flexibility works at the cellular level. Developmental biologist Goro Yoshizaki of Tokyo University of Marine Science and Technology in Japan and his colleagues set out to see whether spermatogonia--male germ cells that give rise to sperm--might contain a subpopulation of cells with stem cell-like activity that can develop into either sperm or eggs.
Yoshizaki's team first performed some germ cell transplant operations. They removed spermatogonia labeled with a fluorescent marker from a mature male trout and injected the cells into the abdomens of newly hatched male and female trout fry. In 39% of the male hatchlings, some of the germ cells became sperm; the rest didn't appear to develop at all. But to the researchers' surprise, in 37% of the female recipients, some of the donated germ cells became eggs. To make sure that the eggs were viable, the team artificially inseminated these fluorescently labeled eggs with sperm. The females gave birth to normal hatchlings, the team reports 6 February in Proceedings of the National Academy of Sciences.
Because male testes harbor spermatogonia throughout life, Yoshizaki says the finding means that a single male--or even just his testes, which researchers are now trying to preserve cryogenically--and a single female could repopulate a threatened fish species, without using mature sperm. The technique would be a last resort, Yoshizaki admits, because such a pairing would decimate the species' genetic diversity.
Stem cell scientist Guang Zhao of the University of Texas Southwestern Medical Center in Dallas notes that the germ-cell gender-switching might not work easily in other animals, such as mammals. But Zhao is excited by the evidence for the germ cells' stem cell-like versatility. "Those fish sperm stem cells may be able to become other types of cells in addition to germ cells," he says.