Production of live fish derived from frozen germ cells via germ cell transplantation.

The conservation of endangered fish is an urgent issue. Although cryo-banking of fish gametes might ultimately help conserve endangered fish, cryopreservation of fish eggs or embryos is still not possible due to their large size and high yolk content. Therefore, as an alternative, we focused on undifferentiated germ cells, such as primordial germ cells, spermatogonia, and oogonia, as materials for cryopreservation. Transplantation of cryopreserved germ cells into the body cavity of allogeneic or xenogeneic recipients sterilized by triploidization or endogenous germ cell ablation resulted in migration of the transplanted germ cells toward the recipient genital ridges, where they were eventually incorporated. The donor-derived germ cells initiated either spermatogenesis or oogenesis in the recipient gonads, depending on the sex of the recipient. Furthermore, by mating the male and female recipients, viable offspring derived from the frozen germ cells were produced. Although this technology was established using salmonid fish, we found that it is applicable to a wide range of fish species. Thus, this method might represent a "silver bullet" for preserving the valuable genetic resources of endangered fish species.