Generation of juvenile rainbow trout derived from cryopreserved whole ovaries by intraperitoneal transplantation of ovarian germ cells

Cryopreservation of fish sperm offers the practical applications in the selective breeding and biodiversity conservation. However, because of the lack of cryopreservation methods for fish eggs and embryos, maternally inherited cytoplasmic compartments cannot be successfully preserved. We previously developed an alternative method to derive functional eggs and sperm from cryopreserved whole testis by transplanting testicular cells into female and male recipients. However, if target fish had ovaries, the previous method employing male-derived germ cells would be ineffective. Here, we aimed to generate functional gametes from cryopreserved whole ovaries by transplanting ovarian germ cells into peritoneal cavity of sterile hatchlings. Cryopreservation conditions for rainbow trout ovaries (1.0 M DMSO, 0.1 M trehalose, and 10% egg yolk) were optimized by testing several different cryoprotective agents. Ovarian germ cells from thawed ovaries were intraperitoneally transplanted into allogeneic triploid hatchlings. Transplanted germ cells migrated toward and were incorporated into recipient gonads, where they underwent gametogenesis. Transplantation efficiency of ovarian germ cells remained stable after cryopreservation period up to 1185 days. Although all triploid recipients that did not undergo transplantation were functionally sterile, 5 of 25 female recipients and 7 of 25 male recipients reached sexual maturity at 2.5 years post-transplantation. Inseminating the resultant eggs and sperm generated viable offspring displaying the donor characteristics of orange body color, green fluorescence, and chromosome numbers. This method is thus a breakthrough tool for the conservation of endangered fish species that are crucial to cryopreserve the genetic resources of female fish.