Production of donor-derived Atlantic salmon progeny using allogeneic surrogate broodstock technology

Abstract

Surrogate broodstock technology enables upscaled production of fish carrying desirable genetics via transplantation of germline stem cells (GSCs) into recipients (the surrogates), which will produce progeny from viable and functional donor-derived gametes. This technology constitutes an important tool for aquaculture and conservation biology, allowing for propagation of limited and valuable genetic resources. In this study, gonadal cell suspensions containing GSCs were prepared from sexually immature female and male Atlantic salmon (Salmo salar) and transplanted separately into mixed-sex triploid hatchlings of the same species. One year post-transplantation, two sampled females exhibited one or both ovaries fully populated with developing perinucleolar oocytes, unlike control triploid ovaries showing undifferentiated oogonial nests only. Male gonads were indistinguishable between groups, filled with type A spermatogonia; however, in a separate experiment, gonadal donor cells labeled with PKH26 were still observed within the surrogate's testes 160 days post-transplantation. Later examinations revealed three additional female surrogates sexually maturing displaying ovaries with oocytes at primary growth or late vitellogenic stages. Three years post-transplantation, five male surrogates matured, and the sperm produced was used to fertilize eggs collected from a standard diploid female. Ploidy and microsatellite analyses confirmed that diploid progeny were generated from two of the surrogates. Finally, four years post-transplantation, three female surrogates ovulated eggs that were fertilized with standard haploid sperm, resulting in diploid donor-derived progeny. As juveniles, all sampled progeny from the three crosses were confirmed diploid donor-derived, showing optimal growth and normal gonadal development. Interestingly, all donor-derived progeny originated from surrogates transplanted with male GSCs, which indicates that male GSCs adapted to the female somatic environment, changed their fate and underwent oogenesis, resulting eventually in the production of functional X:Y eggs. Fertilization of X:Y eggs with standard X:Y sperm resulted in the expected 3:1 male to female sex ratios and the production of ∼1/3rd YY super-males within the F1 offspring. In summary, we demonstrate for the first time that donor-derived functional gametes and progeny can be produced from Atlantic salmon through gonadal cell transplantation into sterile triploid surrogates of the same species. Our results also suggest that both salmon sexes are suitable as surrogates, although higher progeny numbers and survival rates were achieved when using female surrogate parents. Implementing such technology represents a powerful addition to existing breeding programs, allowing more efficient selection and upscaled production of key genetic traits to overcome sustainability challenges in salmon aquaculture.