Surrogate broodstock technology (SBT) enables production of donor-derived gametes, and subsequently, offspring via surrogates. Germline stem cells (GSCs) can be harvested from donor fish and transplanted into multiple surrogate fish. Subsequently, the transplanted GSCs proliferate, differentiate and develop into functional eggs or sperm, depending on the sex of the surrogate. This allows upscaled production of salmon strains of interest, for instance threatened strains (for conservation purposes) or elite individuals with high robustness (in aquaculture). Furthermore, using cryopreserved GSCs as donor cells opens the possibility to utilize and propagate (long-term) valuable, stored genetic material. In addition, surrogates with shorter generation intervals (early maturing strains or species closely related to the GSC donors) can be used to shorten the time required to produce donor-derived offspring. If implemented in conservation or breeding programs, SBT may become an important tool to achieve more efficient and targeted production of valuable salmon strains.
So far, SBT has been successfully applied to several fish species for the preservation of genetic resources. However, in the case of commercial application technical and operational challenges require attention, for instance the unpredictable success rates of GSC transplantation. Several factors may influence GSC transplantation success, such as the age of donors and surrogates, endogenous germ cells competing with transplanted GSCs, and GSC quantity and purity. The use of immature donors, germ cell-free surrogates, and in vitro propagated GSCs, may contribute to increased transplantation success. Nevertheless, each protocol needs to be optimized for each fish species of interest. SBT protocols remain to be developed for Atlantic salmon, an endangered and at the same time highly valuable commercial species. In the “CELLS4TRAITS” project, we aim to optimize protocols for cultivation, propagation, mass-production and preservation of GSCs carrying desired genetic traits (disease resistance and sterility), suitable for mitigating sustainability issues in Atlantic salmon aquaculture. As part of the project, we are currently testing protocols for enrichment and propagation of salmon GSCs in vitro, either through 3D or 2D cell culture systems, to obtain a higher supply of GSCs and improved transplantation success.