Who is the best surrogate for germ stem cell transplantation in fish?

Surrogate broodstock technology in fishes has potential for aquaculture as well as endangered species conservation and propagation. Species with some unfavourable biological characteristics for culturing such as a late maturation or a large body size are ideal candidates for surrogate reproduction using smaller and faster-maturing hosts. One of the general prerequisites for pure donor-derived gamete production is the sterility of the host. Various sterilization methods have been developed and used in fish surrogacy; however, a direct comparison of available methods is missing. Such a knowledge gap hinders choice for the surrogate in various fish species, including those in high commercial demand such as tuna or sturgeons. There is a particular limitation from the point of the live material availability and difficulty to perform a high throughput assessment of different surrogates. Yet, large sturgeons or tuna species are one of the most prominent candidates for surrogacy. Zebrafish (Danio rerio) was utilized in this study as a model species to answer whether and to what extent different sterilization strategies can affect the surrogacy. Germ cell-depleted recipients (produced using knockdown of dead end gene), triploid recipients, and zebrafish x pearl danio (Danio albolineatus) hybrid recipients were tested as they represent the most frequently used types of surrogates. Spermatogonia isolated from the vas::EGFP transgenic strain were intraperitoneally transplanted into swim-up 5 day old zebrafish. Colonization rate of transplanted germ cells, survival, gonadal development, and reproductive output of the fish were analysed. Germ cell-depleted recipients with normal somatic cells were identified as the most convenient among tested sterilization methods considering adult germline chimera rate and their reproductive output. The present study provides a significant aid for selecting suitable surrogates in fishes.