Histological Identification of Germline Stem Cells’ Recipients for Conservation of Balkan Trout Fish Species by Transplantation Technology

Abstract

Combining cryopreservation of germline stem cells (GSCs) with their subsequent transplantation into recipient fish is a powerful technique for long-term conservation of the genetic resources of threatened fish species. However, transplantation of GSCs has been limited by a failure to identify an appropriate recipient for surrogate production of donor-derived gametes. In the worst-case scenario, recipients may completely reject the transplanted GSCs due to an immunological reaction. This limitation can be overcome by specifying the gonadal status of both GSC donor and recipient species. This study used histological methods to identify possible GSC recipients for rescuing endangered trout fish species on the Balkan Peninsula. The study specified the gonadal status of brown trout, triploid rainbow trout, and tiger trout. The findings revealed that both triploid rainbow trout and tiger trout are sterile. The fish's testes contained only early-stage germ cells and did not develop past type-B spermatogonia, whereas the ovaries contained only oogonia. Both spermatogenesis and oogenesis were incomplete in these fish, and the gametes, i.e., functional spermatozoa and eggs, could not be formed. As a result, these fish make excellent recipients for GSC transplantation. Because most Balkan trout belong to the genus Salmo and closely related species, tiger trout would be a more feasible recipient of GSCs due to the small phylogenetic distance, as it is a hybrid of brown trout female and brook trout male.