Common carp belongs into species with highest worldwide production from aquaculture. Similarly, carp in the Czech Republic is the most cultured fish species. The important position of carp is conditioned by long-term breeding efforts when preservation of specific lines and breeds is crucial for further production of hybrids. Cryobanks with sperm collections are serving as a backup which can be utilized to recover valuable genotypes. Recent progress in reproductive biotechnologies developed an alternative strategy for preservation and recovery of genetic resources in fish. All sexually reproducing organism have germ stem cells, female oogonia and male spermatogonia. Germ stem cells are capable of self-renewal but also to differentiate into eggs and sperm. Researches from Faculty of Fisheries and Protection of Waters developed protocols for cryopreservation and recovery of germ stem cells in common carp under the course of Aquaexcel2020 project funded by the European Union. Developed strategy allows to cryopreserve valuable genetic material using efficient protocols for a virtually indefinite period while ensuring 40-65% post-thaw viability. Both testicular and ovarian tissue was cryopreserved having immense potential especially for female genetic resources because egg cryopreservation in fish is far from facilitation to real application.

Cryopreserved germ cells are thawed and recovered by transplantation into recipients. This technology is not far from surrogacy known in humans to tackle infertility issues. However, there is a difference when germ stem cell transplantation in fish results in self-sustaining system when continuous production of sperm and eggs is achieved. Generally, as a recipient can be chosen different species with more favourable characteristics eg. smaller body size, earlier maturation, resistance to disease – goldfish is fulfilling all mentioned advantages to carp while it is still very ordinary species known to anyone from aquaria or garden ponds.

Sterilization of recipients is next important prerequisite in surrogate reproduction. Sterile recipients ensure no competition between endogenous and exogenous cells after transplantation and produced donor-derived gametes are not contaminated by the gametes of the recipient. Knock-down of gene which is crucial for early germ cell survival is most frequently used methods for sterilization. Recipients are ready for transplantation when swim-up stage is reached. Then, gonadal tissue is digested into single cell suspension. Transplantation into goldfish was performed with glass microcapillary filled with cell suspension. About thousands of cells are injected directly into the body cavity of the recipient. Transplanted cells colonized the gonad of the recipient and started proliferation within few days post-transplantation. Presence of donor-derived germ cells in ⁓50% of the recipients was confirmed three months post-transplantation using molecular analysis. Donor-derived carp sperm after oogonia transplantation has been obtained already from goldfish surrogates. This unique ability is given by the stemness of germ stem cells when transdifferentiation and production of gametes of the opposite sex is further opening possibility to produce eggs and sperm even from a single germ cell donor.

Further information can be found in research articles:

- Franěk, R., Marinović, Z., Lujić, J., Urbányi, B., Fučíková, M., Kašpar, V., Pšenička, M., Horváth, Á. Cryopreservation and transplantation of common carp spermatogonia. PLoS ONE 14: e0205481.
- Franěk, R., Tichopád, T., Steinbach, C., Xie, X., Lujić, J., Marinović, Z., Horváth, Á, Kašpar, V., Pšenička M. Preservation of female genetic resources of common carp through oogonial stem cell manipulation. Cryobiology 87:78-85.

Intraperitoneal transplantation into larvae of surrogate goldfish. Cells suspension is stained with methylene blue for better clarity.

Common carp spermatogonia with fluorescently labelled membranes before transplantation.

Detection of fluorescently labelled germ cells of common carp in the goldfish gonads two months post transplantation. White rectangle depicts magnified view on the proliferating germ cells indicated by white arrowheads.