Characterization of potential spermatogonia biomarker genes in the European eel (Anguilla anguilla).

Identification of specific molecular markers for spermatogonial stem cells in teleost is crucial for enhancing the efficacy of reproductive biotechnologies in aquaculture, such as transplantation and surrogate production in fishes. Since it is not yet possible to distinguish spermatogonial stem cells of European eel (Anguilla anguilla) using specific molecular markers, we isolated spermatogonial cells from immature European eels to find these potential markers. We attempted this by studying three candidate genes: vasa, nanos2, and dnd1. Two vasa (vasa1 and vasa2) genes, nanos2, and dnd1 were identified, characterized, and studied in the muscle, testis, and isolated spermatogonia. Our results showed that vasa1 and vasa2 had the highest levels of expression when measured by qPCR. In situ hybridization and immunochemistry assays showed that the four genes were localized explicitly in type A spermatogonia. However, vasa1 and vasa2 exhibited stronger signals in the immature testicular tissue than the other two potential markers. According to this, vasa1 and vasa2 were found to be the most effective markers for spermatogonial cells in the European eel.

The effects of cryopreservation and cold storage on sperm subpopulation structure of common carp (Cyprinus carpio L.).

The objectives of this study were to identify the presence of different spermatozoa subpopulations (SPs) according to their kinematic characteristics in the sperm of common carp and to test the effects of cryopreservation and prolonged (6-day) storage at room temperature (RT; 23 °C) and 4 °C on spermatozoa motility and subsequently on SP dynamics. Two-step clustering analyses identified three motile SPs based on their kinematic properties: SP1 contained spermatozoa with low velocity and low/moderate STR/LIN values (slow non-linear SP); SP2 was comprised of spermatozoa with high velocities and high STR/LIN values (fast linear SP); SP3 was characterized with high VCL, and moderate LIN/STR (fast non-linear SP); and an additional SP0 was added comprising immotile spermatozoa. Total motility, progressive motility and VCL decreased after cryopreservation to approximately 50% of their value in fresh sperm, while the frequency of SPs characterized by high values of motility parameters declined in favor of those with low motility values and SP0. Motility values of fresh and cryopreserved spermatozoa which were washed with fresh extender after thawing decreased significantly after 24 h of storage at RT and after 72 h of storage at 4 °C, while cryopreserved sperm which remained in the original cryomedium faced a steep decline in motility after only 2 h of storage. As subpopulation frequencies followed this dynamic, this indicates that cryopreserved sperm should be washed with fresh extender in order to obtain favorable sperm kinematic properties after freezing.

Cyanobacteria, cyanotoxins, and their histopathological effects on fish tissues in Fehérvárcsurgó reservoir, Hungary.

Cyanobacteria are important members of lake plankton, but they have the ability to form blooms and produce cyanotoxins and thus cause a number of adverse effects. Freshwater ecosystems around the world have been investigated for the distribution of cyanobacteria and their toxins and the effects they have on the ecosystems. Similar research was performed on the Fehérvárcsurgó reservoir in Hungary during 2018. Cyanobacteria were present and blooming, and the highest abundance was recorded in July (2,822,000 cells/mL). The species present were Aphanizomenon flos-aquae, Microcystis flos-aquae, Microcystis wesenbergii, Cuspidothrix issatschenkoi, Dolichospermum flos-aquae, and Snowella litoralis. In July and September, the microcystin encoding gene mcyE and the saxitoxin encoding gene sxtG were amplified in the biomass samples. While a low concentration of microcystin-RR was found in one water sample from July, analyses of Abramis brama and Carassius gibelio caught from the reservoir did not show the presence of the investigated microcystins in the fish tissue. However, several histopathological changes, predominantly in gills and kidneys, were observed in the fish, and the damage was more severe during May and especially July, which coincides with the increase in cyanobacterial biomass during the summer months. Cyanobacteria may thus have adverse effects in this ecosystem.

A novel strategy for conservation of European eel (Anguilla anguilla) genetic resources: Cryopreservation of ovarian stem cells.

The aim of this study was to develop short- and long-term preservation protocols for European eel ovarian stem cells (OSCs) through hypothermic storage and cryopreservation of ovarian fragments that will assist in current conservation programs of this critically endangered species. Firstly, a freezing procedure was developed by testing different cryomedia and technical aspects of freezing. Utilization of 1.5 M of dimethyl sulfoxide (Me2SO), 0.1 M glucose and 1.5% BSA yielded optimal OSCs survival. Additionally, equilibration of 50-mg ovarian fragments for 30 min and plunging into lN2 at -80 °C displayed the highest OSC viability. Different cooling rates ranging from -1 to -40 °C/min did not significantly affect OSC viability when thawing in a 10 °C water bath. In addition, application of needle-immersed vitrification (NIV), combining ES3 (1.5 M PG and 1.5 M Me2SO) with VS3 (3 M PG and 3 M Me2SO) yielded the highest viability rates. Finally, hypothermic storage (4 °C) of ovarian fragments and ovarian cell suspensions displayed favorable viability of ~90% after 48 h of storage and ~65% after 72 h of storage. The development of OSC preservation methods presents an onset of further development of germline stem cell (GSC) manipulation techniques in this species. Cryopreservation of OSCs can enable a continuous supply of cells for either transplantation or in vitro cell culture thus enabling new and improved management and conservation strategies for this endangered species.

Preservation of female genetic resources of common carp through oogonial stem cell manipulation.

Several experiments were conducted in order to develop an optimal protocol for slow-rate freezing (-1 °C/min) and short-term storage (-80 or 4 °C) of common carp ovarian tissue fragments with an emphasis on oogonial stem cells (OSCs). Dimethyl sulfoxide (Me2SO) with concentration of 1.5 M was identified as the best cryoprotectant in comparison to propylene glycol and methanol. When comparing supplementation of sugars (glucose, trehalose, sucrose) in different concentrations (0.1, 0.3, 0.5 M), glucose and trehalose in 0.3 M were identified as optimal. Short-term storage options for ovarian tissue pieces at -80 °C and 4 °C were tested as alternatives to cryopreservation and storage in liquid nitrogen. The presence of OSCs was confirmed by immunocytochemistry and viability after storage was determined by the trypan blue exclusion test. This study identified the optimal protocol for OSC cryopreservation using slow rate freezing resulting in ∼65% viability. The frozen/thawed OSCs were labelled by PKH-26 and transplanted into goldfish recipients. The success of the transplantation was confirmed by presence of fluorescent cells in the recipient gonad and later on by RT-PCR with carp dnd1 specific primers. The results of this study can facilitate long-term preservation of common carp germplasm which can be recovered in a surrogate recipient through interspecific germ cell transplantation.

Preservation of common carp germ cells under hypothermic conditions: Whole tissue vs isolated cells.

The aim of this study was to optimize the conditions for hypothermic storage of spermatogonial stem cells (SSCs) and oogonial stem cells (OSCs) of common carp Cyprinus carpio. This was conducted by storing gonadal tissue or isolated cells for 24 hr under hypothermic conditions in the first experiment and by testing two different storage media (L-15 or DMEM supplemented with 10% FBS and 25 mM HEPES) and regular medium change (every 4 days) during two weeks of hypothermic storage in the second experiment. During the first 24 hr, isolated cells showed no decrease in viability, while cells obtained from hypothermically stored tissues displayed significantly lower viability after only 6 hr (Tukey's HSD, p < 0.01) indicating that hypothermic storage of isolated cells is superior to storing tissue pieces. The 2-week trial demonstrated that storage media have a profound influence, while regular medium exchange does not have a positive effect on cell viability. Viability of SSCs and OSCs after two weeks was approximately 40% and 25%, respectively; however, survival of ~70% was obtained after 10 days of storage for SSCs and 7 days for OSCs. Hypothermic storage developed in this study has many practical applications during the development of surrogate broodstock technologies for common carp, but also in carp hatcheries and for the conservation of genetic resources of closely related cyprinid species.

Interspecific germ cell transplantation: a new light in the conservation of valuable Balkan trout genetic resources?

Interspecific transplantation of germ cells from the brown trout Salmo trutta m. fario and the European grayling Thymallus thymallus into rainbow trout Oncorhynchus mykiss recipients was carried out in order to improve current practices in conservation of genetic resources of endangered salmonid species in the Balkan Peninsula. Current conservation methods mainly include in situ efforts such as the maintenance of purebred individuals in isolated streams and restocking with purebred fingerlings; however, additional ex situ strategies such as surrogate production are needed. Steps required for transplantation such as isolation of high number of viable germ cells and fluorescent labeling of germ cells which are to be transplanted have been optimized. Isolated and labeled brown trout and grayling germ cells were intraperitoneally transplanted into 3 to 5 days post hatch rainbow trout larvae. Survival of the injected larvae was comparable to the controls. Sixty days after transplantation, fluorescently labeled donor cells were detected within the recipient gonads indicating successful incorporation of germ cells (brown trout spermatogonia and oogonia-27%; grayling spermatogonia-28%; grayling oogonia-23%). PCR amplification of donor mtDNA CR fragments within the recipient gonads additionally corroborated the success of incorporation. Overall, the transplantation method demonstrated in this study presents the first step and a possible onset of the application of the germ cell transplantation technology in conservation and revitalization of genetic resources of endangered and endemic species or populations of salmonid fish and thus give rise to new or improved management strategies for such species.

Optimisation of sodium and potassium concentrations and pH in the artificial seminal plasma of common carp Cyprinus carpio L.

The effect of sodium and potassium concentrations as well as optimal pH on the motility of common carp Cyprinus carpio L. sperm during short-term storage in artificial seminal plasma (ASP) was investigated. Sperm was collected from individual males (n = 5) and each sample diluted tenfold (1:9) in ASP (sperm:extender) containing 2 mM CaCl2, 1 mM Mg2SO4 and 20 mM Tris at pH 8.0 and supplemented by the following concentrations of sodium and potassium (mM/mM): 0/150, 20/130, 40/110, 75/75, 110/40, 130/20 and 150/0. The osmolality of all ASP variants was set at 310 mOsm kg-1. Sperm motility was measured using a CASA system during 72 h of storage. Immediately after dilution, sperm motility was high (90%) both in each variant and in the control group (fresh sperm). After 72-h storage, the highest sperm motility was noted in ASP containing 110 mM NaCl and 40 mM KCl. No differences were found in the motility of samples preserved within the pH range of 7.0-9.0. Our data suggest that for the short-term storage of common carp sperm, whereas the pH of the solution does not play a crucial role, a specific potassium concentration of around 40 mM is required.

Development of sperm vitrification protocols for two endangered salmonid species: the Adriatic grayling, Thymallus thymallus, and the marble trout, Salmo marmoratus.

Vitrification was applied to the sperm of two endangered fish species of Soca River basin in Slovenia, the Adriatic grayling (Thymallus thymallus) and marble trout (Salmo marmoratus) following testing different cooling devices and vitrifying media. Sperm was collected, diluted in species-specific non-activating media containing cryoprotectants, and vitrified by plunging directly into liquid nitrogen without pre-cooling. Progressive motility, curvilinear velocity, and straightness of fresh and vitrified-warmed sperm were evaluated with computer-assisted sperm analysis (CASA). Fertilization trials were carried out to test the effectiveness of vitrification in the case of grayling. A protocol utilizing a glucose-based extender, 30% cryoprotectants (15% methanol + 15% propylene glycol), 1:1 dilution ratio, and droplets of 2 µl on a Cryotop as cooling device yielded the highest post-thaw motility values for both Adriatic grayling (7.5 ± 6.5%) and marble trout (26.6 ± 15.8%). Viable embryos were produced by fertilizing eggs with vitrified grayling sperm (hatching 13.1 ± 11.7%, control hatching 73.9 ± 10.4%). The vitrification protocol developed in this study can be utilized in the conservation efforts for the two species as an alternative to slow-rate freezing when working in field conditions or when specific equipment necessary for slow-rate freezing is not available.

Cryosurvival of isolated testicular cells and testicular tissue of tench Tinca tinca and goldfish Carassius auratus following slow-rate freezing.

Experiments were carried out to test the efficiency of cryopreservation of whole testicular tissue in tench Tinca tinca and goldfish Carassius auratus and compare it to cryopreservation of isolated testicular cells. Additionally, effects of three cryoprotectants (dimethyl sulphoxyde - Me2SO, methanol - MeOH and ethylene glycol - EG) at three concentrations (1M, 2M and 3M) on post-thaw cell viability were assessed. Tissue pieces/isolated testicular cells were diluted in cryomedia and cryopreserved by slow-rate freezing (1°C/min to -80°C followed by a plunge into the liquid nitrogen). In both species Me2SO and EG generally yielded higher cryosurvival of early-stage germ cells than MeOH, while spermatozoa of neither species displayed such a pattern. In most cases a 3M>2M>1M viability pattern emerged in both species for both sample types regardless of the cryoprotectant used. Sample type (dissociated testicular cells vs testicular tissue) did not seem to affect viability rates of tench early-stage germ cells and goldfish spermatozoa, while the opposite was observed for tench spermatozoa and goldfish early-stage germ cells. Additionally, through histological analysis we displayed that tissue structure mainly remained unaltered after thawing in goldfish. These results indicate that cryopreservation of whole testicular tissue is indeed a valid alternative method to cryopreservation of dissociated testicular cells. Early-stage germ cells obtained from cryopreserved testis can be further used in different purposes such as transplantation into suitable donors while viable sperm might be used for fertilization when feasible.

Development of sperm vitrification protocols for freshwater fish (Eurasian perch, Perca fluviatilis) and marine fish (European eel, Anguilla anguilla).

Vitrification was successfully applied to the sperm of two fish species, the freshwater Eurasian perch (Perca fluviatilis) and marine European eel (Anguilla anguilla). Sperm was collected, diluted in species-specific non-activating media and cryoprotectants and vitrified by plunging directly into liquid nitrogen without pre-cooling in its vapor. Progressive motility of fresh and vitrified-thawed sperm was evaluated with computer-assisted sperm analysis (CASA). Additional sperm quality parameters such as sperm head morphometry parameters (in case of European eel) and fertilizing capacity (in case of Eurasian perch) were carried out to test the effectiveness of vitrification. The vitrification method for Eurasian perch sperm resulting the highest post-thaw motility (14±1.6%) was as follows: 1:5 dilution ratio, Tanaka extender, 30% cryoprotectant (15% methanol+15% propylene-glycol), cooling device: Cryotop, 2µl droplets, and for European eel sperm: dilution ratio 1:1, with 40% cryoprotectant (20% MeOH and 20% PG), and 10% FBS, cooling device: Cryotop, with 2µl of sperm suspension. Viable embryos were produced by fertilization with vitrified Eurasian perch sperm (neurulation: 2.54±1.67%). According to the ASMA analysis, no significant decrease in head area and perimeter of vitrified European eel spermatozoa were found when compared to fresh spermatozoa.

First successful vitrification of salmonid ovarian tissue.

Due to a lack of cryopreservation protocols for fish eggs and embryos, alternative techniques which will enable storage of female genetic resources are crucial for future development of reproduction management in conservation biology and aquaculture. Experiments were conducted to develop an optimal vitrification protocol for cryopreservation of brown trout Salmo trutta juvenile ovarian tissue. Needle immersed vitrification (NIV) method was used where ovaries were pinned on an acupuncture needle, passaged through equilibration and vitrification solutions containing different combinations and concentrations of methanol (MeOH), propylene glycol (PG) and dimethyl sulfoxide (Me2SO) and subsequently plunged into liquid nitrogen. Vitrification solutions containing equal cryoprotectant concentrations (3M Me2SO and 3M PG) yielded the highest oogonia survival rates (up to 40%) and qualitatively and quantitatively unaltered perinucleolar follicles. The method developed for brown trout could be applied to the conservation of female genetic resources of other salmonid species, including endangered and endemic species or populations.

Toxicopathology induced by microcystins and nodularin: a histopathological review.

Cyanobacteria are present in all aquatic ecosystems throughout the world. They are able to produce toxic secondary metabolites, and microcystins are those most frequently found. Research has displayed a negative influence of microcystins and closely related nodularin on fish, and various histopathological alterations have been observed in many organs of the exposed fish. The aim of this article is to summarize the present knowledge of the impact of microcystins and nodularin on the histology of fish. The observed negative effects of cyanotoxins indicate that cyanobacteria and their toxins are a relevant medical (due to irritation, acute poisoning, tumor promotion, and carcinogenesis), ecotoxicological, and economic problem that may affect both fish and fish consumers including humans.

Vitrification of the ovarian tissue in sturgeons.

The aim of this study was to test whether vitrification of sterlet Acipenser ruthenus and Russian sturgeon Acipenser gueldenstaedtii ovarian tissue through needle-immersed vitrification (NIV) is an efficient strategy for the preservation of oogonia (OOG) in order to supplement the current conservation efforts for these endangered fish species. Histological analyses of the gonads displayed that the ovaries of both species were immature and contained predominantly OOG and primary oocytes. The germline origin of these cells was verified by localization of the vasa protein through immunocytochemistry. NIV protocol was optimized by testing different equilibration (ES) and vitrification solutions (VS) containing various concentrations of dimethyl sulfoxide (Me2SO), propylene glycol (PG) or methanol (MeOH). In sterlet, the highest average viability (55.7 ± 11.5%) was obtained by using a combination of 1.5 M PG and 1.5 M Me2SO in the ES, and 1.5 M MeOH and 5.5 M Me2SO in the VS. In Russian sturgeon, the highest average viability (49.4 ± 17.1%) was obtained by using a combination of 1.5 M MeOH and 1.5 M Me2SO in the ES, and 3 M PG and 3 M Me2SO in the VS. To test whether vitrified/warmed OOG are functional, we have conducted an intra-specific transplantation assay to verify whether transplanted sterlet OOG will colonize the gonads of recipient fish. Fluorescently labelled cells were detected within recipient gonads at 2 and 3 months post-fertilization (mpf). Colonization rates of vitrified/warmed OOG (70% at 2 mpf and 61% at 3 mpf) were similar to those of fresh OOG (80% at 2 mpf and 70% at 3 mpf). This study has demonstrated that vitrification of ovarian tissue is an effective method for the preservation of OOG, and that the vitrified/warmed cells are functional and are able to colonize recipient gonads after transplantation similarly to the fresh cells. Since the vitrification procedure displayed in this study is simple and does not require complex and expensive laboratory equipment, it can be readily applied in field conditions, and therefore it can be invaluable for the conservation efforts of the critically endangered sturgeon species. However, care needs to be taken that despite the research conducted so far, donor-derived progeny was not yet obtained in sturgeons.

Cryopreservation and Transplantation of Spermatogonial Stem Cells.

Cryopreservation as a method that enables long-term storage of biological material has long been used for the conservation of valuable zebrafish genetic resources. However, currently, only spermatozoa of zebrafish can be successfully cryopreserved, while protocols for cryopreservation of eggs and embryos have not yet been fully developed. Transplantation of germline stem cells (GSCs) has risen as a favorable method that can bypass the current problem in cryopreservation of female genetic resources and can lead to reconstitution of fish species and lines through surrogate production. Here, we describe essential steps needed for the cryopreservation of spermatogonial stem cells (SSCs) and their utilization in the conservation of zebrafish genetic resources through SSC transplantation and surrogate production.

Does the Kis-Balaton Water Protection System (KBWPS) Effectively Safeguard Lake Balaton from Toxic Cyanobacterial Blooms?

Lake Balaton is the largest shallow lake in Central Europe. Its water quality is affected by its biggest inflow, the Zala River. During late 20th century, a wetland area named the Kis-Balaton Water Protection System (KBWPS) was constructed in the hopes that it would act as a filter zone and thus ameliorate the water quality of Lake Balaton. The aim of the present study was to test whether the KBWPS effectively safeguards Lake Balaton against toxic cyanobacterial blooms. During April, May, July and September 2018, severe cyanobacterial blooming was observed in the KBWPS with numbers reaching up to 13 million cells/mL at the peak of the bloom (July 2018). MC- and STX-coding genes were detected in the cyanobacterial biomass. Five out of nine tested microcystin congeners were detected at the peak of the bloom with the concentrations of MC-LR reaching 1.29 µg/L; however, accumulation of MCs was not detected in fish tissues. Histopathological analyses displayed severe hepatopancreas, kidney and gill alterations in fish obtained throughout the investigated period. In Lake Balaton, on the other hand, cyanobacterial numbers were much lower; more than 400-fold fewer cells/mL were detected during June 2018 and cyanotoxins were not detected in the water. Hepatic, kidney and gill tissue displayed few alterations and resembled the structure of control fish. We can conclude that the KBWPS acts as a significant buffering zone, thus protecting the water quality of Lake Balaton. However, as MC- and STX-coding genes in the cyanobacterial biomass were detected at both sites, regular monitoring of this valuable ecosystem for the presence of cyanobacteria and cyanotoxins is of paramount importance.

Cryopreservation and transplantation of common carp spermatogonia.

Common carp (Cyprinus carpio) is one of the most cultured fish species over the world with many different breeds and plenty of published protocols for sperm cryopreservation. However, data regarding preservation of gonadal tissue and surrogate production is still missing. A protocol for freezing common carp spermatogonia was developed through varying different factors along a set of serial subsequent experiments. Among the six cryoprotectants tested, the best survival was achieved with dimethyl sulfoxide (Me2SO). In the next experiment, a wide range of cooling rates (0.5-10°C/min) and different concentrations of Me2SO were tested resulting in the highest survival achieved using 2 M Me2SO and cooling rate of -1°C/min. When testing different tissue sizes and incubation times in the cryomedia, the highest viability was observed when incubating 100 mg tissue fragments for 30 min. Finally, sugar supplementation did not yield significant differences. When testing different equilibration (ES) and vitrification solutions (VS) used for needle-immersed vitrification, no significant differences were observed between the tested groups. Additionally, varied exposure time to VS did not improve the vitrification outcome where the viability was 4-fold lower than that of freezing. The functionality of cryopreserved cells was tested by interspecific transplantation into sterilized goldfish recipients. The exogenous origin of the germ cells in gonads of goldfish recipient was confirmed by molecular markers and incorporation rate was over 40% at 3 months post-transplantation. Results of this study can serve for long-term preservation of germplasm in carp which can be recovered in a surrogate recipient.

Artificial insemination of African catfish (Clarias gariepinus) using cryopreserved sperm.

In this study, we aimed to develop a practical protocol for using cryopreserved sperm for induced/wild/tank spawning of fish species with external fertilization. Experiments were carried out on African catfish (Clarias gariepinus) as a model species. Sperm was collected for cryopreservation and diluted with the cryomedium (266 mM fructose, 20% methanol) at a ratio of 1:1 with a final methanol concentration of 2.47 M pH7.73. Diluted sperm was loaded into 0.5-ml straws and cryopreserved by conventional protocol. Samples were prepared for insemination 24 h later, by thawing for 13 s in a 40 °C water bath, and centrifuged at 500 × g for 10 min at 20 °C. The seminal plasma, extender and external cryoprotectant were removed from the concentrated spermatozoa. The pellet was then resuspended in common carp (Cyprinus carpio) seminal plasma to reconstitute the lost volume. Sperm samples were then injected by a catheter into the ovarian cavity through the oviduct of the experimental females by the so-called ovarian lavage method in parallel with the intramuscular hormonal administration (5 mg carp pituitary/kg bw). Inseminated females (n = 9) were monitored for 10 h and ovulated eggs and spermatozoa stored in in the ovary were stripped. Stripped gamete samples were divided into two batches: (1) the first batch contained only the previously injected spermatozoa and was activated by aerated water (WA) immediately after stripping; (2) in case of the second batch additional, freshly stripped sperm was added as positive control to the stripped eggs before water activation (PC). Furthermore, five females were propagated by using the dry fertilization method (in vitro fertilization) as negative control (NC). All sperm and hormone injected females produced fertilised eggs with a hatching rate of 17.7 ±â€¯13.2%, 12.5 ±â€¯9.3%, and 61 ±â€¯11.5% for WA, PC and NC respectively. These results indicate that artificial insemination based on using cryopreserved sperm with ovarian lavage can be a viable alternative to in vitro fertilization in a catfish species. Thus, we describe a proof of principle for a practical protocol for the induced/wild/tank spawning of an externally fertilising fish species with economical importance and propose that the protocol could be also applied to endangered marine or fresh fish species.

Preservation of zebrafish genetic resources through testis cryopreservation and spermatogonia transplantation.

Zebrafish is one of the most commonly used model organisms in biomedical, developmental and genetic research. The production of several thousands of transgenic lines is leading to difficulties in maintaining valuable genetic resources as cryopreservation protocols for eggs and embryos are not yet developed. In this study, we utilized testis cryopreservation (through both slow-rate freezing and vitrification) and spermatogonia transplantation as effective methods for long-term storage and line reconstitution in zebrafish. During freezing, utilization of 1.3 M of dimethyl sulfoxide (Me2SO) displayed the highest spermatogonia viability (~60%), while sugar and protein supplementation had no effects. Needle-immersed vitrification also yielded high spermatogonia viability rates (~50%). Both optimal slow-rate freezing and vitrification protocols proved to be reproducible in six tested zebrafish lines after displaying viability rates of >50% in all lines. Both fresh and cryopreserved spermatogonia retained their ability to colonize the recipient gonads after intraperitoneal transplantation of vasa::egfp and actb:yfp spermatogonia into wild-type AB recipient larvae. Colonization rate was significantly higher in dnd-morpholino sterilized recipients than in non-sterilized recipients. Lastly, wild-type recipients produced donor-derived sperm and donor-derived offspring through natural spawning. The method demonstrated in this study can be used for long-term storage of valuable zebrafish genetic resources and for reconstitution of whole zebrafish lines which will greatly improve the current preservation practices.