Generation of live fry from intraperitoneally transplanted primordial germ cells in rainbow trout

Gametes of semelparous salmon are repeatedly produced by surrogate rainbow trout

Most Pacific salmon species grow in the ocean, return to their native rivers to reproduce, and then die (semelparous type). However, rainbow trout survive after spawning and reproduce repeatedly until the end of their lives (iteroparous type). Little is known about how germline stem cells behave during gametogenesis in the two types of Pacific salmon. In this study, we show that all germline stem cells disappear after the first gametogenesis in Chinook and Kokanee salmon, whereas germline stem cells are maintained in rainbow trout. However, the germline stem cells of Chinook and Kokanee salmon transplanted into rainbow trout survive even after their spawning seasons and supply salmon gametes for multiple years. These results indicate that the behavior of the germline stem cells is mainly regulated by the somatic environment.

Immune Rejection Mediated by prf1 and gzmb Affects the Colonization of Fat Greenling (Hexagrammos otakii) Spermatogonia in Heterotransplantation

Fish germ cell transplantation holds great potential for conserving endangered species, improving cultured fish breeds, and exploring reproductive techniques. However, low transplantation efficiency is a common issue in heterotransplantation. This study transplanted fat greenling (Hexagrammos otakii) spermatogonia into the testes of spotted sea bass (Lateolabrax maculatus) to investigate factors that might affect the colonization and fixation of heterologous transplanted germ cells. Results indicated that transplanted fat greenling spermatogonia cells were successfully detected in the early transplantation phase in spotted sea bass. Their numbers gradually decreased over time, and after 10 days post-transplantation, more than 90% of the transplanted cells underwent apoptosis. Transcriptome sequencing analysis of the testes of spotted sea bass and fat greenling spermatogonia on days 1 and 10 post-transplantation revealed that this apoptosis process involved many immune-related genes and their associated signaling pathways. Acute immune rejection marker genes prf1 and gzmb were detected in the spotted sea bass testes, while immune tolerance genes lck and zap-70 were expressed in the fat greenling spermatogonia. Additionally, differential expression of prf1 and gzmb genes was screened from spotted sea bass, with experimental evidence indicating that PRF1 and GZMB protein from spotted sea bass primarily induce apoptosis in transplanted fat greenling spermatogonia via the mitochondrial apoptosis pathway, at the protein level. This suggests that the difficulties in heterotransplantation are primarily related to acute immune rejection, with PRF1 and GZMB playing significant roles.

Time-efficient germ cell transplantation from goldfish (Carassius auratus) into adult common carp (Cyprinus carpio)

Germ cell transplantation in fish is a promising technique for surrogate broodstock parents with broader application in aquaculture and conserving endangered and valuable genetic resources. Herein, we describe the establishment of an intrapapillary xenogeneic transplant of germ cells from sexually mature goldfish (C. auratus) males into common carp (C. carpio) males cytoablated with a thermochemical treatment (two doses of busulfan at 40 mg/kg at 35°C). To analyze the presence and development of donor germ cells in recipient testes, donor germ cells were labeled with PKH26, a fluorescent cell membrane dye, before transplantation. Our results demonstrated that thermochemical treatment caused effective spermatogenesis suppression and pronounced germ cell loss. Moreover, transplanted spermatogonial cells were able to colonize the recipients' testes, resume spermatogenesis, and generate spermatozoa within eight weeks after germ cell transplantation. These findings suggested that recipient testes provided suitable conditions for the survival, colonization, proliferation, and differentiation of donor spermatogonia from a related species. This study indicated that recipients' testes exhibited a high degree of plasticity to accept and support xenogeneic donor germ cells, which were able to form sperm in a short time frame. This approach has significant implications for assisted animal reproduction, biotechnology, conservation, and the production of valuable genetic resources and endangered fish species.

Glial cell line-derived neurotrophic factor (GDNF) is essential for colonization and expansion of turbot (Scophthalmus maximus) germ cells in recipients and in vitro culture

Germ cell transplantation (GCT) is a promising biotechnology that enables the production of donor-derived gametes in surrogate recipients. It plays a crucial role in the protection of endangered species, the propagation of elite species with desired traits, and long-term preservation of genetic resources. This significance is particularly pronounced when GCT is synergistically employed with cryopreservation techniques. However, GCT often encounters challenges due to low colonization rates and, in some cases, complete loss of donor cells in recipients. Glial cell line-derived neurotrophic factor (GDNF) plays a pivotal role in sustaining the self-renewal of spermatogonial stem cells (SSCs) in mammals. Additionally, it has been shown to promote the proliferation of spermatogonia in vitro cultures in certain animal species. In turbot (Scophthalmus maximus), we found that the expressions of gdnf and gfrα1a were predominantly observed in spermatogonia rather than somatic cells, which differed from their expression patterns in mammals. The efficiency of exogenous spermatogonia transplantation in Japanese flounder (Paralichthys olivaceus) larvae could be substantially enhanced by incubating donor cells from turbot with 100 ng/ml GDNF prior to transplantation. This led to a noteworthy increase in the colonization rate, rising from 33%-50%-61.5%. Additionally, the addition of 20 ng/ml GDNF in cell medium could also promote the proliferation of turbot germ cells in vitro. These results demonstrated the gdnf in turbot testis expression characteristics and suggested that addition of GNDF could be an effective way to improve the GCT efficiency and promote the germ cells expansion during in vitro culture.

Traceability of primordial germ cells in three neotropical fish species aiming genetic conservation actions

Primordial germ cells (PGCs) are embryonic pluripotent cells that can differentiate into spermatogonia and oogonia, and therefore, PGCs are a genetic source for germplasm conservation through cryobanking and the generation of germline chimeras. The knowledge of PGC migration routes is essential for transplantation studies. In this work, the mRNA synthesized from the ddx4 3'UTR sequence of Pseudopimelodus mangurus, in fusion with gfp or dsred, was microinjected into zygotes of three neotropical species (P. mangurus, Astyanax altiparanae, and Prochilodus lineatus) for PGC labeling. Visualization of labeled PGCs was achieved by fluorescence microscopy during embryonic development. In addition, ddx4 and dnd1 expressions were evaluated during embryonic development, larvae, and adult tissues of P. mangurus, to validate their use as a PGC marker. As a result, the effective identification of presumptive PGCs was obtained. DsRed-positive PGC of P. mangurus was observed in the hatching stage, GFP-positive PGC of A. altiparanae in the gastrula stage, and GFP-positive PGCs from P. lineatus were identified at the segmentation stage, with representative labeling percentages of 29% and 16% in A. altiparanae and P. lineatus, respectively. The expression of ddx4 and dnd1 of P. mangurus confirmed the specificity of these genes in germ cells. These results point to the functionality of the P. mangurus ddx4 3'UTR sequence as a PGC marker, demonstrating that PGC labeling was more efficient in A. altiparanae and P. lineatus. The procedures used to identify PGCs in P. mangurus consolidate the first step for generating germinal chimeras as a conservation action of P. mangurus.

Germ cell recovery, cryopreservation and transplantation in the California white sturgeon, Acipenser transmontanus

The white sturgeon (Acipenser transmontanus) is the largest freshwater fish in North America. Because of the unique life history characteristics of sturgeon, including longevity, late maturation and long spawning intervals, their aquaculture can be a significant investment of resources. As a result of habitat loss and overharvesting, natural populations of white sturgeon are threatened and there is a growing effort to improve conservation aquaculture programs. Germ cell transplantation is an innovative technology previously demonstrated in a variety of fish species to be able to produce a surrogate broodstock. The technique relies upon optimal donor germ cell recovery and transplantation into a recipient fish. In this study, we developed and optimized the harvest of donor cells for germline transplantation and evaluated methods for ovary cryopreservation for the first time in the white sturgeon. We found that harvesting gonads from juveniles between the ages of 1.5 and 2.5-years resulted in reliably high proportions of pre-meiotic cells regardless of sex, a critical feature for using white sturgeon for transplantation studies since the species shows no distinguishing external sex characteristics. From the viable cells, we identified germline cells using immunolabeling with the antibody DDX4, a marker specific to the germline. For in vivo tracking of donor cells during transplantations, gonadal cells were stained with a long half-life non-toxic cell membrane dye, PKH26, and microinjected into the peritoneal cavity of newly hatched white sturgeon larvae. Larvae were reared until 3 months post-transplantation to monitor for colonization and proliferation of PKH26-labeled cells within the recipient larval gonads. Furthermore, viable cell detection, assessment of germline-specificity, and transplantation was determined for cells recovered from cryopreserved ovarian tissue from sexually immature females. Transplantations using cells cryopreserved with media supplemented with dimethyl sulfoxide (DMSO) rather than ethylene glycol (EG) demonstrated the highest number of PKH26-labeled cells distributed along the gonadal ridges of the larval recipient. Determining optimal methods of tissue cryopreservation, and germ cell recovery and transplantation are foundational to the future development of germ cell transplantation as a strategy to improve the aquaculture and conservation of this species. Our study demonstrates that conservation actions, such as surrogate breeding, could be utilized by hatcheries to retain or improve natural gamete production without genetic modification, and provide an encouraging approach to the management of threatened sturgeon species.

Generation of surrogate goldfish Carassius auratus progeny from common carp Cyprinus carpio parents

Surrogate broodstock technology can increase the production efficiency of commercially important fishes that are difficult to breed in confinement and aid the propagation and recovery of endangered populations. In this study, we report the application of germ cell (GC) transplantation (GCT) for increasing the numbers of progeny produced by small-bodied ornamental fishes by using sexually mature adult fish as recipients. The GCs isolated from prepubertal male goldfish (Carassius auratus) donors (n = 5) were transplanted through the genital papilla into the gonads of adult common carp (Cyprinus carpio) recipients. The endogenous GCs of the recipient were depleted using busulfan (40 mg/kg body weight [BW]; in five doses at 2-week intervals) and high-temperature (38 °C) treatments. Within 4 months after GCT, the donor GCs recolonised the recipients' gonads and resumed gametogenesis. The presence of donor-derived gametes was confirmed through polymerase chain reaction-restriction fragment length polymorphism analysis in all the surrogate common carp males and females. Artificial fertilisation and induced spawning between surrogate males and females yielded pure goldfish progeny; the fertilisation and hatching rates were similar to those of the controls. These results suggest that GCT could also be potentially applied in commercial aquaculture, mainly to increase the numbers of progeny obtained from small-bodied fishes those having low gamete counts.

Establishing a model fish for the Neotropical region: The case of the yellowtail tetra Astyanax altiparanae in advanced biotechnology

The use of model organisms is important for basic and applied sciences. Several laboratory species of fishes are used to develop advanced technologies, such as the zebrafish (Danio rerio), the medaka (Oryzias latipes), and loach species (Misgurnus spp.). However, the application of these exotic species in the Neotropical region is limited due to differences in environmental conditions and phylogenetic distances. This situation emphasizes the establishment of a model organism specifically for the Neotropical region with the development of techniques that may be applicable to other Neotropical fish species. In this work, the previous research efforts are described in order to establish the yellowtail tetra Astyanax altiparanae as a model laboratory species for both laboratory and aquaculture purposes. Over the last decade, starting with artificial fertilization, the yellowtail tetra has become a laboratory organism for advanced biotechnology, such as germ cell transplantation, chromosome set manipulation, and other technologies, with applications in aquaculture and conservation of genetic resources. Nowadays, the yellowtail tetra is considered the most advanced fish with respect to fish biotechnology within the Neotropical region. The techniques developed for this species are being used in other related species, especially within the characins class.

Embryo manipulation in neotropical characiform fish: incubation system, anaesthetic, and PGC transplantation in Prochilodus lineatus

Primordial germ cells transplantation is a unique approach for conservation and reconstitution of endangered fish species. This study aimed to establish techniques to culture dechorionated embryos in different incubation systems and also to determine anaesthetic concentration for fish recipients in the larval stage for subsequent primordial germ cell transplantation. Intact and dechorionated embryos were divided into three incubation systems: (1) a control group with manual replacement of the solution; (2) a closed environment with high oxygen with manual replacement of the solution; and (3) constant solution recirculation. This combination resulted in six treatments. For the evaluation of anaesthetics for larvae, the concentrations evaluated were 19.5 mM, 24.4 mM, 29.3 mM, and 34.2 mM of 2-phenoxyethanol. Anaesthesia concentration and recovery at different stages were evaluated. For transplantation, primordial germ cells of Astyanax altiparanae were transplanted into anaesthetised larvae (1 dph) of Prochilodus lineatus. Better results were obtained in the recirculation system for dechorionated embryos of P. lineatus for hatching (54.18%) and normal morphology (50.06%). The 2-phenoxyethanol anaesthetic with a dose of 29.3 mM resulted in shorter induction times, in addition to the recovery time between 5 and 10 min. By using this anaesthetic concentration at transplantation, GFP-positive cells were seen in two recipients, but the cells did not proliferate. This study established an effective incubation system for the development of the dechorionated embryo and determined an effective anaesthetic concentration for P. lineatus larvae. In addition, micromanipulation and transplantation of primordial germ cells in neotropical species were conducted for the first time.

Allogeneic testes transplanted into partially castrated adult medaka (Oryzias latipes) can produce donor-derived offspring by natural mating over a prolonged period

Generally, successful testis transplantation has been considered to require immune suppression in the recipient to avoid rejection of the transplanted tissue. In the present study, we demonstrate in medaka that allogeneic adult testicular tissue will engraft in adult recipients immediately after partial castration without the use of immunosuppressive drugs. The allografted testes are retained in the recipient's body for at least 3 months and are able to produce viable sperm that yield offspring after natural mating. Some recipients showed a high frequency (over 60%) of offspring derived from spermatozoa produced by the transplanted testicular tissue. Histological analyses showed that allografted testicular tissues included both germ cells and somatic cells that had become established within an immunocompetent recipient testis. The relative simplicity of this testis transplantation approach will benefit investigations of the basic processes of reproductive immunology and will improve the technique of gonadal tissue transplantation.

Advantages, Factors, Obstacles, Potential Solutions, and Recent Advances of Fish Germ Cell Transplantation for Aquaculture-A Practical Review

Simple Summary

This review aims to provide practical information and viewpoints regarding fish germ cell transplantation for enhancing its commercial applications. We reviewed and summarized the data from more than 70 important studies and described the advantages, obstacles, recent advances, and future perspectives of fish germ cell transplantation. We concluded and proposed the critical factors for achieving better success and various options for germ cell transplantation with their pros and cons. Additionally, we discussed why this technology has not actively been utilized for commercial purposes, what barriers need to be overcome, and what potential solutions can advance its applications in aquaculture.

Abstract

Germ cell transplantation technology enables surrogate offspring production in fish. This technology has been expected to mitigate reproductive barriers, such as long generation time, limited fecundity, and complex broodstock management, enhancing seed production and productivity in aquaculture. Many studies of germ cell transplantation in various fish species have been reported over a few decades. So far, surrogate offspring production has been achieved in many commercial species. In addition, the knowledge of fish germ cell biology and the related technologies that can enhance transplantation efficiency and productivity has been developed. Nevertheless, the commercial application of this technology still seems to lag behind, indicating that the established models are neither beneficial nor cost-effective enough to attract potential commercial users of this technology. Furthermore, there are existing bottlenecks in practical aspects such as impractical shortening of generation time, shortage of donor cells with limited resources, low efficiency, and unsuccessful surrogate offspring production in some fish species. These obstacles need to be overcome through further technology developments. Thus, we thoroughly reviewed the studies on fish germ cell transplantation reported to date, focusing on the practicality, and proposed potential solutions and future perspectives.

Role and relevance of fish cell lines in advanced in vitro research

Introduction

Cell line derived from fish has been established as a promising tool for studying many key issues of aquaculture covering fish growth, disease, reproduction, genetics, and biotechnology. In addition, fish cell lines are very useful in vitro models for toxicological, pathological, and immunological studies. The easier maintenance of fish cell lines in flexible temperature regimes and hypoxic conditions make them preferable in vitro tools over mammalian cell lines. Great excitement has been observed in establishing and characterizing new fish cell lines representing diverse fish species and tissue types. The well-characterized and authenticated cell lines are of utmost essential as these represent cellular functions very similar to in vivo state of an organism otherwise it would affect the reproducibility of scientific research.

Conclusion

The fish cell lines have exhibited encouraging results in several key aspects of in vitro research in aquaculture including virology, nutrition and metabolism, production of vaccines, and transgenic fish production. The review paper reports the cell lines developed from fish, their characterization, and biobanking along with their potential applications and challenges in in vitro research.

Successful Spermatogonial Stem Cells Transplantation within Pleuronectiformes: First Breakthrough at inter-family Level in Marine Fish

As a promising biotechnology, fish germ cell transplantation shows potentials in conservation germplasm resource, propagation of elite species, and generation of transgenic individuals. In this study, we successfully transplanted the Japanese flounder (P. olivaceus), summer flounder (P. dentatus), and turbot (S. maximus) spermatogonia into triploid Japanese flounder larvae, and achieved high transplantation efficiency of 100%, 75-95% and 33-50% by fluorescence tracking and molecular analysis, respectively. Eventually, donor-derived spermatozoa produced offspring by artificial insemination. We only found male and intersex chimeras in inter-family transplantations, while male and female chimeras in both intra-species and intra-genus transplantations. Moreover, the intersex chimeras could mature and produce turbot functional spermatozoa. We firstly realized inter-family transplantation in marine fish species. These results demonstrated successful spermatogonial stem cells transplantation within Pleuronectiformes, suggesting the germ cells migration, incorporation and maturation within order were conserved across a wide range of teleost species.

Surrogate production of genome-edited sperm from a different subfamily by spermatogonial stem cell transplantation

The surrogate reproduction technique, such as inter-specific spermatogonial stem cells (SSCs) transplantation (SSCT), provides a powerful tool for production of gametes derived from endangered species or those with desirable traits. However, generation of genome-edited gametes from a different species or production of gametes from a phylogenetically distant species such as from a different subfamily, by SSCT, has not succeeded. Here, using two small cyprinid fishes from different subfamilies, Chinese rare minnow (gobiocypris rarus, for brief: Gr) and zebrafish (danio rerio), we successfully obtained Gr-derived genome-edited sperm in zebrafish by an optimized SSCT procedure. The transplanted Gr SSCs supported the host gonadal development and underwent normal spermatogenesis, resulting in a reconstructed fertile testis containing Gr spermatids and zebrafish testicular somatic cells. Interestingly, the surrogate spermatozoa resembled those of host zebrafish but not donor Gr in morphology and swimming behavior. When pou5f3 and chd knockout Gr SSCs were transplanted, Gr-derived genome-edited sperm was successfully produced in zebrafish. This is the first report demonstrating surrogate production of gametes from a different subfamily by SSCT, and surrogate production of genome-edited gametes from another species as well. This method is feasible to be applied to future breeding of commercial fish and livestock.

Preliminary study on testicular germ cell isolation and transplantation in an endangered endemic species Brycon orbignyanus (Characiformes: Characidae)

We aimed to develop a simplified protocol for transplantation of Brycon orbignyanus spermatogonial stem cells (SSCs) into Astyanax altiparanae testes. Brycon orbignyanus testes were enzymatically digested and SSC purified by a discontinuous density gradient. Endogenous spermatogenesis was suppressed in A. altiparanae using busulfan or by incubation at 35 °C water, and SSCs from B. orbignyanus labeled with PKH26 were injected into their testes via the urogenital papilla. Twenty-two hours post-transplantation, labeled spermatogonia were observed in A. altiparanae tubular lumen. After 7 days, spermatogonia proliferated in the epithelium, and 21 days post-transplantation, sperm was observed in the lumen. Of surviving host fish, nearly 67% of those treated with busulfan and 85% of those held in warm water showed labeled cells in host germinal epithelium. The present study standardized, by a simple and accessible method, germ cell transplantation between sexually mature Characiformes fish species. This is the first report of xenogenic SSC transplantation in this fish order.

New directions in assisted breeding techniques for fish conservation

Fish populations continue to decline globally, signalling the need for new initiatives to conserve endangered species. Over the past two decades, with advances in our understanding of fish germ line biology, new exsitu management strategies for fish genetics and reproduction have focused on the use of germ line cells. The development of germ cell transplantation techniques for the purposes of propagating fish species, most commonly farmed species such as salmonids, has been gaining interest among conservation scientists as a means of regenerating endangered species. Previously, exsitu conservation methods in fish have been restricted to the cryopreservation of gametes or maintaining captive breeding colonies, both of which face significant challenges that have restricted their widespread implementation. However, advances in germ cell transplantation techniques have made its application in endangered species tangible. Using this approach, it is possible to preserve the genetics of fish species at any stage in their reproductive cycle regardless of sexual maturity or the limitations of brief annual spawning periods. Combining cryopreservation and germ cell transplantation will greatly expand our ability to preserve functional genetic samples from threatened species, to secure fish biodiversity and to produce new individuals to enhance or restore native populations.

Germ cell-less hybrid fish: ideal recipient for spermatogonial transplantation for the rapid production of donor-derived sperm†

An interspecific hybrid marine fish that developed a testis-like gonad without any germ cells, i.e., a germ cell-less gonad, was produced by hybridizing a female blue drum Nibea mitsukurii with a male white croaker Pennahia argentata. In this study, we evaluated the suitability of the germ cell-less fish as a recipient by transplanting donor testicular cells directly into the gonads through the urogenital papilla. The donor testicular cells were collected from hemizygous transgenic, green fluorescent protein (gfp) (+/-) blue drum, and transplanted into the germ cell-less gonads of the 6-month-old adult hybrid croakers. Fluorescent and histological observations showed the colonization, proliferation, and differentiation of transplanted spermatogonial cells in the gonads of hybrid croakers. The earliest production of spermatozoa in a hybrid recipient was observed at 7 weeks post-transplantation (pt), and 10% of the transplanted recipients produced donor-derived gfp-positive spermatozoa by 25 weeks pt. Sperm from the hybrid recipients were used to fertilize eggs from wild-type blue drums, and approximately 50% of the resulting offspring were gfp-positive, suggesting that all offspring originated from donor-derived sperm that were produced in the transplanted gfp (+/-) germ cells. To the best of our knowledge, this is the first report of successful spermatogonial transplantation using a germ cell-less adult fish as a recipient. This transplantation system has considerable advantages, such as the use of comparatively simple equipment and procedures, and rapid generation of donor-derived spermatogenesis and offspring, and presents numerous applications in commercial aquaculture.

Resumption of donor-origin spermatogenesis in senescent goldfish Carassius auratus (Linnaeus, 1758) following spermatogonial cell therapy

Stem cell research has come into prominence because of its applications in assisted reproductive technology and the treatment of deadly diseases. In teleost fishes, spermatogonial stem cells have been effectively used to produce surrogate gametes and progeny through germ cell transplantation technique. The present study is the first report of an innovative application of stem cell therapy in fish species for revitalising the reproductive competence of senescent individuals. Senescent male goldfish, Carassius auratus, approximately 10 years of age were procured from a fish-breeding farm and were reared locally in the lab for an additional two years. The senescence of the individuals was then evaluated and confirmed using histological analysis, gonadal index assessment, and germ-cell specific vasa gene expression. Analyses revealed absence of spermatogonial cells and other germ cells in the testes of the senescent fish (n = 5). Spermatogonial cells from sexually immature C. auratus male donor were isolated using discontinuous percoll gradients, labelled with the fluorescent dye PKH-26, and transplanted into the gonads of senescent C. auratus males through urogenital papilla. Six months after the transplant, spermatozoa were collected through applying gentle manual pressure on the abdomen and were observed under a microscope. All C. auratus males with the transplant had produced spermatozoa from the transplanted cells. This was confirmed by the retention of PKH-26 in the spermatozoa and diagnostic SSR locus. Gravid C. auratus females were artificially inseminated with the spermatozoa of those senescent males and natural spawning was allowed. As a result viable progeny were produced. These observation suggests that the reproductive competence of senescent male fishes can be revitalised through spermatogonial stem cell therapy to produce functional gametes.

In vivo phagocytosis and hematology in Astyanax altiparanae, a potential model for surrogate technology

Abstract Although the potential of surrogate propagation technology for aquaculture and conservation of Neotropical fish, the poor understanding of the host immune system may results in rejection and destruction of the donor material. Thus, it is necessary to study and to develop methods to evaluate the effects of immunosuppressive drugs employment and to evaluate the immunocompatibility between donor and receptor. Thus, the present study aimed to optimize a methodology to assess in vivo phagocytosis in Astyanax altiparanae using Saccharomyces cerevisiae and to evaluate their hematological response resultant from the inflammatory induction. To this, S. cerevisiae were labeled with Congo red and injected in the coelomic cavity of A. altiparanae at the concentration of 2.5 x 106 cells mL-1. A PBS solution and a non-injected group were kept as control. Fish blood was sampled and the phagocytic capacity and index were determined at 1, 2, 3 and 6 h post-injection (hpi). The yeast injection successfully stimulated phagocytosis, with the best result for phagocytosis assessment after 2 hpi. Moreover, it was achieved a high traceability of phagocytized and non-phagocytized yeast under optic microscopy analysis due to the Congo red labeling. The hematological profile was similar to usually observed in early infections, indicating lymphocyte migration to inflammatory site and increase in number of circulating phagocytes due to natural response to inflammatory stimulus. In conclusion, our method was efficient to assess in vivo phagocytosis in A. altiparanae and will be an important tool to evaluate the efficacy of immunosuppressive drugs in this species. Additionally, these results may serve as support for further studies in fish immunocompetence, both in laboratory and in field conditions.

Depletion of endogenous germ cells in striped catfish Pangasianodon hypophthalmus (Sauvage, 1878) by heat-chemical treatments

Germ cell (GC) transplantation (GCT) is a proven powerful reproductive technique to enhance the production efficiency of domesticated animals and aid to the recovery of endangered germ lines. In mammals, several methods have been adopted for the eradication of GCs such as treatment with cytotoxic drugs, irradiation, cold ischaemia and hyperthermic treatment. Some of these methods have also been tried in fishes, and conditions for sterilization of gonads have been established. Here, we report the production of GC-depleted male striped catfish Pangasianodon hypophthalmus in 12 weeks by the combination of heat and chemical treatments. The cytotoxic drug busulphan (40 mg/kg) was intraperitoneally injected into the animals at 2-week intervals (six doses in total) and maintained in water at 38°C between weeks 1 and 12. The effectiveness of the treatment was assessed using gonadal index and histology. At the end of 12 weeks, very severe gonadal degeneration was observed in fish treated with the heat-chemical combination, and 100% of sampled fish (n = 5) were found devoid of endogenous GCs. On contrary, high temperature alone caused minor gonadal degeneration. Results obtained in this study suggest that endogenous GCs of large-bodied fish such as P. hypophthalmus can also be sterilized by heat and chemical treatments within a considerably short period.

The Role of Reproductive Sciences in the Preservation and Breeding of Commercial and Threatened Teleost Fishes

The teleost fishes are the largest and most diverse vertebrate group, accounting for nearly half of all known vertebrate species. Teleost fish exhibit greater species diversity than any other group of vertebrates and this is reflected in the unique variety of different reproductive strategies displayed by fish. Fish have always been an important resource for humans worldwide, especially as food. While wild capture fisheries have historically been the main source of fish, the farming of fish (aquaculture) is increasingly becoming the more dominant source of food fish, and is predicted to account for 60% of total global fish production by 2030.Fishes are increasingly threatened by a wide range of anthropogenic impacts, including loss of habitat, pollution, invasive species and over-exploitation. In addition, climate change, especially the consequences of global warming, can impact fish at all levels of biological organization from the individual to the population level, influencing both physiological and ecological processes in a variety of direct and indirect ways. As such, there is an urgent need to protect and conserve the huge genetic diversity offered by this diverse vertebrate group, not just as a source of genes for contemporary breeding and for protection against the consequences of climate change and disease, but also as part of our national heritage. While the cryopreservation of reproductive cells is a means of achieving these objectives, currently only fish sperm can be successfully frozen. Due to their large size, large yolk compartment, low membrane permeability and high chilling sensitivity, successful and reproducible protocols for the cryopreservation of fish oocytes and embryos still remains elusive. However, significant advances have been made in the cryopreservation of primordial germ cells as an alternative means of conserving both paternal and maternal genomes. Although more research needs to be carried out on how these cells can be optimally applied to emerging reproductive technologies, including transplantation techniques and surrogate broodstock technologies, the successful cryopreservation of fish germ cells, and the establishment of genetic resource banks, offers the possibility of both conserving and restoring threatened species. Further, current and future conservation efforts need to consider the impact of climate change in both in situ conservation and reintroduction efforts.In conclusion, it is anticipated that the successful cryopreservation of fish germplasm will result in a range of economic, ecological and societal benefits. In partnership with emerging assisted reproductive technologies, the successful cryopreservation of fish germplasm will lead to more efficient reproduction in aquaculture, assist selective breeding programmes, and be of crucial importance to future species conservation actions.

Biotechnology applied to fish reproduction: tools for conservation

This review discusses the new biotechnological tools that are arising and promising for conservation and enhancement of fish production, mainly regarding the endangered and the most economically important species. Two main techniques, in particular, are available to avoid extinction of endangered fish species and to improve the production of commercial species. Germ cell transplantation technology includes a number of approaches that have been studied, such as the transplantation of embryo-to-embryo blastomere, embryo-to-embryo differentiated PGC, larvae to larvae and embryo differentiated PGC, transplantation of spermatogonia from adult to larvae or between adults, and oogonia transplantation. However, the success of germ cell transplantation relies on the prior sterilization of fish, which can be performed at different stages of fish species development by means of several protocols that have been tested in order to achieve the best approach to produce a sterile fish. Among them, fish hybridization and triploidization, germline gene knockdown, hyperthermia, and chemical treatment deserve attention based on important results achieved thus far. This review currently used technologies and knowledge about surrogate technology and fish sterilization, discussing the stronger and the weaker points of each approach.

Characterization and localization of primordial germ cells in Totoaba macdonaldi

The totoaba, Totoaba macdonaldi, is an endangered fish of the Gulf of California with high economic and ecological potential. Therefore, our purpose was to characterize the Primordial Germ Cells (PGCs) of this Sciaenid with two objectives: (1) to provide the basis for PGCs cryopreservation to preserve the genetic resources and (2) to take the first step to know the gonadal genesis and sex differentiation of totoaba. Immunofluorescence analysis performed from 2-cell stage to 8-day after hatch (DAH) shows that Vasa protein is specific for PGCs. These cells were first observed in the peripheral and dorsal regions of the blastodisc at approximately the 50%-epiboly stage and migrated to both sides of embryo body during the development. Finally, at 7 DAH the PGCs of the hatching embryo reached the place where the gonad will be developed. Histology analysis of larvae showed a genital ridge with enclosed PGCs on the dorsal side of the peritoneum at 9 DAH, gonadal primordium growth was observed at 11 DAH as a result of the interaction between PGCs and somatic cells derived from the peritoneum. Results of qPCR showed that vasa expression was restricted to the embryonic and early larval development, highest values were observed in 2-cell and mid-blastula stage suggesting the maternal inheritance of vasa mRNA. These findings support the hypothesis of preformation in T. macdonaldi PGCs. The migration pattern of PGCs allow us to recommend the isolation and subsequent cryopreservation of these cells before 7 DAH when the embryonic and larval development is given at 21 °C.

Testicular germ line cell identification, isolation, and transplantation in two North American catfish species

Our aim was to transplant blue catfish germ line stem cells into blastulae of triploid channel catfish embryos to produce interspecific xenogenic catfish. The morphological structure of the gonads of blue catfish (Ictalurus furcatus) in ~ 90- to 100-day-old juveniles, two-year-old juveniles, and mature adults was studied histologically. Both oogonia (12-15 μm, diameter with distinct nucleus 7-8 μm diameter) and spermatogonia (12-15 μm, with distinct nucleus 6-7.5 μm diameter) were found in all ages of fish. The percentage of germ line stem cells was higher in younger blue catfish of both sexes. After the testicular tissue was trypsinized, a discontinuous density gradient centrifugation was performed using 70, 45, and 35% Percoll to enrich the percentage of spermatogonial stem cells (SSCs). Four distinct cell bands were generated after the centrifugation. It was estimated that 50% of the total cells in the top band were type A spermatogonia (diameter 12-15 μm) and type B spermatogonia (diameter 10-11 μm). Germ cells were confirmed with expression of vasa. Blastula-stage embryos of channel catfish (I. punctatus) were injected with freshly dissociated blue catfish testicular germ cells as donor cells for transplantation. Seventeen days after the transplantation, 33.3% of the triploid channel catfish fry were determined to be xenogenic catfish. This transplantation technique was efficient, and these xenogenic channel catfish need to be grown to maturity to verify their reproductive capacity and to verify that for the first time SSCs injected into blastulae were able to migrate to the genital ridge and colonize. These results open the possibility of artificially producing xenogenic channel catfish males that can produce blue catfish sperm and mate with normal channel catfish females naturally. The progeny would be all C × B hybrid catfish, and the efficiency of hybrid catfish production could be improved tremendously in the catfish industry.

Synchronizing developmental stages in Neotropical catfishes for application in germ cell transplantation

The aim of this study was to describe the effect of temperature on the fertilization, early developmental stages, and survival rate of two Neotropical catfishes Pimelodus maculatus and Pseudopimelodus mangurus. After fertilization, the eggs were incubated at 22°C, 26°C, and 30°C, which resulted in fertilization rates of 96.95 ± 1.79%, 98.74 ± 0.76%, and 98.44 ± 0.19% for P. maculatus and 96.10 ± 1.58%, 98.00 ± 0.63%, and 94.60 ± 2.09% for P. mangurus, respectively. For P. maculatus, hatching occurred after 22 h 30 min post-fertilization at 22°C, 16 h 30 min at 26°C, and 11 h 20 min at 30°C, and the hatching rates were 43.87 ± 7,46%, 57.57 ± 17.49%, and 53.63 ± 16.27%, respectively. For P. mangurus, hatching occurred after 28 h 30 min post-fertilization at 22°C and 17 h 30 min at 26°C with respective hatching rates of 45.4 ± 21.02% and 68.1 ± 12.67%. For this species, all embryos incubated at 30°C died before hatching. Additionally, for P. maculatus, the larvae from the lower (22°C) and higher temperatures (30°C) presented increased abnormality rates, as observed in the head, tail and yolk regions. The lowest abnormality rate was detected at 26°C, which was considered the optimal incubation temperature for both species. The developed protocol enables the manipulation of embryonic development, which is important for the application of reproductive biotechniques, including chimerism and chromosome-set manipulation. The data obtained here are also important for the surrogate propagation of this species as P. mangurus was recently categorized as an endangered fish species.

Characterization of vasa homolog in a neotropical catfish, Jundiá (Rhamdia quelen): Molecular cloning and expression analysis during embryonic and larval development

We have characterized the full-length vasa cDNA from Jundiá, Rhamdia quelen (Heptapteridae, Siluriformes). vasa encodes a member of the DEAD-box protein family of ATP-dependent RNA helicases. This protein is highly conserved among different organisms and its role is associated with RNA metabolism. In the majority of the investigated species, vasa is restricted to the germ cell lineage and its expression has been used to study germline development in many organisms, including fish. The deduced R. quelen vasa amino acid sequence displayed high similarity with Vasa protein sequences from other organisms, and did not cluster with PL10 or P68 DEAD-box protein subfamilies. We also reported that there is no other isoform for vasa mRNA in R. quelen gonads. Expression analysis by RT-PCR and qPCR showed vasa transcripts exclusively expressed in the germ cells of R. quelen gonads. R. quelen vasa mRNA was maternally inherited, and was detected in the migrating primordial germ cells (PGCs) until 264 h post-fertilization during embryonic and larval development. This work has characterized for the first time the full-length R. quelen vasa cDNA, and describes its expression patterns during R. quelen embryonic and larval development. Our results will contribute to the basic reproductive biology of this native species, and will support studies using vasa as a germ cell marker in different biotechnological studies, such as germ cell transplantation.

Application of dead end-knockout zebrafish as recipients of germ cell transplantation

Germ cell transplantation is a promising technology for the propagation of endangered or valuable fishes. In this technique, sterile male and female recipient fish are injected with donor germ cells so they can produce viable gametes derived from the donor cells. The dead end (dnd) gene is involved in the migration of primordial germ cells; therefore, dnd-knockout zebrafish are expected to be germ-cell-free, making them suitable recipients for germ cell transplantation. dnd mutants were produced by microinjecting 2 nl of 10 ng/μl cRNAs encoding zinc finger nucleases against dnd into the blastodisc of zebrafish embryos before the cell- cleavage stage. One of the resulting founder males was mated with a wild-type female, and produced heterozygous mutants in the F1 generation. Mating of these F1 mutants produced an F2 generation with approximately 25% of the clutch being homozygous mutant (dnd-knockout) male, and lacking germ cells (as confirmed by expression analyses of vasa). The resulting dnd-knockout zebrafish males were tested for suitability as germ cell transplantation recipients by intraperitoneal transplantation of testicular cells prepared from vasa-gfp zebrafish. GFP-positive germ cells incorporated into the germ-cell-free gonads of the dnd-knockout recipients matured into functional sperm. Progeny tests revealed that the sperm from these dnd-knockout recipients were derived entirely from donor cells. Thus, we demonstrated that homozygous dnd mutants became germ-cell-free males that are able to nurse donor-derived germ cells.

Divergent Expression Patterns and Function Implications of Four nanos Genes in a Hermaphroditic Fish, Epinephelus coioides

Multiple nanos genes have been characterized in several fishes, but the functional implications of their various expression patterns remain unclear. In this study, we identified and characterized four nanos genes from a hermaphroditic fish orange-spotted grouper, Epinephelus coioides. Ecnanos1a and Ecnanos1b show divergent expression patterns, and the dynamic expression change of Ecnanos1a in pituitaries during sex change is associated with testis differentiation and spermatogenesis. Ecnanos2 and Ecnanos3 might be germline stem cells (GSCs) and primordial germ cells (PGCs)-specific markers, respectively. Significantly, Ecnanos3 3'-untranslated region (UTR) is necessary for PGC specific expression, where a non-canonical "GCACGTTT" sequence is required for miR-430-mediated repression of Ecnanos3 RNA. Furthermore, grouper Dead end (Dnd) can relieve miR-430 repression in PGCs by associating with a 23 bp U-rich region (URR) in Ecnanos3 3'-UTR. The current study revealed the functional association of multiple nanos genes with PGC formation and germ cell development in orange-spotted grouper, and opened up new possibilities for developing biotechnologies through utilizing the associations between Ecnanos3 and PGCs or between Ecnanos2 and GSCs in the hermaphroditic fish.

Gene delivery to Nile tilapia cells for transgenesis and the role of PI3K-c2α in angiogenesis

Microinjection is commonly performed to achieve fish transgenesis; however, due to difficulties associated with this technique, new strategies are being developed. Here we evaluate the potential of lentiviral particles to genetically modify Nile tilapia cells to achieve transgenesis using three different approaches: spermatogonial stem cell (SSC) genetic modification and transplantation (SC), in vivo transduction of gametes (GT), and fertilised egg transduction (ET). The SC protocol using larvae generates animals with sustained production of modified sperm (80% of animals with 77% maximum sperm fluorescence [MSF]), but is a time-consuming protocol (sexual maturity in Nile tilapia is achieved at 6 months of age). GT is a faster technique, but the modified gamete production is temporary (70% of animals with 52% MSF). ET is an easier way to obtain mosaic transgenic animals compared to microinjection of eggs, but non-site-directed integration in the fish genome can be a problem. In this study, PI3Kc2α gene disruption impaired development during the embryo stage and caused premature death. The manipulator should choose a technique based on the time available for transgenic obtainment and if this generation is required to be continuous or not.

Production of the medaka derived from vitrified whole testes by germ cell transplantation

The medaka (Oryzias latipes) is a teleost model distinguished from other model organisms by the presence of inbred strains, wild stocks, and related species. Cryopreservation guarantees preservation of these unique biological resources. However, because of their large size, cryopreservation techniques for their eggs and embryos have not been established. In the present study, we established a methodology to produce functional gametes from cryopreserved testicular cells (TCs). Whole testes taken from medaka were cryopreserved by vitrification. After thawing, the cells dissociated from cryopreserved testicular tissues were intraperitoneally transplanted into sterile triploid hatchlings. Some cells, presumably spermatogonial stem cells, migrated into the genital ridges of recipients and resulted in the production of eggs or sperm, based on sex of the recipient. Mating of recipients resulted in successful production of cryopreserved TC-derived offspring. We successfully produced individuals from the Kaga inbred line, an endangered wild population in Tokyo, and a sub-fertile mutant (wnt4b^−/−) from cryopreserved their TCs. This methodology facilitates semi-permanent preservation of various medaka strains.

Establishment of intraperitoneal germ cell transplantation for critically endangered Chinese sturgeon Acipenser sinensis

Recent progress in germ cell transplantation techniques in fish has paved the way for the conservation of endangered species. Here, we developed an intraperitoneal germ cell transplantation procedure using Chinese and Dabry's sturgeon as donor and recipient species, respectively. Histological analysis revealed that primordial germ cells migrated on the peritoneal wall at 16 days post-hatch (dph) in Dabry's sturgeon. The genital ridges of Dabry's sturgeon (recipient) first formed at 28 dph, suggesting that for successful colonization of donor germ cells in the recipient gonads, the transplantation should be performed earlier than this age. Sexual dimorphism of gonadal structure was first observed at 78 dph. Gonadal germ cell proliferation was not seen in either sex during this period. Immunohistochemistry using the anti-Vasa antibody found that donor testes from 2-year-old Dabry's sturgeon mainly consisted of single- or paired-type A spermatogonia, while donor ovaries from 11.5-year-old Chinese sturgeon had perinucleolus stage oocytes and clusters of oogonia. Donor cells isolated from Dabry's sturgeon testes or Chinese sturgeon ovary labeled with PKH26 fluorescent dye were transplanted into the peritoneal cavity of the 7- or 8-dph Dabry's sturgeon larvae. More than 90% and 70% of transplanted larvae survived after 2 days post-transplantation (dpt) and 51 dpt, respectively. At 51 dpt, PKH26-labeled cells exhibiting germ cell-specific nuclear morphology and diameter were observed in excised recipient gonads by fluorescent and confocal microscopy. The colonization rate of allogeneic testicular germ cell transplantation (Group 1) was 70%, while that of two batches of xenogeneic ovarian germ cell transplantation (Group 2 and Group 3) were 6.7% and 40%, respectively. The ratio of colonized germ cells to endogenous germ cells was 11.96%, 5.35% and 3.56% for Group 1, Group 2 and Group 3, respectively. Thus, we established a germ cell transplantation technique for the critically endangered Chinese sturgeon using the most closely related species as a recipient and demonstrated the successful preparation of transplantable female germ cells from aged adult Chinese sturgeon.

Characterization of a vasa homolog in the brown-marbled grouper (Epinephelus fuscoguttatus) and its expression in gonad and germ cells during larval development

The vasa gene is specifically expressed in the germ cell lineage, and its expression has been used to study germline development in many organisms, including fishes. In this study, we cloned and characterized vasa as Efu-vasa in the brown-marbled grouper (Epinephelus fuscoguttatus). Efu-vasa contained predicted regions that shared consensus motifs with the vasa family in teleosts, including arginine- and glycine-rich repeats, ATPase motifs, and a DEAD box. Phylogenetic-tree construction using various DEAD-box proteins confirmed that Efu-vasa was clustered in the vasa family. Efu-vasa mRNA was detectable only in gonads, by reverse transcription polymerase chain reaction. Primordial germ cells (PGCs) during early gonad development in larvae were characterized by histological examination and in situ hybridization using an Efu-vasa antisense probe. Migrating PGCs were found in larvae at 9-21 days post-hatching, and rapid proliferation of PGCs was initiated in 36 days post-hatching. These findings provide a valuable basis for optimizing the developmental stages for germ cell transplantation in order to produce surrogate broodstock, which may help in the production of larvae of large and endangered grouper species.

Successful xenogeneic germ cell transplantation from Jundia catfish (Rhamdia quelen) into adult Nile tilapia (Oreochromis niloticus) testes

Fish germ cell transplantation presents several important potential applications for aquaculture, including the preservation of germplasm from endangered fish species with high genetic and commercial values. Using this technique in studies developed in our laboratory with adult male Nile tilapias (Oreochromis niloticus), all the necessary procedures were successfully established, allowing the production of functional sperm and healthy progeny approximately 2months after allogeneic transplantation. In the present study, we evaluated the viability of the adult Nile tilapia testis to generate sperm after xenogeneic transplant of germ cells from sexually mature Jundia catfish (Rhamdia quelen) that belong to a different taxonomic order. Therefore, in order to investigate at different time-periods post-transplantation, the presence and development of donor PKH26 labeled catfish germ cells were followed in the tilapia seminiferous tubules. From 7 to 20days post-transplantation, only PKH26 labeled spermatogonia were observed, whereas spermatocytes at different stages of development were found at 70days. Germ cell transplantation success and progression of spermatogenesis were indicated by the presence of labeled PKH26 spermatids and sperm on days 90 and 120 post-transplantation, respectively. Confirming the presence of the catfish genetic material in the tilapia testis, all recipient tilapias evaluated (n=8) showed the genetic markers evaluated. Therefore, we demonstrated for the first time that the adult Nile tilapia testis offers the functional conditions for development of spermatogenesis with sperm production from a fish species belonging to a different order, which provides an important new venue for aquaculture advancement.

The effects of temperature and busulfan (Myleran) on the yellowtail tetra Astyanax altiparanae (Pisces, Characiformes) spermatogenesis

We aimed to standardize a protocol to suppress spermatogenesis in the characiform fish, Astyanax altiparanae, for future use as a host in germ cell transplant research, opening opportunities for a range of studies, such as spermatogenesis analyses and transgenesis because this species presents livestock characteristics to be used as a biological model. The effects of the chemotherapeutic busulfan (formulated as Myleran), which is used as medicine, therefore not as toxic to humans manipulation as analytical grade busulfan (Fluka) used in previous studies, were evaluated at physiological temperature of 28 °C, ideal for growth and reproduction of A altiparanae, and also at increased temperature 35 °C. The temperature groups were divided into three treatment groups: busulfan, DMSO only, and an untreated control. Macroscopic, histologic, stereological, and ultrastructure analysis showed that, at 28 °C, busulfan did not cause depletion of germ cells in A altiparanae. However, at 35 °C, sterilization was observed 3 weeks after the initial application. Similar results were obtained with maintenance of fish at 35 °C for a longer period with no accompanying Myleran treatment. This procedure allows reduction in stress and lower mortality resulting from manipulation during busulfan injection and is also suitable for mass treatment because large numbers of fish can be incubated in warm water.

Development and application of biological technologies in fish genetic breeding

Fish genetic breeding is a process that remolds heritable traits to obtain neotype and improved varieties. For the purpose of genetic improvement, researchers can select for desirable genetic traits, integrate a suite of traits from different donors, or alter the innate genetic traits of a species. These improved varieties have, in many cases, facilitated the development of the aquaculture industry by lowering costs and increasing both quality and yield. In this review, we present the pertinent literatures and summarize the biological bases and application of selection breeding technologies (containing traditional selective breeding, molecular marker-assisted breeding, genome-wide selective breeding and breeding by controlling single-sex groups), integration breeding technologies (containing cross breeding, nuclear transplantation, germline stem cells and germ cells transplantation, artificial gynogenesis, artificial androgenesis and polyploid breeding) and modification breeding technologies (represented by transgenic breeding) in fish genetic breeding. Additionally, we discuss the progress our laboratory has made in the field of chromosomal ploidy breeding of fish, including distant hybridization, gynogenesis, and androgenesis. Finally, we systematically summarize the research status and known problems associated with each technology.

Maintenance of potential spermatogonial stem cells in vitro by GDNF treatment in a chondrichthyan model (Scyliorhinus canicula L.)

Previous work in dogfish, Scyliorhinus canicula, has identified the testicular germinative area as the spermatogonial stem cell niche. In the present study, an in vitro co-culture system of spermatogonia and somatic cells from the germinative area was developed. Long-term maintenance of spermatogonia has been successful, and addition of GDNF has promoted the development of clones of spermatogonia expressing stem cell characteristics such as alkaline phosphatase activity and has allowed maintenance of self-renewal in spermatogonia for at least 5 mo under culture conditions, notably by decreasing cell apoptosis. Furthermore, clones of spermatogonia expressed the receptor of GDNF, GFRalpha1, which is consistent with the effect of GDNF on cells despite the lack of identification of a GDNF sequence in the dogfish's transcriptome. However, a sequence homologous to artemin has been identified, and in silico analysis supports the hypothesis that artemin could replace GDNF in the germinative area in dogfish. This study, as the first report on long-term in vitro maintenance of spermatogonia in a chondrichthyan species, suggests that the GFRalpha1 signaling function in self-renewal of spermatogonial stem cells is probably conserved in gnathostomes.

Intraperitoneal germ cell transplantation in the Nile tilapia Oreochromis niloticus

Germ cell transplantation offers promising applications in finfish aquaculture and the preservation of endangered species. Here, we describe an intraperitoneal spermatogonia transplantation procedure in the Nile tilapia Oreochromis niloticus. Through histological analysis of early gonad development, we first determined the best suitable stage at which exogenous germ cells should be transplanted into the recipients. For the transplantation procedure, donor testes from a transgenic Nile tilapia strain carrying the medaka β-actin/enhanced green fluorescent protein (EGFP) gene were subjected to enzymatic dissociation. These testicular cells were then stained with PKH26 and microinjected into the peritoneal cavity of the recipient fish. To confirm colonization of the donor-derived germ cells, the recipient gonads were examined by fluorescent and confocal microscopy. PKH26-labeled cells exhibiting typical spermatogonial morphology were incorporated into the recipient gonads and were not rejected within 22 days posttransplantation. Long-term survival of transgenic donor-derived germ cells was then verified in the gonads of 5-month-old recipients and in the milt and vitelogenic oocytes of 1-year-old recipients, by means of PCR using EGFP-specific primers. EGFP-positive milt from adult male recipients was used to fertilize non-transgenic oocytes and produced transgenic offspring expressing the donor-derived phenotype. These results imply that long-term survival, proliferation, and differentiation of the donor-derived spermatogonia into vitelogenic oocytes and functional spermatozoa are all possible. Upon further improvements in the transplantation efficiency, this intraperitoneal transplantation system could become a valuable tool in the conservation of genetic resources for cichlid species.

Enrichment of spermatogonial stem cells using side population in teleost

Spermatogenesis originates from a small population of spermatogonial stem cells; this population can maintain continuous sperm production throughout the life of fish via self-renewal and differentiation. Despite their biological importance, spermatogonial stem cells are not thoroughly characterized because they are difficult to distinguish from their progeny cells that become committed to differentiation. We previously established a novel technique for germ cell transplantation to identify spermatogonial stem cells based on their colonizing activity and their ability to initiate donor-derived gametogenesis in the rainbow trout (Oncorhynchus mykiss). Although spermatogonial stem cells can be retrospectively identified after transplantation, there is currently no technique to prospectively enrich for or purify spermatogonial stem cells. Here, we describe a method for spermatogonial stem cell enrichment using a side population. With optimized Hoechst 33342 staining conditions, we successfully identified side-population cells among type A spermatogonia. Side-population cells were transcriptomically and morphologically distinct from non-side-population cells. To functionally determine whether the transplantable spermatogonial stem cells were enriched in the side-population fraction, we compared the colonization activity of side-population cells with that of non-side-population cells. Colonization efficiency was significantly higher with side-population cells than with non-side-population cells or with total type A spermatogonia. In addition, side-population cells could produce billions of sperm in recipients. These results indicated that transplantable spermatogonial stem cells were enriched in the side-population fraction. This method will provide biological information that may advance our understanding of spermatogonial stem cells in teleosts. Additionally, this technique will increase the efficiency of germ cell transplantation used in surrogate broodstock technology.

Surrogate production of eggs and sperm by intrapapillary transplantation of germ cells in cytoablated adult fish

Germ cell transplantation (GCT) is a promising assisted reproductive technology for the conservation and propagation of endangered and valuable genetic resources. In teleost fish, GCT in adult gonads has been achieved only in male recipients, limiting greatly the usefulness of this technique in situations where both sexes need equal and timely attention for conservation and/or propagation. Here we describe a simplified GCT approach that ultimately leads to production of donor-derived eggs and sperm in considerably short time. Donor germ cells isolated from young pejerrey Odontesthes bonariensis (Atherinopsidae) were transplanted non-surgically through the genital papilla into the sexually mature gonads of Patagonian pejerrey O. hatcheri recipients whose gonads have been depleted of endogenous GCs by heat (26°C) and chemical treatment (four doses of Busulfan at 30 mg/kg and 40 mg/kg for females and males, respectively). Transplanted spermatogonial and oogonial cells were able to recolonize the recipients' gonads and produce functional donor origin eggs and sperm within 7 months from the GCT. We confirmed the presence of donor-derived gametes by PCR in 17% and 5% of the surrogate O. hatcheri fathers and mothers, respectively. The crosses between surrogate fathers and O. bonariensis mothers yielded 12.6-39.7% pure O. bonariensis and that between a surrogate mother and an O. bonariensis father yielded 52.2% pure O. bonariensis offspring. Our findings confirm that transplantation of germ cells into sexually competent adult fish by non-surgical methods allows the production of functional donor-derived eggs and sperm in a considerably short time. The methods described here could play a vital role in conservation and rapid propagation of endangered fish genetic resources.

Spermatogonial stem cell quest: nanos2, marker of a subpopulation of undifferentiated A spermatogonia in trout testis

Continuous or cyclic production of spermatozoa throughout life in adult male vertebrates depends on a subpopulation of undifferentiated germ cells acting as spermatogonial stem cells (SSCs). What makes these cells self-renew or differentiate is barely understood, in particular in nonmammalian species, including fish. In the highly seasonal rainbow trout, at the end of the annual spermatogenetic cycle, tubules of the spawning testis contain only spermatozoa, with the exception of scarce undifferentiated spermatogonia that remain on the tubular wall and that will support the next round of spermatogenesis. Taking advantage of this model, we identified putative SSCs in fish testis using morphological, molecular, and functional approaches. In all stages, large spermatogonia with ultrastructural characteristics of germinal stem cells were found, isolated or in doublet. Trout homologues of SSC and/or immature progenitor markers in mammals-nanos2 and nanos3, pou2, plzf, and piwil2-were preferentially expressed in the prepubertal testis and in the undifferentiated A spermatogonia populations purified by centrifugal elutriation. This expression profile strongly suggests that these genes are functionally conserved between fish and mammals. Moreover, transplantation into embryonic recipients of the undifferentiated spermatogonial cells demonstrated their high "stemness" efficiency in terms of migration into gonads and the ability to give functional gametes. Interestingly, we show that nanos2 expression was restricted to a subpopulation of undifferentiated spermatogonia (less than 20%) present as isolated cells or in doublet in the juvenile and in the maturing trout testis. In contrast, nanos2 transcript was detected in all the undifferentiated spermatogonia remaining in the spawning testis. Plzf was also immunodetected in A-Spg from spawning testis, reinforcing the idea that these cells are stem cells. From those results, we hypothesize that the subset of undifferentiated A spermatogonia expressing nanos2 transcript are putative SSC in trout.

Sexually dimorphic expression of vasa isoforms in the tongue sole (Cynoglossus semilaevis)

The vasa gene encodes an ATP-dependent RNA helicase of the DEAD box protein family that functions in a broad range of molecular events involving duplex RNA. In most species, the germline specific expression of vasa becomes a molecular marker widely used in the visualization and labeling of primordial germ cells (PGCs) and a tool in surrogate broodstock production through PGC transplantation. The vasa gene from tongue sole (Cynoglossus semilaevis) was characterized to promote the development of genetic breeding techniques in this species. Three C. semilaevis vasa transcripts were isolated, namely vas-l, vas-m, and vas-s. Quantitative real-time PCR results showed that C. semilaevis vasa transcripts were prevalently expressed in gonads, with very weak expression of vas-s in other tissues. Embryonic development expression profiles revealed the onset of zygotic transcription of vasa mRNAs and the maternal deposit of the three transcripts. The genetic ZW female juvenile fish was discriminated from genetic ZZ males by a pair of female specific primers. Only the expression of vas-s can be observed in both sexes during early gonadal differentiation. Before PGCs started mitosis, there was sexually dimorphic expression of vas-s with the ovary showing higher levels and downward trend. The results demonstrated the benefits of vasa as a germline specific marker for PGCs during embryonic development and gonadal differentiation. This study lays the groundwork for further application of C. semilaevis PGCs in fish breeding.

Application of testis germ cell transplantation in breeding systems of food producing species: a review

A major benefit of advanced reproduction technologies (ART) in animal breeding is the ability to produce more progeny per individual parent. This is particularly useful with animals of high genetic merit. Testis germ cell transplantation (TGCT) is emerging as a novel reproductive technology with application in animal breeding systems, including the potential for use as an alternative to artificial insemination (AI), an alternative to transgenesis, part of an approach to reducing generation intervals, or an approach toward development of interspecies hybrids. There is one major difference in TGCT between rodents and some other species associated with immunotolerance in heterologous transplantation. In particular, livestock and aquatic species do not require an immunesuppression procedure to allow donor cell survival in recipient testis. Testicular stem cells from a genetically elite individual transplanted into others can develop and produce a surrogate male-an animal that produces the functional sperm of the original individual. Spermatozoa produced from testis stem cells are the only cells in the body of males that can transmit genetic information to the offspring. The isolation and genetic manipulation of testis stem cells prior to transplantation has been shown to create transgenic animals. However, the current success rate of the transplantation procedure in livestock and aquatic species is low, with a corresponding small proportion of donor spermatozoa in the recipient's semen. The propagation of donor cells in culture and preparation of recipient animals are the two main factors that limit the commercial application of this technique. The current paper reviews and compares recent progress and examines the difficulties of TGCT in both livestock and aquatic species, thereby providing new insights into the application of TGCT in food producing animals.

Development of interspecies testicular germ-cell transplantation in flatfish

Interspecific testicular germ cell (TGC) transplantation was investigated in two commercial flatfish species. Testes from donor species (Senegalese sole) were evaluated using classical histological techniques (haematoxylin–eosin staining and haematoxylin–light green–orange G–acid fuchsine staining), in situ hybridisation and immunohistochemical analysis. Both Ssvasa1–2 mRNAs and SsVasa protein allowed the characterisation of TGCs, confirming the usefulness of the vasa gene in the detection of Senegalese sole TGCs. Xenogenic transplants were carried out using TGCs from one-year-old Senegalese sole into turbot larvae. Propidium iodide–SYBR-14 and 4′,6′-diamidino-2-phenylindole (DAPI) staining showed that 87.98% of the extracted testicular cells were viable for microinjection and that 15.63% of the total recovered cells were spermatogonia. The vasa gene was characterised in turbot recipients using cDNA cloning. Smvasa mRNA was confirmed as a germ cell-specific molecular marker in this species. Smvasa expression analysis during turbot ontogeny was carried out before Senegalese sole TGC transplants into turbot larvae. Turbot larvae at 18 days after hatching (DAH) proved to be susceptible to manipulation procedures. High survival rates (83.75 ± 15.90 – 100%) were obtained for turbot larvae at 27, 34 and 42 DAH. These data highlight the huge potential of this species for transplantation studies. Quantitative PCR was employed to detect Senegalese sole vasa mRNAs (Ssvasa1–2) in the recipient turbot larvae. The Ssvasa mRNAs showed a significant increase in relative expression in 42-DAH microinjected larvae three weeks after treatment, showing the proliferation of Senegalese sole spermatogonia in transplanted turbot larvae.