Generation of viable fish from cryopreserved primordial germ cells

Post-thaw quality assessment of testicular fragments as a source of spermatogonial cells for surrogate production in the flatfish Solea senegalensis

Cryopreservation of germ cells would facilitate the availability of cells at any time allowing the selection of donors and maintaining quality control for further applications such as transplantation and germline recovery. In the present study, we analyzed the efficiency of four cryopreservation protocols applied either to isolated cell suspensions or to testes fragments from Senegalese sole. In testes fragments, the quality of cryopreserved germ cells was analyzed in vitro in terms of cell recovery, integrity and viability, DNA integrity (fragmentation and apoptosis), and lipid peroxidation (malondialdehyde levels). Transplantation of cryopreserved germ cells was performed to check the capacity of cells to in vivo incorporate into the gonadal primordium of Senegalese sole early larval stages (6 days after hatching (dah), pelagic live), during metamorphosis (10 dah) and at post-metamorphic stages (16 dah and 20 dah, benthonic life). Protocols incorporating dimethyl sulfoxide (DMSO) as a cryoprotectant showed higher number of recovered spermatogonia, especially in samples cryopreserved with L-15 + DMSO (0.39 ± 0.18 × 106 cells). Lipid peroxidation and DNA fragmentation were also significantly lower in this treatment compared with other treatments. An important increase in oxidation (MDA levels) was detected in samples containing glycerol as a cryoprotectant, reflected also in terms of DNA damage. Transplantation of L-15 + DMSO cryopreserved germ cells into larvae during early metamorphosis (10 dah, 5.2 mm) showed higher incorporation of cells (27.30 ± 5.27%) than other larval stages (lower than 11%). Cryopreservation of germ cells using testes fragments frozen with L-15 + DMSO was demonstrated to be a useful technique to store Senegalese sole germline.

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.

Opportunities and Limitations for Reproductive Science in Species Conservation

Reproductive science in the context of conservation biology is often understood solely in terms of breeding threatened species. Although technologies developed primarily for agriculture or biomedicine have a potentially important role in species conservation, their effectiveness is limited if we regard the main objective of animal conservation as helping to support populations rather than to breed a small number of individuals. The global threats facing wild species include the consequences of climate change, population growth, urbanization, atmospheric and water pollution, and the release of chemicals into the environment, to cite but a few. Reproductive sciences provide important and often unexpected windows into many of these consequences, and our aim here is both to demonstrate the breadth of reproductive science and the importance of basic knowledge and to suggest where some of the insights might be useful in mitigating the problems. Expected final online publication date for the Annual Review of Animal Biosciences, Volume 10 is February 2022. Please see http://www.annualreviews.org/page/journal/pubdates for revised estimates.

Surrogate broodstock to enhance biotechnology research and applications in aquaculture

Aquaculture is playing an increasingly important role in meeting global demands for seafood, particularly in low and middle income countries. Genetic improvement of aquaculture species has major untapped potential to help achieve this, with selective breeding and genome editing offering exciting avenues to expedite this process. However, limitations to these breeding and editing approaches include long generation intervals of many fish species, alongside both technical and regulatory barriers to the application of genome editing in commercial production. Surrogate broodstock technology facilitates the production of donor-derived gametes in surrogate parents, and comprises transplantation of germ cells of donors into sterilised recipients. There are many successful examples of intra- and inter-species germ cell transfer and production of viable offspring in finfish, and this leads to new opportunities to address the aforementioned limitations. Firstly, surrogate broodstock technology raises the opportunity to improve genome editing via the use of cultured germ cells, to reduce mosaicism and potentially enable in vivo CRISPR screens in the progeny of surrogate parents. Secondly, the technology has pertinent applications in preservation of aquatic genetic resources, and in facilitating breeding of high-value species which are otherwise difficult to rear in captivity. Thirdly, it holds potential to drastically reduce the effective generation interval in aquaculture breeding programmes, expediting the rate of genetic gain. Finally, it provides new opportunities for dissemination of tailored, potentially genome edited, production animals of high genetic merit for farming. This review focuses on the state-of-the-art of surrogate broodstock technology, and discusses the next steps for its applications in research and production. The integration and synergy of genomics, genome editing, and reproductive technologies have exceptional potential to expedite genetic gain in aquaculture species in the coming decades.

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Genetic improvement in aquaculture species has a major role in global food security. Advances in biotechnology provide new opportunities to support aquaculture breeding.

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Advances in biotechnology provide new opportunities to support aquaculture breeding.

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Donor-derived gametes can be produced from surrogate broodstock of several aquaculture species.

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Surrogate broodstock technology provides new opportunities for application of genome editing.

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Surrogate broodstock can accelerate genetic gain, and improve dissemination of elite germplasm.

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.

Aging- and temperature-related activity of spermatogonial stem cells for germ cell transplantation in medaka

Spermatogonial transplantation can contribute to developing a novel method of producing seedlings for both aquaculture and biotic conservation. This study's purpose was to investigate aging- and temperature-related changes in the numbers and stem cell functions of type-A spermatogonia (ASG) in the model fish medaka (Oryzias latipes). The ASG numbers in medaka of different ages were quantified via histological observation and enzymatic dissociation of vasa-Gfp medaka testes. The ASG numbers were higher in eight-month-old medaka (maturation) than in four-month-old medaka (the onset of maturation). However, ASG numbers decreased in 18-month-old medaka (senescence). Low water temperature appeared to slow down both testis development and aging processes. To study the effects of aging on ASG stem cell activity, testicular cell suspensions containing GFP-expressed ASG were prepared from vasa-Gfp medaka donors at 4 and 18 months of age and transplanted into recipient hybrid larvae of medaka (O. latipes x O. curvinotus), which provided young stem-cell-niches. The findings revealed no significant differences in ASG colonization rates isolated from medaka of different ages. Each group displayed similar rates of germ-line transmission. Furthermore, water temperature had no significant effects on each ASG's stem cell activity. Taken together, these results indicated that aging and temperature affect ASG numbers. However, ASG isolated from medaka with different ages were transplanted into gonads with a young niche microenvironment, and there was no evidence of donor aging on stem cell activity.

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.

Enrichment of transplantable germ cells in salmonids using a novel monoclonal antibody by magnetic-activated cell sorting

In the fish germ cell transplantation system, only type A spermatogonia (ASGs) and oogonia are known to be incorporated into the recipient genital ridges, where they undergo gametogenesis. Therefore, high colonization efficiency can be achieved by enriching undifferentiated germ cells out of whole testicular cells. In this study, we used magnetic-activated cell sorting (MACS) for enriching undifferentiated germ cells of rainbow trout using a monoclonal antibody that recognizes a specific antigen located on the germ cell membrane. We screened the antibodies to be used for MACS by performing immunohistochemistry on rainbow trout gonads. Two antibodies, nos. 172 and 189, showed strong signals for ASGs and oogonia. Next, we performed MACS with antibody no. 172 using gonadal cells isolated from vasa-gfp rainbow trout showing GFP in undifferentiated germ cells. We found that GFP-positive cells are highly enriched in antibody no. 172-positive fractions. Finally, to examine the transplantability of MACS-enriched cells, we intraperitoneally transplanted sorted or unsorted cells into recipient larvae. We observed that transplantability of sorted cells, particularly ovarian cells, were significantly higher than that of unsorted cells. Therefore, MACS with antibody no. 172 could enrich ASGs and oogonia and become a powerful tool to improve transplantation efficiency in salmonids.

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.

Reproduction biotechnologies in germplasm banking of livestock species: a review

Many biotechnologies are currently used in livestock breeding with the aim of improving reproductive efficiency and increasing the rate of genetic progress in production animals. Semen cryopreservation is the most widely used cryobiotechnology, although vitrification techniques now allow embryos and oocytes to be banked in ever-increasing numbers. Cryopreservation of other types of germplasm (reproductive tissue in general) is also possible, although the techniques are still in the early stages of development for use in livestock species. Although still in their infancy, these techniques are increasingly being used in aquaculture. Germplasm conservation enables reproductive tissues from both animals and fish to be preserved to generate offspring in the future without having to maintain large numbers of living populations of these species. However, such measures need careful planning and coordination. This review explains why the preservation of genetic diversity is needed for livestock and fish, and describes some of the issues involved in germplasm banking. Furthermore, some recent developments in semen handling leading to improved semen cryopreservation and biosecurity measures are also discussed.

Progress and biotechnological prospects in fish transgenesis

The history of transgenesis is marked by milestones such as the development of cellular transdifferentiation, recombinant DNA, genetic modification of target cells, and finally, the generation of simpler genetically modified organisms (e.g. bacteria and mice). The first transgenic fish was developed in 1984, and since then, continuing technological advancements to improve gene transfer have led to more rapid, accurate, and efficient generation of transgenic animals. Among the established methods are microinjection, electroporation, lipofection, viral vectors, and gene targeting. Here, we review the history of animal transgenesis, with an emphasis on fish, in conjunction with major developments in genetic engineering over the past few decades. Importantly, spermatogonial stem cell modification and transplantation are two common techniques capable of revolutionizing the generation of transgenic fish. Furthermore, we discuss recent progress and future biotechnological prospects of fish transgenesis, which has strong applications for the aquaculture industry. Indeed, some transgenic fish are already available in the current market, validating continued efforts to improve economically important species with biotechnological advancements.

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.

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.

Production of germ cell-deficient salmonids by dead end gene knockdown, and their use as recipients for germ cell transplantation

We previously established a spermatogonial transplantation model in fish using triploid recipients. Although triploid salmonids are sterile, they carry a limited number of immature triploid germ cells that potentially compete with the donor-derived germ cells for their niche. We therefore assessed the biological characteristics of germ cell-deficient gonads in rainbow trout for their suitability as recipients for germ cell transplantation in this study. Antisense morpholino oligonucleotides against the dead end gene were microinjected into the fertilized eggs of rainbow trout to eliminate endogenous germ cells, leaving only their supporting cells. Unlike similar approaches performed in zebrafish and medaka, these germ cell-deficient rainbow trout did not show a male-biased sex ratio. Approximately 30,000 spermatogonia were then transplanted into the body cavities of both germ cell-deficient and control recipients. The donor-derived germ cells showed significantly higher proliferation in the gonads of germ cell-deficient recipients than those in the gonads of the control recipients. Finally, the applicability of the germ cell-deficient recipients for xenogeneic transplantation was evaluated by transplanting rainbow trout spermatogonia into germ cell-deficient masu salmon recipients. The resulting recipient salmon matured normally and produced trout gametes, and early survival of the resulting trout offspring was as high as that of the control offspring. Thus, dead end-knockdown salmonids appear to be ideal recipients for the intraperitoneal transplantation of spermatogonia.

The genomic and genetic toolbox of the teleost medaka (Oryzias latipes)

The Japanese medaka, Oryzias latipes, is a vertebrate teleost model with a long history of genetic research. A number of unique features and established resources distinguish medaka from other vertebrate model systems. A large number of laboratory strains from different locations are available. Due to a high tolerance to inbreeding, many highly inbred strains have been established, thus providing a rich resource for genetic studies. Furthermore, closely related species native to different habitats in Southeast Asia permit comparative evolutionary studies. The transparency of embryos, larvae, and juveniles allows a detailed in vivo analysis of development. New tools to study diverse aspects of medaka biology are constantly being generated. Thus, medaka has become an important vertebrate model organism to study development, behavior, and physiology. In this review, we provide a comprehensive overview of established genetic and molecular-genetic tools that render medaka fish a full-fledged vertebrate system.

Medium composition for effective slow freezing of embryonic cell lines derived from marine medaka (Oryzias dancena)

This study was conducted to identify optimal medium composition for freezing Oryzias dancena embryonic cell lines. Different freezing media consisting of various concentration of dimethyl sulfoxide (DMSO), fetal bovine serum (FBS), and trehalose were prepared and long-term cultured embryonic cell line was frozen in each freezing medium by conventional slow freezing program for 7 days. Through measurement of viability and growth of post-thaw cells frozen in each freezing medium, it was determined that optimal composition of three components was 10 % DMSO, 20 % FBS, and 0.1 M trehalose. The post-thaw cells frozen in optimal freezing medium showed similar morphology and growth rate with non-frozen cells. Next, this condition was applied to two different sets of experiment; (1) freezing of the same cells during expanded period (57 days) and (2) freezing of short-term cultured cells from other batches for 7 days. The viability of post-thaw cells was significantly low and comparable in set 1 and 2, respectively, when compared with the result of long term-cultured cells frozen in optimal freezing medium for 7 days and similar morphology and growth rate with non-frozen counterparts were detected in the post-thaw cells from both sets. In conclusion, this study first reports the optimal medium composition for freezing O. dancena embryonic cells, which can contribute to fish species preservation as well as improvement of cell-based biotechnology by providing stable cell storage.

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.

Molecular characterization, sexually dimorphic expression, and functional analysis of 3'-untranslated region of vasa gene in half-smooth tongue sole (Cynoglossus semilaevis)

Vasa is a highly conserved ATP-dependent RNA helicase expressed mainly in germ cells. The vasa gene plays a crucial role in the development of germ cell lineage and has become an excellent molecular marker in identifying germ cells in teleosts. However, little is known about the structure and function of the vasa gene in flatfish. In this study, the vasa gene (Csvasa) was isolated and characterized in half-smooth tongue sole (Cynoglossus semilaevis), an economically important flatfish in China. In the obtained 6425-bp genomic sequence, 23 exons and 22 introns were identified. The Csvasa gene encodes a 663-amino acid protein, including highly conserved domains of the DEAD-box protein family. The amino acid sequence also shared a high homology with other teleosts. Csvasa expression was mainly restricted to the gonads, with little or no expression in other tissues. Real-time quantitative polymerase chain reaction analysis revealed that Csvasa expression levels decreased during embryonic and early developmental stages and increased with the primordial germ cell proliferation. A typical sexually dimorphic expression pattern of Csvasa was observed during early development and sex differentiation, suggesting that the Csvasa gene might play a differential role in the proliferation and differentiation of male and female primordial germ cells (PGCs). Csvasa mRNA expression levels in neomales were significantly lower than those in normal males and females, indicating that the Csvasa gene might be implicated in germ cell development after sex reversal by temperature treatment. In addition, medaka (Oryzias latipes) PGCs could be transiently labeled by microinjection of synthesized mRNA containing the green fluorescence protein gene and 3'-untranslated region of Csvasa, which confirmed that the Csvasa gene has the potential to be used as a visual molecular marker of germ cells and laid a foundation for manipulation of PGCs in tongue sole reproduction.

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.

Recent advances and prospects in germplasm preservation of rare and endangered species

Fertility preservation strategies using cryopreservation have enormous potential for helping sustain and protect rare and endangered species, especially to assist managing or 'rescuing' the genomes of genetically valuable individuals. However, wide-scale applications are still limited by significant physiological variations among species and a sheer lack of fundamental knowledge about basic reproductive traits as well as in germplasm cryobiology. Cryo-studies have been conducted in more species (mainly vertebrates) in the recent years but a vast majority still remains un-studied. Semen cryopreservation represents the most extensive effort with live births reported in more and more species after artificial insemination. Oocyte freezing remains challenging and unsuccessful in wild species and will require more research before becoming a standard procedure. As an alternative to fully grown gametes, gonadal tissue preservation has become a promising option in vertebrates. Yet, more fertility preservation options are necessary to save species so a change in strategy might be required. It is worthwhile thinking beyond systematic characterizations and considering the application of cutting edge approaches to universally preserve the fertility of a vast array of species.

Current and future assisted reproductive technologies for fish species

The Food and Agriculture Organization of the United Nations (FAO) estimates that in 2012 aquaculture production of fish will meet or exceed that of the capture fisheries for the first time. Thus, we have just turned the corner from a predominantly hunting gathering approach to meeting our nutritional needs from fish, to a farming approach. In 2012, 327 finfish species and five hybrids were covered by FAO aquaculture statistics, although farming of carps, tilapias, salmonids, and catfishes account for most of food-fish production from aquaculture. Although for most major species at least part of production is based on what might be considered domesticated animals, only limited production in most species is based on farming of improved lines of fish or is fully independent of wild seedstock. Consistent with the infancy of most aquaculture industries, much of the development and implementation of reproductive technologies over the past 100 years has been directed at completion of the life cycle in captivity in order to increase seed production and begin the process of domestication. The selection of species to farm and the emphasis of selective breeding must also take into account other ways to modify performance of an animal. Reproductive technologies have also been developed and implemented to affect many performance traits among fishes. Examples include technologies to control gender, alter time of sexual maturation, and induce sterilization. These technologies help take advantage of sexually dimorphic growth, overcome problems with growth performance and flesh quality associated with sexual maturation, and genetic containment. Reproductive technologies developed to advance aquaculture and how these technologies have been implemented to advance various sectors of the aquaculture industry are discussed. Finally, we will present some thoughts regarding future directions for reproductive technologies and their applications in finfish aquaculture.

Solea senegalensis vasa transcripts: molecular characterisation, tissue distribution and developmental expression profiles

The Vasa protein is an RNA helicase belonging the DEAD (Asp-Glu-Ala-Asp)-box family. The crucial role played by the vasa gene in the germ-cell lineage of both vertebrates and invertebrates has made this gene a useful molecular marker for germinal cells and a useful tool in surrogate broodstock production using primordial germ cell transplantation. With the aim of establishing a novel approach to improving Solea senegalensis broodstock management, the vasa gene in this species was characterised. Four S. senegalensis vasa transcripts were isolated: Ssvasa1, Ssvasa2, Ssvasa3 and Ssvasa4. Their phylogenetic relationship with other vasa homologues was determined confirming the high degree of conservation of this helicase throughout evolution. Our qPCR results showed that S. senegalensis vasa transcripts are prevalently expressed in gonads, with ovary-specific expression for Ssvasa3 and Ssvasa4. During embryonic and larval development, a switch between the longest and the shortest transcripts was observed. While Ssvasa1 and Ssvasa2 were maternally supplied, Ssvasa3 and Ssvasa4 depended on the de novo expression program of the growing juveniles, suggesting that vasa mRNA could be involved in Senegalese sole gonad differentiation. In situ hybridisation and immunohistochemical analysis performed in 150-days after hatching (DAH) larvae showed vasa product expression in the germinal region of early gonads. In our work we demonstrated the usefulness of Ssvasa mRNAs as molecular markers for primordial germ cells and germinal cells during embryonic development, larval ontogenesis and gonad differentiation. Furthermore, our results confirmed the potential of vasa to help investigate germinal cell biotechnology for Senegalese sole reproduction.

Germ cell transplantation as a potential biotechnological approach to fish reproduction

Although the use of germ cell transplantation has been relatively well established in mammals, the technique has only been adapted for use in fish after entering the 2000s. During the last decade, several different approaches have been developed for germ cell transplantation in fish using recipients of various ages and life stages, such as blastula-stage embryos, newly hatched larvae and sexually mature specimens. As germ cells can develop into live organisms through maturation and fertilization processes, germ cell transplantation in fish has opened up new avenues of research in reproductive biotechnology and aquaculture. For instance, the use of xenotransplantation in fish has lead to advances in the conservation of endangered species and the production of commercially valuable fish using surrogated recipients. Further, this could also facilitate the engineering of transgenic fish. However, as is the case with mammals, knowledge regarding the basic biology and physiology of germline stem cells in fish remains incomplete, imposing a considerable limitation on the application of germ cell transplantation in fish. Furthering our understanding of germline stem cells would contribute significantly to advances regarding germ cell transplantation in fish.

Generation of functional eggs and sperm from cryopreserved whole testes

The conservation of endangered fish is of critical importance. Cryobanking could provide an effective backup measure for use in conjunction with the conservation of natural populations; however, methodology for cryopreservation of fish eggs and embryos has not yet been developed. The present study established a methodology capable of deriving functional eggs and sperm from frozen type A spermatogonia (ASGs). Whole testes taken from rainbow trout were slowly frozen in a cryomedium, and the viability of ASGs within these testes did not decrease over a 728-d freezing period. Frozen-thawed ASGs that were intraperitoneally transplanted into sterile triploid hatchlings migrated toward, and were incorporated into, recipient genital ridges. Transplantability of ASGs did not decrease after as much as 939 d of cryopreservation. Nearly half of triploid recipients produced functional eggs or sperm derived from the frozen ASGs and displayed high fecundity. Fertilization of resultant gametes resulted in the successful production of normal, frozen ASG-derived offspring. Feasibility and simplicity of this methodology will call for an immediate application for real conservation of endangered wild salmonids.

Germ line specific expression of a vasa homologue gene in turbot (Scophthalmus maximus): evidence for vasa localization at cleavage furrows in euteleostei

Specification of primordial germ cells during early embryogenesis is a critical biological issue in reproduction and development. Yet, little is known in marine economic fish species. Vasa, a component of germ plasm, is the most-documented germ cell marker in teleosts. We isolated a full-length vasa cDNA (Smvas) from turbot (Scophthalmus maximus), a marine Euteleostei species, and investigated its expression patterns by RT-PCR and in situ hybridization during embryogenesis and gametogenesis to identify the germ cell lineage in this species. The deduced amino acid sequence of the isolated cDNA shared typical characteristics of Vasa protein and high identity to Vasa homologues in medaka (76.9%) and zebrafish (68.5%). The Smvas transcripts were exclusively detected in germ cells of testis and ovary, and exhibited an interesting dynamic localization pattern during oogenesis. The distribution pattern of Smvas during embyogenesis in this Euteleostei closely resembled the pattern observed in zebrafish (belonging to Osteriophysans) rather than medaka (belonging to Euteleostei). Thus, it is concluded that Smvas isolated in this study is a germ cell specific molecular marker in turbot. Furthermore, we hypothesize that Euteleostei could localize vasa mRNA by a special mode. The results not only facilitate the germ cell manipulation of the turbot, but also improve our understanding of germline development and evolution of vasa localization in teleost.

Comparative cryobiological traits and requirements for gametes and gonadal tissues collected from wildlife species

A major challenge to retaining viability of frozen gametes and reproductive tissues is to understand and overcome species-specificities, especially because there is substantial diversity in cryobiological properties and requirements among cell types and tissues. Systematic studies can lead to successful post-thaw recovery, especially after determining: 1) membrane permeability to water and cryoprotectant, 2) cryoprotectant toxicity, 3) tolerance to osmotic changes, and 4) resistance to cooling and freezing temperatures. Although species-dependency ultimately dictates the ability of specific cells and tissues to survive freeze-thawing, there are commonalities between taxa that allow a protocol developed for one species to be useful information for another. This is the reason for performing comparative cryopreservation studies among diverse species. Our laboratory has compared cellular cryotolerance, especially in spermatozoa, in a diverse group of animals-from corals to elephants-for more than 30 yrs. Characterizing the biophysical traits of gametes and tissues is the most efficient way to develop successful storage and recovery protocols, but, such data are only available for a few laboratory, livestock, and fish species, with virtually all others (wild mammals, birds, reptiles, and amphibians) having gone unstudied. Nonetheless, when a rare animal unexpectedly dies, there is no time to understand the fundamentals of biophysics. In these emergencies, it is necessary to rely on experience and the best data from taxonomically-related species. Fortunately, there are some general similarities among most species, which, for example, allow adequate post-thaw viability. Regardless, there is a priority for more information on biophysical traits and freezing tolerance of distinctive biomaterials, especially for oocytes and gonadal tissues, and even for common, domesticated animals. Our colleague, Dr John Critser was a pioneer in cryobiology, earning that moniker because of his advocacy and devotion to understanding the differences (and similarities) among species to better store living genetic material.

Production of donor-derived offspring by allogeneic transplantation of spermatogonia in the yellowtail (Seriola quinqueradiata)

Although the yellowtail (Seriola quinqueradiata) is the fish most commonly farmed in Japan, breeding of this species has not yet started. This is primarily due to the lack of sufficiently sophisticated methods for manipulating gametogenesis, which makes it difficult to collect gametes from specific dams and sires. If it were possible to produce large numbers of surrogate fish by transplanting germ cells isolated from donor individuals harboring desirable genetic traits, then the probability of acquiring gametes carrying the donor-derived haplotype would increase, and breeding programs involving this species might increase as a result. As a first step, we established a method for the allogeneic transplantation of yellowtail spermatogonia and the production of donor-derived offspring. Donor cells were collected from immature (10-month-old) yellowtail males with testes containing abundant type A spermatogonia, labeled with PKH26 fluorescent dye, and transferred into the peritoneal cavities of 8-day-old larvae. Fluorescence observation at 28 days post-transplantation revealed that PKH26-labeled cells were incorporated into recipients' gonads. To assess whether donor-derived spermatogonia could differentiate into functional gametes in the allogeneic recipient gonads, gametes collected from nine male and four female adult recipients were fertilized with wild-type eggs and milt. Analysis of microsatellite DNA markers confirmed that some of the first filial (F(1)) offspring were derived from donor fish, with the average contribution of donor-derived F(1) offspring being 66% and the maximum reaching 99%. These findings confirmed that our method was effective for transplanting yellowtail spermatogonia into allogeneic larvae to produce donor-derived offspring.

A new and fast technique to generate offspring after germ cells transplantation in adult fish: the Nile tilapia (Oreochromis niloticus) model

BACKGROUND

Germ cell transplantation results in fertile recipients and is the only available approach to functionally investigate the spermatogonial stem cell biology in mammals and probably in other vertebrates. In the current study, we describe a novel non-surgical methodology for efficient spermatogonial transplantation into the testes of adult tilapia (O. niloticus), in which endogenous spermatogenesis had been depleted with the cytostatic drug busulfan.

METHODOLOGY/PRINCIPAL FINDINGS

Using two different tilapia strains, the production of fertile spermatozoa with donor characteristics was demonstrated in adult recipient, which also sired progeny with the donor genotype. Also, after cryopreservation tilapia spermatogonial cells were able to differentiate to spermatozoa in the testes of recipient fishes. These findings indicate that injecting germ cells directly into adult testis facilitates and enable fast generation of donor spermatogenesis and offspring compared to previously described methods.

CONCLUSION

Therefore, a new suitable methodology for biotechnological investigations in aquaculture was established, with a high potential to improve the production of commercially valuable fish, generate transgenic animals and preserve endangered fish species.

Spermatogonial transplantation in fish: A novel method for the preservation of genetic resources

Recent progress in genome-based breeding has created various fish strains carrying desirable genetic traits; however, methods for the long-term preservation of their genetic resources have not yet been developed, mainly due to the lack of cryopreservation techniques for fish eggs and embryos. Recently, we established an alternative cryopreservation technique for fish spermatogonia using a slow-freezing method. Furthermore, we developed a transplantation system to produce functional eggs and sperm derived from spermatogonia. Spermatogonia isolated from the testes of vasa-green fluorescent protein (Gfp) transgenic rainbow trout (Oncorhynchus mykiss) were transplanted into the peritoneal cavity of triploid masu salmon (Oncorhynchus masou) hatchlings of both genders. The transplanted trout spermatogonia migrated towards the gonadal anlagen of the recipient salmon, into which they were subsequently incorporated. We confirmed that the donor-derived spermatogonia resumed gametogenesis, and produced sperm and eggs in male and female recipient salmon, respectively. Fertilization of the resultant eggs and sperm produced only rainbow trout in the first filial (F₁) generation, suggesting that the sterile triploid recipient salmon produced functional eggs and sperm derived from the trout donors. A combination of spermatogonial transplantation and cryopreservation could be a powerful tool for preserving valuable fish strains with desirable genetic traits and endangered species.

Production of fertile zebrafish (Danio rerio) possessing germ cells (gametes) originated from primordial germ cells recovered from vitrified embryos

This study aimed to produce fertile zebrafish (Danio rerio) possessing germ cells (gametes) that originated from cryopreserved primordial germ cells (PGCs). First, to improve the vitrification procedure of PGCs in segmentation stage embryos, dechorionated yolk-intact and yolk-removed embryos, the PGCs of which were labeled with green fluorescent protein, were cooled rapidly after serial exposures to equilibration solution (ES) and vitrification solution (VS), which contained ethylene glycol, DMSO, and sucrose. Yolk removal well prevented ice formation in the embryos during cooling and improved the viability of cryopreserved PGCs. The maximum recovery rate of live PGCs in the yolk-removed embryos vitrified after optimum exposure to ES and VS was estimated to be about 90%, and about 50% of the live PGCs showed pseudopodial movement. Next, to elucidate the ability of cryopreserved PGCs to differentiate into functional gametes, PGCs recovered from the yolk-removed embryos (striped-type) that were vitrified under the optimum exposure to ES and VS were transplanted individually into 218 sterilized recipient blastulae (golden-type). Two days after the transplantation, 7.5% (14/187) of morphologically normal embryos had PGC(s) in the genital ridges. Six (5 males and 1 female) of the 14 recipient embryos developed into mature fish and generated progeny with characteristics inherited from PGC donors. In conclusion, we demonstrated the successful cryopreservation of PGCs by vitrification of yolk-removed embryos and the production of fertile zebrafish possessing germ cells that originated from the PGCs in vitrified embryos.

An overview of functional and stereological evaluation of spermatogenesis and germ cell transplantation in fish

Although there are almost thirty-thousand species of fish living in a great variety of habitats and utilizing vast reproductive strategies, our knowledge of morphofunctional and quantitative aspects of testis structure and spermatogenesis is still incipient for this group of vertebrates. In this review, we discuss aspects that are important to better understanding of testis structure and function, and of the development of germ cells (GC) during spermatogenesis. To achieve this, we have recently completed a number of studies presenting morphometric and functional data related to the numbers of GC and Sertoli cells (SC) per each type of spermatogenic cyst, the number of spermatogonial generations, the SC efficiency, and the magnitude of GC loss that normally occurs during spermatogenesis. We also investigated SC proliferation and the relationship of this important event to early spermatogenic cysts. The available data strongly suggest that SC proliferation in sexually mature tilapia is the primary factor responsible for the increase in testis size and for determination of the magnitude of sperm production. The influence of temperature on the duration of spermatogenesis in tilapia was also evaluated and we have used this knowledge to deplete endogenous spermatogenesis in this teleost, in order to develop an experimental system for GC transplantation. This exciting technique results in new possibilities for investigation of spermatogenesis and spermatogonial stem cell biology, creating also an entirely new and promising scenario in biotechnology-transgenic animal production and the preservation of the genetic stocks of valuable animals or endangered species.

Experimentally induced depletion of germ cells in sub-adult Patagonian pejerrey (Odontesthes hatcheri)

Germ cell (GC) transplantation (GCT) is a novel reproductive technology with application in seed production and conservation of endangered species. This study examined the suitability of treatment with Busulfan, a cytotoxic agent, and warm water, known to cause GC degeneration, for depletion of endogenous GCs in sub-adult Patagonia pejerrey Odontesthes hatcheri intended as hosts in GCT. In two experiments, fish were treated with six combinations of temperature (intermediate and high, 20 and 25 degrees C, respectively) and Busulfan (0, 20, and 40 mg/kg body weight), given intraperitoneally (ip) as a single (0 week) or repeated (0 and 4 week) dose. The effectiveness of the treatments was assessed by gonado-somatic index, histology, and (germ cell-specific) vasa gene expression after 8 weeks. Fish were allowed to recover at 17 degrees C for 4-8 weeks after the treatments to ascertain the permanency of the effects. The high temperature (25 degrees C) alone induced only incipient gonadal degeneration and germ cell loss, but was highly effective in combination with double administration of 40 mg/kg Busulfan. Males tolerated Busulfan better and were more easily depleted of germ cells than females. Animals treated for 8 weeks were severely devoid of germ cells, but were still capable of gametogenesis. Thus, the combination of Busulfan and high water temperature appeared to be efficient for depletion of GCs in adult fish; and the treated gonads retained the ability to support GC proliferation and differentiation. Furthermore, quantitative analysis of vasa transcript levels was found to be an useful to monitor the degree of gonad sterility during treatment.

Novel gamete storage

The aim of this review is to outline recent advances in gamete storage that are beneficial for rescuing endangered species or for the breeding of companion animals. Much more information is available on the technical resolutions and practical applications of sperm cryopreservation in various species than of female gametes, reproductive tissues or organs. Mammalian sperm cryopreservation often works relatively efficiently; however, the ability of female gametes to be cryopreserved and still be viable for fertilisation is also essential for rescuing endangered species. For a proper evaluation of gamete cryopreservation possibilities in a given species, it is essential to understand the basic mechanism affecting the survival of cryopreserved cells, the technical and physical limitations, the available techniques and the new avenues to resolve the specific problems in that species. This paper is aimed to provide some help for this process. The limited length of this paper resulted in the omission of information on many important areas, including most data on teleosts, amphibian and insect cryopreservation.

Manipulation of Fish Germ Cell: Visualization, Cryopreservation and Transplantation

Germ-cell transplantation has many applications in biology and animal husbandry, including investigating the complex processes of germ-cell development and differentiation, producing transgenic animals by genetically modifying germline cells, and creating broodstock systems in which a target species can be produced from a surrogate parent. The germ-cell transplantation technique was initially established in chickens using primordial germ cells (PGCs), and was subsequently extended to mice using spermatogonial stem cells. Recently, we developed the first germ-cell transplantation system in lower vertebrates using fish PGCs and spermatogonia. During mammalian germ-cell transplantation, donor spermatogonial stem cells are introduced into the seminiferous tubules of the recipient testes. By contrast, in the fish germ-cell transplantation system, donor cells are microinjected into the peritoneal cavities of newly hatched embryos; this allows the donor germ cells to migrate towards, and subsequently colonize, the recipient genital ridges. The recipient embryos have immature immune systems, so the donor germ cells can survive and even differentiate into mature gametes in their allogeneic gonads, ultimately leading to the production of normal offspring. In addition, implanted spermatogonia can successfully differentiate into sperm and eggs, respectively, in male and female recipients. The results of transplantation studies in fish are improving our understanding of the development of germ-cell systems during vertebrate evolution.