Biotechnology applied to fish reproduction: tools for conservation

Advancing aquaculture: Production of xenogenic catfish by transplanting blue catfish (Ictalurus furcatus) and channel catfish (I. punctatus) stem cells into white catfish (Ameiurus catus) triploid fry

Xenogenesis has been recognized as a prospective method for producing channel catfish, Ictalurus punctatus ♀ × blue catfish, I. furcatus ♂ hybrids. The xenogenesis procedure can be achieved by transplanting undifferentiated stem cells derived from a donor fish into a sterile recipient. Xenogenesis for hybrid catfish embryo production has been accomplished using triploid channel catfish as a surrogate. However, having a surrogate species with a shorter maturation period, like white catfish (Ameiurus catus), would result in reduced feed costs, labor costs, and smaller body size requirements, making it a more suitable species for commercial applications where space is limited, and as a model species. Hence, the present study was conducted to assess the effectiveness of triploid white catfish as a surrogate species to transplant blue catfish stem cells (BSCs) and channel catfish stem cells (CSCs). Triploid white catfish fry were injected with either BSCs or CSCs labeled with PKH 26 fluorescence dye from 0 to 12 days post hatch (DPH). No significant differences in weight and length of fry were detected among BSCs and CSCs injection times (0 to 12 DPH) when fry were sampled at 45 and 90 DPH (P > 0.05). The highest survival was reported when fry were injected between 4.0 to 5.5 DPH (≥ 81.2%). At 45 and 90 DPH, cell and cluster area increased for recipients injected from 0 to 5.2 DPH, and the highest cluster area values were reported between 4.0 to 5.2 DPH. Thereafter, fluorescent cell and cluster area in the host declined with no further decrease after 10 DPH. At 45 DPH, the highest percentage of xenogens were detected when fry were injected with BSCs between 4.0 to 5.0 and CSCs between 3.0 to 5.0 DPH. At 90 DPH, the highest number of xenogens were detected from 4.0 to 6.0 DPH when injected with either BSCs or CSCs. The current study demonstrated the suitability of white catfish as a surrogate species when BSCs and CSCs were transplanted into triploid white catfish between 4.0 to 6.0 DPH (27.4 ± 0.4°C). Overall, these findings allow enhanced efficiency of commercializing xenogenic catfish carrying gametes of either blue catfish or channel catfish.

Modeling the SDF-1/CXCR4 protein using advanced artificial intelligence and antagonist screening for Japanese anchovy

SDF-1/CXCR4 chemokine signaling are indispensable for cell migration, especially the Primordial Germ Cell (PGC) migration towards the gonadal ridge during early development. We earlier found that this signaling is largely conserved in the Japanese anchovy (Engraulis japonicus, EJ), and a mere treatment of CXCR4 antagonist, AMD3100, leads to germ cell depletion and thereafter gonad sterilization. However, the effect of AMD3100 was limited. So, in this research, we scouted for CXCR4 antagonist with higher potency by employing advanced artificial intelligence deep learning-based computer simulations. Three potential candidates, AMD3465, WZ811, and LY2510924, were selected and in vivo validation was conducted using Japanese anchovy embryos. We found that seven transmembrane motif of EJ CXCR4a and EJ CXCR4b were extremely similar with human homolog while the CXCR4 chemokine receptor N terminal (PF12109, essential for SDF-1 binding) was missing in EJ CXCR4b. 3D protein analysis and cavity search predicted the cavity in EJ CXCR4a to be five times larger (6,307 Å³) than that in EJ CXCR4b (1,241 Å³). Docking analysis demonstrated lower binding energy of AMD3100 and AMD3465 to EJ CXCR4a (Vina score -9.6) and EJ CXCR4b (Vina score -8.8), respectively. Furthermore, we observed significant PGC mismigration in microinjected AMD3465 treated groups at 10, 100 and 1 × 105 nM concentration in 48 h post fertilized embryos. The other three antagonists showed various degrees of PGC dispersion, but no significant effect compared to their solvent control at tested concentrations was observed. Cumulatively, our results suggests that AMD3645 might be a better candidate for abnormal PGC migration in Japanese anchovy and warrants further investigation.

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.

Establishment of a Coilia nasus Spermatogonial Stem Cell Line Capable of Spermatogenesis In Vitro

The process by which spermatogonial stem cells (SSCs) continuously go through mitosis, meiosis, and differentiation to produce gametes that transmit genetic information is known as spermatogenesis. Recapitulation of spermatogenesis in vitro is hindered by the challenge of collecting spermatogonial stem cells under long-term in vitro culture conditions. Coilia nasus is a commercially valuable anadromous migrant fish found in the Yangtze River in China. In the past few decades, exploitation and a deteriorating ecological environment have nearly caused the extinction of C. nasus's natural resources. In the present study, we established a stable spermatogonial stem cell line (CnSSC) from the gonadal tissue of the endangered species C. nasus. The cell line continued to proliferate and maintain stable cell morphology, a normal diploid karyotype, and gene expression patterns after more than one year of cell culture (>80 passages). Additionally, CnSSC cells could successfully differentiate into sperm cells through a coculture system. Therefore, the establishment of endangered species spermatogonial stem cell lines is a model for studying spermatogenesis in vitro and a feasible way to preserve germplasm resources.

Identification and Expressional Analysis of Putative PRDI-BF1 and RIZ Homology Domain-Containing Transcription Factors in Mulinia lateralis

Mollusca represents one of the ancient bilaterian groups with high morphological diversity, while the formation mechanisms of the precursors of all germ cells, primordial germ cells (PGCs), have not yet been clarified in mollusks. PRDI-BF1 and RIZ homology domain-containing proteins (PRDMs) are a group of transcriptional repressors, and PRDM1 (also known as BLIMP1) and PRDM14 have been reported to be essential for the formation of PGCs. In the present study, we performed a genome-wide retrieval in Mulinia lateralis and identified 11 putative PRDMs, all of which possessed an N-terminal PR domain. Expressional profiles revealed that all these prdm genes showed specifically high expression levels in the given stages, implying that all PRDMs played important roles during early development stages. Specifically, Ml-prdm1 was highly expressed at the gastrula stage, the key period when PGCs arise, and was specifically localized in the cytoplasm of two or three cells of blastula, gastrula, or trochophore larvae, matching the typical characteristics of PGCs. These results suggested that Ml-prdm1-positive cells may be PGCs and that Ml-prdm1 could be a candidate marker for tracing the formation of PGCs in M. lateralis. In addition, the expression profiles of Ml-prdm14 hinted that it may not be associated with PGCs of M. lateralis. The present study provides insights into the evolution of the PRDM family in mollusks and offers a better understanding of the formation of PGCs in mollusks.

Identification of germ cells in large yellow croaker (Larimichthys crocea) and yellow drum (Nibea albiflora) using RT-PCR and in situ hybridization analyses

Ocean-caught large yellow croaker (Larimichthys crocea) represents an important germplasm resource for the breeding of this species; however, these fish tend to show poor survival in captivity and would be unsuitable breeding purposes. As an alternative to the use of wild-caught croakers, germ cell transplantation has been proposed using the L. crocea specimens as donors and yellow drum (Nibea albiflora) as recipients. In this regard, the identification of L. crocea and N. albiflora germ cells is an essential prerequisite for establishing a germ cell transplantation protocol for these fish. In this study, we cloned the 3' untranslated regions (UTR) of the vasa, dnd, and nanos2 genes in N. albiflora using the rapid amplification of cDNA ends (RACE) method and then aligned and analyzed the sequences of the corresponding genes in L. crocea and N. albiflora. On the basis of gene sequence differences, we designed species-specific primers and probes for RT-PCR analysis and in situ hybridization. RT-PCR analysis revealed that these species-specific primers exclusively amplified DNA from gonads of the respective species, thus confirming that we had six specific primer pairs that could be used to distinguish the germ cells in L. crocea and N. albiflora. Using in situ hybridization analysis, we established that whereas Lcvasa and Nadnd probes showed high species specificity, the probes for Navasa and Lcdnd showed a less specificity. In situ hybridization using Lcvasa and Nadnd thus enabled us to visualize the germ cells in these two species. Using these species-specific primers and probes, we can reliably distinguish the germ cells of L. crocea and N. albiflora, thereby establishing an effective approach for the post-transplantation identification of germ cells when using L. crocea and N. albiflora as donors and recipients, respectively.

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.

Discovery of Nanos1 and Nanos2/3 as Germ Cell Markers During Scallop Gonadal Development

Nanos are conserved genes involved in germline cell specification and differentiation. However, little is known about the role of different members of Nanos family in germ cell development in mollusks. In the present study, we conducted genome-wide identification of Nanos family in an economically important scallop Patinopecten yessoensis, and detected their expression in adult tissues and during early development. Two Nanos genes (PyNanos1, PyNanos2/3) were identified, both of which have the N-terminal NOT1-interacting motif and C-terminal (CCHC)₂ zinc finger domain. Expression profiles showed that PyNanos1 and PyNanos2/3 were primarily expressed in the gonads, with PyNanos1 being localized in the oogonia, oocytes, and spermatogonia, while PyNanos2/3 being specifically in spermatogonia. The results suggest that PyNanos are germ cell specific and may play crucial roles in gametogenesis in the scallop. PyNanos1 is a maternal gene, which is distributed uniformly at early cleavage, and restricted to 2-3 cell clusters from blastulae to trochophore larvae, suggesting its potential role in the formation of PGCs. Zygotically expressed PyNanos2/3 displayed a similar signal with PyNanos1 in the trochophore larvae, suggesting it may also participate in the formation and/or maintenance of PGCs. This study will benefit germplasm exploitation and conservation in bivalves, and facilitate a better understanding of the evolution of Nanos family and the role of different Nanos in germ cell development in mollusks.

Cryopreservation and Flow Cytometric Analysis of Ovarian Tissue in Murray River Rainbowfish, Melanotaenia fluviatilis

Simple Summary

Freshwater fish populations are in global decline, with many Australian freshwater species expected to become extinct in the next twenty years. The storage of reproductive cells and tissues at extremely cold temperatures in bio-banks known as “Frozen Zoos”, allows for the indefinite storage of genetic material, meaning that in the event of an extinction, we have a genetic blueprint available to produce new individuals and reintroduce a species into the wild. Here we have developed a cryopreservation protocol for the storage of ovarian tissue from the threatened Murray River Rainbowfish. Many Australian freshwater fish species are threatened with extinction, our methodology provides a framework for the conservation of other fish species in Australia and globally.

Abstract

Freshwater fish populations are declining with many small, Australian fish species at risk of extinction within the next twenty-years. Cryopreservation of reproductive cells and tissues makes it possible to reproduce individuals from a species even after they are extinct in the wild. We describe the successful cryopreservation of ovarian tissue in the Murray River Rainbowfish, Melanotaenia fluviatilis (Order: Atheriniformes). Histology showed that oogonia are 13.70 µm ± 1.75 µm in size, stain positive for germ-line marker Vasa, and represent approximately 2.29 ± 0.81% of cells in the ovary. Flow cytometry was used to analyse ovarian cell suspensions, requiring an optimised tissue digestion protocol. We found that 0.25% trypsin with 1.13 mM EDTA produced cell suspensions with the highest viability (76.28 ± 4.64%) and the highest number of cells recovered per gram of tissue (1.2 × 10⁸ ± 4.4 × 10⁷ cells/g). Subsequent sorting of ovarian cell suspensions by flow cytometry increased oogonial cells in suspension from 2.53 ± 1.31% in an unsorted sample to 5.85 ± 4.01% in a sorted sample (p = 0.0346). Cryopreservation of ovarian tissue showed DMSO-treated samples had higher cell viability post-thaw (63.5 ± 18.2%) which was comparable to fresh samples (82.5 ± 7.1%; p = 0.36). Tissue cryopreserved in 2.0 M DMSO had the highest cell viability overall (76.07 ± 3.89%). This protocol could be applied to bio-banking programs for other species in the Melanotaeniidae, and perhaps species in other families and orders of Australian fish.

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.

Heat-induced triploids in Brycon amazonicus: a strategic fish species for aquaculture and conservation

Triploidization plays an important role in aquaculture and surrogate technologies. In this study, we induced triploidy in the matrinxã fish (Brycon amazonicus) using a heat-shock technique. Embryos at 2 min post fertilization (mpf) were heat shocked at 38°C, 40°C, or 42°C for 2 min. Untreated, intact embryos were used as a control. Survival rates during early development were monitored and ploidy status was confirmed using flow cytometry and nuclear diameter analysis of erythrocytes. The hatching rate reduced with heat-shock treatment, and heat-shock treatments at 42°C resulted in no hatching events. Optimal results were obtained at 40°C with 95% of larvae exhibiting triploidy. Therefore, we report that heat-shock treatments of embryos (2 mpf) at 40°C for 2 min is an effective way to induce triploid individuals in B. amazonicus.

Effects of busulfan on somatic cells after inhibiting germ cells in the gonads of the young olive flounder Paralichthys olivaceus

Busulfan is widely used in some species to inhibit germ cell proliferation. This study was conducted to evaluate effects of busulfan on germ and somatic cells in gonads of olive flounder, Paralichthys olivaceus, one of the most economically important mariculture fish species. After intraperitoneal injection with 80 (80B) or 120 (120B) mg/kg busulfan, both gonads were atrophied, and ovaries were discolored with adhesion to the visceral mass. Histological results indicated that germ cells in the gonads were detached, and there was a larger nucleus size and smaller cytoplasmic volume in spermatogonia. Numbers of oocytes and somatic cells in the ovary were both less (P < 0.05), while in the testis, numbers of spermatogonia and somatic cells were markedly lesser and greater, respectively (P < 0.05). In ovaries of the flounder treated with 80B and 120B, relative abundance of vasa and cyp19a1a mRNA transcripts was very small in the cytoplasm of oocytes, while the cyp19a1a transcript was still present in theca cells. In the testis of flounder treated with 80B and 120B, abundance of vasa was markedly less (P < 0.05) with there being very little vasa in spermatogonia and disruption of the spermatogonium structure. In the 80B treatment group, amh was in lesser abundance with there being very little amh in spermatogonia, however, with the 120B treatment there was a large amh abundance in spermatogonium with there being disruption of structure of these germ cells and Sertoli cells. Busulfan, therefore, might inhibit the development of spermatogonia in the flounder testis.

Production of donor-derived eggs after ovarian germ cell transplantation into the gonads of adult, germ cell-less, triploid hybrid fish†

In animals, spermatogonial transplantation in sterile adult males is widely developed; however, despite its utility, ovarian germ cell transplantation is not well developed. We previously showed that the interspecific hybrid offspring of sciaenid was a suitable model for germ cell transplantation studies as they have germ cell-less gonads. However, all these gonads have testis-like characteristics. Here, we tested whether triploidization in hybrid embryos could result in germ cell-less ovary development. Gonadal structure dimorphism and sex-specific gene expression patterns were examined in 6-month-old triploid hybrids (3nHybs). Thirty-one percent of 3nHybs had germ cell-less gonads with an ovarian cavity. cyp19a1a and foxl2, ovarian differentiation-related genes, were expressed in these gonads, whereas dmrt1 and vasa were not expressed, suggesting ovary-like germ cell-less gonad development. Some (26%) 3nHybs had testis-like germ cell-less gonads. Ovarian germ cells collected from homozygous green fluorescent protein (GFP) transgenic blue drum (BD) (Nibea mitsukurii) were transplanted into 6-month-old 3nHybs gonads via the urogenital papilla or oviduct. After 9 months, the recipients were crossed with wild type BD. Among the six 3nHyb recipients that survived, one female and one male produced fertile eggs and motile sperm carrying gfp-specific DNA sequences. Progeny tests revealed that all F1 offspring possessed gfp-specific DNA sequences, suggesting that these recipients produced only donor-derived eggs or sperm. Histological observation confirmed donor-derived gametogenesis in the 3nHyb recipients' gonads. Overall, triploidization reduces male-biased sex differentiation in germ cell-less gonads. We report, for the first time, donor-derived egg production in an animal via direct ovarian germ cell transplantation into a germ cell-less ovary.

Spermatogonial Stem Cells in Fish: Characterization, Isolation, Enrichment, and Recent Advances of In Vitro Culture Systems

Spermatogenesis is a continuous and dynamic developmental process, in which a single diploid spermatogonial stem cell (SSC) proliferates and differentiates to form a mature spermatozoon. Herein, we summarize the accumulated knowledge of SSCs and their distribution in the testes of teleosts. We also reviewed the primary endocrine and paracrine influence on spermatogonium self-renewal vs. differentiation in fish. To provide insight into techniques and research related to SSCs, we review available protocols and advances in enriching undifferentiated spermatogonia based on their unique physiochemical and biochemical properties, such as size, density, and differential expression of specific surface markers. We summarize in vitro germ cell culture conditions developed to maintain proliferation and survival of spermatogonia in selected fish species. In traditional culture systems, sera and feeder cells were considered to be essential for SSC self-renewal, in contrast to recently developed systems with well-defined media and growth factors to induce either SSC self-renewal or differentiation in long-term cultures. The establishment of a germ cell culture contributes to efficient SSC propagation in rare, endangered, or commercially cultured fish species for use in biotechnological manipulation, such as cryopreservation and transplantation. Finally, we discuss organ culture and three-dimensional models for in vitro investigation of fish spermatogenesis.

Production and use of triploid zebrafish for surrogate reproduction

We report for the first time, a comparison of two approaches for artificially induced triploidy in zebrafish (Danio rerio) using cold shock and heat shock treatments. Of the two methods, heat shock treatment proved more effective with a triploid production rate of 100% in particular females. Subsequently, triploid zebrafish larvae were used as recipients for intraperitoneal transplantation of ovarian and testicular cells originating from vas:EGFP strain in order to verify their suitability for surrogate reproduction. Production of donor-derived sperm was achieved in 23% of testicular cell recipients and 16% of ovarian cell recipients, indicating the suitability of triploids as surrogate hosts for germ cell transplantation. Success of the transplantation was confirmed by positive GFP signal detected in gonads of dissected fish and stripped sperm. Germline transmission was confirmed by fertilization tests followed by PCR analysis of embryos with GFP specific primers. Reproductive success of germline chimera triploids evaluated as fertilization rate and progeny development was comparable to control groups.

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.