Dissecting the dynamic cellular transcriptional atlas of adult teleost testis development throughout the annual reproductive cycle

Teleost testis development during the annual cycle involves dramatic changes in cellular compositions and molecular events. In this study, the testicular cells derived from adult black rockfish at distinct stages - regressed, regenerating and differentiating - were meticulously dissected via single-cell transcriptome sequencing. A continuous developmental trajectory of spermatogenic cells, from spermatogonia to spermatids, was delineated, elucidating the molecular events involved in spermatogenesis. Subsequently, the dynamic regulation of gene expression associated with spermatogonia proliferation and differentiation was observed across spermatogonia subgroups and developmental stages. A bioenergetic transition from glycolysis to mitochondrial respiration of spermatogonia during the annual developmental cycle was demonstrated, and a deeper level of heterogeneity and molecular characteristics was revealed by re-clustering analysis. Additionally, the developmental trajectory of Sertoli cells was delineated, alongside the divergence of Leydig cells and macrophages. Moreover, the interaction network between testicular micro-environment somatic cells and spermatogenic cells was established. Overall, our study provides detailed information on both germ and somatic cells within teleost testes during the annual reproductive cycle, which lays the foundation for spermatogenesis regulation and germplasm preservation of endangered species.

Reproductive Characteristics and Suitability of Sterile dead end Knockout Nibe Croaker as a Recipient for Intraperitoneal Germ Cell Transplantation

The use of sterile recipients is crucial for efficiently producing donor-derived offspring through surrogate broodstock technology for practical aquaculture applications. Although knockout (KO) of the dead end (dnd) gene has been used in previous studies as a sterilization method, it has not been reported in marine fish. In this study, nibe croaker was utilized as a model for marine teleosts that produce small pelagic eggs, and the clustered regularly interspaced short palindromic repeat (CRISPR)/CRISPR-associated protein 9 (CRISPR/Cas9) system was utilized to produce dnd KO fish. The F1 generation, which carried a nonsense mutation in the dnd gene, was produced by mating founder individuals with wild-type counterparts. Subsequently, the F2 generation was produced by mating the resulting males and females. Among the F2 generations, 24.0% consisted of homozygous KO individuals. Histological analysis revealed that primordial germ cells (PGCs) were present in homozygous KO individuals at 10 days post-hatching (dph), similar to wild-type individuals. However, by 20 dph, PGCs were absent in KO individuals. Furthermore, no germ cells were observed in the gonads of both sexes of homozygous KO individuals at 6 months old, which is the typical maturity age for wild-type individuals of both sexes. In addition, when cryopreserved donor nibe croaker testicular cells were transplanted, only donor-derived offspring were successfully obtained through the spontaneous mating of homozygous KO recipient parents. Results indicate that dnd KO nibe croaker lacks germ cells and can serve as promising recipients, producing only donor-derived gametes as surrogate broodstock.

The volume and shape of bitterling eggs are more strongly influenced by germ cell autonomy than by the surrounding somatic cells

There is great variation in the size and shape of teleost eggs from species to species. The size of the teleost egg depends on the amount of yolk accumulated in the egg, which is an important factor directly affecting the survival of hatchlings. Egg shape also contributes significantly to spawning ecology and survival during the prehatching stage. In this study, we used bitterlings, which show a wide variety of egg volumes and shapes, to elucidate whether these factors are determined by germ cells or somatic cells. Reciprocal transplantations of germ cells between two bitterling species revealed that the egg volume was identical to that of the germ cell donor species in both combinations. The egg shape was also very similar to that of the species providing the germ cells. These results suggest that the volume and shape of teleost eggs are greatly influenced by germ cell autonomy.

Enhancement of zebrafish sperm production via a large body-sized surrogate with germ cell transplantation

Zebrafish (Danio rerio) is a commonly-used vertebrate model species for many research areas. However, its low milt volume limits effective cryopreservation of sperm from a single individual and often precludes dividing a single semen sample to conduct multiple downstream procedures such as genomic DNA/RNA extraction and in-vitro fertilization. Here, we apply germ stem cell transplantation to increase zebrafish sperm production in a closely related larger species from the same subfamily, giant danio Devario aequipinnatus. The endogenous germ cell of the host is depleted by dead-end morpholino antisense oligonucleotide. Histology of the sterile gonad and quantitative PCR of gonadal tissue reveals all sterile giant danio develop the male phenotype. Spermatogonial cells of Tg(ddx4:egfp) transgenic zebrafish are transplanted into sterile giant danio larvae, and 22% of recipients (germline chimera) produce donor-derived sperm at sexual maturation. The germline chimera produce approximately three-fold the volume of sperm and 10-fold the spermatozoon concentration of the donor. The donor-derived sperm is functional and gives rise to viable progeny upon fertilization of donor oocytes. We show that the issue of low milt volume can be effectively addressed by employing a larger surrogate parent.

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

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

Production of Germ Cell-Less Rainbow Trout by dead end Gene Knockout and their Use as Recipients for Germ Cell Transplantation

In germ cell transplantation experiments, the use of sterile recipients that do not produce their own gametes is an important prerequisite. Triploidization and dnd gene knockdown (KD) methods have been widely used to produce sterile fish. However, triploidization does not produce complete sterility in some fish species, and gene KD is labor and time intensive since it requires microinjection into individual fertilized eggs. To overcome these problems, in this study, we generated homozygous mutants of the dead end (dnd) gene in rainbow trout (Oncorhynchus mykiss) using the clustered regulatory interspaced short palindromic repeats/CRISPR-associated protein 9 (CRISPR/Cas9) system, analyzed their reproductive capacity, and evaluated their suitability as recipients for germ cell transplantation. By crossing F1 heterozygous mutants produced from founders subjected to genome editing, an F2 generation consisting of approximately 1/4 homozygous knockout mutants (dnd KO) was obtained. The dnd KO hatchlings retained the same number of primordial germ cells (PGCs) as the wild-type (WT) individuals, after which the number gradually decreased. At 1 year of age, germ cells were completely absent in all analyzed individuals. To evaluate the dnd KO individuals as recipients for germ cell transplantation, germ cells prepared from donor individuals were transplanted into the abdominal cavity of dnd KO hatchlings. These cells migrated to the recipient gonads, where they initiated gametogenesis. The mature recipient individuals produced only donor-derived sperm and eggs in equivalent numbers to WT rainbow trout. These results indicate that dnd KO rainbow trout are suitable recipient candidates possessing a high capacity to nurse donor-derived germ cells.

Characterization of a vasa homolog in Mekong giant catfish (Pangasianodon gigas): Potential use as a germ cell marker

For the long-term preservation of the genetic resources of endangered fish species, a combination of germ cell cryopreservation and transplantation can be an effective technique. To optimize these techniques, it is important to identify undifferentiated germ cells possessing transplantability, such as primordial germ cells, type A spermatogonia (ASGs), and oogonia. In this study, a homolog of vasa cDNA in Mekong giant catfish (MGC-vasa) (Pangasianodon gigas), which is an endangered species inhabiting the Mekong river, was cloned and characterized for use as a putative germ cell marker. Results indicate that MGC-Vasa contained all of the consensus motifs, including the arginine-glycine and arginine-glycine-glycine motifs, as well as the nine conserved motifs belonging to the DEAD-box family of proteins. Results from phylogenetic analysis indicated MGC-vasa also grouped with Vasa and was clearly distinguishable from Pl10 in other teleosts. Results from analysis of abundance of mRNA transcripts using reverse transcription-polymerase chain reaction and in situ hybridization performed on immature Mekong giant catfish testis indicated vasa was present specifically in germ cells, with large abundances of the relevant mRNA in spermatogonia and spermatocytes. Sequence similarity and the specific localization of MGC-vasa in these germ cells suggest that the sequence ascertained in this study was a vasa homolog in Mekong giant catfish. Furthermore, vasa-positive cells were detected in prepared smears of testicular cells, indicating that it may be a useful germ cell marker for enzymatically dissociated cells used for transplantation studies.

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

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

Gonadal sex differentiation and early ovarian/testicular development in cultured Pacific bluefin tuna, Thunnus orientalis (Temminck et Schlegel)

Pacific bluefin tuna (PBT), Thunnus orientalis, is one of the most important species for aquaculture in Japan. Recently, the reduction in muscle fat content associated with sexual maturation in farmed PBT has become a serious problem. To develop technologies for inducing sterility, detailed and reliable data on gonadal development in PBT are needed. Here, we demonstrated the process of gonadal sex differentiation, and of early ovarian and testicular development during the immature stages in PBT. Gonadal sex differentiation was first characterized by the formation of the ovarian cavity in female and of the efferent ducts in male 57 days post hatching (dph). The gonads then differentiated into ovaries or testes according to the genotypic sex until 83 dph. During this period, primordial germ cells, oogonia, and type-A spermatogonia were solitarily distributed in the gonads, and the number of germ cells did not differ between sexes. After gonadal sex differentiation, gonads of PBTs developed in a sexually dimorphic manner: proliferation and differentiation of germ cells occurred earlier in the ovaries than in the testes. The oogonia in ovaries formed cysts at 185 dph, but the type-A spermatogonia were solitarily distributed in testes at this stage, and cysts of type-A spermatogonia were first observed at 247 dph. Moreover, the oogonia entered meiosis and differentiated into chromatin-nucleolus stage oocytes until 247 dph, and subsequently into peri-nucleolus stage oocytes until 285 dph, whereas the type-A spermatogonia differentiated into type-B spermatogonia, spermatocytes, spermatids, and spermatozoa from 446 dph onwards. We believe the results of this study provide the necessary basis for future studies on sterile PBT production.

Allelic diversification after transposable element exaptation promoted gsdf as the master sex determining gene of sablefish

Concepts of evolutionary biology suggest that morphological change may occur by rare punctual but rather large changes, or by more steady and gradual transformations. It can therefore be asked whether genetic changes underlying morphological, physiological, and/or behavioral innovations during evolution occur in a punctual manner, whereby a single mutational event has prominent phenotypic consequences, or if many consecutive alterations in the DNA over longer time periods lead to phenotypic divergence. In the marine teleost, sablefish (Anoplopoma fimbria), complementary genomic and genetic studies led to the identification of a sex locus on the Y Chromosome. Further characterization of this locus resulted in identification of the transforming growth factor, beta receptor 1a (tgfbr1a) gene, gonadal somatic cell derived factor (gsdf), as the main candidate for fulfilling the master sex determining (MSD) function. The presence of different X and Y Chromosome copies of this gene indicated that the male heterogametic (XY) system of sex determination in sablefish arose by allelic diversification. The gsdfY gene has a spatio-temporal expression profile characteristic of a male MSD gene. We provide experimental evidence demonstrating a pivotal role of a transposable element (TE) for the divergent function of gsdfY. By insertion within the gsdfY promoter region, this TE generated allelic diversification by bringing cis-regulatory modules that led to transcriptional rewiring and thus creation of a new MSD gene. This points out, for the first time in the scenario of MSD gene evolution by allelic diversification, a single, punctual molecular event in the appearance of a new trigger for male development.

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.

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.

Suitability of hybrid mackerel (Scomber australasicus × S. japonicus) with germ cell-less sterile gonads as a recipient for transplantation of bluefin tuna germ cells

We aim to establish a small-bodied surrogate broodstock, such as mackerel, which produces functional bluefin tuna gametes by spermatogonial transplantation. When reproductively fertile fish are used as recipients, endogenous gametogenesis outcompetes donor-derived gametogenesis, and recipient fish predominantly produce their gametes. In this study, we assessed fertility of hybrid mackerel, Scomber australasicus × S. japonicus, and its suitability as a recipient for transplantation of bluefin tuna germ cells. Hybrid mackerel were produced by artificially inseminating S. australasicus eggs with S. japonicus spermatozoa. Cellular DNA content and PCR analyses revealed that F1 offspring were diploid carrying both paternal and maternal genomes. Surprisingly, histological observations found no germ cells in hybrid mackerel gonads at 120 days post-hatch (dph), although they were present in the gonad of 30- and 60-dph hybrid mackerel. The frequency of germ cell-less fish was 100% at 120-dph, 63.1% at 1-year-old, and 81.8% at 2-year-old. We also confirmed a lack of expression of germ cell marker (DEAD-box helicase 4, ddx4) in the germ cell-less gonads of hybrid mackerel. By contrast, expression of Sertoli cell marker (gonadal soma-derived growth factor, gsdf) and of Leydig cell marker (steroid 11-beta-hydroxlase, cyp11b1) were clearly detected in hybrid mackerel gonads. Together these results showed that most of the hybrid gonads were germ cell-less sterile, but still possessed supporting cells and steroidogenic cells, both of which are indispensable for nursing donor-derived germ cells. To determine whether hybrid gonads could attract and incorporate donor bluefin tuna germ cells, testicular cells labeled with PKH26 fluorescent dye were intraperitoneally transplanted. Fluorescence observation of hybrid recipients at 14 days post-transplantation revealed that donor cells had been incorporated into the recipient's gonads. This suggests that hybrid mackerel show significant promise for use as a recipient to produce bluefin tuna gametes.

Production of Chinese rosy bitterling offspring derived from frozen and vitrified whole testis by spermatogonial transplantation

Bitterling is a small cyprinid fish facing an increasing risk of extinction owing to habitat destruction and decreasing freshwater mussel population that are used as their spawning substrates. Owing to their large size and high yolk contents, methods for cryopreservation of their eggs or embryos, which is a promising method for long-term preservation of their genetic resources, are still not available. We conducted this study to evaluate the feasibility of gamete production by transplanting cryopreserved testicular cells into germ cell-less recipients that were produced by knockdown of dead end gene. Immature testes isolated from recessive albino Chinese rosy bitterlings were cryopreserved by slow freezing or vitrification. Approximately 3000 slow-frozen or vitrified cells were transplanted into the peritoneal cavity of 4-day-old germ cell-less wild-type Chinese rosy bitterlings. We observed no significant differences in the incorporation rates of the slow-frozen and vitrified cells into the genital ridges of recipients compared with those of freshly prepared cells. When the recipients matured, almost half of the male or female recipients that received freshly prepared, slow-frozen, or vitrified cells produced gametes derived from donor cells, with no significant differences in their fecundity among the 3 groups. Moreover, fertilization of the resulting eggs and sperm produced donor-derived offspring exhibiting the albino phenotype. Therefore, the abovementioned methods could be used as a powerful and practical method for long-term preservation of bitterling genetic resources for biotic conservation.

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.

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.