Hybrid Sterility in Fish Caused by Mitotic Arrest of Primordial Germ Cells

Embryonic Lethality, Juvenile Growth Variation, and Adult Sterility Correlate With Phylogenetic Distance of Danionin Hybrids

Hybrid incompatibility, which plays a pivotal role in speciation, is expected to correlate with greater phylogenetic distance. Here, we investigate the fitness of interspecies hybrids within the Danionin subfamily, which includes the model species, Danio rerio, and its relatives - Danio kyathit, Danio albolineatus, Danio margaritatus, and Devario aequipinnatus. We generated hybrids through in vitro fertilization, using Danio rerio as the maternal species, with normal fertilization rates showing no incompatibilities in sperm-egg interactions within these two genera. Generally, all hybrids exhibit normal patterns and timelines in early developmental transitions, from cleavage stages to the initiation of epiboly, although inter-genera Danio-Devario hybrids subsequently exhibit fully penetrant embryonic lethality. Intra-genus Danio hybrids, on the other hand, can survive through embryogenesis and into adulthood. However, rates of survival during these stages diminish according to phylogenetic distance, with increasing early lethality in hybrids from more distantly related species. Additionally, Danio hybrids exhibit increased growth rate variability during juvenile stages. All Danio hybrids have reduced testes sizes, sperm counts, and sperm viabilities, with sperm displaying defects in flagellum formation and integrity. Adult male intra-genus hybrids are invariably sterile, except in the case of Danio rerio hybrids with the closely related Danio kyathit, which produced a backcrossed F2 generation that did not survive juvenile stages. Our studies highlight a loss of hybrid compatibility at various life stages in the Danio and Devario genera, based on deleterious effects and reduced developmental robustness, emphasizing a correlation between the severity of incompatibility outcomes and the degree of phylogenetic relatedness.

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.

Single-cell RNA sequencing revealed the roles of macromolecule epidermal growth factor receptor (EGFR) in the hybrid sterility of hermaphroditic Argopecten scallops

The macromolecule epidermal growth factor receptor (EGFR) is a transmembrane glycoprotein that belongs to the protein kinase superfamily, which plays versatile functions in cell proliferation, development and fertility regulation. Almost all F1 hybrids obtained from the hermaphroditic bay scallops and Peruvian scallops exhibit infertility, and the genetic mechanism remains unclear. In this study, the comprehensive scRNA-seq was first conducted in the gonads of hybrid scallops, deducing the developmental sequence of germ cells and identifying the critical regulators in hybrid sterility: epidermal growth factor receptor. During the development from oogenesis phase germ cells to oocytes, the expression of the EGFR gene gradually decreased in sterile hybrids but increased in fertile hybrids. The significantly lower EGFR expression and ATP content, but higher ROS production rate was detected in the gonad of sterile hybrids than that in fertile hybrids, which might cause slow development of oocytes, stagnation of cell cycle, insufficient energy supply, high level of apoptosis and final sterility. Specific knock-down of EGFR gene led to decreased ATP content, increased ROS production rate, and inhibited oocyte maturation and gonadal development. These findings provide new insights into the roles of EGFR in hybrid infertility of bivalves and the healthy development of scallop breeding.

Genome-wide sequence divergence of satellite DNA could underlie meiotic failure in male hybrids of bighead catfish and North African catfish (Clarias, Clariidae)

Hybrid sterility, a hallmark of postzygotic isolation, arises from parental genome divergence disrupting meiosis. While chromosomal incompatibility is often implicated, the underlying mechanisms remain unclear. This study investigated meiotic behavior and genome-wide divergence in bighead catfish (C. macrocephalus), North African catfish (C. gariepinus), and their sterile male hybrids (important in aquaculture). Repetitive DNA analysis using bioinformatics and cytogenetics revealed significant divergence in satellite DNA (satDNA) families between parental species. Notably, one hybrid exhibited successful meiosis and spermatozoa production, suggesting potential variation in sterility expression. Our findings suggest that genome-wide satDNA divergence, rather than chromosome number differences, likely contributes to meiotic failure and male sterility in these catfish hybrids.

Integrated miRNA and mRNA Sequencing Reveals the Sterility Mechanism in Hybrid Yellow Catfish Resulting from Pelteobagrus fulvidraco (♀) × Pelteobagrus vachelli (♂)

The hybrid yellow catfish exhibits advantages over pure yellow catfish in terms of fast growth, fast development, a high feeding rate, and strong immunity; additionally, it is almost sterile, thus ensuring the conservation of the genetic stock of fish populations. To investigate the sterility mechanism in hybrid yellow catfish (P. fulvidraco (♀) × P. vachelli (♂)), the mRNA and miRNA of the gonads of P. fulvidraco, P. vachelli, and a hybrid yellow catfish were analyzed to characterize the differentially expressed genes; this was carried out to help establish gene expression datasets to assist in the further determination of the mechanisms of genetic sterility in hybrid yellow catfish. In total, 1709 DEGs were identified between the hybrid and two pure yellow catfishes. A KEGG pathway analysis indicated that several genes related to reproductive functions were upregulated, including those involved in the cell cycle, progesterone-mediated oocyte maturation, and oocyte meiosis, and genes associated with ECM-receptor interaction were downregulated. The spermatogenesis-related GO genes CFAP70, RSPH6A, and TSGA10 were identified as being downregulated DEGs in the hybrid yellow catfish. Sixty-three DEmiRNAs were identified between the hybrid and the two pure yellow catfish species. The upregulated DEmiRNAs ipu-miR-194a and ipu-miR-499 were found to target the spermatogenesis-related genes CFAP70 and RSPH6A, respectively, playing a negative regulatory role, which may underscore the miRNA-mRNA regulatory mechanism of sterility in hybrid yellow catfish. The differential expression of ipu-miR-196d, ipu-miR-125b, and ipu-miR-150 and their target genes spidr, cep85, and kcnn4, implicated in reproductive processes, was verified via qRT-PCR, consistent with the transcriptome sequencing expression trends. This study provides deep insights into the mechanism of hybrid sterility in vertebrate groups, thereby contributing to achieving a better understanding and management of fish conservation related to hybrid sterility.

Karyotypic changes and diversification time in Epinephelidae groupers (Perciformes). Implications on reproductive isolation

Groupers (Epinephelidae and Serranidae) have attracted special attention to fish farming, and their species offer good opportunities for successful hybridizations. Cytogenetic data allow a better understanding of the role of karyotypic diversification in the acquisition of post-zygotic reproductive isolation (RI). Thus, chromosomal analyses were performed on E. striatus (Caribbean Sea), E. coioides and E. tauvina (Indo-Pacific Region), using standard procedures and mapping of six repetitive DNA classes by the in situ hybridization. The three species have 2n=48 chromosomes. The karyotypes of E. coioides and E. striatus are composed only of acrocentric chromosomes (FN=48), while E. tauvina has 8 submetacentric chromosomes (FN=56). Heterochromatin has a preferential centromeric distribution, and the microsatellite repeats are dispersed throughout the chromosomes of all species. The 18S and 5S rDNA sites are unique but show a colocalization arrangement in E. tauvina and E. striatus. The chromosomal organization suggests that the three species still maintain a significant amount of syntenic regions. The range of the karyotype divergence and the RI levels showed low, but goes turn proportionally greater in relation to the divergence time between the parental species. The slow acquisition of postzygotic RI is consistent with the high karyotype homogeneity presented by Epinephelidae family.

Genome-wide comparative methylation analysis reveals the fate of germ stem cells after surrogate production in teleost

BACKGROUND

Surrogate production by germline stem cell transplantation is a powerful method to produce donor-derived gametes via a host, a practice known as surrogacy. The gametes produced by surrogates are often analysed on the basis of their morphology and species-specific genotyping, which enables conclusion to be drawn about the donor's characteristics. However, in-depth information, such as data on epigenetic changes, is rarely acquired. Germ cells develop in close contact with supporting somatic cells during gametogenesis in vertebrates, and we hypothesize that the recipient's gonadal environment may cause epigenetic changes in produced gametes and progeny. Here, we extensively characterize the DNA methylome of donor-derived sperm and their intergenerational effects in both inter- and intraspecific surrogates.

RESULTS

We found more than 3000 differentially methylated regions in both the sperm and progeny derived from inter- and intraspecific surrogates. Hypermethylation in the promoter regions of the protocadherin gamma gene in the intraspecific surrogates was found to be associated with germline transmission. On the contrary, gene expression level and the embryonic development of the offspring remained unaffected. We also discovered MAPK/p53 pathway disruption in interspecific surrogates due to promoter hypermethylation and identified that the inefficient removal of meiotic-arrested endogenous germ cells in hybrid gonads led to the production of infertile spermatozoa.

CONCLUSIONS

Donor-derived sperm and progeny from inter- and intraspecific surrogates were more globally hypermethylated than those of the donors. The observed changes in DNA methylation marks in the surrogates had no significant phenotypic effects in the offspring.

Production of all-male non-transgenic zebrafish by conditional primordial germ cell ablation

Many fish species exhibit remarkable sexual dimorphism, with males possessing numerous advantageous traits for commercial production by aquaculture such as faster growth rate, more efficient food energy utilization for muscle development, and better breeding performance. Several studies have shown that a decrease in the number of primordial germ cells (PGCs) during early development leads predominantly to male progeny. In this study, we developed a method to obtain all-male zebrafish (Danio rerio) by targeted PGC ablation using the nitroreductase/metronidazole (NTR/Mtz) system. Embryos generated by female heterozygous Tg(nanos3:nfsB-mCherry-nanos3 3'UTR) and male wild-types (WTs) were treated with vehicle or Mtz. Compared to vehicle-treated controls, 5.0 and 10.0 mM Mtz treatment for 24 h significantly reduced the number of PGCs and yielded an exclusively male phenotype in adulthood. The gonads of offspring treated with 5.0 mM Mtz exhibited relatively normal morphology and histological characteristics. Furthermore, these males were able to chase females, spawn, and produce viable offspring, while about 20.0% of males treated with 10.0 mM Mtz were unable to produce viable offspring. The 5.0 mM Mtz treatment protocol may thus be suitable for large-scale production of fertile male offspring. Moreover, about half of these males were WT as evidenced by the absence of nfsB gene expression. It may thus be possible to breed an all-male WT fish population by Mtz-mediated PGC ablation.

Genetic and karyotype divergence between parents affect clonality and sterility in hybrids

Asexual reproduction can be triggered by interspecific hybridization, but its emergence is supposedly rare, relying on exceptional combinations of suitable genomes. To examine how genomic and karyotype divergence between parental lineages affect the incidence of asexual gametogenesis, we experimentally hybridized fishes (Cobitidae) across a broad phylogenetic spectrum, assessed by whole exome data. Gametogenic pathways generally followed a continuum from sexual reproduction in hybrids between closely related evolutionary lineages to sterile or inviable crosses between distant lineages. However, most crosses resulted in a combination of sterile males and asexually reproducing females. Their gametes usually experienced problems in chromosome pairing, but females also produced a certain proportion of oocytes with premeiotically duplicated genomes, enabling their development into clonal eggs. Interspecific hybridization may thus commonly affect cell cycles in a specific way, allowing the formation of unreduced oocytes. The emergence of asexual gametogenesis appears tightly linked to hybrid sterility and constitutes an inherent part of the extended speciation continuum.

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.

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.

Delete and survive: strategies of programmed genetic material elimination in eukaryotes

Genome stability is a crucial feature of eukaryotic organisms because its alteration drastically affects the normal development and survival of cells and the organism as a whole. Nevertheless, some organisms can selectively eliminate part of their genomes from certain cell types during specific stages of ontogenesis. This review aims to describe the phenomenon of programmed DNA elimination, which includes chromatin diminution (together with programmed genome rearrangement or DNA rearrangements), B and sex chromosome elimination, paternal genome elimination, parasitically induced genome elimination, and genome elimination in animal and plant hybrids. During programmed DNA elimination, individual chromosomal fragments, whole chromosomes, and even entire parental genomes can be selectively removed. Programmed DNA elimination occurs independently in different organisms, ranging from ciliate protozoa to mammals. Depending on the sequences destined for exclusion, programmed DNA elimination may serve as a radical mechanism of dosage compensation and inactivation of unnecessary or dangerous genetic entities. In hybrids, genome elimination results from competition between parental genomes. Despite the different consequences of DNA elimination, all genetic material destined for elimination must be first recognised, epigenetically marked, separated, and then removed and degraded.

Unraveling patterns of disrupted gene expression across a complex tissue

Whole tissue RNASeq is the standard approach for studying gene expression divergence in evolutionary biology and provides a snapshot of the comprehensive transcriptome for a given tissue. However, whole tissues consist of diverse cell types differing in expression profiles, and the cellular composition of these tissues can evolve across species. Here, we investigate the effects of different cellular composition on whole tissue expression profiles. We compared gene expression from whole testes and enriched spermatogenesis populations in two species of house mice, Mus musculus musculus and M. m. domesticus, and their sterile and fertile F1 hybrids, which differ in both cellular composition and regulatory dynamics. We found that cellular composition differences skewed expression profiles and differential gene expression in whole testes samples. Importantly, both approaches were able to detect large-scale patterns such as disrupted X chromosome expression, although whole testes sampling resulted in decreased power to detect differentially expressed genes. We encourage researchers to account for histology in RNASeq and consider methods that reduce sample complexity whenever feasible. Ultimately, we show that differences in cellular composition between tissues can modify expression profiles, potentially altering inferred gene ontological processes, insights into gene network evolution, and processes governing gene expression evolution.

Interaction between sex-determining genes from two species: clues from Xenopus hybrids

Hybrids provide an interesting model to study the evolution of sex-determining genes and sex chromosome systems as they offer the opportunity to see how independently evolving sex-determining pathways interact in vivo. In this context, the genus Xenopus represents a stimulating model, since species with non-homologous sex chromosomes and different sex-determining genes have been identified. In addition, the possibility of interspecies breeding is favoured in this group, which arose by alloploidization events, with species ploidy ranging from 2n = 2x = 20 in X. tropicalis (the only diploid representative of the genus) to 2n = 12x = 108 in X. ruwenzoriensis. To study how two sex-determining genes interact in vivo, X. laevis × X. tropicalis hybrids were produced. Gonadal differentiation in these hybrids revealed that the dm-w gene is dominant over X. tropicalis male-determining sex chromosomes (Y or Z), even though the Y chromosome is dominant in X. tropicalis (Y > W>Z). In the absence of the dm-w gene (the Z chromosome from X. laevis is present), the W chromosome from X. tropicalis is able to trigger ovarian development. Testicular differentiation will take place in the absence of W chromosomes from any of the parental species. The dominance/recessivity relationships between these sex-determining loci in the context of either parental genome remains unknown. This article is part of the theme issue 'Challenging the paradigm in sex chromosome evolution: empirical and theoretical insights with a focus on vertebrates (Part II)'.

Sperm motility analysis of Cobitis hankugensis, Iksookimia longicorpa (Teleostei, Cypriniformes, Cobitidae) and their unisexual natural hybrids

Hybrid sterility is an inevitable phenomenon in the speciation process to avoid indiscriminate increases in species, but it is not always unconditional. We used computer assisted sperm analysis (CASA) to analyze sperm motility of Cobitis hankugensis, Iksookimia longicorpa, and their unisexual natural hybrids. In parental species, the sperm concentrations of C. hankugensis and I. longicorpa were 11.6 ± 4.8 × 10⁹ and 16.5 ± 6.8 × 10⁹ , respectively. For sperm motility, the total motility was higher in the parental species (C. hankugensis, 91.3%; I. longicorpa, 87.5%) than other hybrids. After 1 min, the motility duration was reduced to 14% in C. hankugensis and 3.3% in I. longicorpa. This result could indicate that the duration of sperm motility of C. hankugensis is longer than that of I. longicorpa up to 1 min after spermatozoa activation. All of the hybrids had a low concentration and it was distinct from their parent species. Total motility and other velocity parameters also showed significantly lower values except for the HHL (one from the C. hankugensis genome with two from the I. longicorpa genome) type motility measurement (13.6%). These results suggest that the hybrids derived from C. hankugensis and I. longicorpa, are not completely infertile, contrary to histological observations.

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 functional sperm from cryopreserved testicular germ cells following intraperitoneal transplantation into allogeneic surrogate in yellowtail (Seriola quinqueradiata)

The aim of this study was to establish a method for the cryopreservation of spermatogonia of the yellowtail (Seriola quinqueradiata), which is the most commonly farmed fish in Japan. Testicular cells were prepared by enzymatic dissociation of testicular fragments containing an abundance of type A spermatogonia and were added to cryomedium containing dimethyl sulfoxide (DMSO), ethylene glycol, glycerol, or propylene glycol at concentrations of 0.5-2.5 M. The cells were then frozen and stored in liquid nitrogen for 3 days. After thawing, their survival and transplantability were evaluated. Testicular cells were most successfully cryopreserved in 1.0 M DMSO as indicated by survival of 34% of cells. Furthermore, in situ hybridization using the yellowtail vasa probe showed that these recovered cells contained a similar proportion of germ cells to fresh testicular cells before freezing. Transplantation of the recovered cells into the peritoneal cavities of allogeneic larvae resulted in 94% of surviving recipients having donor-derived germ cells in their gonads after 28 days. Sperm were then collected from seven randomly selected recipients once they reached 2 years of age and used to fertilize wild-type eggs, which led to an average of 26% of the first filial (F1) offspring being derived from donor fish, as confirmed through the use of microsatellite markers. Thus, we successfully cryopreserved yellowtail spermatogonia and produced functional sperm via intraperitoneal transplantation into allogeneic recipients.

Genomic evidence supports the introgression between two sympatric stickleback species inhabiting the White Sea basin

Interspecies hybridization is driven by a complex interplay of factors where introgression plays an important role. In the present study, the transfer of genetic material, between two quite distant fish species from different genera, through spontaneous hybridization was documented with dedicated molecular and bioinformatics tools. We investigate the genomic landscape of putative stickleback-relative introgression by carefully analyzing the tractable transposable elements (TE) on the admixed genome of some individuals of two sympatric stickleback species inhabiting northwestern Russia, namely the three-spined (Gasterosteus aculeatus) and the nine-spined (Pungitius pungitius) sticklebacks. Our data revealed that unique TE amplification types exist, supporting our proposed hypothesis that infers on the interspecific introgression. By running a restriction site-associated DNA sequencing (RAD-Seq) with eight samples of G. aculeatus and P. pungitius and subjecting further the results to a contrasting analysis by variated bioinformatic tools, we identified the related introgression-linked markers. The admixture nature observed in a single sample of the nine-spined stickleback demonstrated the possible traces of remote introgression between these two species. Our work reveals the potential that introgression has on providing particular variants at a high-frequency speed while linking blocks of sequence with multiple functional mutations. However, even though our results are of significant interest, an increased number of samples displaying the introgression are required to further ascertain our conclusions.

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.

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

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

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.

Assessment of Yangtze sturgeon as recipient for the production of American paddlefish gametes through spermatogonia transplantation

The Chinese paddlefish (Psephurus gladius), one of the world's largest freshwater fish, was last seen alive in 2003; they are presumed now to be extinct. In fish, germ cell transplantation is currently known as one of the most powerful assisted reproductive technologies for the conservation of endangered species. In the event that a Chinese paddlefish is unexpectedly caught in the near future, we aimed to develop an experimental strategy to produce paddlefish gametes in the gonads of surrogate sturgeon. Spermatogonia were collected from the testes of 2.5-year-old immature male American paddlefish (Polyodon spathula), the species most closely related to the Chinese paddlefish, by Percoll gradient centrifugation, and transplanted into the peritoneal cavity of Yangtze sturgeon (Acipenser dabryanus) larvae at 7-8 days post-hatch. At two months post-transplantation, donor-derived spermatogonia had efficiently colonized in the recipient gonads and proliferated. A PCR analysis developed to detect xenogenic donor-derived mtDNA sequences in recipient gonads revealed that American paddlefish germ cells survived for at least seven months after transplantation in the gonads of Yangtze sturgeon recipients. These results show that the somatic microenvironment of Yangtze sturgeon gonads was able to support the colonization, proliferation, and survival of xenogeneic germ cells from a different taxonomic family. This study provides key information that could lead to future restoration of Chinese paddlefish using germ cell transplantation.

Developmental potential of somatic and germ cells of hybrids between Carassius auratus females and Hemigrammocypris rasborella males

The cause of hybrid sterility and inviability has not been analyzed in the fin-fish hybrid, although large numbers of hybridizations have been carried out. In this study, we produced allo-diploid hybrids by cross-fertilization between female goldfish (Carassius auratus) and male golden venus chub (Hemigrammocypris rasborella). Inviability of these hybrids was due to breakage of the enveloping layer during epiboly or due to malformation with serious cardiac oedema around the hatching stage. Spontaneous allo-triploid hybrids with two sets of the goldfish genome and one set of the golden venus chub genome developed normally and survived beyond the feeding stage. This improved survival was confirmed by generating heat-shock-induced allo-triploid hybrids that possessed an extra goldfish genome. When inviable allo-diploid hybrid cells were transplanted into goldfish host embryos at the blastula stage, these embryos hatched normally, incorporating the allo-diploid cells. These allo-diploid hybrid cells persisted, and were genetically detected in a 6-month-old fish. In contrast, primordial germ cells taken from allo-diploid hybrids and transplanted into goldfish hosts at the blastula stage had disappeared by 10 days post-fertilization, even under chimeric conditions. In allo-triploid hybrid embryos, germ cells proliferated in the gonad, but had disappeared by 10 weeks post-fertilization. These results showed that while hybrid germ cells are inviable even in chimeric conditions, hybrid somatic cells remain viable.

Parthenogenesis as a Solution to Hybrid Sterility: The Mechanistic Basis of Meiotic Distortions in Clonal and Sterile Hybrids

Hybrid sterility is a hallmark of speciation, but the underlying molecular mechanisms remain poorly understood. Here, we report that speciation may regularly proceed through a stage at which gene flow is completely interrupted, but hybrid sterility occurs only in male hybrids whereas female hybrids reproduce asexually. We analyzed gametogenic pathways in hybrids between the fish species Cobitis elongatoides and C. taenia, and revealed that male hybrids were sterile owing to extensive asynapsis and crossover reduction among heterospecific chromosomal pairs in their gametes, which was subsequently followed by apoptosis. We found that polyploidization allowed pairing between homologous chromosomes and therefore partially rescued the bivalent formation and crossover rates in triploid hybrid males. However, it was not sufficient to overcome sterility. In contrast, both diploid and triploid hybrid females exhibited premeiotic genome endoreplication, thereby ensuring proper bivalent formation between identical chromosomal copies. This endoreplication ultimately restored female fertility but it simultaneously resulted in the obligate production of clonal gametes, preventing any interspecific gene flow. In conclusion, we demonstrate that the emergence of asexuality can remedy hybrid sterility in a sex-specific manner and contributes to the speciation process.

Hybrid between Danio rerio female and Danio nigrofasciatus male produces aneuploid sperm with limited fertilization capacity

In the Danio species, interspecific hybridization has been conducted in several combinations. Among them, only the hybrid between a zebrafish (D. rerio) female and a spotted danio (D. nigrofasciatus) male was reported to be fertile. However, beyond these investigations, by means of reproductive biology, gametes of the hybrid have also not been investigated genetically. For this study, we induced a hybrid of the D. rerio female and D. nigrofasciatus male in order to study its developmental capacity, reproductive performance and gametic characteristics. Its hybrid nature was genetically verified by polymerase chain reaction-restriction fragment length polymorphism (PCR-RFLP) analysis of the rhodopsin gene. Almost all the hybrids (36/37) were males, and only one was female. Developing oocytes were observed in the hybrid female, but ovulated eggs have not been obtained thus far. Microscopic observation revealed various head sizes of sperm in the hybrid males. Flow cytometry showed that the hybrid males generated aneuploid sperm with various ploidy levels up to diploidy. In backcrosses between D. rerio females and hybrid males, fertilization rates were significantly lower than the control D. rerio, and most resultant progeny with abnormal appearance exhibited various kinds of aneuploidies ranging from haploidy to triploidy, but only one viable progeny, which survived more than four months, was triploid. This suggested the contribution of fertile diploid sperm of the hybrid male to successful fertilization and development.

Novel method for mass producing genetically sterile fish from surrogate broodstock via spermatogonial transplantation†

A stable system for producing sterile domesticated fish is required to prevent genetic contamination to native populations caused by aquaculture escapees. The objective of this study was to develop a system to mass produce stock for aquaculture that is genetically sterile by surrogate broodstock via spermatogonial transplantation (SGTP). We previously discovered that female medaka carrying mutations on the follicle-stimulating hormone receptor (fshr) gene become sterile. In this study, we demonstrated that sterile hybrid recipient females that received spermatogonia isolated from sex-reversed XX males (fshr (-/-)) recovered their fertility and produced only donor-derived fshr (-) X eggs. Natural mating between these females and fshr (-/-) sex-reversed XX males successfully produced large numbers of sterile fshr (-/-) female offspring. In conclusion, we established a new strategy for efficient mass production of sterile fish. This system can be applied to any aquaculture species for which SGTP and methods for producing sterile recipients can be established.

The expression of histone methyltransferases and distribution of selected histone methylations in testes of yak and cattle-yak hybrid

Interspecies hybridization exists widely in nature and plays an important role in animal evolution and adaptation. It is commonly recognized that male offspring of interspecies hybrid are often sterile, which presents a crucial way of reproductive isolation. Currently, the mechanisms underlying interspecies hybrid male sterility are not well understood. Cattle-yak, progeny of yak (Bos grunniens) and cattle (Bos taurus) cross, is a unique animal model for investigating hybrid male sterility. Because histone modifications are vital for spermatogenesis, herein, we examined expressions of histone methyltransferases (HMTs) and distributions of histone methylations in the yak and cattle-yak testis. Histological examination of seminiferous tubules revealed that gonocytes and spermatocytes were established normally, however, spermatogenesis was arrested at the meiosis phase began at 10 months after birth in the hybrids. SUV420H1 was the only HMT examined showing a significant enrichment in cattle-yak testes at 3 months. Relative expressions of MLL5, SETDB1 and SUV420H1 were increased while SETDB2 and EZH2 were decreased in cattle-yak testes at 10 months. Relative concentrations of MLL5 and SUV420H1 were again increased while EHMT2 and PRDM9 expressions were decreased at 24 months. Immunofluorescent detection of selected histone methylations in cross-sections of testicular tissues or meiotic chromosomes demonstrated that depletion of H3K4me3 and significant enrichment of H3K27me3 and H4K20me3 were observed in Sertoli cells of cattle-yak. Levels and localizations of H3K4me3, H3K9me1, H3K9me3 and H4K20me3 were strikingly different in meiotic chromosomes of cattle-yak spermatocytes. These results highlighted the potential roles of histone methylations in spermatogenic failure and hybrid male sterility.

Analysis on the Meiosis-Related Gene (Dmc1, Ph1) Expression in Autotriploid Carassius auratus

Triploid is usually considered to be unable to perform normal meiosis due to the abnormal behavior of the three sets of chromosomes. But autotriploid Carassius auratus in the Dongting water system (3n = 150, abbreviated as 3nCC) can perform normal meiosis. In artificial autotriploid Carassius auratus (3n = 150, abbreviated as 3nRR), female individuals undergo normal meiosis and produce mature gametes, while male individuals cannot. To better understand the effects of triploidization on meiosis in fish, we study the structure, methylation level, and expression level of meiosis-related genes (Dmc1, Ph1) in diploid Carassius auratus (2n = 100, abbreviated as 2nCC), Carassius auratus red var.(2n = 100, abbreviated as RCC), 3nCC and 3nRR. The results show that, compared with their diploid ancestors (2nCC and RCC), Dmc1 and Ph1 genes are hypomethylated in all 3nCC and female 3nRR, while are hypermethylated in male 3nRR. Correspondingly, Dmc1 and Ph1 genes are highly expressed in all 3nCC and female 3nRR, while are lowly expressed in male 3nRR. These results indicate that high expression of meiosis-related genes can contribute to restoration of bivalent pairing during meiosis in autotriploid Carassius auratus. This study provides new insights into the effect of DNA methylation on the fertility in triploid fish.

Biotechnology applied to fish reproduction: tools for conservation

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