Medaka vasa is required for migration but not survival of primordial germ cells

Germline replacement by blastula cell transplantation in the fish medaka

Primordial germ cell (PGC) specification early in development establishes the germline for reproduction and reproductive technologies. Germline replacement (GR) is a powerful tool for conservation of valuable or endangered animals. GR is achievable by germ cell transplantation into the PGC migration pathway or gonads. Blastula cell transplantation (BCT) can also lead to the chimeric germline containing PGCs of both donor and host origins. It has remained largely unknown whether BCT is able to achieve GR at a high efficiency. Here we report efficient GR by BCT into blastula embryos in the fish medaka (Oryzias latipes). Specifically, dnd depletion completely ablated host PGCs and fertility, and dnd overexpression remarkably boosted PGCs in donor blastulae. BCT between normal donor and host produced a germline transmission rate of ~4%. This rate was enhanced up to ~30% upon PGC boosting in donors. Most importantly, BCT between PGC-boosted donors and PGC-ablated hosts led to more than 90% fertility restoration and 100% GR. Therefore, BCT features an extremely high efficiency of fertility recovery and GR in medaka. This finding makes medaka an ideal model to analyze genetic and physiological donor-host compatibilities for BCT-mediated surrogate production and propagation of endangered lower vertebrates and biodiversity.

Molecular characterization and expression pattern of a germ cell marker gene dnd in gibel carp (Carassius gibelio)

As a germ cell marker gene, Dead end (dnd) has been identified and characterized in many vertebrates. Recently, we created a complete germ cell-depleted gonad model by the dnd-specific morpholino-mediated knockdown approach, and revealed sex-biased gene expression alteration through utilizing unisexual gynogenetic superiority in polyploid gibel carp. However, dnd and its expression pattern are still unclear in the gibel carp. In this study, we further analyzed molecular characterization of gibel carp dnd and its dynamic expression pattern during gametogenesis and embryogenesis. Similar to other homologs in vertebrates, gibel carp dnd contains a conserved RRM motif and five other motifs, and is highly evolutionary conserved in genomic organization and neighborhood gene synteny. RT-PCR and Western blot analyses showed its gonad-specific expression intensively in testis and ovary. Section in situ hybridization (SISH) and immunofluorescence localization revealed its dynamic expression pattern specific to oogenic cells and spermatogenetic cells during oogenesis and spermatogenesis. Moreover, its temporal and spatial distribution specific to PGCs were also demonstrated by RT-PCR and whole mount in situ hybridization (WISH) during embryogenesis. Therefore, gibel carp Dnd is a conserved germ cell marker during gametogenesis, and its maternal transcript is also a useful marker for tracing PGC specification and migration.

An unregulated regulator: Vasa expression in the development of somatic cells and in tumorigenesis

Growing evidence in diverse organisms shows that genes originally thought to function uniquely in the germ line may also function in somatic cells, and in some cases even contribute to tumorigenesis. Here we review the somatic functions of Vasa, one of the most conserved "germ line" factors among metazoans. Vasa expression in somatic cells is tightly regulated and often transient during normal development, and appears to play essential roles in regulation of embryonic cells and regenerative tissues. Its dysregulation, however, is believed to be an important element of tumorigenic cell regulation. In this perspectives paper, we propose how some conserved functions of Vasa may be selected for somatic cell regulation, including its potential impact on efficient and localized translational activities and in some cases on cellular malfunctioning and tumorigenesis.

Dazl is a critical player for primordial germ cell formation in medaka

The DAZ family genes boule, daz and dazl have conserved functions in primordial germ cell (PGC) migration, germ stem cell proliferation, differentiation and meiosis progression. It has remained unknown whether this family is required for PGC formation in developing embryos. Our recent study in the fish medaka (Oryzias latipes) has defined dnd as the critical PGC specifier and predicted the presence of additional factors essential for PGC formation. Here we report that dazl is a second key player for medaka PGC formation. Dazl knockdown did not prevent PGC formation even in the absence of normal somatic structures. It turned out that a high level of Dazl protein was maternally supplied and persisted until gastrulation, and hardly affected by two antisense morpholino oligos targeting the dazl RNA translation. Importantly, microinjection of a Dazl antibody remarkably reduced the number of PGCs and even completely abolished PGC formation without causing detectable somatic abnormality. Therefore, medaka PGC formation requires the Dazl protein as maternal germ plasm component, offering first evidence that dazl is a critical player in PGC formation in vivo. Our results demonstrate that antibody neutralization is a powerful tool to study the roles of maternal protein factors in PGC development in vivo.

Dnd Is a Critical Specifier of Primordial Germ Cells in the Medaka Fish

Primordial germ cell (PGC) specification occurs early in development. PGC specifiers have been identified in Drosophila, mouse, and human but remained elusive in most animals. Here we identify the RNA-binding protein Dnd as a critical PGC specifier in the medaka fish (Oryzias latipes). Dnd depletion specifically abolished PGCs, and its overexpression boosted PGCs. We established a single-cell culture procedure enabling lineage tracing in vitro. We show that individual blastomeres from cleavage embryos at the 32- and 64-cell stages are capable of PGC production in culture. Importantly, Dnd overexpression increases PGCs via increasing PGC precursors. Strikingly, dnd RNA forms prominent particles that segregate asymmetrically. Dnd concentrates in germ plasm and stabilizes germ plasm RNA. Therefore, Dnd is a critical specifier of fish PGCs and utilizes particle partition as a previously unidentified mechanism for asymmetric segregation. These findings offer insights into PGC specification and manipulation in medaka as a lower vertebrate model.

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The medaka RNA-binding protein Dnd specifies primordial germ cells

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Cells from medaka cleavage embryos can be singly cultured for lineage tracing

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The dnd RNA forms particles as a new mechanism for asymmetric segregation

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These findings offer new insights into PGC specification and manipulation

Autosomal gsdf acts as a male sex initiator in the fish medaka

Sex is pivotal for reproduction, healthcare and evolution. In the fish medaka, the Y-chromosomal dmy (also dmrt1bY) serves the sex determiner, which activates dmrt1 for male sex maintenance. However, how dmy makes the male decision via initiating testicular differentiation has remained unknown. Here we report that autosomal gsdf serves a male sex initiator. Gene addition and deletion revealed that gsdf was necessary and sufficient for maleness via initiating testicular differentiation. We show that gsdf transcription is activated directly by dmy. These results establish the autosomal gsdf as the first male sex initiator. We propose that dmy determines maleness through activating gsdf and dmrt1 without its own participation in developmental processes of sex initiation and maintenance. gsdf may easily become a sex determiner or other autosomal genes can be recruited as new sex determiners to initiate gsdf expression. Our findings offer new insights into molecular mechanisms underlying sex development and evolution of sex-controlling genes in vertebrates.

Medaka Oct4 is essential for pluripotency in blastula formation and ES cell derivation

The origin and evolution of molecular mechanisms underlying cellular pluripotency is a fundamental question in stem cell biology. The transcription factor Oct4 or Pou5f1 identified in mouse features pluripotency expression and activity in the inner cell mass and embryonic stem (ES) cells. Pou2 identified in zebrafish is the non-mammalian homolog prototype of mouse Oct4. The genes oct4 and pou2 have reportedly evolved by pou5 gene duplication in the common ancestor of vertebrates. Unlike mouse oct4, however, zebrafish pou2 lacks pluripotency expression and activity. Whether the presence of pluripotency expression and activity is specific for mammalian Oct4 or common to the ancestor of vertebrate Oct4 and Pou2 proteins has remained to be determined. Here we report that Oloct4, the medaka oct4/pou2, is essential for early embryogenesis and pluripotency maintenance. Oloct4 exists as a single copy gene and is orthologous to pou2 by sequence and chromosome synteny. Oloct4 expression occurs in early embryos, germ stem cells and ES cells like mouse oct4 but also in the brain and tail bud like zebrafish pou2. Importantly, OlOct4 depletion caused blastula lethality or blockage. We show that Oloct4 depletion abolishes ES cell derivation from midblastula embryos. Thus, Oloct4 has pluripotency expression and is essential for early embryogenesis and pluripotency maintenance. Our results demonstrate the conservation of pluripotency expression and activity in vertebrate Oct4 and Pou2 proteins. The finding that Oloct4 combines the features of mouse oct4 and zebrafish pou2 in expression and function suggests that Oloct4 might represent the ancestral prototype of vertebrate oct4 and pou2 genes.

Gonadal transcriptomic analysis and differentially expressed genes in the testis and ovary of the Pacific white shrimp (Litopenaeus vannamei)

Background

The Pacific white shrimp (Litopenaeus vannamei) is the world’s most prevalent cultured crustacean species. However, the supply of high-quality broodstocks is limited and baseline information related to its reproductive activity and molecular issues related to gonad development are scarce. In this study, we performed transcriptome sequencing on the gonads of adult male and female L. vannamei to identify sex-related genes.

Results

A total of 25.16 gigabases (Gb) of sequences were generated from four L. vannamei gonadal tissue libraries. After quality control, 24.11 Gb of clean reads were selected from the gonadal libraries. De-novo assembly of all the clean reads generated a total of 65,218 unigenes with a mean size of 1021 bp and a N50 of 2000 bp. A search of all-unigene against Nr, SwissProt, KEGG, COG and NT databases resulted in 26,482, 23,062, 20,659, 11,935 and 14,626 annotations, respectively, providing a total of 30,304 annotated unigenes. Among annotated unigenes, 12,320 unigenes were assigned to gene ontology categories and 20,659 unigenes were mapped to 258 KEGG pathways. By comparing the ovary and testis libraries, 19,279 testicular up-regulated and 3,529 ovarian up-regulated unigenes were identified. Enrichment analysis of differentially expressed unigenes resulted in 1060 significantly enriched GO terms and 34 significantly enriched KEGG pathways. Nine ovary-specific, 6 testis-specific, 45 testicular up-regulated and 39 ovarian up-regulated unigenes were then confirmed by semi-quantitative PCR and quantitative real-time PCR. In addition, using all-unigenes as a reference, a total of 13,233 simple sequence repeats (SSRs) were identified in 10,411 unigene sequences.

Conclusions

The present study depicts the first large-scale RNA sequencing of shrimp gonads. We have identified many important sex-related functional genes, GO terms and pathways, all of which will facilitate future research into the reproductive biology of shrimp. We expect that the SSRs detected in this study can then be used as genetic markers for germplasm evaluation of breeding and imported populations.

Electronic supplementary material

The online version of this article (doi:10.1186/s12864-015-2219-4) contains supplementary material, which is available to authorized users.

Suppression and restoration of primordial germ cell marker gene expression in channel catfish, Ictalurus punctatus, using knockdown constructs regulated by copper transport protein gene promoters: Potential for reversible transgenic sterilization

Complementary DNA overexpression and short hairpin RNA interference approaches were evaluated for decreasing expression of primordial germ cell (PGC) marker genes and thereby sterilizing channel catfish, Ictalurus punctatus, by delivering knockdown constructs driven by a constitutive promoter from yeast and a copper transport protein gene into fish embryos by electroporation. Two PGC marker genes, nanos and dead end, were the target knockdown genes, and their expressions, along with that of an off-target gene, vasa, were evaluated temporally using real-time polymerase chain reaction. Copper sulfate was evaluated as a repressor compound. Some of the constructs knocked down PGC marker gene expression, and some of the constructs were partially repressed by application of 0.1-ppm copper sulfate. When the rate of sexual maturity was compared for three-year-old broodfish that had been exposed to the sterilizing constructs during embryologic development and controls that had not been exposed, several treatments had reduced sexual maturity for the exposed fish. Of two promoter systems evaluated, the one which had been designed to be less sensitive to copper generally was more effective at achieving sterilization and more responsive to repression. Knockdown constructs based on 3' nanos short hairpin RNA interference appeared to result in the best repression and restoration of normal sexual maturity. We conclude that these copper-based systems exhibited good potential for repressible transgenic sterilization. Optimization of this system could allow environmentally safe application of transgenic technology and might be applicable to other applications for aquatic organisms.

Efficient knockout of transplanted green fluorescent protein gene in medaka using TALENs

Transcription activator-like effector nucleases (TALENs) are used for gene knockout and genome-editing studies in zebrafish, and these techniques have the potential to be applied to other fish species. Here, we show that TALENs can directly knock out a green fluorescent protein (GFP) transgene in medaka by affecting translation and synthesis of the GFP. We constructed a transgenic plasmid (pGFP-RFP) carrying the GFP and red fluorescent protein (RFP) genes, and used a modified TALEN method to assemble a pair of TALENs for the core chromophore Y66 region of GFP. Embryo toxicity of TALEN messenger RNA (mRNA) was far lower than the linearized plasmid; meanwhile, 76.3 % embryos, green fluorescence of embryos decreased significantly after co-injection of TALEN mRNA and the linearized plasmid, but red fluorescence showed no significant change. Real-time quantitative polymerase chain reaction and sequencing results showed that nearly 100 % mutated GFP position was disrupted at the Y66 region of GFP in the co-injected medaka embryos, caused by TALENs. This led to random insertion-deletion of nucleotides, which affected the translation of GFP and disrupted GFP synthesis. This provides new experimental evidence for designing TALEN sites in genes for which only key functional domains are known. Our results show that a modified TALEN method can efficiently and specifically mediate a transgene knockout in medaka. This report may promote the application of TALENs in gene-editing studies of fish species other than zebrafish.

Gene replacement by zinc finger nucleases in medaka embryos

Gene replacement (GR) via homologous recombination is a powerful tool for genome editing. Recently, direct GR is achieved successfully by coinjection of mRNAs for engineered endonucleases such as zinc finger nucleases (ZFNs) and donor DNA in developing embryos of diverse organisms. Here, we report the procedures and efficiency for direct GR by using ZFNs in the fish medaka. Upon zygotic coinjection of mRNAs encoding ZFNs that target the gonad-specifically expressed gsdf locus, linear DNA of GR vector pGRgsdf containing the red fluorescent protein (rfp) gene flanked by two homology arms of ~1-kb each underwent GR via homologous recombination. Specifically, 15 of 231 adults from manipulated embryos contained a GR allele in the caudal fin, producing an efficiency of ~7 % for somatic GR. Progeny test revealed that two out of nine fertile fish containing the GR allele in the fin were capable of transmitting the GR allele to ~6 % of F1 generation at adulthood, generating an efficiency of ~22 % for germline transmission. Sequencing and Southern blotting validated precise GR. We show that the GR allele expressed a chimeric gsdf:rfp RNA between gsdf and cointegrated rfp specifically in the gonad, demonstrating recapitulation of endogenous RNA expression as predicted for the defined GR allele. Most importantly, RFP expression coincides faithfully with the gonad-specific gsdf expression in developing embryos and adults. These results demonstrate, for the first time, the feasibility and efficiency of ZFN-mediated precise GR directly in the developing embryo of medaka as a lower vertebrate model.

Formation and cultivation of medaka primordial germ cells

Primordial germ cell (PGC) formation is pivotal for fertility. Mammalian PGCs are epigenetically induced without the need for maternal factors and can also be derived in culture from pluripotent stem cells. In egg-laying animals such as Drosophila and zebrafish, PGCs are specified by maternal germ plasm factors without the need for inducing factors. In these organisms, PGC formation and cultivation in vitro from indeterminate embryonic cells have not been possible. Here, we report PGC formation and cultivation in vitro from blastomeres dissociated from midblastula embryos (MBEs) of the fish medaka (Oryzias latipes). PGCs were identified by using germ-cell-specific green fluorescent protein (GFP) expression from a transgene under the control of the vasa promoter. Embryo perturbation was exploited to study PGC formation in vivo, and dissociated MBE cells were cultivated under various conditions to study PGC formation in vitro. Perturbation of somatic development did not prevent PGC formation in live embryos. Dissociated MBE blastomeres formed PGCs in the absence of normal somatic structures and of known inducing factors. Most importantly, under culture conditions conducive to stem cell derivation, some dissociated MBE blastomeres produced GFP-positive PGC-like cells. These GFP-positive cells contained genuine PGCs, as they expressed PGC markers and migrated into the embryonic gonad to generate germline chimeras. Our data thus provide evidence for PGC preformation in medaka and demonstrate, for the first time, that PGC formation and derivation can be obtained in culture from early embryos of medaka as a lower vertebrate model.

Parameters and efficiency of direct gene disruption by zinc finger nucleases in medaka embryos

Zinc finger nucleases (ZFNs) can generate targeted gene disruption (GD) directly in developing embryos of zebrafish, mouse and human. In the fish medaka, ZFNs have been attempted on a transgene. Here, we developed procedures and parameters for ZFN-mediated direct GD on the gonad-specifically expressed gsdf locus in medaka. A pair of ZFNs was designed to target the first exon of gsdf and their synthetic mRNAs were microinjected into 1-cell stage embryos. We reveal dose-dependent survival rate and GD efficiency. In fry, ZFN mRNA injection at 10 ng/μl led to a GD efficiency of 30 %. This value increased up to nearly 100 % when the dose was enhanced to 40 ng/μl. In a typical series of experiments of ZFN mRNA injection at 10 ng/μl, 420 injected embryos developed into 94 adults, 4 of which had altered gsdf alleles. This leads to a GD efficacy of ∼4 % in the adulthood. Sequencing revealed a wide variety of subtle allelic alterations including additions and deletions of 1∼18 bp in length in ZFN-injected samples. Most importantly, one of the 4 adults examined was capable of germline transmission to 15.2 % of its F1 progeny. Interestingly, ontogenic analyses of the allelic profile revealed that GD commenced early in development, continued during subsequent stages of development and in primordia for different adult organs of the three germ layers. These results demonstrate the feasibility and--for the first time to our knowledge--the efficacy of ZFN-mediated direct GD on a chromosomal gene in medaka embryos.

Vasa identifies germ cells and critical stages of oogenesis in the Asian seabass

Germ cells produce sperm and eggs for reproduction and fertility. The Asian seabass (Lates calcarifer), a protandrous marine fish, undergoes male-female sex reversal and thus offers an excellent model to study the role of germ cells in sex differentiation and sex reversal. Here we report the cloning and expression of vasa as a first germ cell marker in this organism. A 2241-bp cDNA was cloned by PCR using degenerate primers of conserved sequences and gene-specific primers. This cDNA contains a polyadenylation signal and a full open reading frame for 645 amino acid residues, which was designated as Lcvasa for the seabass vasa, as its predicted protein is homologous to Vasa proteins. The Lcvasa RNA is maternally supplied and specific to gonads in adulthood. By chromogenic and fluorescent in situ hybridization we revealed germ cell-specific Lcvasa expression in both the testis and ovary. Importantly, Lcvasa shows dynamic patterns of temporospatial expression and subcellular distribution during gametogenesis. At different stages of oogenesis, for example, Lcvasa undergoes nuclear-cytoplasmic redistribution and becomes concentrated preferentially in the Balbiani body of stage-II~III oocytes. Thus, the vasa RNA identifies both female and male germ cells in the Asian seabass, and its expression and distribution delineate critical stages of gametogenesis.

Germline-specific and sexually dimorphic expression of a dead end gene homologue in turbot (Scophthalmus maximus)

Germ cells are indispensable for gonadal development and fertility. However, the physiological mechanisms regulating germ cell development in marine fish are poorly understood due to a lack of germ cell markers. The dead end (dnd) gene is a vertebrate-specific component of germplasm crucial for primordial germ cells (PGCs) migration and development in teleosts. In this study, we identified a dnd homologue (Smdnd) in turbot (Scophthalmus maximus) and investigated its expression pattern during embryogenesis and gonadal development. The deduced amino acid sequence of Smdnd shared several conserved motifs of Dnd homologues as well as high identity to other Dnd proteins. Phylogenetic analysis revealed that the SmDnd was closely related to its teleost counterparts. Reverse transcription polymerase chain reaction (RT-PCR) and in situ hybridization revealed that Smdnd transcripts could be exclusively detected in germ cells, including presumptive PGC and adult male and female germ cells. In addition, an interesting sexually dimorphic expression of Smdnd during gonadal development was observed by real-time PCR. Female turbot showed greater (P < 0.05) Smdnd expression than male before sex maturation. This difference reduced gradually due to the upregulation of Smdnd in the male during the period corresponding to spermatogonia proliferation and meiosis. These results indicate that Smdnd can be used as a germ cell marker in turbot. In addition, the temporal and sex differences in Smdnd expression indicate that this gene may play different roles in gonadal development in both sexes.

Benzo[a]pyrene decreases global and gene specific DNA methylation during zebrafish development

DNA methylation is important for gene regulation and is vulnerable to early-life exposure to environmental contaminants. We found that direct waterborne benzo[a]pyrene (BaP) exposure at 24μg/L from 2.5 to 96hpf to zebrafish embryos significantly decreased global cytosine methylation by 44.8% and promoter methylation in vasa by 17%. Consequently, vasa expression was significantly increased by 33%. In contrast, BaP exposure at environmentally relevant concentrations did not change CpG island methylation or gene expression in cancer genes such as ras-association domain family member 1 (rassf1), telomerase reverse transcriptase (tert), c-jun, and c-myca. Similarly, BaP did not change gene expression of DNA methyltransferase 1 (dnmt1) and glycine N-methyltransferase (gnmt). While total DNMT activity was not affected, GNMT enzyme activity was moderately increased. In summary, BaP is an epigenetic modifier for global and gene specific DNA methylation status in zebrafish larvae.

Germ cell specification requires zygotic mechanisms rather than germ plasm in a basally branching insect

BACKGROUND

Primordial germ cell (PGC) specification is a universal process across animals, but the molecular mechanisms specifying PGCs are remarkably diverse. In Drosophila, PGCs are specified by maternally provided, asymmetrically localized cytoplasmic factors (germ plasm). In contrast, historical literature on most other arthropods reports that PGCs arise from mesoderm during midembryogenesis, suggesting that an arthropod last common ancestor may have specified PGCs via zygotic mechanisms. However, there has been no direct experimental evidence to date for germ plasm-independent arthropod PGC specification.

RESULTS

Here we show that in a basally branching insect, the cricket Gryllus bimaculatus, conserved germ plasm molecules are ubiquitously, rather than asymmetrically, localized during oogenesis and early embryogenesis. Molecular and cytological analyses suggest that Gryllus PGCs arise from abdominal mesoderm during segmentation, and twist RNAi embryos that lack mesoderm fail to form PGCs. Using RNA interference we show that vasa and piwi are not required maternally or zygotically for PGC formation but rather are required for primary spermatogonial divisions in adult males.

CONCLUSIONS

These observations suggest that Gryllus lacks a maternally inherited germ plasm, in contrast with many holometabolous insects, including Drosophila. The mesodermal origin of Gryllus PGCs and absence of instructive roles for vasa and piwi in PGC formation are reminiscent of mouse PGC specification and suggest that zygotic cell signaling may direct PGC specification in Gryllus and other Hemimetabola.

Evidence against a germ plasm in the milkweed bug Oncopeltus fasciatus, a hemimetabolous insect

Primordial germ cell (PGC) formation in holometabolous insects like Drosophila melanogaster relies on maternally synthesised germ cell determinants that are asymmetrically localised to the oocyte posterior cortex. Embryonic nuclei that inherit this "germ plasm" acquire PGC fate. In contrast, historical studies of basally branching insects (Hemimetabola) suggest that a maternal requirement for germ line genes in PGC specification may be a derived character confined principally to Holometabola. However, there have been remarkably few investigations of germ line gene expression and function in hemimetabolous insects. Here we characterise PGC formation in the milkweed bug Oncopeltus fasciatus, a member of the sister group to Holometabola, thus providing an important evolutionary comparison to members of this clade. We examine the transcript distribution of orthologues of 19 Drosophila germ cell and/or germ plasm marker genes, and show that none of them localise asymmetrically within Oncopeltus oocytes or early embryos. Using multiple molecular and cytological criteria, we provide evidence that PGCs form after cellularisation at the site of gastrulation. Functional studies of vasa and tudor reveal that these genes are not required for germ cell formation, but that vasa is required in adult males for spermatogenesis. Taken together, our results provide evidence that Oncopeltus germ cells may form in the absence of germ plasm, consistent with the hypothesis that germ plasm is a derived strategy of germ cell specification in insects.

p53 gene targeting by homologous recombination in fish ES cells

Background

Gene targeting (GT) provides a powerful tool for the generation of precise genetic alterations in embryonic stem (ES) cells to elucidate gene function and create animal models for human diseases. This technology has, however, been limited to mouse and rat. We have previously established ES cell lines and procedures for gene transfer and selection for homologous recombination (HR) events in the fish medaka (Oryzias latipes).

Methodology and Principal Findings

Here we report HR-mediated GT in this organism. We designed a GT vector to disrupt the tumor suppressor gene p53 (also known as tp53). We show that all the three medaka ES cell lines, MES1∼MES3, are highly proficient for HR, as they produced detectable HR without drug selection. Furthermore, the positive-negative selection (PNS) procedure enhanced HR by ∼12 folds. Out of 39 PNS-resistant colonies analyzed, 19 (48.7%) were positive for GT by PCR genotyping. When 11 of the PCR-positive colonies were further analyzed, 6 (54.5%) were found to be bona fide homologous recombinants by Southern blot analysis, sequencing and fluorescent in situ hybridization. This produces a high efficiency of up to 26.6% for p53 GT under PNS conditions. We show that p53 disruption and long-term propagation under drug selection conditions do not compromise the pluripotency, as p53-targeted ES cells retained stable growth, undifferentiated phenotype, pluripotency gene expression profile and differentiation potential in vitro and in vivo.

Conclusions

Our results demonstrate that medaka ES cells are proficient for HR-mediated GT, offering a first model organism of lower vertebrates towards the development of full ES cell-based GT technology.

Identification and migration of primordial germ cells in Atlantic salmon, Salmo salar: characterization of vasa, dead end, and lymphocyte antigen 75 genes

No information exists on the identification of primordial germ cells (PGCs) in the super-order Protacanthopterygii, which includes the Salmonidae family and Atlantic salmon (Salmo salar L.), one of the most commercially important aquatic animals worldwide. In order to identify salmon PGCs, we cloned the full-length cDNA of vasa, dead end (dnd), and lymphocyte antigen 75 (ly75/CD205) genes as germ cell marker candidates, and analyzed their expression patterns in both adult and embryonic stages of Atlantic salmon. Semi-quantitative RT-PCR results showed that salmon vasa and dnd were specifically expressed in testis and ovary, and vasa, dnd, and ly75 mRNA were maternally deposited in the egg. vasa mRNA was consistently detected throughout embryogenesis while dnd and ly75 mRNA were gradually degraded during cleavages. In situ analysis revealed the localization of vasa and dnd mRNA and Ly75 protein in PGCs of hatched larvae. Whole-mount in situ hybridization detected vasa mRNA during embryogenesis, showing a distribution pattern somewhat different to that of zebrafish; specifically, at mid-blastula stage, vasa-expressing cells were randomly distributed at the central part of blastodisc, and then they migrated to the presumptive region of embryonic shield. Therefore, the typical vasa localization pattern of four clusters during blastulation, as found in zebrafish, was not present in Atlantic salmon. In addition, salmon PGCs could be specifically labeled with a green fluorescence protein (GFP) using gfp-rt-vasa 3′-UTR RNA microinjection for further applications. These findings may assist in understanding PGC development not only in Atlantic salmon but also in other salmonids.

Identification of the RNAs for transcription factor Mitf as a component of the Balbiani body

Balbiani body (BB) is a large distinctive organelle aggregate uniquely present in developing oocytes of diverse animal species. BB is thought as a stage-specific structure that resembles germ plasm, the cytoplasmic organelle of germ cells. The role and function of BB have remained speculative because of a highly dynamic structure and a lack of genetic and molecular data. BB has been found to contain proteins and RNAs, none of them--except the zebrafish foxH1 RNA, is or encodes a transcription factor. Here we report in the fish medaka (Oryzias latipes) that RNAs encoding microphthalmia-associated transcription factor (Mitf) are prominent components of the BB. By fluorescence in situ hybridization on ovarian section, we revealed that the transcripts of both mitf1 and mitf2 genes concentrated in the BB, in which they co-localized with the dazl RNA, a definitive BB marker highly conserved in vertebrates. Therefore, the mitf product may play dual roles in germ gene transcription and BB formation and/or function in this organism. Our data provide the second evidence that the RNA of a transcription factor can be a prominent component of the BB in a vertebrate.

oct4-EGFP reporter gene expression marks the stem cells in embryonic development and in adult gonads of transgenic medaka

Maintenance of pluripotency in stem cells is tightly regulated among vertebrates. One of the key genes in this process is oct4, also referred to as pou5f1 in mammals and pou2 in teleosts. Pou5f1 evolved by duplication of pou2 early in the tetrapod lineage, but only monotremes and marsupials retained both genes. Either pou2 or pou5f1 was lost from the genomes of the other tetrapods that have been analyzed to date. Consequently, these two homologous genes are often designated oct4 in functional studies. In most vertebrates oct4 is expressed in pluripotent cells of the early embryo until the blastula stage, and later persist in germline stem cells until adulthood. The isolation and analysis of stem cells from embryo or adult individuals is hampered by the need for reliable markers that can identify and define the cell populations. Here, we report the faithful expression of EGFP under the control of endogenous pou2/oct4 promoters in transgenic medaka (Oryzias latipes). In vivo imaging in oct4-EGFP transgenic medaka reveals the temporal and spatial expression of pou2 in embryos and adults alike. We describe the temporal and spatial patterns of endogenous pou2 and oct4-EGFP expression in medaka with respect to germline and adult stem cells, and discuss applications of oct4-EGFP transgenic medaka in reproductive and stem cell biology.

Identification and migration of primordial germ cells in Atlantic salmon, Salmo salar: characterization of vasa, dead end, and lymphocyte antigen 75 genes

No information exists on the identification of primordial germ cells (PGCs) in the super-order Protacanthopterygii, which includes the Salmonidae family and Atlantic salmon (Salmo salar L.), one of the most commercially important aquatic animals worldwide. In order to identify salmon PGCs, we cloned the full-length cDNA of vasa, dead end (dnd), and lymphocyte antigen 75 (ly75/CD205) genes as germ cell marker candidates, and analyzed their expression patterns in both adult and embryonic stages of Atlantic salmon. Semi-quantitative RT-PCR results showed that salmon vasa and dnd were specifically expressed in testis and ovary, and vasa, dnd, and ly75 mRNA were maternally deposited in the egg. vasa mRNA was consistently detected throughout embryogenesis while dnd and ly75 mRNA were gradually degraded during cleavages. In situ analysis revealed the localization of vasa and dnd mRNA and Ly75 protein in PGCs of hatched larvae. Whole-mount in situ hybridization detected vasa mRNA during embryogenesis, showing a distribution pattern somewhat different to that of zebrafish; specifically, at mid-blastula stage, vasa-expressing cells were randomly distributed at the central part of blastodisc, and then they migrated to the presumptive region of embryonic shield. Therefore, the typical vasa localization pattern of four clusters during blastulation, as found in zebrafish, was not present in Atlantic salmon. In addition, salmon PGCs could be specifically labeled with a green fluorescence protein (GFP) using gfp-rt-vasa 3'-UTR RNA microinjection for further applications. These findings may assist in understanding PGC development not only in Atlantic salmon but also in other salmonids.

Multiple regulatory regions control the transcription of medaka germ gene vasa

Numerous regulatory DNA regions and trans-acting protein factors controlling transcription have been characterized for many genes that are expressed in somatic cells. Little is known about the transcriptional control of germ genes, and no cell culture system has been explored for quantitative reporter assay of germ gene transcription in vitro. Here we report the development of such an in vitro system and the identification of regulatory regions in the medaka germ gene vasa. We established the medaka germ cell line SG3 as a suitable in vitro system for analyzing germ gene transcription. Transgenic production revealed that VAS, a 5.1-kb genomic fragment of medaka vasa, possessed regulatory regions essential for germ cell-specific transcription. Importantly, reporter assays revealed 11 positive and negative regulatory regions alternatively positioned throughout VAS including the first intron. Strikingly, the regulatory regions may act in additive, non-additive and dependent manners. We show that a 39-bp element within one regulatory region is able to interact with the nuclear factor(s) of vasa-expressing embryos and testes. These results demonstrate the complexity of transcriptional control of medaka vasa and provide important insights into opposing mechanisms underlying germ gene transcription.

Efficient detection, quantification and enrichment of subtle allelic alterations

Gene targeting (GT) can introduce subtle alterations into a particular locus and represents a powerful tool for genome editing. Engineered zinc finger nucleases (ZFNs) are effective for generating minor allelic alterations. Efficient detection of such minor alterations remains one of the challenges in ZFN-mediated GT experiments. Here, we report the establishment of procedures allowing for efficient detection, quantification and enrichment of such subtle alterations. In a biallelic model, polyacrylamide gel electrophoresis (PAGE) is capable of detecting rare allelic variations in the form of DNA heteroduplexes at a high efficiency of ~0.4% compared with ~6.3% by the traditional T7 endonuclease I-digestion and agarose gel electrophoresis. In a multiple allelic model, PAGE could discriminate different alleles bearing addition or deletion of 1-18 bp as distinct bands that were easily quantifiable by densitometry. Furthermore, PAGE enables enrichment for rare alleles. We show for the first time that direct endogenous GT is possible in medaka by ZFN RNA injection, whereas PAGE allows for detection and cloning of ZFN-targeted alleles in adults arising from ZFN-injected medaka embryos. Therefore, PAGE is effective for detection, quantification and enrichment of multiple fine allelic differences and thus offers a versatile tool for screening targeted subtle gene alterations.

Ol4E-T, a eukaryotic translation initiation factor 4E-binding protein of medaka fish (Oryzias latipes), can interact with nanos3 and vasa in vitro

Maternal factors have essential roles in the specification and development of germ cells in metazoans. In Drosophila, a number of genes such as oskar, vasa, nanos, and tudor are required for specific steps in pole cell formation and further germline development. Drosophila cup, another maternal factor, is confirmed as a main factor in normal oogenesis, maintenance, and survival of female germ-line stem cells by interaction with Nanos. Through searching for the homolog of Drosophila cup in the medaka, the homolog of eukaryotic translation initiation factor 4E (eIF4E)-transporter, named Ol4E-T, was identified. Reverse transcription-polymerase chain reaction (RT-PCR) and in situ hybridization revealed that Ol4E-T is maternally deposited in the embryo and Ol4E-T expression is maintained throughout embryogenesis. Ol4E-T is predominantly expressed in the adult gonads. In the testes, Ol4E-T is expressed in the same regions where medaka vasa, named olvas is expressed. In the ovary, expression of Ol4E-T conforms to that of nanos3 and olvas. Ol4E-T harbors a well-conserved eIF4E-binding motif, YTKEELL, by which Ol4E-T interacts with eIF4E in medaka. Additionally, Ol4E-T can interact with medaka Nanos3 and Olvas, as shown by yeast two hybridization. The spatial expression and interactions between Ol4E-T with germ cell markers Olvas and Nanos3 suggest a role for Ol4E-T in germ-line development in medaka.

Expression of vasa and nanos3 during primordial germ cell formation and migration in Atlantic cod (Gadus morhua L.)

Primordial germ cells (PGCs), progenitors of gametes, are specified very early in embryonic development and undergo an active migration to the site where the future gonads will form. While the developmental pattern of PGCs during embryogenesis has been documented in few model teleost fishes, there is currently no information available for any representative of Superorder Paracanthopterygii. This includes Atlantic cod (Gadus morhua), which is a historically important food fish in both fisheries and aquaculture industries. In the present study, we cloned and characterized vasa and nanos3 and used them as germ cell markers in Atlantic cod. Sequencing results showed prospective vasa and nanos3 mRNA contained the domains used to describe their respective protein family. Furthermore, phylogenetic analysis using the amino acid sequence placed Atlantic cod Vasa distinct from representatives of three other taxonomic Superorders. Atlantic cod Nanos3 was placed with other homologues from the Nanos3 subfamily. Expression of both genes was detected from the first cleavage division; both were specifically expressed in Atlantic cod PGCs from the 32-cell stage. While nanos3 expression ceased during early somitogenesis, vasa was strongly expressed throughout embryonic development. Using vasa as a marker, we described the Atlantic cod PGC migration pattern. We demonstrated that Atlantic cod PGCs migrate ventral to the trunk mesoderm. With the exception of Pacific herring (Clupea pallasii), PGCs in other described teleost fishes migrate lateral to the trunk. The results from this study are the first step toward understanding germ line formation in Atlantic cod.

Interordinal chimera formation between medaka and zebrafish for analyzing stem cell differentiation

Chimera formation is a standard test for pluripotency of stem cells in vivo. Interspecific chimera formation between distantly related organisms offers also an attractive approach for propagating endangered species. Parameters influencing interspecies chimera formation have remained poorly elucidated. Here, we report interordinal chimera formation between medaka and zebrafish, which separated ∼320 million years ago and exhibit a more than 2-fold difference in developmental speed. We show that, on transplantation into zebrafish blastulae, both noncultivated blastomeres and long-term cultivated embryonic stem (ES) cells of medaka adopted the zebrafish developmental program and differentiated into physiologically functional cell types including pigment cells, blood cells, and cardiomyocytes. We also show that medaka ES cells express differentiation gene markers during chimeric embryogenesis. Therefore, the evolutionary distance and different embryogenesis speeds do not produce donor-host incompatibility to compromise chimera formation between medaka and zebrafish, and molecular markers are valuable for analyzing lineage commitment and cell differentiation in interspecific chimeric embryos.

Fusion gene vectors allowing for simultaneous drug selection, cell labeling, and reporter assay in vitro and in vivo

Vector systems allowing simultaneously for rapid drug selection, cell labeling, and reporter assay are highly desirable in biomedical research including stem cell biology. Here, we present such a vector system including pCVpf or pCVpr, plasmids that express pf or pr, a fusion protein between puromycin acetyltransferase and green or red fluorescent protein from CV, the human cytomegalovirus enhancer/promoter. Transfection with pCVpf or pCVpr produced a ∼10% efficiency of gene transfer. A 2-day pulse puromycin selection resulted in ∼13-fold enrichment for transgenic cells, and continuous puromycin selection produced stable transgenic stem cell clones with retained pluripotency. Furthermore, we developed a PAC assay protocol for quantification of transgene expression. To test the usefulness for cell labeling and PAC assay in vivo, we constructed pVASpf containing pf linked to the regulatory sequence of medaka germ gene vasa and generated transgenic fish with visible GFP expression in germ cells. PAC assay revealed the highest expression in the testis. Interestingly, PAC activity was also detectable in somatic organs including the eye, which was validated by fluorescence in situ hybridization. Therefore, the pf and pr vectors provide a useful system for simultaneous drug selection, live labeling, and reporter assay in vitro and in vivo.

Sperm nuclear transfer and transgenic production in the fish medaka

Sperm nuclear transfer or intracytoplasmic sperm injection (ICSI) is a powerful assisted reproductive technology (ART) for treating human male infertility. Controversial reports of increased birth defects have raised concerns about the ART's safety. The cause for birth defects, however, has remained elusive for analysis in human because of the sample size, male infertility genetics, physiological heterogeneity and associated procedures such as embryo manipulations. Animal models are required to evaluate factors leading to the increased birth defects. Here we report the establishment of medakafish model for ICSI and transgenic production. This small laboratory fish has high fecundity and easy embryology. We show that ICSI produced a 5% high percentage of fertile animals that exhibited both paternal and maternal contribution as evidenced by the pigmentation marker. Furthermore, when sperm were pre-incubated with a plasmid ubiquitously expressing RFP and subjected to ICSI, 50% of sperm nuclear transplants showed germline transmission. We conclude that medaka is an excellent model for ICSI to evaluate birth defects and that sperm nuclear transfer can mediate stable gene transfer at high efficiency. Although more demanding for experimentation, sperm-mediated transgenesis should be particularly applicable for aquaculture species with a lengthy generation time and/or a large adult body size.

Medaka tert produces multiple variants with differential expression during differentiation in vitro and in vivo

Embryonic stem (ES) cells have immortality for self-renewal and pluripotency. Differentiated human cells undergo replicative senescence. In human, the telomerase reverse transcriptase (Tert), namely the catalytic subunit of telomerase, exhibits differential expression to regulate telomerase activity governing cellular immortality or senescence, and telomerase activity or tert expression is a routine marker of pluripotent ES cells. Here we have identified the medaka tert gene and determined its expression and telomerase activity in vivo and in vitro. We found that the medaka tert locus produces five variants called terta to terte encoding isoforms TertA to TertE. The longest TertA consists of 1090 amino acid residues and displays a maximum of 34% identity to the human TERT and all the signature motifs of the Tert family. TertB to TertE are novel isoforms and have considerable truncation due to alternative splicing. The terta RNA is ubiquitous in embryos, adult tissues and cell lines, and accompanies ubiquitous telomerase activity in vivo and in vitro as revealed by TRAP assays. The tertb RNA was restricted to the testis, absent in embryos before gastrulation and barely detectable in various cell lines The tertc transcript was absent in undifferentiated ES cells but became evident upon ES cell differentiation, in vivo it was barely detectable in early embryos and became evident when embryogenesis proceeds. Therefore, ubiquitous terta expression correlates with ubiquitous telomerase activity in medaka, and expression of other tert variants appears to delineate cell differentiation in vitro and in vivo.

Identification of pluripotency genes in the fish medaka

Stem cell cultures can be derived directly from early developing embryos and indirectly from differentiated cells by forced expression of pluripotency transcription factors. Pluripotency genes are routinely used to characterize mammalian stem cell cultures at the molecular level. However, such genes have remained unknown in lower vertebrates. In this regard, the laboratory fish medaka is uniquely suited because it has embryonic stem (ES) cells and genome sequence data. We identified seven medaka pluripotency genes by homology search and expression in vivo and in vitro. By RT-PCR analysis, the seven genes fall into three groups of expression pattern. Group I includes nanog and oct4 showing gonad-specific expression; Group II contains sall4 and zfp281 displaying gonad-preferential expression; Group III has klf4, ronin and tcf3 exhibiting expression also in several somatic tissues apart from the gonads. The transcripts of the seven genes are maternally supplied and persist at a high level during early embryogenesis. We made use of early embryos and adult gonads to examine expression in stem cells and differentiated derivatives by in situ hybridization. Strikingly, nanog and oct4 are highly expressed in pluripotent blastomeres of 16-cell embryos. In the adult testis, nanog expression was specific to spermatogonia, the germ stem cells, whereas tcf3 expression occurred in spermatogonia and differentiated cells. Most importantly, all the seven genes are pluripotency markers in vitro, because they have high expression in undifferentiated ES cells but dramatic down-regulation upon differentiation. Therefore, these genes have conserved their pluripotency-specific expression in vitro from mammals to lower vertebrates.

Medaka cleavage embryos are capable of generating ES-like cell cultures

Mammalian embryos at the blastocyst stage have three major lineages, which in culture can give rise to embryonic stem (ES) cells from the inner cell mass or epiblast, trophoblast stem cells from the trophectoderm, and primitive endoderm stem cells. None of these stem cells is totipotent, because they show gene expression profiles characteristic of their sources and usually contribute only to the lineages of their origins in chimeric embryos. It is unknown whether embryos prior to the blastocyst stage can be cultivated towards totipotent stem cell cultures. Medaka is an excellent model for stem cell research. This laboratory fish has generated diploid and even haploid ES cells from the midblastula embryo with ~2000 cells. Here we report in medaka that dispersed cells from earlier embryos can survive, proliferate and attach in culture. We show that even 32-cells embryos can be dissociated into individual cells capable of producing continuously growing ES-like cultures. Our data point to the possibility to derive stable cell culture from cleavage embryos in this organism.

Fish stem cell cultures

Stem cells have the potential for self-renewal and differentiation. First stem cell cultures were derived 30 years ago from early developing mouse embryos. These are pluripotent embryonic stem (ES) cells. Efforts towards ES cell derivation have been attempted in other mammalian and non-mammalian species. Work with stem cell culture in fish started 20 years ago. Laboratory fish species, in particular zebrafish and medaka, have been the focus of research towards stem cell cultures. Medaka is the second organism that generated ES cells and the first that gave rise to a spermatogonial stem cell line capable of test-tube sperm production. Most recently, the first haploid stem cells capable of producing whole animals have also been generated from medaka. ES-like cells have been reported also in zebrafish and several marine species. Attempts for germline transmission of ES cell cultures and gene targeting have been reported in zebrafish. Recent years have witnessed the progress in markers and procedures for ES cell characterization. These include the identification of fish homologs/paralogs of mammalian pluripotency genes and parameters for optimal chimera formation. In addition, fish germ cell cultures and transplantation have attracted considerable interest for germline transmission and surrogate production. Haploid ES cell nuclear transfer has proven in medaka the feasibility of semi-cloning as a novel assisted reproductive technology. In this special issue on "Fish Stem Cells and Nuclear Transfer", we will focus our review on medaka to illustrate the current status and perspective of fish stem cells in research and application. We will also mention semi-cloning as a new development to conventional nuclear transfer.

Germ cell sex prior to meiosis in the rainbow trout

Germ cells make two major decisions when they move from an indeterminate state to their final stage of gamete production. One decision is sexual commitment for sperm or egg production, and the other is to maintain mitotic division or entry into meiosis. It is unclear whether the two decisions are made as a single event or separate events, because there has been no evidence for the presence of germ cell sex prior to meiosis. Here we report direct evidence in the fish rainbow trout that gonia have distinct sexuality. We show that dazl expression occurs in both male and female gonia but exhibits differential intracellular distribution. More strikingly, we show that boule is highly expressed in male gonia but absent in female gonia. Therefore, mitotic gonia possess sex, sperm/egg decision and mitosis/meiosis decision are two independent events, and sperm/egg decision precedes mitosis/meiosis decision in rainbow trout, making this organism a unique vertebrate model for mechanistic understanding of germ cell sex differentiation and relationship between the two decisions.

Nanog regulates primordial germ cell migration through Cxcr4b

Gonadal development in vertebrates depends on the early determination of primordial germ cells (PGCs) and their correct migration to the sites where the gonads develop. Several genes have been implicated in PGC specification and migration in vertebrates. Additionally, some of the genes associated with pluripotency, such as Oct4 and Nanog, are expressed in PGCs and gonads, suggesting a role for these genes in maintaining pluripotency of the germ lineage, which may be considered the only cell type that perpetually maintains stemness properties. Here, we report that medaka Nanog (Ol-Nanog) is expressed in the developing PGCs. Depletion of Ol-Nanog protein causes aberrant migration of PGCs and inhibits expression of Cxcr4b in PGCs, where it normally serves as the receptor of Sdf1a to guide PGC migration. Moreover, chromatin immunoprecipitation analysis demonstrates that Ol-Nanog protein binds to the promoter region of Cxcr4b, suggesting a direct regulation of Cxcr4b by Ol-Nanog. Simultaneous overexpression of Cxcr4b mRNA and depletion of Ol-Nanog protein in PGCs rescues the migration defective phenotype induced by a loss of Ol-Nanog, whereas overexpression of Sdf1a, the ligand for Cxcr4b, does not restore proper PGC migration. These results indicate that Ol-Nanog mediates PGC migration by regulating Cxcr4b expression.

Evolution and spermatogenesis

Sexual reproduction depends on the production of haploid gametes, and their fusion to form diploid zygotes. Here, we discuss sperm production and function in a molecular and functional evolutionary context, drawing predominantly from studies in model organisms (mice, Drosophila, Caenorhabditis elegans). We consider the mechanisms involved in establishing and maintaining a germline stem cell population in testes, as well as the factors that regulate their contribution to the pool of differentiating cells. These processes involve considerable interaction between the germline and the soma, and we focus on regulatory signalling events in a variety of organisms. The male germline has a unique transcriptional profile, including expression of many testis-specific genes. The evolutionary pressures associated with gene duplication and acquisition of testis function are discussed in the context of genome organization and transcriptional regulation. Post-meiotic differentiation of spermatids involves very dramatic changes in cell shape and acquisition of highly specialized features. We discuss the variety of sperm motility mechanisms and how various reproductive strategies are associated with the diversity of sperm forms found in animals.

Medaka fish stem cells and their applications

Stem cells are present in developing embryos and adult tissues of multicellular organisms. Owing to their unique features, stem cells provide excellent opportunities for experimental analyses of basic developmental processes such as pluripotency control and cell fate decision and for regenerative medicine by stem cell-based therapy. Stem cell cultures have been best studied in 3 vertebrate organisms. These are the mouse, human and a small laboratory fish called medaka. Specifically, medaka has given rise to the first embryonic stem (ES) cells besides the mouse, the first adult testis-derived male stem cells spermatogonia capable of test-tube sperm production, and most recently, even haploid ES cells capable of producing Holly, a semi-cloned fertile female medaka from a mosaic oocyte created by microinjecting a haploid ES cell nucleus directly into a normal oocyte. These breakthroughs make medaka a favoring vertebrate model for stem cell research, the topic of this review.

Fish germ cells

Fish, like many other animals, have two major cell lineages, namely the germline and soma. The germ-soma separation is one of the earliest events of embryonic development. Germ cells can be specifically labeled and isolated for culture and transplantation, providing tools for reproduction of endangered species in close relatives, such as surrogate production of trout in salmon. Haploid cell cultures, such as medaka haploid embryonic stem cells have recently been obtained, which are capable of mimicking sperm to produce fertile offspring, upon nuclear being directly transferred into normal eggs. Such fish originated from a mosaic oocyte that had a haploid meiotic nucleus and a transplanted haploid mitotic cell culture nucleus. The first semi-cloned fish is Holly. Here we review the current status and future directions of understanding and manipulating fish germ cells in basic research and reproductive technology.

Genetic basis and breeding application of clonal diversity and dual reproduction modes in polyploid Carassius auratus gibelio

A unisexual species is generally associated with polyploidy, and reproduced by a unisexual reproduction mode, such as gynogenesis, hybridogenesis or parthenogenesis. Compared with other unisexual and polyploid species, gibel carp (Carassius auratus gibelio) has a higher ploidy level of hexaploid. It has undergone several successive rounds of genome polyploidy, and experienced an additional, more recent genome duplication event. More significantly, the dual reproduction modes, including gynogenesis and sexual reproduction, have been demonstrated to coexist in the polyploid gibel carp. This article reviews the genetic basis concerning polyploidy origin, clonal diversity and dual reproduction modes, and outlines the progress in new variety breeding and gene identification involved in the reproduction and early development. The data suggests that gibel carp are under an evolutionary trajectory of diploidization. As a novel evolutionary developmental (Evo-Devo) biology model, this work highlights future perspectives about the functional divergence of duplicated genes and the sexual origin of vertebrate animals.

Accessibility of host cell lineages to medaka stem cells depends on genetic background and irradiation of recipient embryos

Chimera formation is a powerful tool for analyzing pluripotency in vivo. It has been widely accepted that host cell lineages are generally accessible to embryonic stem (ES) cells with the actual contribution depending solely on the intrinsic pluripotency of transplanted donor cells. Here, we show in the fish medaka (Oryzias latipes) that the host accessibility to ES cell contribution exhibits dramatic differences. Specifically, of three albino host strains tested (i (1) , i (3) and af), only strain i (1) generated pigmented chimeras. Strikingly, this accessibility is completely lost in i (1) but acquired in i (3) after host gamma-irradiation. Host irradiation also differentially affected ES cell contribution to somatic organs and gonad. Therefore, the accessibility of various host cell lineages can vary considerably depending on host strains and cell lineages as well as on irradiation. Our findings underscore the importance of host genotypes for interpreting donor cell pluripotency and for improving ES-derived chimera production.