A controllable on-off strategy for the reproductive containment of fish

Application of Repetitive Sequences in Fish Cell Depletion as a Target for the CRISPR/Cas9 System

Specific cell depletion is a common means to study the physiological function of cell lineages and tissue regeneration. However, 100% depletion is difficult to achieve with existing cell depletion strategies. With the increasing maturity of CRISPR/Cas9 technology, it is increasingly used for the depletion of various cells. However, even with this technology, it is difficult to complete the depletion of specific gene knockout cells. For this reason, cell depletion with the use of repetitive sequences as the target of CRISPR/Cas9 was explored using zebrafish. All cells were used as the target cells for the first set of experiments. The results showed that injection of a mixture of DANA-gRNA and Cas9 mRNA into zygotes resulted in substantial cell apoptosis. Cells are almost invisible in the embryonic animal pole during the dome stage. The activities of the caspase-3 and caspase-9 proteins and the mRNA level of the P53 gene were significantly increased. Then, primordial germ cells (PGCs) in embryos were used as the target cells in subsequent experiments. To specifically knock out PGCs, we injected the mix of DANA-gRNA, pkop: Cas9 plasmid (the kop promotor allows Cas9 expression only in PGCs), and eGFP-nos3'UTR mRNA into zebrafish fertilized eggs. The results revealed that the activity of the caspase-3 protein was significantly increased, and the mRNA levels of P53, ku70, and ku80 were significantly upregulated, while the number of PGCs decreased gradually. Few PGCs labeled with GFP could be seen 20 h post-fertilization (hpf), and no PGCs could be seen at the germinal ridge 24 hpf. Therefore, the combination of CRISPR/Cas9 technology and repetitive sequences can achieve efficient cell depletion regardless of whether there is generalized expression or expression in specific cells. These results indicate that it is feasible to eliminate cells by using repeat sequences as CRISPR/Cas9 system target sites.

Insights into Early Ontogenesis of Salmo salar: RNA Extraction, Housekeeping Gene Validation and Transcriptional Expression of Important Primordial Germ Cell and Sex-Determination Genes

The challenge in extracting high-quality RNA impedes the investigation of the transcriptome of developing salmonid embryos. Furthermore, the mRNA expression pattern of important PGC and SD genes during the initial embryonic development of Salmo salar is yet to be studied. So, in the present study, we aimed to isolate high-quality RNA from eggs and developing embryos to check vasa, dnd1, nanos3a, sdf1, gsdf, amh, cyp19a, dmrt1 and foxl2 expression by qPCR. Additionally, four HKGs (GAPDH, UB2L3, eEf1a and β-actin) were validated to select the best internal control for qPCR. High-quality RNA was extracted, which was confirmed by spectrophotometer, agarose gel electrophoresis and Agilent TapeStation analysis. UB2L3 was chosen as a reference gene because it exhibited lower intra- and inter-sample variation. vasa transcripts were expressed in all the developmental stages, while dnd1 was expressed only up to 40 d°C. Nanos3a was expressed in later stages and remained at its peak for a shorter period, while sdf1 showed an irregular pattern of mRNA expression. The mRNA expression levels of SD genes were observed to be upregulated during the later stages of development, prior to hatching. This study presents a straightforward methodology for isolating high-quality RNA from salmon eggs, and the resulting transcript profiles of significant PGC and SD genes in S. salar could aid in improving our comprehension of reproductive development in this commercially important species.

Rescue of germ cells in dnd crispant embryos opens the possibility to produce inherited sterility in Atlantic salmon

Genetic introgression of escaped farmed Atlantic salmon (Salmo salar) into wild populations is a major environmental concern for the salmon aquaculture industry. Using sterile fish in commercial aquaculture operations is, therefore, a sustainable strategy for bio-containment. So far, the only commercially used methodology for producing sterile fish is triploidization. However, triploid fish are less robust. A novel approach in which to achieve sterility is to produce germ cell-free salmon, which can be accomplished by knocking out the dead-end (dnd) gene using CRISPR-Cas9. The lack of germ cells in the resulting dnd crispants, thus, prevents reproduction and inhibits subsequent large-scale production of sterile fish. Here, we report a rescue approach for producing germ cells in Atlantic salmon dnd crispants. To achieve this, we co-injected the wild-type (wt) variant of salmon dnd mRNA together with CRISPR-Cas9 constructs targeting dnd into 1-cell stage embryos. We found that rescued one-year-old fish contained germ cells, type A spermatogonia in males and previtellogenic primary oocytes in females. The method presented here opens a possibility for large-scale production of germ-cell free Atlantic salmon offspring through the genetically sterile broodstock which can pass the sterility trait on the next generation.

Gnrh3 Regulates PGC Proliferation and Sex Differentiation in Developing Zebrafish

Gonadotropin-releasing hormone (Gnrh) plays important roles in reproduction by stimulating luteinizing hormone release, and subsequently ovulation and sperm release, ultimately controlling reproduction in many species. Here we report on a new role for this decapeptide. Surprisingly, Gnrh3-null zebrafish generated by CRISPR/Cas9 exhibited a male-biased sex ratio. After the dome stage, the number of primordial germ cells (PGCs) in gnrh3-/- fish was lower than that in wild-type, an effect that was partially rescued by gnrh3 overexpression. A terminal deoxynucleotidyl transferase dUTP nick-end labeling (TUNEL) analysis revealed no detectable apoptosis of PGCs in gnrh3-/- embryos. Proliferating PGCs could be detected in wild-type embryos, while there was no detectable signal in gnrh3-/- embryos. Compared with wild type, the phosphorylation of AKT was not significantly different in gnrh3-/- embryos, but the phosphorylation of ERK1/2 decreased significantly. Treatment with a Gnrh analog (Alarelin) induced ERK1/2 phosphorylation and increased PGC numbers in both wild-type and gnrh3-/- embryos, and this was blocked by the MEK inhibitor PD0325901. The relative expression of sox9a, amh, and cyp11b were significantly upregulated, while cyp19a1a was significantly downregulated at 18 days post-fertilization in gnrh3-/- zebrafish. Taken together, these results indicate that Gnrh3 plays an important role in early sex differentiation by regulating the proliferation of PGCs through a MAPK-dependent path.

Potential of Genome Editing to Improve Aquaculture Breeding and Production

Aquaculture is the fastest growing food production sector and is rapidly becoming the primary source of seafood for human diets. Selective breeding programs are enabling genetic improvement of production traits, such as disease resistance, but progress is limited by the heritability of the trait and generation interval of the species. New breeding technologies, such as genome editing using CRISPR/Cas9 have the potential to expedite sustainable genetic improvement in aquaculture. Genome editing can rapidly introduce favorable changes to the genome, such as fixing alleles at existing trait loci, creating de novo alleles, or introducing alleles from other strains or species. The high fecundity and external fertilization of most aquaculture species can facilitate genome editing for research and application at a scale that is not possible in farmed terrestrial animals.

Dnd is required for primordial germ cell specification in Oryzias celebensis

Dead end (dnd) is a germ plasm component that plays an essential role for primordial germ cell (PGC) development in vertebrates. Previously, we have found that dnd is the first fish PGC specifier in medaka. Here, we present an additional evidence that dnd is the determinant for PGC specification in Oryzias celebensis. In adult tissues, the O. celebensis dnd (Ocdnd) RNA shows germ cells specific expression in gonads. In the testis, Ocdnd RNA is strongly detected in spermatogonia and meiotic cells and gradually decreases during the spermatogenesis. In the ovary, Ocdnd RNA is present throughout oogenesis. In the embryos, Ocdnd RNA is maternally provided and asymmetrically localized to prominent particles of presumptive PGCs before gastrulation stage and restricted to PGCs subsequently. In addition, Ocdnd 3' UTR can induce specific and stabilized GFP reporter expression in PGCs. Furthermore, knockdown of Ocdnd by morpholino (MO) injection abolishes the PGCs formation and this can be rescued by co-injection of medaka dnd (Oldnd) mRNA. More importantly, overexpression of Oldnd mRNA surprisingly boosts PGCs number. These results provide insights into function of dnd as a conserved specifier of PGCs in the genus Oryzias.

Generation of all-male-like sterile zebrafish by eliminating primordial germ cells at early development

Production of all-male and sterile fish may not only substantially improve yield but also be crucial for the application of genome modified species in aquaculture. Previously, it was reported that the fish lacking primordial germ cells (PGCs) becomes infertile, and nitroreductase, an enzyme converting non-toxic metronidazole (MTZ) into toxic metabolites, induces targeted toxicity to kill the cells expressing it. In this study, we generated a transgenic zebrafish line of Tg(nanos3:nfsB-mCherry-nanos3 3'UTR) in which the NfsB nitroreductase is solely expressed in PGCs. Treating the embryos derived from the female transgenic zebrafish with MTZ from 0 through 2 dpf (days post fertilization), we found that the germ cells were completely eliminated in the ones older than 2.5 dpf. At 20 dpf, the MTZ-treated juvenile had no germ cells in their gonads. At 100 dpf, the MTZ-treated adult exhibited male-like morphology and showed normal mating behaviors although they had no germ cells but only supporting cells in their gonads. Taken together, our results demonstrated that conditional elimination of PGCs during early development make the zebrafish male-like and infertile. It may provide an alternative strategy to make sterile and all-male farmed fish that is good for increasing aquaculture yield and preventing the genome modified species from potential ecological risks.

Mettl3 Mutation Disrupts Gamete Maturation and Reduces Fertility in Zebrafish

N⁶-methyladenosine (m⁶A), catalyzed by Mettl3 methyltransferase, is a highly conserved epigenetic modification in eukaryotic messenger RNA (mRNA). Previous studies have implicated m⁶A modification in multiple biological processes, but the in vivo function of m⁶A has been difficult to study, because mettl3 mutants are embryonic lethal in both mammals and plants. In this study, we have used transcription activator-like effector nucleases and generated viable zygotic mettl3 mutant, Zmettl3^m/m , in zebrafish. We find that the oocytes in Zmettl3^m/m adult females are stalled in early development and the ratio of full-grown stage (FG) follicles is significantly lower than that of wild type. Human chorionic gonadotropin-induced ovarian germinal vesicle breakdown in vitro and the numbers of eggs ovulated in vivo are both decreased as well, while the defects of oocyte maturation can be rescued by sex hormone in vitro and in vivo In Zmettl3^m/m adult males, we find defects in sperm maturation and sperm motility is significantly reduced. Further study shows that 11-ketotestosterone (11-KT) and 17β-estradiol (E2) levels are significantly decreased in Zmettl3^m/m , and defective gamete maturation is accompanied by decreased overall m⁶A modification levels and disrupted expression of genes critical for sex hormone synthesis and gonadotropin signaling in Zmettl3^m/m Thus, our study provides the first in vivo evidence that loss of Mettl3 leads to failed gamete maturation and significantly reduced fertility in zebrafish. Mettl3 and m⁶A modifications are essential for optimal reproduction in vertebrates.

Genetic analysis of the reproductive axis in fish using genome-editing nucleases

Reproduction in fish is controlled by the brain-pituitary-gonad reproductive axis. Although genes of the reproductive axis are conserved from fish to humans, their in vivo functions are less clear in fish. Mutant lines of the reproductive axis have been systematically investigated in zebrafish and medaka using recently developed genome-editing nucleases. Here, we review recent progress in the genetic analysis of the reproductive axis in fish as well as the opportunities and challenges of applying genome-editing nucleases in fisheries.

Gene editing nuclease and its application in tilapia

Gene editing nucleases including zinc-finger nucleases (ZFNs), transcription activator like effector nucleases (TALENs) and clustered regularly interspaced short palindromic repeats (CRISPR)-CRISPR-associated (Cas) system (CRISPR/Cas9) provide powerful tools that improve our ability to understand the physiological processes and their underlying mechanisms. To date, these approaches have already been widely used to generate knockout and knockin models in a large number of species. Fishes comprise nearly half of extant vertebrate species and provide excellent models for studying many aspects of biology. In this review, we present an overview of recent advances in the use of gene editing nucleases for studies of fish species. We focus particularly on the use of TALENs and CRISPR/Cas9 genome editing for studying sex determination in tilapia.

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.

Synthetic bugs on the loose: containment options for deeply engineered (micro)organisms

Synthetic Biology (SynBio) has brought up again questions on the environmental fate of microorganisms carrying genetic modifications. The growing capacity of editing genomes for deployment of man-made programs opens unprecedented biotechnological opportunities. But the same exacerbate concerns regarding fortuitous or deliberate releases to the natural medium. Most approaches to tackle these worries involve endowing SynBio agents with containment devices for halting horizontal gene transfer and survival of the live agents only at given times and places. Genetic circuits and trophic restraint schemes have been proposed to this end in the pursuit of complete containment. The most promising include adoption of alternative genetic codes and/or dependency on xenobiotic amino acids and nucleotides. But the field has to still overcome serious bottlenecks.