Advances in genetic engineering of the avian genome: "Realising the promise"

Overview of Avian Sex Reversal

Sex determination and differentiation are processes by which a bipotential gonad adopts either a testicular or ovarian cell fate, and secondary sexual characteristics adopt either male or female developmental patterns. In birds, although genetic factors control the sex determination program, sex differentiation is sensitive to hormones, which can induce sex reversal when disturbed. Although these sex-reversed birds can form phenotypes opposite to their genotypes, none can experience complete sex reversal or produce offspring under natural conditions. Promising evidence indicates that the incomplete sex reversal is associated with cell autonomous sex identity (CASI) of avian cells, which is controlled by genetic factors. However, studies cannot clearly describe the regulatory mechanism of avian CASI and sex development at present, and these factors require further exploration. In spite of this, the abundant findings of avian sex research have provided theoretical bases for the progress of gender control technologies, which are being improved through interdisciplinary co-operation and will ultimately be employed in poultry production. In this review, we provide an overview of avian sex determination and differentiation and comprehensively summarize the research progress on sex reversal in birds, especially chickens. Importantly, we describe key issues faced by applying gender control systems in poultry production and chronologically summarize the development of avian sex control methods. In conclusion, this review provides unique perspectives for avian sex studies and helps scientists develop more advanced systems for sex regulation in birds.

Formation, Application, and Significance of Chicken Primordial Germ Cells: A Review

Chicken is one of the most widely consumed sources of protein globally. Primordial germ cells (PGCs) are the precursors for ova and sperm. One of the early embryogenesis events in most animals is the segregation of the somatic and germ lineages. PGC cultures occur in the germline, and PGCs are less studied in many species. It is relatively challenging to separate, cultivate, and genetically alter chicken without mutating the basic germline. The present study aims to gather previous research about chicken PGCs and provide a customized review of studies and developments in the field of PGCs, especially for avian species. Furthermore, we show that the propagation of chicken PGCs into embryonic germ cells that contribute to somatic tissues may be produced in vitro. Primordial germ cells offer an ideal system in developmental biology, as these cells play a vital role in the genetic modification and treatment of infertility. Cryopreservation helps to maintain genetic resources and sustainable production in the poultry industry. Keeping in mind the significance of cryopreservation for storage and gametogenesis, we discuss its role in the preservation of primordial germ cells. Transgenesis and genetic modifications in chicken lead to the development of various medicinal chicken varieties and aid in improving their production and quality for consumption purposes. Additionally, these characteristics open up new possibilities for modifying the chicken genome for agricultural and medical purposes.

ATAC-seq and RNA-seq analysis unravel the mechanism of sex differentiation and infertility in sex reversal chicken

BACKGROUND

Sex determination and differentiation are complex and delicate processes. In female chickens, the process of sex differentiation is sensitive and prone to be affected by the administration of aromatase inhibitors, which result in chicken sex reversal and infertility. However, the molecular mechanisms underlying sex differentiation and infertility in chicken sex reversal remain unclear. Therefore, we established a sex-reversed chicken flock by injecting an aromatase inhibitor, fadrozole, and constructed relatively high-resolution profiles of the gene expression and chromatin accessibility of embryonic gonads.

RESULTS

We revealed that fadrozole affected the transcriptional activities of several genes, such as DMRT1, SOX9, FOXL2, and CYP19A1, related to sex determination and differentiation, and the expression of a set of gonadal development-related genes, such as FGFR3 and TOX3, by regulating nearby open chromatin regions in sex-reversed chicken embryos. After sexual maturity, the sex-reversed chickens were confirmed to be infertile, and the possible causes of this infertility were further investigated. We found that the structure of the gonads and sperm were greatly deformed, and we identified several promising genes related to spermatogenesis and infertility, such as SPEF2, DNAI1, and TACR3, through RNA-seq.

CONCLUSIONS

This study provides clear insights into the exploration of potential molecular basis underlying sex differentiation and infertility in sex-reversed chickens and lays a foundation for further research into the sex development of birds.

Transcriptome Sequencing Analysis of circRNA in Skeletal Muscle between Fast- and Slow-Growing Chickens at Embryonic Stages

Skeletal muscle growth has always been the focus of the broiler industry, and circRNAs play a significant role in this process. We collected leg muscles of slow- and fast-growing Bian chicken embryos in the study at 14 (S14 and F14) and 20 (S20 and F20) days for RNA-seq. Finally, 123 and 121 differentially expressed circRNAs (DECs) were identified in S14 vs. F14 and S20 vs. F20, respectively. GO enrichment analysis for DECs obtained important biological process (BP) terms including nicotinate nucleotide biosynthetic process, nicotinate nucleotide salvage, and NAD salvage in S20 vs. F20 and protein mannosylation in S14 vs. F14. KEGG pathway analysis showed Wnt signaling pathway, Tight junction, Ubiquitin mediated proteolysis, and Notch signaling pathway were enriched in the top 20. Based on the GO and KEGG analysis results, we found some significant host genes and circRNAs such as NAPRT and novel_circ_0004547, DVL1 and novel_circ_0003578, JAK2 and novel_circ_0010289, DERA and novel_circ_0003082, etc. Further analysis found 19 co-differentially expressed circRNAs between the two comparison groups. We next constructed a circRNA-miRNA network for them, and some candidate circRNA-miRNA pairs related to skeletal muscle were obtained, such as novel_circ_0002153-miR-12219-5p, novel_circ_0003578-miR-3064-3p, and novel_circ_0010661-miR-12260-3p. These results would help to reveal the mechanism for circRNAs in skeletal muscle and also provide some guidance for the breeding of broilers.

Ethical Eggs: Can Synthetic Biology Disrupt the Global Egg Production Industry?

Commercial egg production relies on the industry-accepted practice of culling day-old male chicks, which are a live by-product of the egg production industry. Researchers are exploring the use of a transgenic marker gene to allow early identification of male embryos in ovo at the point of lay, rather than upon hatching. Here we examine social acceptability of this biotechnology-enabled solution to sex selection, which addresses the key ethical issue of culling and improved sustainability of food systems. A national online survey (N = 1148) measured psychological factors influencing public support for the development of the technology and willingness to purchase eggs derived from the novel process. Most participants expressed at least a moderate intention to support the development of gene marking technology, with 1 in 5 people expressing strong support. Participants expressed moderate to high agreement that gene marking of chickens would: (a) help reduce or eliminate the practice of culling male chicks in the egg-laying industry (response efficacy), and; (b) that this new synbio approach to sex selection may be better than current methods of identifying and removing male chicks during egg production (relative advantage). Of those participants who consumed eggs, almost 60% reported they would be moderately to strongly willing to purchase eggs derived from the gene marking process. A partially-mediated path model comprising both intention to support and willingness to buy eggs (R2 = 0.78) showed that key factors involved in decision-making, in addition to response efficacy and relative advantage, were evaluative attitudes toward the technology (e.g., was the technology bad/good, risky/safe, unethical/ethical) and emotional reactions. These results suggest that consumers may be primarily basing their decisions and behavioral choices on how valuable they perceive the novel gene marking solution, reflecting on how it compares favorably to current culling practices, yielding a range of benefits such as higher animal welfare, improved sustainability, and reduced waste.

Identification of crucial circRNAs in skeletal muscle during chicken embryonic development

Background

Chicken provides humans with a large amount of animal protein every year, in which skeletal muscle plays a leading role. The embryonic skeletal muscle development determines the number of muscle fibers and will affect the muscle production of chickens. CircRNAs are involved in a variety of important biological processes, including muscle development. However, studies on circRNAs in the chicken embryo muscle development are still lacking.

Results

In the study, we collected chicken leg muscles at 14 and 20-day embryo ages both in the fast- and slow-growing groups for RNA-seq. We identified 245 and 440 differentially expressed (DE) circRNAs in the comparison group F14vsF20 and S14vsS20 respectively. GO enrichment analysis for the host genes of DE circRNAs showed that biological process (BP) terms in the top 20 related to growth in F14vsF20 were found such as positive regulation of transcription involved in G1/S phase of mitotic cell cycle, multicellular organismal macromolecule metabolic process, and multicellular organismal metabolic process. In group S14vsS20, we also found some BP terms associated with growth in the top 20 including actomyosin structure organization, actin cytoskeleton organization and myofibril assembly. A total of 7 significantly enriched pathways were obtained, containing Adherens junction and Tight junction. Further analysis of those pathways found three crucial host genes MYH9, YBX3, IGF1R in both fast- and slow-growing groups, three important host genes CTNNA3, AFDN and CREBBP only in the fast-growing group, and six host genes FGFR2, ACTN2, COL1A2, CDC42, DOCK1 and MYL3 only in the slow-growing group. In addition, circRNA-miRNA network also revealed some key regulation pairs such as novel_circ_0007646-miR-1625-5p, novel_circ_0007646-miR-1680-5p, novel_circ_0008913-miR-148b-5p, novel_circ_0008906-miR-148b-5p and novel_circ_0001640-miR-1759-3p.

Conclusions

Comprehensive analysis of circRNAs and their targets would contribute to a better understanding of the molecular mechanisms in poultry skeletal muscle and it also plays an important guiding role in the next research.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12864-022-08588-4.

Towards progressive regulatory approaches for agricultural applications of animal biotechnology

Traditional breeding techniques, applied incrementally over thousands of years, have yielded huge benefits in the characteristics of agricultural animals. This is a result of significant, measurable changes to the genomes of those animal species and breeds. Genome editing techniques may now be applied to achieve targeted DNA sequence alterations, with the potential to affect traits of interest to production of agricultural animals in just one generation. New opportunities arise to improve characteristics difficult to achieve or not amenable to traditional breeding, including disease resistance, and traits that can improve animal welfare, reduce environmental impact, or mitigate impacts of climate change. Countries and supranational institutions are in the process of defining regulatory approaches for genome edited animals and can benefit from sharing approaches and experiences to institute progressive policies in which regulatory oversight is scaled to the particular level of risk involved. To facilitate information sharing and discussion on animal biotechnology, an international community of researchers, developers, breeders, regulators, and communicators recently held a series of seven virtual workshop sessions on applications of biotechnology for animal agriculture, food and environmental safety assessment, regulatory approaches, and market and consumer acceptance. In this report, we summarize the topics presented in the workshop sessions, as well as discussions coming out of the breakout sessions. This is framed within the context of past and recent scientific and regulatory developments. This is a pivotal moment for determination of regulatory approaches and establishment of trust across the innovation through-chain, from researchers, developers, regulators, breeders, farmers through to consumers.

Transcriptomic Analysis of MSTN Knockout in the Early Differentiation of Chicken Fetal Myoblasts

In mammals, Myostatin (MSTN) is a known negative regulator of muscle growth and development, but its role in birds is poorly understood. To investigate the molecular mechanism of MSTN on muscle growth and development in chickens, we knocked out MSTN in chicken fetal myoblasts (CFMs) and sequenced the mRNA transcriptomes. The amplicon sequencing results show that the editing efficiency of the cells was 76%. The transcriptomic results showed that 296 differentially expressed genes were generated after down-regulation of MSTN, including angiotensin I converting enzyme (ACE), extracellular fatty acid-binding protein (EXFABP) and troponin T1, slow skeletal type (TNNT1). These genes are closely associated with myoblast differentiation, muscle growth and energy metabolism. Subsequent enrichment analysis showed that DEGs of CFMs were related to MAPK, Pl3K/Akt, and STAT3 signaling pathways. The MAPK and Pl3K/Akt signaling pathways are two of the three known signaling pathways involved in the biological effects of MSTN in mammals, and the STAT3 pathway is also significantly enriched in MSTN knock out chicken leg muscles. The results of this study will help to understand the possible molecular mechanism of MSTN regulating the early differentiation of CFMs and lay a foundation for further research on the molecular mechanism of MSTN involvement in muscle growth and development.

Effective MSTN Gene Knockout by AdV-Delivered CRISPR/Cas9 in Postnatal Chick Leg Muscle

Muscle growth and development are important aspects of chicken meat production, but the underlying regulatory mechanisms remain unclear and need further exploration. CRISPR has been used for gene editing to study gene function in mice, but less has been done in chick muscles. To verify whether postnatal gene editing could be achieved in chick muscles and determine the transcriptomic changes, we knocked out Myostatin (MSTN), a potential inhibitor of muscle growth and development, in chicks and performed transcriptome analysis on knock-out (KO) muscles and wild-type (WT) muscles at two post-natal days: 3d (3-day-old) and 14d (14-day-old). Large fragment deletions of MSTN (>5 kb) were achieved in all KO muscles, and the MSTN gene expression was significantly downregulated at 14d. The transcriptomic results indicated the presence of 1339 differentially expressed genes (DEGs) between the 3d KO and 3d WT muscles, as well as 597 DEGs between 14d KO and 14d WT muscles. Many DEGs were found to be related to cell differentiation and proliferation, muscle growth and energy metabolism. This method provides a potential means of postnatal gene editing in chicks, and the results presented here could provide a basis for further investigation of the mechanisms involved in muscle growth and development.

Potential benefits of gene editing for the future of poultry farming

The chicken is an exemplar of efficient intensive animal agriculture and provides two valuable food products, chicken meat and eggs. Only aquaculture is better, by efficiency, but poultry is still top, by mass of animal protein produced as food in the global context. However this efficiency and intensive production comes with a number of challenges. Though the genetics of selective breeding have led to dramatic improvements in yield, efficiency and product quality, traits that relate to disease and welfare outcomes have not been so tractable. These two issues are major impacts to the industry in terms of production and in terms of public perception. Both transgenic technology and genome editing have clear potential for impact in these two important areas. The reproductive biology of birds requires techniques very specific to birds to achieve heritable (germline) edited traits. These are quite involved and, even though they are now well-defined and reliable, there is room for improvement and advances can be expected in the future. Currently the key targets for this technology are modifying chicken genes involved in virus-receptor interactions and cellular response involved in infection. For the egg industry the technology is being applied to the issue of sex-selection for layer hens (and the removal of males), removal of allergens from egg white and the tailoring of eggs system to enhance the yield of influenza vaccine doses. Regulation and trading of the animals generated, and resulting food products, will significantly impact the value and future development of genome editing for poultry.

Embryonic Chicken (Gallus gallus domesticus) as a Model of Cardiac Biology and Development

Cardiovascular disease remains one of the top contributors to morbidity and mortality in the United States. Increasing evidence suggests that many processes, pathways, and programs observed during development and organogenesis are recapitulated in adults in the face of disease. Therefore, a heightened understanding of cardiac development and organogenesis will help increase our understanding of developmental defects and cardiovascular diseases in adults. Chicks have long served as a model system in which to study developmental problems. Detailed descriptions of morphogenesis, low cost, accessibility, ease of manipulation, and the optimization of genetic engineering techniques have made chicks a robust model for studying development and make it a powerful platform for cardiovascular research. This review summarizes the cardiac developmental milestones of embryonic chickens, practical considerations when working with chicken embryos, and techniques available for use in chicks (including tissue chimeras, genetic manipulations, and live imaging). In addition, this article highlights examples that accentuate the utility of the embryonic chicken as model system in which to study cardiac development, particularly epicardial development, and that underscore the importance of how studying development informs our understanding of disease.

Comparison of the MicroRNA Expression Profiles of Male and Female Avian Primordial Germ Cell Lines

Primordial germ cells (PGCs) are the precursors of adult germ cells, and among the embryonic stem-like cells in the bird embryo, only they can transmit the genetic information to the next generation. Despite the wide range of applications, very little is known about the mechanism that governs primordial germ cell self-renewal and differentiation. As a first step, we compared 12 newly established chicken PGC lines derived from two different chicken breeds, performing CCK-8 proliferation assay. All of the lines were derived from individual embryos. A significant difference was found among the lines. As microRNAs have been proved to play a key role in the maintenance of pluripotency and the cell cycle regulation of stem cells, we continued with a complex miRNA analysis. We could discover miRNAs expressing differently in PGC lines with high proliferation rate, compared to PGC lines with low proliferation rate. We found that gga-miR-2127 expresses differently in female and male cell lines. The microarray analysis also revealed high expression level of the gga-miR-302b-3p strand (member of the miR-302/367 cluster) in slowly proliferating PGC lines compared to the gga-miR-302b-5p strand. We confirmed that the inhibition of miR-302b-5p significantly increases the doubling time of the examined PGC lines. In conclusion, we found that gga-miR-181-5p, gga-miR-2127, and members of the gga-miR-302/367 cluster have a dominant role in the regulation of avian primordial germ cell proliferation.

Surgical Extraction of Viable Hen ( Gallus gallus domesticus) Follicles for In Vitro Fertilization

Assisted reproduction techniques in birds have been developed for zootechnical purposes and have been adapted for use in conservation of wild bird species. To develop a technique for obtaining follicles in live hens, 5 Rhode Island red hens ( Gallus gallus domesticus) were anesthetized, and abdominal ultrasound was performed to confirm the presence of ovarian follicles. A left celiotomy then was performed to obtain follicles in different stages of maturation for in vitro fertilization. The follicles were located by digital exploration, then extracted by isolating each follicle with the index finger of each hand, holding it by the stigma, and then applying slight traction towards the exterior of the coelomic cavity until the follicle separated from the ovary. In total, 18 of 30 (60%) follicles obtained were suitable for in vitro fertilization, but only 3 (16%) were fertilized successfully. All birds recovered from the procedure and remained in good condition postoperatively. Perfecting assisted reproduction technique holds potential benefits for determining sex of embryos by blastomeres sexing, supporting the conservation efforts of avian species, and benefiting research areas, such as genetic and biopharmaceutical research.

Regulation and function of avian selenogenome

BACKGROUND

Selenium (Se) is an essential micronutrient required by avian species. Dietary Se/vitamin E deficiency induces three classical diseases in chicks: exudative diathesis, nutritional pancreatic atrophy, and nutritional muscular dystrophy.

SCOPE OF REVIEW

This review is to summarize and analyze the evolution, regulation, and function of avian selenogenome and selenoproteome and their relationship with the three classical Se/vitamin E deficiency diseases.

MAJOR CONCLUSIONS

There are 24 selenoproteins confirmed in chicks, with two avian-specific members (SELENOU and SELENOP2) and two missing mammalian members (GPX6 and SELENOV). There are two forms of SELENOP containing 1 or 13 selenocysteine residues. In addition, a Gallus gallus gene was conjectured to be the counterpart of the human SEPHS2. Expression of selenoprotein genes in the liver, pancreas, and muscle of chicks seemed to be highly responsive to dietary Se changes. Pathogeneses of the Se/vitamin E deficient diseases in the chicks were likely produced by missing functions of selected selenoproteins in regulating cellular and tissue redox balance and inhibiting oxidative/reductive stress-induced cell death.

GENERAL SIGNIFICANCE

Gene knockout models, similar to those of rodents, will help characterize the precise functions of avian selenoproteins and their comparisons with those of mammalian species.

Molecular signatures of epithelial oviduct cells of a laying hen (Gallus gallus domesticus) and quail (Coturnix japonica)

Background

In this work we have determined molecular signatures of oviduct epithelial and progenitor cells. We have proposed a panel of selected marker genes, which correspond with the phenotype of oviduct cells of a laying hen (Gallus gallus domesticus) and quail (Coturnix japonica). We demonstrated differences in characteristics of those cells, in tissue and in vitro, with respect to different anatomical and functional parts of the oviduct (infundibulum (INF), distal magnum (DM, and proximal magnum (PM)). The following gene expression signatures were studied: (1) oviduct markers (estrogen receptor 1, ovalbumin, and SPINK7 - ovomucoid), (2) epithelial markers (keratin 5, keratin 14, and occludin) and (3) stem-like/progenitor markers (CD44 glycoprotein, LGR5, Musashi-1, and sex determining region Y-box 9, Nanog homebox, OCT4/cPOUV gene encoding transcription factor POU5F3).

Results

In chicken, the expression of oviduct markers increased toward the proximal oviduct. Epithelial markers keratin14 and occludin were high in distal oviduct and decreased toward the proximal magnum. In quail oviduct tissue, the gene expression pattern of oviduct/epithelial markers was similar to chicken. The markers of progenitors/stemness in hen oviduct (Musashi-1 and CD44 glycoprotein) had the highest relative expression in the infundibulum and decreased toward the proximal magnum. In quail, we found significant expression of four progenitor markers (LGR5 gene, SRY sex determining region Y-box 9, OCT4/cPOUV gene, and CD44 glycoprotein) that were largely present in the distal oviduct. After in vitro culture of oviduct cells, the gene expression pattern has changed. High secretive potential of magnum-derived cells diminished by using decreased abundance of mRNA. On the other hand, chicken oviduct cells originating from the infundibulum gained ability to express OVM and OVAL. Epithelial character of the cells was maintained in vitro. Among progenitor markers, both hen and quail cells expressed high level of SOX9, LGR5 and Musashi-1.

Conclusion

Analysis of tissue material revealed gradual increase/decrease pattern in majority of the oviduct markers in both species. This pattern changed after the oviductal cells have been cultured in vitro. The results can provide molecular tools to validate the phenotype of in vitro biological models from reproductive tissue.

Electronic supplementary material

The online version of this article (10.1186/s12861-018-0168-2) contains supplementary material, which is available to authorized users.

The novel T755C mutation in <i>BMP15</i> is associated with the litter size of Iranian Afshari, Ghezel, and Shal breeds

Abstract. The present study was conducted to determine the molecular genetic variation in bone morphogenetic protein 15 (BMP15) of some selected Iranian sheep breeds and aims to provide relevant genetic information for twin-birth lambing. Therefore, a total of 44 rams and 213 ewes with single-, twin-, and triplet-birth lambing were used from Shal, Ghezel, Afshari and Lori-Bakhtiari sheep breeds. Three primer pairs were used for amplification of exons 1 and 2 of the BMP15 gene. The sequencing result of distinctive single-strand conformation polymorphism (SSCP) patterns showed segregation of the T755C mutation and the ensuing non-conservative substitution of L252P in the BMP15 propeptide. All three ewes with triplet-birth lambing and two sterile ewes were heterozygotic for this mutation. Association analysis revealed that this mutation was significantly related to the ewe's litter size, so that ewes with the CT genotype had 0.24 and 0.30 more lambs than those with the CC (p < 0.05) and TT (p < 0.01) genotypes, respectively. The mutation effect on the litter size was incomplete dominance in Afshari breed and over-dominance in Shal and Ghezel breeds. In conclusion, the findings in this experiment suggested synergistic effects of the T755C mutation and other unknown mutations in other effective genes which cause infertility and higher litter size in two different ways.

Sex selection in layer chickens

The ability to detect and remove male chicks pre-hatch would be a big step forward to the egg-laying and related industries. The current practice of culling male chicks post-hatch creates a major ethical dilemma for many countries. Hatching out and growing male layer chicks is not a sustainable option for farmers. A genetic based in ovo sex selection application would effectively negate the need to cull or grow out male chickens and would contribute to a more sustainable industry with a view to future food security. Recent advancements in avian gene technology allow specific marking of the sex-determining chromosome in chickens so that the males can be identified before hatching and removed before incubation. This provides a simple solution to meet a pressing need for the industry and a leading opportunity for the adoption of biotechnology in animal agriculture.

The chick eye in vision research: An excellent model for the study of ocular disease

The domestic chicken, Gallus gallus, serves as an excellent model for the study of a wide range of ocular diseases and conditions. The purpose of this manuscript is to outline some anatomic, physiologic, and genetic features of this organism as a robust animal model for vision research, particularly for modeling human retinal disease. Advantages include a sequenced genome, a large eye, relative ease of handling and maintenance, and ready availability. Relevant similarities and differences to humans are highlighted for ocular structures as well as for general physiologic processes. Current research applications for various ocular diseases and conditions, including ocular imaging with spectral domain optical coherence tomography, are discussed. Several genetic and non-genetic ocular disease models are outlined, including for pathologic myopia, keratoconus, glaucoma, retinal detachment, retinal degeneration, ocular albinism, and ocular tumors. Finally, the use of stem cell technology to study the repair of damaged tissues in the chick eye is discussed. Overall, the chick model provides opportunities for high-throughput translational studies to more effectively prevent or treat blinding ocular diseases.

Germline Gene Editing in Chickens by Efficient CRISPR-Mediated Homologous Recombination in Primordial Germ Cells

The CRISPR/Cas9 system has been applied in a large number of animal and plant species for genome editing. In chickens, CRISPR has been used to knockout genes in somatic tissues, but no CRISPR-mediated germline modification has yet been reported. Here we use CRISPR to target the chicken immunoglobulin heavy chain locus in primordial germ cells (PGCs) to produce transgenic progeny. Guide RNAs were co-transfected with a donor vector for homology-directed repair of the double-strand break, and clonal populations were selected. All of the resulting drug-resistant clones contained the correct targeting event. The targeted cells gave rise to healthy progeny containing the CRISPR-targeted locus. The results show that gene-edited chickens can be obtained by modifying PGCs in vitro with the CRISPR/Cas9 system, opening up many potential applications for efficient genetic modification in birds.