A modified method of shell windowing for producing somatic or germline chimeras in fertilized chicken eggs

Avian Embryonic Culture: A Perspective of In Ovo to Ex Ovo and In Vitro Studies

The avian embryos growing outside the natural eggshell (ex ovo) were observed since the early 19th century, and since then chick embryonic structures have revealed reaching an in-depth view of external and internal anatomy, enabling us to understand conserved vertebrate development. However, the internal environment within an eggshell (in ovo) would still be the ideal place to perform various experiments to understand the nature of avian development and to apply other biotechnology techniques. With the advent of genetic manipulation and cell culture techniques, avian embryonic parts were dissected for explant culture to eventually generate expandable cell lines (in vitro cell culture). The expansion of embryonic cells allowed us to unravel the transcriptional network for understanding pluripotency and differentiation mechanism in the embryos and in combination with stem cell technology facilitated the applications of avian culture to the next levels in transgenesis and wildlife conservation. In this review, we provide a panoramic view of the relationship among different cultivation platforms from in ovo studies to ex ovo as well as in vitro culture of cell lines with recent advances in the stem cell fields.

Research Note: Embryonic viability by weight difference between donor and surrogate eggs in a surrogate eggshell incubation system

A surrogate eggshell incubation system is a well-defined method to apply to avian genetic modification. In this study, we tried to investigate whether the egg weight differences between donor and surrogate eggs have an effect on donor viability. The groups were divided by egg weight differences between the donor and surrogate eggs into 4 in each system. The viability at d 4 was evaluated at the end of System II, the embryos alive were transferred into the second surrogate eggshells, and the viability at d 5, 6 was evaluated at early phase of System III. Then, the viability of System III was evaluated at different incubation period: d 6–12, d 13–18, d 19–21, and hatching rate was evaluated at d 22. Although the effect of egg weight differences between the donor and surrogate eggs was not observed, a specific group in System III showed higher survival and hatching rate than other group (P > 0.05).

Designing A Transgenic Chicken: Applying New Approaches toward A Promising Bioreactor

Specific developmental characteristics of the chicken make it an attractive model for the generation of transgenic organisms. Chicken possess a strong potential for recombinant protein production and can be used as a powerful bioreactor to produce pharmaceutical and nutritional proteins. Several transgenic chickens have been generated during the last two decades via viral and non-viral transfection. Culturing chicken primordial germ cells (PGCs) and their ability for germline transmission ushered in a new stage in this regard. With the advent of CRISPR/Cas9 system, a new phase of studies for manipulating genomes has begun. It is feasible to integrate a desired gene in a predetermined position of the genome using CRISPR/Cas9 system. In this review, we discuss the new approaches and technologies that can be applied to generate a transgenic chicken with regards to recombinant protein productions.

Sexing of chicken eggs by fluorescence and Raman spectroscopy through the shell membrane

In order to provide an alternative to day-old chick culling in the layer hatcheries, a noninvasive method for egg sexing is required at an early stage of incubation before onset of embryo sensitivity. Fluorescence and Raman spectroscopy of blood offers the potential for precise and contactless in ovo sex determination of the domestic chicken (Gallus gallus f. dom.) eggs already during the fourth incubation day. However, such kind of optical spectroscopy requires a window in the egg shell, is thus invasive to the embryo and leads to decreased hatching rates. Here, we show that near infrared Raman and fluorescence spectroscopy can be performed on perfused extraembryonic vessels while leaving the inner egg shell membrane intact. Sparing the shell membrane makes the measurement minimally invasive, so that the sexing procedure does not affect hatching rates. We analyze the effect of the membrane above the vessels on fluorescence signal intensity and on Raman spectrum of blood, and propose a correction method to compensate for it. After compensation, we attain a correct sexing rate above 90% by applying supervised classification of spectra. Therefore, this approach offers the best premises towards practical deployment in the hatcheries.

Avian Biotechnology

Primordial germ cells (PGCs) generate new individuals through differentiation, maturation and fertilization. This means that the manipulation of PGCs is directly linked to the manipulation of individuals, making PGCs attractive target cells in the animal biotechnology field. A unique biological property of avian PGCs is that they circulate temporarily in the vasculature during early development, and this allows us to access and manipulate avian germ lines. Following the development of a technique for transplantation, PGCs have become central to avian biotechnology, in contrast to the use of embryo manipulation and subsequent transfer to foster mothers, as in mammalian biotechnology. Today, avian PGC transplantation combined with recent advanced manipulation techniques, including cell purification, cryopreservation, depletion, and long-term culture in vitro, have enabled the establishment of genetically modified poultry lines and ex-situ conservation of poultry genetic resources. This chapter introduces the principles, history, and procedures of producing avian germline chimeras by transplantation of PGCs, and the current status of avian germline modification as well as germplasm cryopreservation. Other fundamental avian reproductive technologies are described, including artificial insemination and embryo culture, and perspectives of industrial applications in agriculture and pharmacy are considered, including poultry productivity improvement, egg modification, disease resistance impairment and poultry gene "pharming" as well as gene banking.

Fluorescent Quail: A Transgenic Model System for the Dynamic Study of Avian Development

Real-time four-dimensional (4D, xyzt) imaging of cultured avian embryos is an ideal method for investigating the complex movements of cells and tissues during early morphogenesis. While methods that transiently label cells, such as electroporation, are highly useful for dynamic imaging, they can also be limiting due to the number and type of cells that can be effectively targeted. In contrast, the heritable, stable, and long-term expression of a fluorescent protein driven by the exogenous promoter of a transgene overcomes these challenges. We have used lentiviral vectors to produce several novel transgenic quail lines that express fluorescent proteins either ubiquitously or in a cell-specific manner. These lines have proven to be useful models for dynamic imaging and analysis. Here, we provide detailed protocols for generating transgenic quail with the emphasis on producing high titer lentivirus , effectively introducing it into the early embryo and efficiently screening for G1 founder birds .

A comparative analysis of chick culturing methods on skeletogenesis

Chick embryos are desirable models for the study of developmental biology. Despite this, there are very few studies that examine the effect of different culturing methods on skeletogenesis, specifically, intramembranous and endochondral bones. This study presents a detailed description of these effects by comparing two different culturing methods: windowed (in the shell) eggs and ex-ovo or shell-less culturing to normal development. Using whole mount bone staining, we determined that there is no significant difference in the length of the ossified region of intramembranous and endochondral bones in control versus window cultured embryos. However, these bones are significantly underossified in shell-less embryos. Shell-less embryos also exhibit abnormalities in endochondral bones. Intramembranous bones, interestingly, are morphologically normal in shell-less embryos. This study provides the first detailed description of ossification in window (in-ovo) and shell-less (ex-ovo) cultured embryos compared with controls (in-ovo). Patterning of the skeleton is unaffected regardless of culturing method. We conclude that studies involving endochondral bones should not utilise shell-less culturing methods. This data has been lacking in the literature and will serve as an important resource for those using cultured chick embryos in the study of skeletogenesis.

Comparative analysis of temporal gene expression patterns in the developing ovary of the embryonic chicken

Many genes participate in the process of ovarian germ cell development, while the combined action mechanisms of these molecular regulators still need clarification. The present study was focused on determination of differentially expressed genes and gene functions at four critical time points in chicken ovarian development. Comparative transcriptional profiling of ovaries from embryonic day 5.5 (E5.5), E12.5, E15.5 and E18.5 was performed using an Affymetrix GeneChip chicken genome microarray. Differential expression patterns for genes specifically depleted and enriched in each stage were identified. The results showed that most of the up- and downregulated genes were involved in the metabolism of retinoic acid (RA) and synthesis of hormones. Among them, a higher number of up- and downregulated genes in the E15.5 ovary were identified as being involved in steroid biosynthesis and retinol metabolism, respectively. To validate gene changes, expressions of twelve candidate genes related to germ cell development were examined by real-time PCR and found to be consistent with the of GeneChip data. Moreover, the immunostaining results suggested that ovarian development during different stages was regulated by different genes. Furthermore, a Raldh2 knockdown chicken model was produced to investigate the fundamental role of Raldh2 in meiosis initiation. It was found that meiosis occurred abnormally in Raldh2 knockdown ovaries, but the inhibitory effect on meiosis was reversed by the addition of exogenous RA. This study offers insights into the profile of gene expression and mechanisms regulating ovarian development, especially the notable role of Raldh2 in meiosis initiation in the chicken.

G protein-coupled receptor 30 mediates estrogen-induced proliferation of primordial germ cells via EGFR/Akt/β-catenin signaling pathway

In vertebrates, estrogens are required for the normal development and function of postnatal gonads. However, it remains unclear whether estrogens are able to modulate development of the fetal germ cells. Here, we show that, unexpectedly, chicken primordial germ cells (PGC) lacking estrogen receptor α/β still proliferate in response to 17β-estradiol (E(2)). This is due to the capacity of G protein-coupled receptor 30 (GPR30), existing on PGC, to directly bind E(2). Knockdown experiments suggest that GPR30 is required for E(2)-stimulated PGC proliferation. Furthermore, this estrogen-induced activation of GPR30 is revealed to occur through the Gβγ-subunit protein-dependent and through the matrix metalloproteinase-dependent transactivation of the epidermal growth factor receptor. Epidermal growth factor receptor activation results in a series of intracellular events, including activation of the phosphatidylinositol 3-kinase/serine-threonine kinase/β-catenin pathway, which are followed by the induction of c-fos, c-myc, cyclin D1/E, and B-cell lymphoma 2 expression, and the inhibition of B-cell lymphoma 2-associated X protein expression and caspase3/9 activity. This eventually leads to decreased apoptosis and increased PGC proliferation. Collectively, these findings offer novel insights into the dynamic mechanism of estrogen action on PGC proliferation and suggest that E(2)/GPR30 signaling might play an important role in regulating fetal germ cell development, particularly at the stage before sexual differentiation.

Gamma-irradiation depletes endogenous germ cells and increases donor cell distribution in chimeric chickens

The production of chimeric birds is an important tool for the investigation of vertebrate development, the conservation of endangered birds, and the development of various biotechnological applications. This study examined whether gamma (γ)-irradiation depletes endogenous primordial germ cells and enhances the efficiency of somatic chimerism in chickens. An optimal irradiation protocol for stage X embryos was determined after irradiation at various doses (0, 100, 300, 500, 600, 700, and 2,000 rad). Exposure to 500 rad of γ-irradiation for 73 s significantly decreased the number of primordial germ cells (P < 0.0001). Somatic chimera hatchlings were then produced by transferring blastodermal cells from a Korean Oge into either an irradiated (at 500 rad) or intact stage X White Leghorn embryo. An analysis of feather color pattern and polymerase chain reaction-based species-specific amplification of various tissues of the hatchlings confirmed chimerism in most organs of the chick produced from the irradiated recipient; a lesser degree of chimerism was observed in the non-irradiated control recipient. In conclusion, the exposure of chick embryos to an optimized dose of γ-irradiation effectively depleted germ cells and yielded greater somatic chimerism than non-irradiated control embryos. This technique can be applied to interspecies reproduction or the production of transgenic birds.

Effects of 17beta-estradiol on distribution of primordial germ cell migration in male chicks

AIM

To assess whether exogenous estradiol has any effect on migration of primordial germ cells (PGCs) in the chick.

METHODS

Fertilized eggs were treated with 17beta-estradiol (E(2)) (80 microg/egg) at stage X (day 0 of incubation), stages 8-10 (incubation 30 h) and 13-15 (incubation 55 h). Controls received vehicle (emulsion) only. Changes in PGC number were measured on different days according to developmental stages.

RESULTS

In male right gonads, but not in female left gonads, at stages 28-30 (incubation 132 h) significant decreases in the mean number of PGCs aggregating were observed compared with the controls (P < 0.05) while the total PGC number in the right and left gonads at each stage did not change (P > 0.05).

CONCLUSION

The present study provides evidence that E(2) has significant effects on the localization of PGCs in male right, but not female left, gonads of chicken embryos at stages 28-30, compared with controls.

Generation of transgenic birds with replication-deficient lentiviruses

Birds are of great interest as an animal model in biological research and for commercial applications as a bioreactor. Effective methods for manipulating the avian genome would accelerate progress in fields such as developmental biology and behavioral neurobiology, which traditionally have relied on birds as model systems for biological research. Here we describe a simple and effective protocol for producing transgenic birds using lentiviral vectors that can be used to achieve tissue-specific transgene expression at high levels. The time allotted for the procedure depends upon the species of bird; adult transgenic quails can be generated in approximately 5 months.

Production of germline transgenic chickens expressing enhanced green fluorescent protein using a MoMLV-based retrovirus vector

The Moloney murine leukemia virus (MoMLV) -based retrovirus vector system has been used most often in gene transfer work, but has been known to cause silencing of the imported gene in transgenic animals. In the present study, using a MoMLV-based retrovirus vector, we successfully generated a new transgenic chicken line expressing high levels of enhanced green fluorescent protein (eGFP). The level of eGFP expression was conserved after germline transmission and as much as 100 microg of eGFP could be detected per 1 mg of tissue protein. DNA sequencing showed that the transgene had been integrated at chromosome 26 of the G1 and G2 generation transgenic chickens. Owing to the stable integration of the transgene, it is now feasible to produce G3 generation of homozygous eGFP transgenic chickens that will provide 100% transgenic eggs. These results will help establish a useful transgenic chicken model system for studies of embryonic development and for efficient production of transgenic chickens as bioreactors.

Progress Toward the Culture and Transformation of Chicken Blastodermal Cells

Chicken blastodermal cells can be cultured for short periods of time and retain the ability to contribute to somatic and germline tissues when injected into gamma-irradiated stage X embryos. Such a method has yet to yield a germline transgenic bird, in part due to the low rate of transgene integration into the avian genome. In addition, the short culture period precludes the identification and expansion of those cells that carry an integrated transgene. In this study, two methods were developed that produced blastodermal cells isolated from stage X Barred Plymouth Rock embryos bearing an integrated transgene. Addition of chick embryo extract to the culture medium enabled expansion of single colonies for multiple passages. Southern blot analysis indicated that the transgenes had integrated as a single copy in most of the clones. Cells from passaged, transgenic embryo cells were injected into irradiated stage X White Leghorn embryos, producing hatched chicks that bore the donor cells in their somatic tissues. Transgene sequences were detected in sperm DNA; however, breeding of chimeras did not result in germline transmission of the transgene, indicating that the contribution of the transgenic cells to the germline was either nonexistent or very low.

Competitive bioreactor hens on the horizon

The hen has long held promise as a low-cost, high-yield bioreactor for the production of human biopharmaceuticals in egg whites using genetic engineering. Two separate reports have recently appeared indicating the production of substantial levels of human monoclonal and single chain antibodies (>3 mg and >150 mg, respectively) in eggs of transgenic hens. These promising findings indicate that the hen is close to becoming a competitive manufacturing platform for the production of human biopharmaceuticals.

Avian Germplasm Preservation: Embryonic Stem Cells or Primordial Germ Cells?

Presently, avian genetic resources are best maintained as living collections of birds. Unfortunately, these stocks have been under constant pressure to be destroyed because of the decline in the number of Poultry Science Departments and pressures to cut costs at land grant institutions. Cryopreservation of semen is often suggested as a means to bank avian germplasm. However, this is only applicable for single-gene traits and does not allow for full reconstitution of the genetics of the original line. Over the last 15 yr, advances in the manipulation of the early chick embryo, manipulation of primordial germ cells (PGC), and the culture of embryonic stem cells (ESC) suggests that cryopreservation of blastodermal cells, ESC, or PGC might offer a means to preserve the entire genome of highly selected, specialized stocks of poultry. Freezing each of these cell types is possible with varying degrees of efficiency. Similarly, the effectiveness of generating germ line chimeras using blastodermal cells, ESC, or PGC also varies greatly. Other factors that must be considered include the choice of the recipient lines to develop the germ line chimeras and the number of individuals needed to reconstitute the line. Finally, the low efficiency rate of reconstitution and the high cost associated with current technologies makes these approaches prohibitive. Significant challenges remain to be overcome before the entire genome of poultry stocks can be routinely cryoperserved and reconstituted.

Culture of chicken embryos in surrogate eggshells

The chick embryo is a classical model to study embryonic development. However, most researchers have not studied the effect of embryonic manipulation on chick hatchability. The objective of this study was to determine the effect of egg orientation and type of sealing film on the hatchability of cultured embryos. Windows were made in the small end of recipient surrogate chicken eggshells, and donor embryos were placed into the recipient eggshell for the first 3 d of incubation. Survival over the first 3 d was maximized (P < 0.05) when windowed eggs sealed with Saran Wrap were positioned with the window-end down compared with window-end up. Three-day-old cultured embryos were transferred into recipient turkey eggshells, sealed with cling film, and cultured until hatch. Water weight loss of the surrogate eggshell cultures regardless of cling film type was not significantly different from control intact eggs. The embryos cultured in turkey eggshells and sealed with Handi Wrap exhibited higher hatchability (75% +/- 10.2%) than cultures sealed with Saran Wrap (45.2% +/- 13.8%). Hatchability of control intact eggs (86.4% +/- 5.3%) was not significantly (P > 0.05) different from the hatchability of eggs sealed with Handi Wrap, which suggested that Handi Wrap was an excellent sealant for chick embryos cultured after 3 d of incubation.

Effects of 2,2',5,5'-tetrachlorobiphenyl (PCB52) on migration of chicken primordial germ cells

Polychlorinated biphenyls cause developmental and physiological anomalies in the reproductive system. This study investigated the effects of 2,22,5,52-tetrachlorobiphenyl (PCB52), which can produce oestrogenic effects on the homeostasis of chicken primordial germ cells from the initial stage until completion of their settlement in the gonadal primordium. The blastoderm of chicken embryos was injected with 1 (1/4)L PCB52 (10 micromol/L) and oestradiol (100 micromol/L) before incubation, and the number of primordial germ cells was determined during their migration and development. The number of primordial germ cells in germinal crescents in PCB52-treated groups was slightly decreased (P = 0.068), but it was reduced significantly at stages 13-15 and 28-30 (P < 0.01, respectively) compared with controls. No obvious effects on primordial germ cell migration were observed with oestradiol treatments. The present results suggest that the influence of PCB52 on chicken primordial germ cell migration and proliferation may be via its toxic effect, not its oestrogen-mimicking effect, and provide information on the sensitivity of primordial germ cells to the direct action of PCB52.

Status of transgenic chicken models for developmental biology

The chick embryo is a classic model that has been used to gain insight into developmental processes and cell fate within the embryo for over a century. For the most part, investigators have implanted quail cells into a chicken embryo. A more powerful tool for developmental biology research than the quail:chick chimera system would be to have lines of transgenic chickens expressing reporter genes that are readily available to the research community. However, avian transgenic technology has been fraught with technical difficulties, and transgenic chickens expressing reporter genes have only recently been developed. The goal of this review is to report the technologies that have been used to generate transgenic chickens and to discuss the challenges in generating avian transgenics for developmental biology research.

Avian pluripotent stem cells

Pluripotent embryonic stem cells are undifferentiated cells capable of proliferation and self-renewal and have the capacity to differentiate into all somatic cell types and the germ line. They provide an in vitro model of early embryonic differentiation and are a useful means for targeted manipulation of the genome. Pluripotent stem cells in the chick have been derived from stage X blastoderms and 5.5 day gonadal primordial germ cells (PGCs). Blastoderm-derived embryonic stem cells (ESCs) have the capacity for in vitro differentiation into embryoid bodies and derivatives of the three primary germ layers. When grafted onto the chorioallantoic membrane, the ESCs formed a variety of differentiated cell types and attempted to organize into complex structures. In addition, when injected into the unincubated stage X blastoderm, the ESCs can be found in numerous somatic tissues and the germ line. The potential give rise to somatic and germ line chimeras is highly dependent upon the culture conditions and decreases with passage. Likewise, PGC-derived embryonic germ cells (EGCs) can give rise to simple embryoid bodies and can undergo some differentiation in vitro. Interestingly, chicken EG cells contribute to somatic lineages when injected into the stage X blastoderm, but only germ line chimeras have resulted from EGCs injected into the vasculature of the stage 16 embryo. To date, no lines of transgenic chickens have been generated using ESCs or EGCs. Nevertheless, progress towards the culture of avian pluripotent stem cells has been significant. In the future, the answers to fundamental questions regarding segregation of the avian germ line and the molecular basis of pluripotency should foster the full use of avian pluripotent stem cells.

Biologically active human interferon alpha-2b produced in the egg white of transgenic hens

We have previously described the expression of a bacterial protein in the egg white of transgenic chickens using a replication-deficient retroviral vector. Here we report the expression of a glycosylated human protein, interferon alpha-2b (hIFN), in the egg white of transgenic hens. The hIFN secreted into the egg white was biologically active as determined by a viral inhibition assay. Purification and carbohydrate analysis of the hIFN expressed in egg white revealed that two of the six major glycosylated hIFN species match the naturally occurring human hIFN glycovariants. These results support the potential of the hen as a bioreactor for the production of commercially valuable, biologically active, and glycosylated proteins in egg white.

Rapid and improved method for windowing eggs accessing the stage X chicken embryo

The chick stage X blastoderm is routinely accessed through a small window in a freshly laid egg. However, windowing severely compromises embryo survival with hatch rates as low as a few percent. We previously reported a simple modification to the standard method that reduced introduction of air into the sealed egg and improved the hatchability to 32%. Here, we describe an even simpler and more rapid method for sealing a windowed egg using hot glue or paraffin in which the hatch rate increased to an average of 63% of the unwindowed control eggs. The primary reason for low hatchability can be attributed to air trapped within the egg during windowing and/or leakage during incubation, as shown by increased lethality by artificially introducing air into windowed and sealed eggs. Although the hatch rate was considerably improved, air can still enter the egg during incubation and is likely to account for less than 100% hatchability of the sealed eggs. The success of this new windowing method will facilitate high throughput for the production of transgenic birds and find use in developmental biology, toxicity testing, and avian disease research.

The embryonated egg: a practical target for genetic based advances to improve poultry production

The embryonated avian egg is an attractive target for applying technology-based advances to improve poultry production. There are a number of reasons for this. First, the egg is immobile and can be easily accessed by high-speed automated equipment such as the commercial egg injection system used for vaccination of broilers worldwide. Second, due to successful breeding techniques, the embryonic period now composes 30 to 40% of a broiler's total lifespan, underscoring the importance of this window in the bird production life cycle. Third, the period of incubation involves rapid development from a ball of 40,000 to 60,000 undifferentiated blastodermal cells to a fully formed chick and associated extra-embryonic compartments in 21 d, allowing development of novel approaches for improving poultry production. Some of these novel approaches will be discussed in this paper and include gender discrimination of embryos and the possibility of changing the breeding paradigm through introduction of undifferentiated cells such as avian blastodermal or embryonic stem cells.

Validating the hen as a bioreactor for the production of exogenous proteins in egg white

Increased demand for the production of human biopharmaceuticals in transgenic organisms has led to an intensive effort to develop the hen as a bioreactor producing exogenous proteins in egg white via transgenesis. To date, however, robust methods for transgenic modification of the avian genome have been lacking. We have used a replication-defective retroviral vector derived from avian leukosis virus (ALV) to generate transgenic chickens expressing bacterial beta-lactamase secreted into serum and egg whites through several generations. Expression was driven by the ubiquitous cytomegalovirus (CMV) promoter. Here we describe results from a transgenic lineage (Harvey et al., 2002a,b) in which (1) the transgene was stably transmitted from a G1 founder male (5657) through several generations without silencing, (2) the protein was biologically active, and (3) the level of expression in egg whites was doubled in a G3 homozygote.

Avian transgenesis: progress towards the promise

The hen has long held promise as a low cost, high-yield bioreactor for the production of human biopharmaceuticals in egg whites. A typical egg white contains 3.5-4.0 grams of protein, more than half of which comes from a single gene (ovalbumin). Harnessing the power of the gene to express a recombinant protein could yield up to a gram or more of the protein in the naturally sterile egg. Accordingly, a major effort has been underway for more than a decade to develop robust methods for modification of the chicken genome. This effort intensified in the mid-1990s when several avian transgenic companies entered the scene. Progress has been made in that time but much remains to be done.

Reconstitution of a chicken breed by inter se mating of germline chimeric birds

Blastoderm cells from chicken embryos of a donor breed (Green-legged Partridgelike; GP) were transferred to embryos of a recipient breed (White Leghorn; WL) to form chimeric progeny that, after inter se mating, permitted successful reconstitution of the donor breed. Among 23 chimeric chicks hatched from WL embryos injected with GP cells, 20 (87%) were raised until maturity, and progeny were tested by mating with GP birds to determine the ability of blastodermal cells to form germline chimeras. Six of the tested birds (30%) produced recipient-derived and donor-derived offspring, indicating that they were germline chimeras. The mean percentages of donor-derived germ cells in these birds were 21.1 (17.6 to 50.0%) and 16.9 (5.3 to 23.1%) in males and females, respectively. Among 477 chicks, resulting from mating the germline chimeric male with four germline chimeric females, 10 chicks (2.1%) exhibited a GP phenotype, indicating that the original donor stock had been reconstituted. Only one germline chimeric hen produced GP offspring, but the expected and calculated percentages of GP offspring were similar (2.99 and 2.08, respectively). Two methods of DNA analyses (RFLP and PCR amplification of polymorphic microsatellite loci) of chimeras and their offspring indicated that through mating of a relatively small number of chimeras it is possible to reconstitute a highly diverse population.

Consistent production of transgenic chickens using replication-deficient retroviral vectors and high-throughput screening procedures

We have developed a novel method of DNA extraction combined with a high-throughput method of gene detection allowing thousands of potentially transgenic chicks to be screened quickly and reliably. By using this method and a replication-deficient retroviral vector based on avian leukosis virus (ALV), we have demonstrated germline transmission of three different transgenes. Several generations of chickens carrying intact transgenes were produced, validating the use of the ALV retroviral vectors for large-scale production of transgenic flocks. Fourth-generation chicks that were nontransgenic, hemizygous, or homozygous for the transgene were identified with the combined genetic screening methods.