Protein expression reveals a molecular sexual identity of avian primordial germ cells at pre-gonadal stages

Sex-specific transcriptome of the chicken chorioallantoic membrane

Dimorphism between male and female embryos has been demonstrated in many animal species, including chicken species. Likewise, extraembryonic membranes such as the chorioallantoic membrane (CAM) are likely to exhibit a sex-specific profile. Analysis of the previously published RNA-seq data of the chicken CAM sampled at two incubation times, revealed 783 differentially expressed genes between the CAM of male and female embryos. The expression of some of these genes is sex-dependant only at one or other stage of development, while 415 genes are sex-dependant at both developmental stages. These genes include well-known sex-determining and sex-differentiation genes (DMRT1, HEGM, etc.), and are mainly located on sex chromosomes. This study provides evidence that gene expression of extra-embryonic membranes is differentially regulated between male and female embryos. As such, a better characterisation of associated mechanisms should facilitate the identification of new sex-specific biomarkers.

Initiative on Avian Primordial Germ Cell Cryobanking in Thailand

Background: Biobanking the reproductive tissues or cells of animals preserves the genetic and reproductive ability of the species in long-term storage and promotes sharing of reproductive materials. In avian species, the primordial germ cell (PGC) is one of the most promising reproductive cells to be preserved in biobanks, due to self-renewal properties and direct access to the germ line mediated by PGC transfer. Methods: To conserve the genetic resource of local chicken breeds that are of conservation importance, we systematically isolated two types of pregonadal PGCs from chicken embryos-circulating and tissue PGCs. PGCs of individual embryos were separately isolated, cultured, and cryopreserved. Characteristics of cultured PGCs are described and evaluated. Results: The efficiency of PGC isolation from individual embryos was 98.9% (660/667). In most cases, both matching circulating and tissue PGC lines were isolated from the same embryo (68.2%, 450/660), whereas the remaining lines were from a single source, being either tissue (30.6%, 202/660) or circulating (1.2%, 8/660). Efficient PGC isolation and proliferation can be expected in cultures of circulating PGCs (68.7% and 64.3%, respectively) and tissue PGCs (97.8% and 80.7%, respectively). Following cryopreservation, recovered cells sustained PGC identities including expression of chicken vasa homolog and deleted in azoospermia-like proteins and migration ability to recipient embryonic gonads. Culture conditions equally supported proliferation of circulating and tissue PGCs from both sexes. Combining tissue PGC culture in the regimen prevented 30.3% loss of PGC cultures in the case where circulating PGC culture was ineffective. Cultured circulating and tissue PGCs were similar in morphology, but optimal culture characteristics were different. Conclusion: We applied the approach of PGC isolation from blood and tissue origins on a wide scale and demonstrated its efficiency for biobanking chicken PGCs. The workflow can be operated effectively almost year-round in a tropical climate. It was also described in ample and practical details, which are suitable for adoption or optimization in other conditions.

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.

The Transcriptome of Chicken Migratory Primordial Germ Cells Reveals Intrinsic Sex Differences and Expression of Hallmark Germ Cell Genes

Primordial germ cells (PGCs) are germline-restricted embryonic cells that form the functional gametes of the adult animal. The use of avian PGCs in biobanking and producing genetically modified birds has driven research on the in vitro propagation and manipulation of these embryonic cells. In avian species, PGCs are hypothesized to be sexually undetermined at an early embryonic stage and undergo differentiation into an oocyte or spermatogonial fate dictated by extrinsic factors present in the gonad. However, chicken male and female PGCs require different culture conditions, suggesting that there are sex-specific differences, even at early stages. To understand potential differences between male and female chicken PGCs during migratory stages, we studied the transcriptomes of circulatory stage male and female PGCs propagated in a serum-free medium. We found that in vitro cultured PGCs were transcriptionally similar to their in ovo counterparts, with differences in cell proliferation pathways. Our analysis also revealed sex-specific transcriptome differences between male and female cultured PGCs, with notable differences in Smad7 and NCAM2 expression. A comparison of chicken PGCs with pluripotent and somatic cell types identified a set of genes that are exclusive to germ cells, enriched in the germplasm, and associated with germ cell development.

Fate Decisions of Chicken Primordial Germ Cells (PGCs): Development, Integrity, Sex Determination, and Self-Renewal Mechanisms

Primordial germ cells (PGCs) are precursor cells of sperm and eggs. The fate decisions of chicken PGCs in terms of their development, integrity, and sex determination have unique features, thereby providing insights into evolutionary developmental biology. Additionally, fate decisions in the context of a self-renewal mechanism have been applied to establish culture protocols for chicken PGCs, enabling the production of genome-edited chickens and the conservation of genetic resources. Thus, studies on the fate decisions of chicken PGCs have significantly contributed to both academic and industrial development. Furthermore, studies on fate decisions have rapidly advanced owing to the recent development of essential research technologies, such as genome editing and RNA sequencing. Here, we reviewed the status of fate decisions of chicken PGCs and provided insight into other important research issues that require attention.

Molecular characterization of Vasa homolog in the pen shell Atrina pectinata: cDNA cloning and expression analysis during gonadal development

Vasa is an ATP-dependent RNA helicase of the DEAD (Asp-Glu-Ala-Asp) box family and a representative component of the germ plasm. In this study, we cloned the full-length vasa homolog in the bivalve Atrina pectinata (psvasa), and performed phylogenetic analysis, mRNA expression analysis for tissue-specific distributions, and immunostaining analysis to reveal its histological localization. The sequence of psvasa was 3587 bp in length and contained a 5' untranslated region of 150 bp, an open reading frame of 2214 bp, and a 3' untranslated region of 1223 bp. The deduced amino acid sequence of psvasa was 737 amino acids long and contained evolutionarily conserved sequences reported in other animals. The mRNA expression analysis showed the highest expression levels in the gonads. Expression was especially high in the ovaries, followed by the testes. The immunostaining analysis showed Vasa-positive cells in the developing gonads, suggesting the presence of putative germ stem cells contributing to the supply of germ cells. Furthermore, characteristic Vasa signals were observed in the basophilic nuclei of the oocytes, suggesting that psvasa plays an important role in the progression of meiosis in oocytes.

Prediction of sex-determination mechanisms in avian primordial germ cells using RNA-seq analysis

In birds, sex is determined through cell-autonomous mechanisms and various factors, such as the dosage of DMRT1. While the sex-determination mechanism in gonads is well known, the mechanism in germ cells remains unclear. In this study, we explored the gene expression profiles of male and female primordial germ cells (PGCs) during embryogenesis in chickens to predict the mechanism underlying sex determination. Male and female PGCs were isolated from blood and gonads with a purity > 96% using flow cytometry and analyzed using RNA-seq. Prior to settlement in the gonads, female circulating PGCs (cPGCs) obtained from blood displayed sex-biased expression. Gonadal PGCs (gPGCs) also exhibited sex-biased expression, and the number of female-biased genes detected was higher than that of male-biased genes. The female-biased genes in gPGCs were enriched in some metabolic processes. To reveal the mechanisms underlying the transcriptional regulation of female-biased genes in gPGCs, we performed stimulation tests. Retinoic acid stimulation of cultured gPGCs derived from male embryos resulted in the upregulation of several female-biased genes. Overall, our results suggest that sex determination in avian PGCs involves aspects of both cell-autonomous and somatic-cell regulation. Moreover, it appears that sex determination occurs earlier in females than in males.

A paternal bias in germline mutation is widespread in amniotes and can arise independently of cell division numbers

In humans and other mammals, germline mutations are more likely to arise in fathers than in mothers. Although this sex bias has long been attributed to DNA replication errors in spermatogenesis, recent evidence from humans points to the importance of mutagenic processes that do not depend on cell division, calling into question our understanding of this basic phenomenon. Here, we infer the ratio of paternal-to-maternal mutations, α, in 42 species of amniotes, from putatively neutral substitution rates of sex chromosomes and autosomes. Despite marked differences in gametogenesis, physiologies and environments across species, fathers consistently contribute more mutations than mothers in all the species examined, including mammals, birds, and reptiles. In mammals, α is as high as 4 and correlates with generation times; in birds and snakes, α appears more stable around 2. These observations are consistent with a simple model, in which mutations accrue at equal rates in both sexes during early development and at a higher rate in the male germline after sexual differentiation, with a conserved paternal-to-maternal ratio across species. Thus, α may reflect the relative contributions of two or more developmental phases to total germline mutations, and is expected to depend on generation time even if mutations do not track cell divisions.

A low-tech, cost-effective and efficient method for safeguarding genetic diversity by direct cryopreservation of poultry embryonic reproductive cells

Chickens are an important resource for smallholder farmers who raise locally adapted, genetically distinct breeds for eggs and meat. The development of efficient reproductive technologies to conserve and regenerate chicken breeds safeguards existing biodiversity and secures poultry genetic resources for climate resilience, biosecurity, and future food production. The majority of the over 1600 breeds of chicken are raised in low and lower to middle income countries under resource-limited, small-scale production systems, which necessitates a low-tech, cost-effective means of conserving diversity is needed. Here, we validate a simple biobanking technique using cryopreserved embryonic chicken gonads. The gonads are quickly isolated, visually sexed, pooled by sex, and cryopreserved. Subsequently, the stored material is thawed and dissociated before injection into sterile host chicken embryos. By using pooled GFP and RFP-labelled donor gonadal cells and Sire Dam Surrogate mating, we demonstrate that chicks deriving entirely from male and female donor germ cells are hatched. This technology will enable ongoing efforts to conserve chicken genetic diversity for both commercial and smallholder farmers, and to preserve existing genetic resources at poultry research facilities.