Overview of chicken embryo genes related to sex differentiation

Sex determination in chickens at an early embryonic stage has been a longstanding challenge in poultry production due to the unique ZZ:ZW sex chromosome system and various influencing factors. This review has summarized the genes related to the sex differentiation of chicken early embryos (mainly Dmrt1, Sox9, Amh, Cyp19a1, Foxl2, Tle4z1, Jun, Hintw, Ube2i, Spin1z, Hmgcs1, Foxd1, Tox3, Ddx4, cHemgn and Serpinb11 in this article), and has found that these contributions enhance our understanding of the genetic basis of sex determination in chickens, while identifying potential gene targets for future research. This knowledge may inform and guide the development of sex screening technologies for hatching eggs and support advancements in gene-editing approaches for chicken embryos. Moreover, these insights offer hope for enhancing animal welfare and promoting conservation efforts in poultry production.

Female-Biased Expression of R-spondin 1 in Chicken Embryonic Gonads Is Estrogen-Dependent

The mechanism of sex determination in chickens, especially the molecular mechanism of female ovarian development, has not yet been fully elucidated. Previous studies have shown that RSPO1, which is associated with ovarian development in mammals, might have a conserved role in chickens. In this study, we systematically investigated the spatiotemporal expression pattern of RSPO1 in various tissues, especially gonads, of male and female chicken embryos using qPCR and Western blotting, and we explored its correlation with the expression of key genes in the estrogen pathway using drug treatment or gene overexpression in vivo and in vitro. Our results reveal that RSPO1 was widely expressed in all examined tissues of chicken embryos, showing a female bias in gonadal tissues at both the mRNA and protein levels. Surprisingly, RSPO1 was not differentially expressed between male and female gonadal cells with fadrozole-induced estrogen pathway blockades, and furthermore, estradiol-induced estrogen stimulation altered the expression of RSPO1. In addition, overexpression of RSPO1 in gonadal cells induced the mRNA expression of its downstream target genes, Wnt family member 4 (WNT4) and Catenin beta 1 (CTNNB1), and that of estrogen receptor α (ERα), an estrogen pathway gene. In summary, this study provided new evidence for elucidating the role of RSPO1 in ovarian development in poultry.

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.

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.

Prenatal aromatase inhibition alters postnatal immunity in domestic chickens (Gallus gallus)

In birds, exposure to exogenous testosterone during embryonic development can suppress measures of immune function; however, it is unclear whether these effects are due to direct or indirect action via aromatization. Estradiol (E₂) is synthesized from testosterone by the enzyme aromatase, and this conversion is a necessary step in many signaling pathways that are ostensibly testosterone-dependent. Many lines of evidence in mammals indicate that E₂ can affect immune function. We tested the hypothesis that some of the immunomodulatory effects observed in response to in ovo testosterone exposure in birds are mediated by conversion to E₂ by aromatase, by using fadrozole to inhibit aromatization of endogenous testosterone during a crucial period of embryonic immune system development in domestic chickens (Gallus gallus). We then measured total IgY antibody count, response to PHA challenge, mass of thymus and bursa of Fabricius, and plasma testosterone post-hatch on days 3 and 18. Because testosterone has a reputation for immunosuppression, we predicted that if modulation of an immune measure by testosterone is dependent on aromatization, then inhibition of estrogen production by fadrozole treatment would lead to elevated measures of that parameter. Conversely, if testosterone inhibits an immune measure directly, then fadrozole treatment would likely not alter that parameter. Fadrozole treatment reduced circulating E₂ in female embryos, but had no effect on males or on testosterone in either sex. Fadrozole-treated chicks had decreased day 3 plasma IgY antibody titers and a strong trend towards increased day 18 thymic mass. Furthermore, fadrozole treatment generated a positive relationship between testosterone and thymic mass in males, and tended to increase day 18 IgY levels for a given bursal mass in females. There was no effect on PHA response, bursal mass, or plasma testosterone at either age post-hatch. The alteration of several indicators of immune function in fadrozole-treated chicks implicates aromatization as a relevant pathway through which developmental exposure to testosterone can affect immunity in birds.

Oestrogen in the chick embryo can induce chromosomally male ZZ left gonad epithelial cells to form an ovarian cortex that can support oogenesis

In chickens, the embryonic ovary differentiates into two distinct domains before meiosis: a steroidogenic core (the female medulla), overlain by the germ cell niche (the cortex). The differentiation of the medulla is a cell-autonomous process based on chromosomal sex identity (CASI). In order to address the extent to which cortex differentiation depends on intrinsic or extrinsic factors, we generated models of gonadal intersex by mixing ZW (female) and ZZ (male) cells in gonadal chimeras, or by altering oestrogen levels of ZW and ZZ embryos. We found that CASI does not apply to the embryonic cortex. Both ZW and ZZ cells can form the cortex and this can happen independently of the phenotypic sex of the medulla as long as oestrogen is provided. We also show that the cortex-promoting activity of oestrogen signalling is mediated via estrogen receptor alpha within the left gonad epithelium. However, the presence of a medulla with an 'intersex' or male phenotype may compromise germ cell progression into meiosis, causing cortical germ cells to remain in an immature state in the embryo.

The Degree of Sex Reversal in Muscovy Ducks (Cairina moschata domestica) Induced by an Aromatase Inhibitor

Under the same feeding conditions, the growth and development of male Muscovy ducks is significantly greater than that of females. Thus, controlling their sex expression can have economic benefits. However, reports on the degree of sex reversal in Muscovy ducks are scarce. In this study, we obtained sex-reversed Muscovy ducks by injecting letrozole before sex differentiation. We analyzed the degree of sex reversal in Muscovy ducks in terms of hormone levels, gonadal tissue development, and growth and found that the estradiol levels of AI-females (letrozole-induced female-to-male sex reversal) were not significantly different from those of normal males (p > 0.05), but the testosterone levels were significantly lower than those in normal males (p < 0.05). AI-female gonad tissue had changed, and the right gonad presented ovotestis tissue. The growth and development of AI-females was significantly less than that of normal males (p < 0.05) but was not significantly different from that of normal females (p > 0.05). Letrozole can induce female Muscovy ducks to convert into males, but the reversal cannot be completed. Thus, further studies are needed to elucidate how to entirely attain the change.

Effect of dietary administration of letrozole and tamoxifen on gonadal development, sex differentiation and biochemical changes in common carp (Cyprinus carpio L.)

The effect of letrozole and tamoxifen on the specific growth rate (SGR; % day(-1)), gonado-somatic index (GSI), total haemoglobin (g%), gonadal and serum protein as well as lipid, sex differentiation and 17β-oestradiol levels were studied in sexually undifferentiated Cyprinus carpio fingerlings 30 days post fertilisation (30 dpf) for 60 days. Results showed decreased GSI with tamoxifen treatment whereas letrozole increased it. There were reduced protein, lipid, triglyceride and cholesterol levels after treatment with tamoxifen and letrozole during gonadal development. Tamoxifen (200mgkg(-1) feed) induced 82.5% masculinisation, whereas letrozole in the same dose produced 98.5% males. Gonadal 17β-oestradiol significantly declined from 86.0±1.41pg per 100mg (control) to 45.5±1.94pg per 100mg with tamoxifen and 36.0±0.72pg per 100mg with letrozole treatment. Similarly, serum 17β-oestradiol levels also decreased after tamoxifen and letrozole treatments. Testicular development in 37.8% of fish treated with tamoxifen and letrozole was found to be more advanced (spermatocytes) than in the control (spermatogonium); however, there was reduced ovarian growth and increased atresia. It was concluded that letrozole and tamoxifen both significantly affect sex differentiation and gonadal maturity in C. carpio leading to the production of sex-reversed males, yet the effect of letrozole was more potent.

The Modulatable Stem Cell Niche: Tissue Interactions during Hair and Feather Follicle Regeneration

Hair and feathers are unique because (1) their stem cells are contained within a follicle structure, (2) they undergo cyclic regeneration repetitively throughout life, (3) regeneration occurs physiologically in healthy individuals and (4) regeneration is also induced in response to injury. Precise control of this cyclic regeneration process is essential for maintaining the homeostasis of living organisms. While stem cells are regulated by the intra-follicle-adjacent micro-environmental niche, this niche is also modulated dynamically by extra-follicular macro-environmental signals, allowing stem cells to adapt to a larger changing environment and physiological needs. Here we review several examples of macro-environments that communicate with the follicles: intradermal adipose tissue, innate immune system, sex hormones, aging, circadian rhythm and seasonal rhythms. Related diseases are also discussed. Unveiling the mechanisms of how stem cell niches are modulated provides clues for regenerative medicine. Given that stem cells are hard to manipulate, focusing translational therapeutic applications at the environments appears to be a more practical approach.

Synergistic effect of fadrozole and insulin-like growth factor-I on female-to-male sex reversal and body weight of broiler chicks

The aim of this study was to investigate the effects of Fadrozole hydrochloride and recombinant human insulin-like growth factor I (rhIGF-I) on female-to-male sex reversal, hatching traits, and body weight of broiler chickens. On the third day of incubation, fertile eggs were randomly assigned to five experimental groups comprising (i) Fadrozole (0.1 mg/egg), (ii) rhIGF-I (100 ng/egg), (iii) Fadrozole (0.1 mg/egg) + rhIGF-I (100 ng/egg), (iv) vehicle injection (10 mM acetic acid and 0.1% BSA), and (v) non-injected eggs. Eggs in the rhIGF-I-injected groups showed the mode of hatching time at the 480th hour of incubation, 12 hours earlier compared to the other groups, with no statistically significant difference in mortality and hatchability. On Day 1 and 42 of production, 90% of genetically female chicks were masculinized using Fadrozole treatment, while 100% female-to-male phenotypic sex reversal was observed in the Fadrozole+rhIGF-I group. Fadrozole equalized the body weight of both genders, although rhIGF-I was effective on the body weight of male chicks only. Interestingly, combined rhIGF-I and Fadrozole could increase the body weight in both sexes compared to the individual injections (P<0.05). These findings revealed that (i) IGF-I-treated chicken embryos were shown to be an effective option for overcoming the very long chicken deprivation period, (ii) the simultaneous treatment with Fadrozole and IGF-I could maximize the female-to-male sex reversal chance, (iii) the increase in the body weight of masculinized chickens via Fadrozole could be equal to their genetically male counterparts, and (iv) the IGF-I effectiveness, specifically along with the application of aromatase inhibitors in female chicks, indicates that estrogen synthesis could be a stumbling block for the IGF-I action mechanism in female embryos.

The molecular genetics of ovarian differentiation in the avian model

In birds as in mammals, sex is determined at fertilization by the inheritance of sex chromosomes. However, sexual differentiation - development of a male or female phenotype - occurs during embryonic development. Sex differentiation requires the induction of sex-specific developmental pathways in the gonads, resulting in the formation of ovaries or testes. Birds utilize a different sex chromosome system to that of mammals, where females are the heterogametic sex (carrying Z and W chromosomes), while males are homogametic (carrying 2 Z chromosomes). Therefore, while some genes essential for testis and ovarian development are conserved, important differences also exist. Namely, the key mammalian male-determining factor SRY does not exist in birds, and another transcription factor, DMRT1, plays a central role in testis development. In contrast to our understanding of testis development, ovarian differentiation is less well-characterized. Given the presence of a female-specific chromosome, studies in chicken will provide insight into the induction and function of female-specific gonadal pathways. In this review, we discuss sexual differentiation in chicken embryos, with emphasis on ovarian development. We highlight genes that may play a conserved role in this process, and discuss how interaction between ovarian pathways may be regulated.

[Mechanism of avian sex determination and differentiation]

Avian sex is determined by genes on the sex chromosomes (ZZ for male and ZW for female). In avian embryo stage, genes on one or two chromosomes control the sex differentiation. Gonad develops to testis in ZZ male and to ovary in ZW female. To date, DMRT1 (Doublesex and mab-3 related transcription factor 1) is considered to be the best candidate gene in controlling the avian gonad differentiation. However, recent study showed that avian sex might be determined by cell autonomous independent of sex hormone signal. Therefore, sex determination gene does not only control the gonadal differentiation, but also control body cells. From this sense, DMRT1 is not the switch gene of avian sex determination. What is the switch factor of avian sex determination, and what is the mechanism of avian sex determination? This review discussed the current progresses on avian sex determination and differentiation from three aspects: W chromosome and ovary development, Z chromosome and testis development, and avian sex determination and cell autonomous.