Expression of chicken steroidogenic factor-1 during gonadal sex differentiation

In ovo injection of betaine promotes adrenal steroidogenesis in pre-hatched chicken fetuses

Corticosterone is critical for the maturation and survival of chicken fetus around hatching. Betaine is used as a feed additive in poultry industry to promote growth and mitigate stress. However, it remains unknown whether betaine could affect adrenal corticosterone synthesis in pre-hatching chicken fetuses. In this study, betaine (2.5 mg/egg) was injected into developing chicken fetuses at d 11 of incubation (E11) and its impact on adrenal steroidogenesis was investigated at day 19 (E19). Plasma corticosterone concentration was significantly (P < 0.05) elevated in betaine-treated fetuses, together with increased adrenal expression of melanocortin 2 receptor and steroidogenic acute regulatory protein. Accordingly, the corticosterone biosynthetic enzymes, such as cytochrome P450 family 11 subfamily A member 1, 3β-hydroxysteroid dehydrogenase and cytochrome P450 family 21 subfamily A member 2, as well as cholesterol biosynthesis or regulation-related genes, such as sterol regulatory element-binding protein 1, 3-hydroxy-3-methyl-glutaryl-coenzyme A reductase and low-density lipoprotein receptor, were all significantly (P < 0.05) upregulated in betaine group. Meanwhile, steroidogenic factor-1 and glucocorticoid receptor were significantly (P < 0.05) enhanced, whereas expression of dosage-sensitive sex reversal-adrenal hypoplasia congenita critical region on the X chromosome gene, a nuclear receptor known as a repressor of adrenal steroidogenesis, was significantly (P < 0.05) downregulated. Betaine significantly (P < 0.05) increased adrenal expression of genes involved in one-carbon metabolism and DNA methylation, such as S-adenosyl homocysteine hydrolase, betaine-homocysteine-methyltransferase, methionine adenosyl transferase and DNA methyltransferases, yet the promoter regions of most steroidogenic genes were significantly (P < 0.05) hypomethylated. These results indicate that in ovo injection of betaine promotes adrenal glucocorticoid synthesis in chicken fetuses before hatching, which involves alterations in DNA methylation.

Insights into Gonadal Sex Differentiation Provided by Single-Cell Transcriptomics in the Chicken Embryo

Although the genetic triggers for gonadal sex differentiation vary across species, the cell biology of gonadal development was long thought to be largely conserved. Here, we present a comprehensive analysis of gonadal sex differentiation, using single-cell sequencing in the embryonic chicken gonad during sexual differentiation. The data show that chicken embryonic-supporting cells do not derive from the coelomic epithelium, in contrast to other vertebrates studied. Instead, they derive from a DMRT1⁺/PAX2⁺/WNT4⁺/OSR1⁺ mesenchymal cell population. We find a greater complexity of gonadal cell types than previously thought, including the identification of two distinct sub-populations of Sertoli cells in developing testes and derivation of embryonic steroidogenic cells from a differentiated supporting-cell lineage. Altogether, these results indicate that, just as the genetic trigger for sex differs across vertebrate groups, cell lineage specification in the gonad may also vary substantially.

The transcriptome of the newt Cynops orientalis provides new insights into evolution and function of sexual gene networks in sarcopterygians

Amphibians evolved in the Devonian period about 400 Mya and represent a transition step in tetrapod evolution. Among amphibians, high-throughput sequencing data are very limited for Caudata, due to their largest genome sizes among terrestrial vertebrates. In this paper we present the transcriptome from the fire bellied newt Cynops orientalis. Data here presented display a high level of completeness, comparable to the fully sequenced genomes available from other amphibians. Moreover, this work focused on genes involved in gametogenesis and sexual development. Surprisingly, the gsdf gene was identified for the first time in a tetrapod species, so far known only from bony fish and basal sarcopterygians. Our analysis failed to isolate fgf24 and foxl3, supporting the possible loss of both genes in the common ancestor of Rhipidistians. In Cynops, the expression analysis of genes described to be sex-related in vertebrates singled out an expected functional role for some genes, while others displayed an unforeseen behavior, confirming the high variability of the sex-related pathway in vertebrates.

Nr5a1b promotes and Nr5a2 inhibits transcription of lhb in the orange-spotted grouper, Epinephelus coioides†

Nr5a1 (Sf-1) up-regulates lhb expression across vertebrates; however, its regulatory roles on fshb remain to be defined. Moreover, the involvement of Nr5a2 in the regulation of gonadotropin expression is not clear either. In the present study, the involvement of Nr5a1b (a homologue of Nr5a1) and Nr5a2 in the regulation of lhb and fshb expression in the orange-spotted grouper was examined. Dual fluorescent immunohistochemistry using homologous antisera showed that in the pituitary of orange-spotted groupers, Lh cells contain both immunoreactive Nr5a1b and Nr5a2 signals, whereas Fsh cells contain neither of them. In LβT2 cells, Nr5a1b up-regulated basal activities of lhb and fshb promoters possibly via Nr5a sites, and synergistically (on lhb promoter) or additively (on fshb promoter) with forskolin. Surprisingly, Nr5a2 inhibited basal activities of lhb promoter possibly via Nr5a sites and attenuated the stimulatory effects of both forskolin and Nr5a1b. In contrast, Nr5a2 had no effects on fshb promoter. Chromatin immunoprecipitation analysis showed that both Nr5a1b and Nr5a2 bound to lhb promoter, but not fshb promoter in the pituitary of the orange-spotted grouper. The abundance of Nr5a1b bound to lhb promoter was significantly higher at the vitellogenic stage than the pre-vitellogenic stage, whereas that of Nr5a2 exhibited an opposite trend. Taken together, data of the present study demonstrated antagonistic effects of Nr5a1b and Nr5a2 on lhb transcription in the orange-spotted grouper and revealed novel regulatory mechanisms of differential expression of lhb and fshb genes through Nr5a homologues in vertebrates.

Limb patterning genes and heterochronic development of the emu wing bud

Background

The forelimb of the flightless emu is a vestigial structure, with greatly reduced wing elements and digit loss. To explore the molecular and cellular mechanisms associated with the evolution of vestigial wings and loss of flight in the emu, key limb patterning genes were examined in developing embryos.

Methods

Limb development was compared in emu versus chicken embryos. Immunostaining for cell proliferation markers was used to analyze growth of the emu forelimb and hindlimb buds. Expression patterns of limb patterning genes were studied, using whole-mount in situ hybridization (for mRNA localization) and RNA-seq (for mRNA expression levels).

Results

The forelimb of the emu embryo showed heterochronic development compared to that in the chicken, with the forelimb bud being retarded in its development. Early outgrowth of the emu forelimb bud is characterized by a lower level of cell proliferation compared the hindlimb bud, as assessed by PH3 immunostaining. In contrast, there were no obvious differences in apoptosis in forelimb versus hindlimb buds (cleaved caspase 3 staining). Most key patterning genes were expressed in emu forelimb buds similarly to that observed in the chicken, but with smaller expression domains. However, expression of Sonic Hedgehog (Shh) mRNA, which is central to anterior–posterior axis development, was delayed in the emu forelimb bud relative to other patterning genes. Regulators of Shh expression, Gli3 and HoxD13, also showed altered expression levels in the emu forelimb bud.

Conclusions

These data reveal heterochronic but otherwise normal expression of most patterning genes in the emu vestigial forelimb. Delayed Shh expression may be related to the small and vestigial structure of the emu forelimb bud. However, the genetic mechanism driving retarded emu wing development is likely to rest within the forelimb field of the lateral plate mesoderm, predating the expression of patterning genes.

Electronic supplementary material

The online version of this article (doi:10.1186/s13227-016-0063-5) contains supplementary material, which is available to authorized users.

Identification of male-biased microRNA-107 as a direct regulator for nuclear receptor subfamily 5 group A member 1 based on sexually dimorphic microRNA expression profiling from chicken embryonic gonads

Several studies indicate that sexual dimorphic microRNAs (miRNAs) in chicken gonads are likely to have important roles in sexual development, but a more global understanding of the roles of miRNAs in sexual differentiation is still needed. To this end, we performed miRNA expression profiling in chicken gonads at embryonic day 5.5 (E5.5). Among the sex-biased miRNAs validated by qRT-PCR, twelve male-biased and six female-biased miRNAs were consistent with the sequencing results. Bioinformatics analysis revealed that some sex-biased miRNAs were potentially involved in gonadal development. Further functional analysis found that over-expression of miR-107 directly inhibited nuclear receptor subfamily 5 group A member 1 (NR5a1), and its downstream cytochrome P450 family 19 subfamily A, polypeptide 1 (CYP19A1). However, anti-Mullerian hormone (AMH) was not directly or indirectly regulated by miR-107. Overall results indicate that miR-107 may specifically mediate avian ovary-development by post-transcriptional regulation of NR5a1 and CYP19A1 in estrogen signaling pathways.

Early Development of the Gonads: Origin and Differentiation of the Somatic Cells of the Genital Ridges

The earliest manifestation of gonadogenesis in vertebrates is the formation of the genital ridges. The genital ridges form through the transformation of monolayer coelomic epithelium into a cluster of somatic cells. This process depends on increased proliferation of coelomic epithelium and disintegration of its basement membrane, which is foreshadowed by the expression of series of regulatory genes. The earliest expressed gene is Gata4, followed by Sf1, Lhx9, Emx2, and Cbx2. The early genital ridge is a mass of somatic SF1-positive cells (gonadal precursor cells) that derive from proliferating coelomic epithelium. Primordial germ cells (PGCs) immigrate to the coelomic epithelium even in the absence of genital ridges, e.g., in mouse null mutants for Gata4. And conversely, the PGCs are not required for the formation of the genital ridges. After reaching genital ridges, the PGCs become enclosed by somatic cells derived from coelomic epithelium. Subsequently, the expression of sex-determining genes begins and the bipotential gonads differentiate into either testes or ovaries. Gonadal precursor cells, derived from coelomic epithelium, give rise to the somatic supporting cells such as Sertoli cells, follicular cells, and probably also peritubular myoid and steroidogenic cells.

Sexual Dimorphism in the Early Embryogenesis in Zebra Finches

Sex-specific gene expression before the onset of gonadogensis has been documented in embryos of mammals and chickens. In several mammalian species, differences in gene expression are accompanied by faster growth of pre-implantation male embryos. Here we asked whether avian embryos before gonadal differentiation are also sex-dimorphic in size and what genes regulate their growth. We used captive zebra finches (Taeniopygia guttata) whose freshly laid eggs were artificially incubated for 36–40 hours. Analyses controlling for the exact time of incubation of 81 embryos revealed that males were larger than females in terms of Hamburger and Hamilton stage and number of somites. Expression of 15 genes involved in cell cycle regulation, growth, metabolic activity, steroidogenic pathway and stress modulation were measured using RT-PCR in 5 male and 5 female embryos incubated for exactly 36 h. We found that in the presence of equal levels of the growth hormone itself, the faster growth of male embryos is most likely achieved by the overexpression of the growth hormone receptor gene and three other genes responsible for cell cycle regulation and metabolism, all of them located on the Z chromosome. Autosomal genes did not show sex-specific expression, except for the steroidogenic factor 1 which was expressed only in female embryos. To our knowledge this is the first report of sexual size dimorphism before gonadogenesis in birds. The finding suggests that faster growth of early male embryos is conserved through the mammalian and bird phyla, irrespective of their differential sex chromosome systems.

Chronology, magnitude and duration of expression of putative sex-determining/differentiation genes in a turtle with temperature-dependent sex determination

The red-eared slider turtle (Trachemys scripta) possesses temperature-dependent sex determination (TSD) in which the incubation temperature determines gonadal sex. Although a number of mammalian gene homologues have been identified in reptiles with TSD, the exact sex-determining trigger(s) is not known. To date, the current study represents the most comprehensive simultaneous evaluation of the chronology of mRNA expression profiles of putative sex-determining/differentiation genes (Dmrt1, Sox9, Amh, Lhx9, and Foxl2) from gonads incubated at male- and female-producing temperatures in T. scripta. Additionally, sex-reversing treatments with 17β-estradiol and letrozole were examined. At a male-producing temperature, Dmrt1 expression was sexually dimorphic by stage 17, Sox9 by 19 and Amh by 21. In contrast, Foxl2 did not significantly increase until after the thermosensitive period at a female-producing temperature. Treatment with 17β-estradiol resulted in reduced gonad size and/or inhibited gonadal development and differentiation. Gene expression was subsequently low in this group. Sex reversal utilizing letrozole failed to produce testes at a female-producing temperature and as such, gene expression was comparable to ovary. These results indicate that Dmrt1 and Sox9 are potential triggers for testis differentiation and Amh, Lhx9 and Foxl2 represent a conserved core set of genes in the sex-determining/differentiation pathway of TSD species.

Identification, expression, and regulation of anti-Müllerian hormone type-II receptor in the embryonic chicken gonad

Anti-Müllerian hormone (AMH) signaling is required for proper development of the urogenital system in vertebrates. In male mammals, AMH is responsible for regressing the Müllerian ducts, which otherwise develop into the fallopian tubes, oviducts, and upper vagina of the female reproductive tract. This role is highly conserved across higher vertebrates. However, AMH is required for testis development in fish species that lack Müllerian ducts, implying that AMH signaling has broader roles in other vertebrates. AMH signals through two serine/threonine kinase receptors. The primary AMH receptor, AMH receptor type-II (AMHR2), recruits the type I receptor, which transduces the signal intracellularly. To enhance our understanding of AMH signaling and the potential role of AMH in gonadal sex differentiation, we cloned chicken AMHR2 cDNA and examined its expression profile during gonadal sex differentiation. AMHR2 is expressed in the gonads and Müllerian ducts of both sexes but is more strongly expressed in males after the onset of gonadal sex differentiation. In the testes, the AMHR2 protein colocalizes with AMH, within Sertoli cells of the testis cords. AMHR2 protein expression is up-regulated in female embryos treated with the estrogen synthesis inhibitor fadrozole. Conversely, knockdown of the key testis gene DMRT1 leads to disruption of AMHR2 expression in the developing seminiferous cords of males. These results indicate that AMHR2 is developmentally regulated during testicular differentiation in the chicken embryo. AMH signaling may be important for gonadal differentiation in addition to Müllerian duct regression in birds.

Just how conserved is vertebrate sex determination?

BACKGROUND

Sex determination in vertebrate embryos has long been equated with gonadal differentiation into testes or ovaries. This view has been challenged over the years by reports of somatic sexual dimorphisms pre-dating gonadal sex differentiation. The recent finding that sex determination in birds is likely to be partly cell autonomous has again called for a broader definition of sex determination. Inherent sexual differentiation in each and every cell may apply widely among vertebrates, and may involve more than one "master sex gene" on a sex chromosome. At the gonadal level, key genes required for proper sexual differentiation are conserved among vertebrates, but their relative positions in the ovarian and testicular cascades differ.

RESULTS

We illustrate these differences by comparing key sex genes in fishes versus birds and mammals, with emphasis on DM domain genes and the SOX9-AMH pathway in the testis and the FOXL2-Aromatase pathway in the ovary. Such comparisons facilitate the identification of ancient versus derived genes involved in gonadal sex determination.

CONCLUSIONS

The data indicate that vertebrate sex-determining cascades are not as conserved as once thought.

The long non-coding RNA, MHM, plays a role in chicken embryonic development, including gonadogenesis

MHM is a chicken Z chromosome-linked locus that is methylated and transcriptionally silent in male cells, but is hypomethylated and transcribed into a long non-coding RNA in female cells. MHM has been implicated in both localised dosage compensation and sex determination in the chicken embryo, but direct evidence is lacking. We investigated the potential role of MHM in chicken embryonic development, using expression analysis and retroviral-mediated mis-expression. At embryonic stages, MHM is only expressed in females. Northern blotting showed that both sense and antisense strands of the MHM locus are transcribed, with the sense strand being more abundant. Whole mount in situ hybridization confirmed that the sense RNA is present in developing female embryos, notably in gonads, limbs, heart, branchial arch and brain. Within these cells, the MHM RNA is localized to the nucleus. The antisense transcript is lowly expressed and has a cytoplasmic localization in cells. Mis-expression of MHM sense and antisense sequences results in overgrowth of tissues in which transcripts are predominantly expressed. This includes altered asymmetric ovarian development in females. In males, MHM mis-expression impairs gonadal expression of the testis gene, DMRT1. Both MHM sense and antisense mis-expression cause brain abnormalities, while MHM sense causes an increase in male-biased embryo mortality. These results indicate that MHM has a role in chicken normal embryonic development, including gonadal sex differentiation.

HOXA13 and HOXD13 expression during development of the syndactylous digits in the marsupial Macropus eugenii

Background

Kangaroos and wallabies have specialised limbs that allow for their hopping mode of locomotion. The hindlimbs differentiate much later in development but become much larger than the forelimbs. The hindlimb autopod has only four digits, the fourth of which is greatly elongated, while digits two and three are syndactylous. We investigated the expression of two genes, HOXA13 and HOXD13, that are crucial for digit patterning in mice during formation of the limbs of the tammar wallaby.

Results

We describe the development of the tammar limbs at key stages before birth. There was marked heterochrony and the hindlimb developed more slowly than the forelimb. Both tammar HOXA13 and HOXD13 have two exons as in humans, mice and chickens. HOXA13 had an early and distal mRNA distribution in the tammar limb bud as in the mouse, but forelimb expression preceded that in the hindlimb. HOXD13 mRNA was expressed earlier in the forelimb than the hindlimb and was predominantly detected in the interdigital tissues of the forelimb. In contrast, the hindlimb had a more restricted expression pattern that appeared to be expressed at discrete points at both posterior and anterior margins of the limb bud, and was unlike expression seen in the mouse and the chicken.

Conclusions

This is the first examination of HOXA and HOXD gene expression in a marsupial. The gene structure and predicted proteins were highly conserved with their eutherian orthologues. Interestingly, despite the morphological differences in hindlimb patterning, there were no modifications to the polyalanine tract of either HOXA13 or HOXD13 when compared to those of the mouse and bat but there was a marked difference between the tammar and the other mammals in the region of the first polyserine tract of HOXD13. There were also altered expression domains for both genes in the developing tammar limbs compared to the chicken and mouse. Together these findings suggest that the timing of HOX gene expression may contribute to the heterochrony of the forelimb and hindlimb and that alteration to HOX domains may influence phenotypic differences that lead to the development of marsupial syndactylous digits.

OVEX1, a novel chicken endogenous retrovirus with sex-specific and left-right asymmetrical expression in gonads

BACKGROUND

In chickens, as in most birds, female gonad morphogenesis is asymmetrical. Gonads appear first rather similarly, but only the left one undergoes full differentiation and gives rise to a functional ovary. The right gonad, in which the cortex does not develop, remains restricted to the medulla and finally regresses. Opportunity was taken of this left-right asymmetry to perform a suppression subtractive hybridization screening to select for transcripts preferentially expressed in the developing left ovary as compared to the right one, and thus identify genes that are potentially involved in the process of ovarian differentiation.

RESULTS

One of these transcripts, named Ovex1 according to its expression profile, corresponds to an endogenous retrovirus that has not been previously characterized. It is transcribed as full-length and singly spliced mRNAs and contains three uninterrupted open reading frames coding potentially for proteins with homology to Gag and Pro-Pol retroviral polyproteins and a third protein showing only a weak similarity with Env glycoproteins. Ovex1 is severely degenerated; it is devoid of typical long terminal repeats and displays some evidence of recombination. An orthologous Ovex1 locus was identified in the genome of zebra finch, a member of a different bird order, and similar sequences were detected in turkey, guinea fowl, and duck DNA. The relationship between these sequences follows the bird phylogeny, suggesting vertical transmission of the endogenous retrovirus for more than 100 million years. Ovex1 is transcribed in chicken gonads with a sex-dependent and left-right asymmetrical pattern. It is first expressed in the cortex of the left indifferent gonads of both sexes. Expression is transient in the left testis and absent in the right one. In developing ovaries, Ovex1 transcription increases sharply in the left cortex and is weakly detected in the medulla. After folliculogenesis, Ovex1-expressing cells constitute the follicular granulosa cell layer. Ovex1 expression highlights a striking desquamation process that leads to profound cortical remodeling associated with follicle morphogenesis.

CONCLUSION

Evidence for a selection pressure at the protein level suggests that this endogenous retrovirus, expressed in the ovarian supporting cell lineage, might play an active role in bird ovarian physiology.

Steroid signaling and temperature-dependent sex determination-Reviewing the evidence for early action of estrogen during ovarian determination in turtles

The developmental processes underlying gonadal differentiation are conserved across vertebrates, but the triggers initiating these trajectories are extremely variable. The red-eared slider turtle (Trachemys scripta elegans) exhibits temperature-dependent sex determination (TSD), a system where incubation temperature during a temperature-sensitive period of development determines offspring sex. However, gonadal sex is sensitive to both temperature and hormones during this period-particularly estrogen. We present a model for temperature-based differences in aromatase expression as a critical step in ovarian determination. Localized estrogen production facilitates ovarian development while inhibiting male-specific gene expression. At male-producing temperatures aromatase is not upregulated, thereby allowing testis development.

Gonadal expression of Sf1 and aromatase during sex determination in the red-eared slider turtle (Trachemys scripta), a reptile with temperature-dependent sex determination

Many egg-laying reptiles have temperature-dependent sex determination (TSD), where the offspring sex is determined by incubation temperature during a temperature-sensitive period (TSP) in the middle third of development. The underlying mechanism transducing a temperature cue into an ovary or testis is unknown, but it is known that steroid hormones play an important role. During the TSP, exogenous application of estrogen can override a temperature cue and produce females, while blocking the activity of aromatase (Cyp19a1), the enzyme that converts testosterone to estradiol, produces males from a female-biased temperature. The production of estrogen is a key step in ovarian differentiation for many vertebrates, including TSD reptiles, and temperature-based differences in aromatase expression during the TSP may be a critical step in ovarian determination. Steroidogenic factor-1 (Sf1) is a key gene in vertebrate sex determination and regulates many steroidogenic enzymes, including aromatase. We find that Sf1 and aromatase are differentially expressed during sex determination in the red-eared slider turtle, Trachemys scripta elegans. Sf1 is expressed at higher levels during testis development while aromatase expression increases during ovary determination. We also assayed Sf1 and aromatase response to sex-reversing treatments via temperature or the modulation of estrogen availability. Sf1 expression was redirected to low-level female-specific patterns with feminizing temperature shift or exogenous estradiol application and redirected to more intense male-specific patterns with male-producing temperature shift or inhibition of aromatase activity. Conversely, aromatase expression was redirected to more intense female-specific patterns with female-producing treatment and redirected toward diffuse low-level male-specific patterns with masculinizing sex reversal. Our data do not lend support to a role for Sf1 in the regulation of aromatase expression during slider turtle sex determination, but do support a critical role for estrogen in ovarian development.

Avian sex determination: what, when and where?

Sex is determined genetically in all birds, but the underlying mechanism remains unknown. All species have a ZZ/ZW sex chromosome system characterised by female (ZW) heterogamety, but the chromosomes themselves can be heteromorphic (in most birds) or homomorphic (in the flightless ratites). Sex in birds might be determined by the dosage of a Z-linked gene (two in males, one in females) or by a dominant ovary-determining gene carried on the W sex chromosome, or both. Sex chromosome aneuploidy has not been conclusively documented in birds to differentiate between these possibilities. By definition, the sex chromosomes of birds must carry one or more sex-determining genes. In this review of avian sex determination, we ask what, when and where? What is the nature of the avian sex determinant? When should it be expressed in the developing embryo, and where is it expressed? The last two questions arise due to evidence suggesting that sex-determining genes in birds might be operating prior to overt sexual differentiation of the gonads into testes or ovaries, and in tissues other than the urogenital system.

Mechanisms of gonadal morphogenesis are not conserved between chick and mouse

To understand mechanisms of sex determination, it is important to know the lineage relationships of cells comprising the gonads. For example, in mice, the Y-linked gene Sry triggers differentiation of Sertoli cells from a cell population originating in the coelomic epithelium overlying the nascent gonad that also gives rise to uncharacterised interstitial cells. In contrast, little is known about origins of somatic cell types in the chick testis, where there is no Sry gene and sex determination depends on a ZZ male/ZW female mechanism. To investigate this, we performed fate mapping experiments in ovo, labelling at indifferent stages the coelomic epithelium by electroporation with a lacZ reporter gene and the underlying nephrogenous (or mesonephric) mesenchyme with chemical dyes. After sex differentiation, LacZ-positive cells were exclusively outside testis cords and were 3betaHSD-negative, indicating that the coelomic epithelium contributes only to non-steroidogenic interstitial cells. However, we detected dye-labelled cells both inside and outside the cords. The former were AMH-positive while some of the latter were 3betaHSD-positive, showing that nephrogenous mesenchyme contributes to both Sertoli cells and steroidogenic cells. This is the first demonstration via lineage analysis that steroidogenic cells originate from nephrogenous mesenchyme, but the revelation that Sertoli cells have different origins between chick and mouse suggests that, during evolution, mechanisms of gonad morphogenesis may diverge alongside those of sex determination.

Comparative gene expression of steroidogenic factor 1 in Chrysemys picta and Apalone mutica turtles with temperature-dependent and genotypic sex determination

Characterizing the molecular network underlying temperature-dependent (TSD) and genotypic (GSD) sex determination, including patterns across closely related taxa, is crucial to elucidate the still enigmatic evolution of sex determining mechanisms in vertebrates. Here we examined the expression of an important gene for sexual differentiation common to both systems, Sf1, at male- and female-producing temperatures, in TSD (Chrysemys picta) and GSD turtles (Apalone mutica). We tested the hypotheses that Sf1 expression responds to temperature consistently across TSD turtles but is unaffected in GSD turtles, and that this differential expression starts no earlier than the onset of the thermosensitive period (TSP). As expected, Sf1 expression was thermally insensitive in A. mutica (GSD). Although Sf1 exhibited a differential expression by temperature in C. picta, the expression pattern differed from other TSD turtles (Trachemys scripta), perhaps reflecting divergence of the gene regulatory networks underlying sex determination over evolutionary time. Most notably, Sf1 was differentially expressed in C. picta (significantly higher at the male-producing temperature) before the onset of the TSP, implying that in TSD taxa significant thermal effects may occur early in development. This result may reconcile field observations where temperatures experienced prior to the TSP have an effect on sex ratios, thus challenging traditional TSP models. Importantly, the molecular factors that render TSD mechanisms thermosensitive remain unknown, and potential candidates are genes that express differentially before the onset of the TSP (genes shaping or opening the TSP-window rather those acting once the TSP window has opened). Therefore, our findings make Sf1 one such potential candidate.

Evidence for activation of Amh gene expression by steroidogenic factor 1

The anti-Müllerian hormone gene (Amh) is responsible for regression in males of the Müllerian ducts. The molecular mechanism of regulation of chicken Amh expression is poorly understood. To investigate the regulation of chicken Amh expression, we have cloned Amh cDNAs from quail and duck as well as the promoter regions of the gene from chicken, quail, and duck. The expression patterns of Amh during embryonic development in these three species were found to be similar, suggesting that the regulatory mechanisms of Amh expression are conserved. The sequence of the proximal promoter of Amh contains a putative binding site for steroidogenic factor 1 (SF1), the protein product of which can up-regulate Amh in mammals. We showed here that SF1 is able to activate the chicken Amh promoter and binds to its putative SF1 binding site. These results suggest that SF1 plays a role in regulation of Amh expression in avian species.

Molecular cloning and tissue distribution of SF-1-related orphan receptors during sexual maturation in female goldfish

The steroidogenic factor (SF)-1 gene is one of a number of orphan nuclear receptors, which is a key transcriptional regulator in vertebrate reproduction. We have isolated the SF-1 homologue cDNA from the goldfish pituitary and designed primers for SF-1 on the basis of the highly conserved regions of various known SF-1 superfamily genes. SF-1 cDNA contained 1,948 nucleotides including an open reading frame predicted to encode a protein of 503 amino acids. The distribution pattern of SF-1 in a variety of tissues during sexual maturation in female goldfish was also examined by RT-PCR. Significant variations in the relative expression of SF-1 were observed in different tissues in immature and mature female goldfish. SF-1 transcript in pituitary was significantly higher than other tissues tested in immature and mature female goldfish. Lower expression of SF-1 was observed in the liver but was not detected in brain and ovary of the immature female goldfish. Presence of SF-1 was the predominant expression in the pituitary and brain of mature female goldfish. Also, in the mature female goldfish, a weak transcript was detected in liver and ovary. Interestingly, RT-PCR analysis revealed that the expression of SF-1 became higher in the brain and weaker in the liver in maturing female goldfish. Thus, SF-1 may be regulated in goldfish brain and/or liver. Thus is also tissue-specific distribution of SF-1 during sexual maturation in female goldfish.

Gene expression of sex-determining factors and steroidogenic enzymes in the chicken embryo: influence of xenoestrogens

Many genes involved in gonadal development have been proposed for mammals. To elucidate if those genes play any critical role in sexual differentiation of the avian gonad, we have examined expressions of the genes for proposed sex-determining factors (SF1, Sox9, DMRT1, Wpkci, and AMH), steroidogenic enzymes (P-450scc, 3beta-HSD, P-450c17, 17beta-HSD and aromatase) and the estrogen receptor in the urogenital system during chicken embryogenesis (days 4-16 of incubation), using a semi-quantitative reverse transcription-polymerase chain reaction. Transcripts of the genes for sex-determining factors except Wpkci and AMH were detected in both sexes but had no sexual dimorphism. Wpkci expression was female specific and constantly high throughout incubation. AMH was expressed in both sexes from the earliest stages but was higher in males than in females after the onset of gonadal differentiation. Expressions of the genes for more downstream enzymes in a steroidogenic pathway, such as P-450c17, 17beta-HSD and aromatase, were clearly higher in females than in males. In particular, 17beta-HSD expression increased in the course of gonadal development in females, whereas it was constantly low in males. Aromatase was highly expressed in females during gonadal differentiation but not in males over the period. In addition, to elucidate the relationship between gene activation during embryogenesis and reproductive abnormalities in wild birds, we examined expressions of these genes in embryos treated with various doses of diethylstilbestrol (DES), as a representative estrogenic compound. DES had no effect on the expressions of all the genes in either sex during the periods of gonadal differentiation (days 8, 12, and 16). Sexual dimorphism of the gene expression for steroidogenic enzymes appeared to be closely related to gonadal development in the chicken embryo, especially in the female. However, all the genes examined here seem unlikely to respond to xenoestrogens.

Lymphoid aggregates in gonads of embryos, hatchlings, and young of turtles with temperature-dependent sex determination

Cellular infiltrations forming lymphoid-like aggregates were previously observed in gonads of two turtle species exhibiting temperature-dependent sex determination (TSD): at hatching in Chelydra serpentina; at and after hatching in Emys orbicularis. We show here that such aggregates are also present in gonads of Testudo graeca by the end of embryonic development, suggesting that their occurrence is general in turtles. Since in C. serpentina, infiltrations were observed mainly in testes exhibiting remnants of the germinal epithelium, it was assumed that their occurrence was an expression of maleness leading to rejection of this epithelium. The generality of this hypothesis was tested in E. orbicularis by looking for lymphoid-like aggregates in three types of gonads (testes, ovotestes, and ovaries) and for the stages at which they occur. Gonads were from embryos, hatchlings, and young incubated at various temperatures. Ovotestes obtained by treatment with an aromatase inhibitor of eggs incubated at female-producing temperature were also examined. In these gonads, the differentiation of Sertoli cells in testicular cords/tubes was ascertained by expression of SOX9. Moreover, the cell composition of aggregates was determined on electron micrographs. Aggregates appear in ovaries and ovotestes by the end of embryonic development and are present in the majority of these gonads at hatching, and at least up to one year after hatching. They are composed mainly of lymphocytes and fibroblasts. Aggregates are not present in typical testes. Since they occur in most ovaries, they cannot be seen as an expression of maleness. Rather, lymphocytic infiltration and formation of lymphoid aggregates in turtle gonads can be seen as components of the immune system, and can be under the control of gonadal endogenous sex steroids.

Sex determination: insights from the chicken

Not all vertebrates share the familiar system of XX:XY sex determination seen in mammals. In the chicken and other birds, sex is determined by a ZZ:ZW sex chromosome system. Gonadal development in the chicken has provided insights into the molecular genetics of vertebrate sex determination and how it has evolved. Such comparative studies show that vertebrate sex-determining pathways comprise both conserved and divergent elements. The chicken embryo resembles lower vertebrates in that estrogens play a central role in gonadal sex differentiation. However, several genes shown to be critical for mammalian sex determination are also expressed in the chicken, but their expression patterns differ, indicating functional plasticity. While the genetic trigger for sex determination in birds remains unknown, some promising candidate genes have recently emerged. The Z-linked gene, DMRT1, supports the Z-dosage model of avian sex determination. Two novel W-linked genes, ASW and FET1, represent candidate female determinants.

Profiles of mRNA expression of genes related to sex differentiation of the gonads in the chicken embryo

Sex is determined genetically in birds. The homogametic sex is male (ZZ), whereas the heterogametic sex is female (ZW). According to the genetic sex, gonads develop into testes or ovary. In this study, we performed experiments to reveal mRNA expression patterns in the gonad between d 5.5 and 8.5 of incubation and examined a possible role of Dss-Ahc critical region on the X chromosome 1 (Dax1), Steroidogenic factor 1 (Sf1), P450aromatase (P450arom), Estrogen receptor alpha (ER alpha), doublesex and mab3 related transcription factor 1 (Dmrt1), Sry-related HMG box gene 9 (Sox9), Gata binding protein 4 (Gata4), and anti-müllerian hormone (Amh) in sex differentiation in chicken embryonic gonads using RNase protection assay. In embryonic chicken gonads, Dax1 mRNA was expressed in both sexes but was higher in females than in males at d 6.5 and 7.5 of incubation. The Sf1 mRNA was expressed in both sexes, but it was expressed more in males at d 5.5 than in females but more in females than in males at d 7.5 and 8.5 of incubation. The P450arom mRNA was expressed only in female gonads from d 5.5 of incubation. The ER alpha mRNA was expressed in both sexes, but it did not show a sex difference. On the other hand, the Dmrt1 mRNA was expressed in both sexes, but it showed a male-specific expression pattern. The male-specific expression pattern was observed in Sox9 mRNA, but it was not expressed in female gonads. The Gata4 mRNA was expressed in both sexes, and sex differences were not revealed throughout the observational period. Amh mRNA was expressed in both sexes, but it had male-specific mRNA expression pattern at d 6.5 to 8.5 of incubation. These results indicate that Dax1, Sf1, and P450arom have possible roles in ovary formation, whereas Dmrt1, Sox9, and Amh are related to testis formation in differentiating chicken gonads at d 5.5 to 8.5 of incubation.

Different patterns of anti-Müllerian hormone expression, as related to DMRT1, SF-1, WT1, GATA-4, Wnt-4, and Lhx9 expression, in the chick differentiating gonads

In mammals, anti-Müllerian hormone (AMH) is produced by Sertoli cells from the onset of testicular differentiation and by granulosa cells after birth. In birds, AMH starts to be expressed in indifferent gonads of both sexes at a similar level and is later up-regulated in males. We previously demonstrated that, unlike in mammals, the onset of AMH expression occurs in chick embryo in the absence of SOX9. We looked for potential factors that might be involved in regulating AMH expression at different stages of chick gonad differentiation by comparing its expression pattern in embryos and young chicken with that of DMRT1, SF-1, WT1, GATA-4, Wnt-4, and Lhx9, by in situ hybridization. The results allowed us to distinguish different phases. (1) In indifferent gonads of both sexes, AMH is expressed in dispersed medullar cells. SF-1, WT1, GATA-4, Wnt-4, and DMRT1 are transcribed in the same region of the gonads, but none of these factors has an expression strictly coincident with that of AMH. Lhx9 is present only in the cortical area. (2) After this period, AMH is up-regulated in male gonads. The up-regulation is concomitant with the beginning of SOX9 expression and a sex dimorphic level of DMRT1 transcripts. It is followed by the aggregation of the AMH-positive cells (Sertoli cells) into testicular cords in which AMH is coexpressed with DMRT1, SF-1, WT1, GATA-4, and SOX9. (3) In the females, the low level of dispersed medullar AMH expression is conserved. With development of the cortex in the left ovary, cells expressing AMH accumulate in the juxtacortical part of the medulla, whereas they remain dispersed in the right ovary. At this stage, AMH expression is not strictly correlated with any of the studied factors. (4) After hatching, the organization of left ovarian cortex is characterized by the formation of follicles. Follicular cells express AMH in conjunction with SF-1, WT1, and GATA-4 and in the absence of SOX9, as in mammals. In addition, they express Lhx9 and Wnt-4, the latter being also found in the oocytes. (5) Moreover, unlike in mammals, the chicken ovary retains a dispersed AMH expression in cortical interstitial cells between the follicles, with no obvious correlation with any of the factors studied. Thus, the dispersed type of AMH expression in indifferent and female gonads appears to be bird-specific and not controlled by the same factors as testicular or follicular AMH transcription.

Sexually dimorphic expression of steroidogenic factor 1 (SF-1) in developing gonads of the American bullfrog, Rana catesbeiana

Genetic sex determination leads to gonadal differentiation and ultimately the differences between the sexes in steroid hormone secretion. Gonadal steroidogenesis is critical for the development of a sexually dimorphic phenotype and adult reproductive function. Control of gonadal development and steroidogenesis is under the regulation, at least in part, of steroidogenic factor-1 (SF-1). We have begun to characterize SF-1 expression in an amphibian to determine the role of this protein in development and reproduction. We have detected a putative SF-1 protein from several tissues in the American bullfrog, Rana catesbeiana, that co-migrates with mouse SF-1 on a Western blot. Our results show that bullfrog SF-1 protein is expressed in steroidogenic and other reproductive tissues in a manner similar to that reported for other species, with high expression in the brain, pituitary, gonad, liver, and interrenal, but little or no expression in non-reproductive tissues such as skin and intestine. Using a quantitative Western blot analysis system, we documented changes in SF-1 protein in the gonads of developing tadpoles. Our results indicate that there is sexually dimorphic expression of SF-1 protein that becomes evident at the time of sexual differentiation of the gonads. In males, the expression of SF-1 decreases following testicular formation and in females the expression increases with the formation of ovaries. This is the first study to investigate changes in SF-1 during development at the protein level. The expression is similar to that reported for changes in SF-1 mRNA expression in chickens and alligators, however, opposite to that seen in mammals and turtles. These results indicate that SF-1 may play a pivotal role in development of the reproductive system in amphibians as it does in other vertebrate groups.

Dynamics of yolk steroid hormones during development in a reptile with temperature-dependent sex determination

Many oviparous reptiles exhibit temperature-dependent sex determination (TSD); i.e., the temperature at which the egg is incubated determines the sex of the offspring. In TSD reptiles, yolk steroids not only may influence sex determination, but also may mediate hormonal effects on subsequent growth and behavior, as in some avian species. We investigated changes in the levels of estradiol (E(2)) and testosterone (T) during development in yolks of snapping turtle eggs, examined how incubation temperature affects hormone levels, and determined how hormones in turtle eggs are influenced by individual females (=clutch effects). Results indicate significant decreases in both hormones (>50% decline) by the end of the sex-determining period, when two-thirds of the development is complete. The declines in both E(2) and T were significantly affected by incubation temperature, but in different ways. Eggs incubated at female-producing temperatures maintained high levels, those incubated at male-producing temperatures had low E(2) values, and eggs incubated at pivotal temperatures had intermediate levels of E(2). At all three temperatures, T values underwent significant but approximately equal declines, except during the developmental stages just after the sex-determining period, when T levels decreased more at the male-producing temperature than at either of the other two temperatures. Initially, there were significant clutch effects in both hormones, but such differences, attributable to individual females, were maintained only for E(2) later in development. Here we report for the first time that incubation temperature significantly affects the hormonal environment of the developing embryo of a turtle with temperature-dependent sex determination. Based on this and related findings, we propose that yolk sex steroids influence sexual differentiation in these TSD species and play a role in sex determination at pivotal temperatures.

Cloning and expression of zebra finch (Taeniopygia guttata) steroidogenic factor 1: overlap with hypothalamic but not with telencephalic aromatase

The zebra finch (Taeniopygia guttata) brain is highly sexually dimorphic. The organization and production of sex-specific song is considerably influenced by estrogens and androgens. Because the brain itself expresses several steroidogenic enzymes, the local production of sex steroids may contribute to sex differences in neural development. Sex steroid production in gonads is directed by a master regulatory factor, steroidogenic factor 1 (SF1). We have identified a cDNA encoding the homologue of SF1 in the zebra finch and utilized reverse transcription-polymerase chain reaction and in situ hybridization to examine early and late developmental expression of SF1 in brain and in early gonadal development. We found that SF1 is expressed early in embryonic development in the Rathke pouch, beginning at stage 15 and extending to at least stage 27 in both males and females. The earliest expression of SF1 in gonads was found at stage 17 for both males and females and extended to at least stage 27. In brain, we assessed SF1 mRNA expression in posthatch and adult telencephalon, and we compared SF1 and aromatase mRNA expression in adult hypothalamus. In the telencephalon and hippocampus, aromatase was expressed independently of SF1, whereas in the hypothalamus, aromatase and SF1 expression were more closely associated. Expression of SF1 and of aromatase overlapped in restricted regions of the hypothalamus, suggesting that SF1 may regulate aromatase expression in these regions. These findings suggest that steroidogenesis in the zebra finch brain may be regulated by both SF1-dependent and SF1-independent mechanisms. No sex differences were detected in SF1 expression in brain.

Sex determination in the chicken embryo

The chicken embryo represents a suitable model for studying vertebrate sex determination and gonadal sex differentiation. While the basic mechanism of sex determination in birds is still unknown, gonadal morphogenesis is very similar to that in mammals, and most of the genes implicated in mammalian sex determination have avian homologues. However, in the chicken embryo, these genes show some interesting differences in structure or expression patterns to their mammalian counterparts, broadening our understanding of their functions. The novel candidate testis-determining gene in mammals, DMRT1, is also present in the chicken, and is expressed specifically in the embryonic gonads. In chicken embryos, DMRT1 is more highly expressed in the gonads and Müllerian ducts of male embryos than in those of females. Meanwhile, expression of the orphan nuclear receptor, Steroidogenic Factor 1 (SF1) is up-regulated during ovarian differentiation in the chicken embryo. This contrasts with the expression pattern of SF1 in mouse embryos, in which expression is down-regulated during female differentiation. Another orphan receptor initially implicated in mammalian sex determination, DAX1, is poorly conserved in the chicken. A chicken DAX1 homologue isolated from a urogenital ridge library lacked the unusual DNA-binding motif seen in mammals. Chicken DAX1 is autosomal, and is expressed in the embryonic gonads, showing somewhat higher expression in female compared to male gonads, as in mammals. However, expression is not down-regulated at the onset of testicular differentiation in chicken embryos, as occurs in mice. These comparative data shed light on vertebrate sex determination in general.

Role of steroidogenic factor 1 and aromatase in temperature-dependent sex determination in the red-eared slider turtle

Red-eared slider turtles are genetically bipotential for sex determination. In this species, as in many other reptiles, incubation temperature of the egg determines gonadal sex. At higher incubation temperatures females are produced and increasing temperature appears to increase estrogen production in the embryonic brain. Treatment of eggs incubating at a male-producing temperature with exogenous estrogen causes ovaries to form. At a female-biased incubation temperature, prevention of estrogen biosynthesis or administration of nonaromatizable androgens results in the development of testes. In mammals, steroidogenic factor 1 (SF-1) regulates most genes required for estrogen biosynthesis, including aromatase. In both mammals and red-eared sliders, SF-1 is differentially expressed in males and females during gonadogenesis. We have examined both SF-1 gene expression and aromatase activity in embryos incubating at different temperatures and after manipulation to change the course of gonadal development. Our findings indicate a central role for SF-1 in enacting the effect of estrogen. Estrogen treatment directly or indirectly downregulates SF-1 and, ultimately, causes development of females. The inhibition of estrogen results in upregulation of SF-1 and male hatchlings. Thus, SF-1 may lie at the center of one molecular crossroad in male versus female differentiation of the red-eared slider.

Sex, genes, and heat: triggers of diversity

In vertebrates, sex is determined by a surprising variety of mechanisms. In many reptiles, the primary testis or ovary-determining trigger is regulated by egg incubation temperature. This temperature dependent sex determining (TSD) mechanism occurs in all crocodilians and marine turtles examined to date and is common in terrestrial turtles and viviparous lizards (Ewert et al. 1994. J Exp Zool 270:3-15; Lang and Andrews. 1994. J Exp Biol 270:28-44; Mrosovsky. 1994. J Exp Zool 270:16-27; Pieau. 1996. Bioessays 18:19-26; Viets et al. 1994. J Exp Zool 270:45-56; Wibbels et al. 1998. J Exp Zool 281:409-416). In contrast, sex in mammals and birds is determined chromosomally (CSD). Despite these differences, morphological development of the gonads in all these vertebrate groups appears to have been conserved through evolution. Therefore, the genetic mechanisms triggering sex determination appear not to have been conserved through evolution, although the basic genetic pathway controlling the morphological differentiation of the gonads appears to have been conserved.

Expression of AMH, SF1, and SOX9 in gonads of genetic female chickens during sex reversal induced by an aromatase inhibitor

Aromatase inhibitors administered prior to histological signs of gonadal sex differentiation can induce sex reversal of genetic female chickens. Under the effects of Fadrozole (CGS 16949A), a nonsteroidal aromatase inhibitor, the right gonad generally becomes a testis, and the left gonad a testis or an ovotestis. We have compared the expression pattern of the genes encoding AMH (the anti-Müllerian hormone), SF1 (steroidogenic factor 1), and SOX9 (a transcription factor related to SRY) in these sex-reversed gonads with that in control testes and ovaries, using in situ hybridization with riboprobes on gonadal sections. In control males, the three genes are expressed in Sertoli cells of testicular cords; however, only SOX9 is male specific, since as observed previously AMH and SF1 but not SOX9 are expressed in the control female gonads. In addition to testicular-like cords, sex-reversed gonads present many lacunae with a composite, thick and flat epithelium. We show that during embryonic and postnatal development, AMH, SF1 and SOX9 are expressed in the epithelium of testicular-like cords and in the thickened part but not in the flattened part of the epithelium of composite lacunae. AMH and SF1 but not SOX9 are expressed in follicular cells of ovotestes. Coexpression of the three genes, of which SOX9 is a specific Sertoli-cell marker, provides strong evidence for the transdifferentiation of ovarian into testicular epithelium in gonads of female chickens treated with Fadrozole.

Expression of steroidogenic factor-1 in frog embryo and developing gonad

Steroidogenic factor-1 (SF-1), originally identified as an orphan nuclear receptor that regulates expression of genes encoding cytochrome P-450 steroid hydroxylases, is an essential transcriptional factor for adrenal and gonadal development in mammals. Since sex steroid hormones have been shown to play important roles in the sex determination of frogs, it is of interest to know how SF-1 gene expression is regulated during the sexual development of this organism. A previous study isolated the cDNA of the frog Rana rugosa SF-1 (rrSF-1) and found sexual differences in its gene expression in adult frogs; positive in testis and negative in ovary (Kawano et al., 1998). This study examined rrSF-1 gene expression in frog embryos and developing and mature gonads by immunohistochemical staining using anti-rrSF-1 protein antibody for protein localization and by in situ hybridization analysis for mRNA transcription. The results obtained in this study indicated that cells expressing SF-1 that originate in the mesoderm and endoderm probably migrate into the developing gonad via the dorsal mesentery, genital ridge, and mesorchium or mesovarium. Thus, SF-1 may play an important role in gonadal development in amphibians.

Developmental expression patterns of FTZ-F1 homologues in zebrafish (Danio rerio)

The fushi tarazu factor 1 (FTZ-F1) gene family constitutes a subgroup of orphan nuclear receptors which can be divided into two groups (LRH/FTF- and SF-1/Ad4BP-like) based on sequence homology, function, and tissue distribution. Analysis of zebrafish FTZ-F1 homologues (zFF1 and ff1b) during embryogenesis indicated distinct expression patterns for both genes. Besides the previously observed expression in pituitary/hypothalamus and mandibular arch, zFF1 transcripts were also detected in domains corresponding to the pronephric duct, somites, liver, and hindbrain. Additionally, ff1b transcripts were detected at other developmental stages than earlier documented. Comparative sequence analysis showed that zFF1 exhibited higher sequence similarity to the LRH/FTF group than the SF-1/Ad4BP group, whereas ff1b was indistinguishable between the groups. These observations, coupled with obtained expression patterns, indicate that zebrafish FTZ-F1 homologues exhibit characteristics that are indicative of both LRH/FTF- and SF-1/Ad4BP-like genes.

Temperature-dependent sex determination in the American alligator: expression of SF1, WT1 and DAX1 during gonadogenesis

Sex determination in mammals and birds is chromosomal, while in many reptiles sex determination is temperature dependent. Morphological development of the gonads in these systems is conserved, suggesting that many of the genes involved in gonad development are also conserved. The genes SF1, WT1 and DAX1 play various roles in the mammalian testis-determining pathway. SF1 and WT1 are thought to interact to cause male-specific gene expression during testis development, while DAX1 is believed to inhibit this male-specific gene expression. We have cloned SF1 and DAX1 from the American alligator, a species with temperature-dependent sex determination (TSD). SF1, DAX1 and WT1 are expressed in the urogenital system/gonad throughout the period of alligator gonadogenesis which is temperature sensitive. SF1 appears to be expressed at a higher level in females than in males. This SF1 expression pattern is concordant with the observed pattern during chicken gonadogenesis, but opposite to that observed during mouse gonadogenesis. Although the observed sexual dimorphism of gonadal SF1 expression in alligators and chickens is opposite that observed in the mouse, it is probable that SF1 is involved in control of gonadal steroidogenesis in all these vertebrates. DAX1 and WT1 are both expressed during stages 22-25 of both males and females. However, there appear to be no sex differences in the expression patterns of these genes. We conclude that DAX1, WT1 and SF1 may be involved in gonadal development of the alligator. These genes may form part of a gonadal-development pathway which has been conserved through vertebrate evolution.

Developmental expression of steroidogenic factor 1 in a turtle with temperature-dependent sex determination

A variety of reptiles possess temperature-dependent sex determination (TSD) in which the incubation temperature of a developing egg determines the gonadal sex. Current evidence suggests that temperature signals may be transduced into steroid hormone signals with estrogens directing ovarian differentiation. Steroidogenic factor 1 (SF-1) is one component of interest because it regulates the expression of steroidogenic enzymes in mammals and is differentially expressed during development of testis and ovary. Northern blot analysis of SF-1 in developing tissues of the red-eared slider turtle (Trachemys scripta), a TSD species, detected a single primary SF-1 transcript of approximately 5.8 kb across all stages of development examined. Analysis by in situ hybridization indicated nearly equivalent SF-1 expression in early, bipotential gonads at male (26 degrees C)- and female (31 degrees C)-producing incubation temperatures. In subsequent stages, as gonadal sex first becomes histologically distinguishable during the temperature-sensitive period, SF-1 expression increased in gonads at a male-producing temperature and decreased at a female-producing temperature, suggesting a role for SF-1 in the sex differentiation pathway. SF-1 message was also found in adrenal and in the periventricular region of the preoptic area and diencephalon, but there was no apparent sex bias in these tissues at any stage examined. The overall developmental pattern of SF-1 mRNA expression in T. scripta appears to parallel that found in mammals, indicating possible homologous functions.

Targeted mutagenesis of the endogenous mouse Mis gene promoter: in vivo definition of genetic pathways of vertebrate sexual development

Mutations were introduced into conserved steroidogenic factor 1 (SF1)- and SOX9-binding sites within the endogenous mouse Mullerian inhibiting substance (Mis) promoter. Male mice homozygous for the mutant SF1-binding site correctly initiated Mis transcription in fetal testes, although at significantly reduced levels. Surprisingly, sufficient MIS was produced to eliminate the MUllerian ducts. In contrast, males homozygous for the mutant SOX9-binding site did not initiate Mis transcription, resulting in pseudohermaphrodites. These studies suggest an essential role for SOX9 in the initiation of Mis transcription, whereas SF1 appears to act as a quantitative regulator of Mis transcript levels, perhaps for influencing non-Mullerian duct tissues. Comparative studies of Mis expression in vertebrates indicate that the Mis promoter receives transcriptional inputs that vary between species but result in the same functional readout.

Incubation temperature influences sex-steroid levels in juvenile red-eared slider turtles, Trachemys scripta, a species with temperature-dependent sex determination

Incubation temperature determines gonadal sex in the red-eared slider turtle, Trachemys scripta. However, little is known about the long-term effects of incubation temperature on traits other than gonadal sex in this species. To investigate the hypothesis that incubation temperature (independent of gonadal sex) influences sex steroid levels after hatching, we incubated eggs of the red-eared slider turtle at three temperatures (26, 28.6, and 31 degrees C). We then measured plasma levels of dihydrotestosterone, estradiol, progesterone, and testosterone in 6-wk-old males from 26 degrees C and 28.6 degrees C eggs, and in 6-wk-old females from 28.6 degrees C and 31 degrees C eggs. We found that dihydrotestosterone levels were not influenced by incubation temperature or gonadal sex. However, progesterone levels were significantly higher in males from 26 degrees C eggs than in males from 28.6 degrees C eggs. In contrast, testosterone levels did not differ between males from 26 degrees C versus males from 28.6 degrees C eggs, but they were significantly higher in females from 28.6 degrees C than in females from 31 degrees C eggs. Progesterone and testosterone levels did not differ between males and females from 28.6 degrees C eggs. Temperature also influenced estradiol levels in both sexes, but the effects were enigmatic. We conclude that incubation temperature has lasting effects on sex steroid levels even after hatching.