SRY Gene

The reproductive system

Barry Mitchell BSc MSc PhD FIBMS FIBiol , Ram Sharma BSc MSc PhD , in Embryology (Second Edition), 2009

Development of the testis (Fig. 9.3)

The sex-determining gene in the Y chromosome produces a protein (testis-determining factor) that promotes the development of a testis: the primitive sex cords proliferate and penetrate into the medulla to form the testicular cords. Some of those cells differentiate into Sertoli cells, whilst the remainder become incorporated into seminiferous tubules. The latter are solid cylinders until after puberty at which time they canalize. The testicular cords anastomose to form the rete testis, which becomes continuous with 15–20 persisting mesonephric tubules, the ductuli efferentes. The testis-determining factor also induces differentiation of gonadal mesenchymal cells into the interstitial Leydig cells.

Read full chapter

URL:

https://www.sciencedirect.com/science/article/pii/B9780702032257500129

Androgens

Anthony W. Norman Ph.D. , Helen L. Henry Ph.D. , in Hormones (Third Edition), 2015

4 SRY or Testis-Determining Factor

The SRY gene product, SRY, is detected in the bipotential gonad of XY individuals at about 42 days. The protein contains an HMG (high mobility group) box of 79 amino acids that binds to specific regions of DNA, causing the DNA molecule to bend. This bending is thought to make these regions more accessible to other transcription-regulating proteins which ultimately bring about the differentiation of the gonadal cells, some to Sertoli cells, some to Leydig cells. SRY also contains nuclear localization signals, phosphorylation sites, and additional protein–protein interaction sites.

As depicted in Figure 12-12, a major transcription factor in testicular development is SOX9 (sex-determining region-box 9) an autosomal gene regulated by the SRY protein. In XY fetus, SOX9 is also positively regulated by FGF9 and the product of prostaglandin D synthase, PGD2 (see Chapter 8). SOX9 also has its own positive feedback loop. Sustained elevated levels of SOX9 lead to normal development of the testes. Since the gene for SOX9 is autosomal, it can also be expressed in XX fetuses and adults. Its expression and consequent inappropriate development of testicular cells in the XX fetus is restrained by the Wnt4/β-catenin signaling pathway. This brake on SOX9 expression is required for maintenance of normal ovarian function in the female throughout reproductive life and this is provided by FOXL2 (Forkhead box L2), among other factors.

Figure 12-12. Signaling pathways in the control of gonad differentiation.

In the XY fetus, SRY (sex determining region of the Y chromosome) activates the expression of SOX9 (SRY-related HMG-box-9; HMG=high mobility group), a potent transcription factor that is required for the expression of the proteins required for fetal testis differentiation. FGF9 (fibroblast growth factor 9) and prostaglandin D2 (produced by PGDS=prostaglandin D synthase) also contribute to the maintenance of SOX9 expression in the male. In the embryonic female SOX9 expression is suppressed by the WNT4/β-catenin pathway and its stimulators. In the adult female, normal ovarian function requires the continued suppression of SOX9, which is mediated by the transcription factor FOXL2 (Forkhead box L2).

By the end of the sixth week after conception the Sertoli cells of the fetal testis have begun to differentiate and secrete anti-Müllerian hormone (see below). Within the next two weeks, the Leydig cells appear and begin to secrete testosterone. Ovarian differentiation follows that of the testis by 4–5 weeks.

Read full chapter

URL:

https://www.sciencedirect.com/science/article/pii/B9780080919065000124

Y Chromosome (Human)

C. Tyler-Smith , in Brenner's Encyclopedia of Genetics (Second Edition), 2013

What Is the Biological Role of the Y Chromosome?

One gene, SRY (for sex-determining region of the Y chromosome), determines sex in humans; although many genes are involved, SRY is the key switch and individuals with SRY develop into males, while those without it develop into females. This mechanism of sex determination requires that SRY is active in half the population and not active in the other half, and this is achieved by its location on a chromosome that is inherited by half the population and not the other half: the Y chromosome. Males produce sperm with an X chromosome or a Y chromosome in equal numbers; eggs fertilized by a Y-bearing sperm develop into males and those fertilized by an X-bearing sperm develop into females. Thus the primary biological role of the Y chromosome is sex determination.

Many of the other special properties of the Y chromosome can be understood as consequences of this primary role. There can be only one copy per cell of the Y, a condition named haploidy. However, successful production of sperm (as of eggs) requires that chromosomes recombine. Autosomes, and the X in females, recombine with their homologue (the corresponding chromosome from the other parent), but a haploid chromosome like the Y needs to recombine with a different chromosome, in this case the X. Such recombination must not disrupt the sex determination mechanism, and this is achieved by partitioning the chromosome into recombining segments located at each end of the X and Y, and a nonrecombining Y-specific segment carrying SRY in between (Figure 1). The recombining segments share many properties with the autosomes and are termed pseudoautosomal: these properties include both the obvious ones of their inheritance pattern and diploidy, and less obvious ones like regular gene density.

Figure 1. Structure of the human Y chromosome. Left to right: scale in Mb (millions of base pairs); regions showing pseudoautosomal or male-specific inheritance; location of functions relevant to medical genetics; cytogenetic banding pattern with the highly repeated long arm sequence block shown in gray; genes mentioned in the text.

The lack of recombination in the Y-specific region means that a slightly deleterious mutation arising on a Y haplotype is a permanent part of that lineage; the slow accumulation of such mutations over evolutionary time since the origin of the X and Y chromosomes from a pair of autosomes >150 million years ago, termed Muller's ratchet, has led to loss of nonessential genes and the current low gene density of the Y. In addition, the number of Y chromosomes in a population is one-quarter of the number of any autosome, and this has consequences for its use in studies of human history, described below.

The following sections concentrate on the Y-specific region of the chromosome.

Read full chapter

URL:

https://www.sciencedirect.com/science/article/pii/B9780123749840016582

Epigenetic Shaping of Sociosexual Interactions

Ryohei Sekido , in Advances in Genetics, 2014

4.5 Sry as an miRNA Sponge

Most Sry genes contain no intron, except for some marsupials, although overall conservation of Sry genomic sequences is very poor apart from the HMG box (Sekido, 2010). In mice, two types of Sry transcripts are generated from proximal and distal promoters (Dolci, Grimaldi, Geremia, Pesce, & Rossi, 1997; Jeske, Bowles, Greenfield, & Koopman, 1995). The proximal promoter gives rise to the linear Sry transcript, i.e., Sry mRNAs, which are translated into SRY protein. The transcript that arises from the distal promoter contains an inverted repeat on both 5′ and 3′ ends, which allows a stem-loop structure to form (Figure 7.2). Since the Sry ORF is flanked by splice acceptor (SA) and splice donor (SD) sequences, the proximity of the SA and SD sites in the loop facilitates pre-mRNA splicing to produce an untranslatable circular transcript (Capel et al., 1993). Despite the fact that both transcripts are expressed not only in the testis but also in other tissues, the roles of Sry in these tissues are largely unknown.

Figure 7.2. The mouse Sry gene produces the protein-coding mRNA and circular transcript.

The short linear transcript or mRNA is translated into SRY protein. A large inverted repeat present at 5′ and 3′ ends of the long linear transcript would allow a stem loop to form. The untranslatable circular transcript that may act as an RNA sponge is generated by pre-mRNA splicing through splice acceptor (SA) and donor (SD) sites. Open box, open reading frame; solid boxes, 5′ and 3′ untranslated regions.

High-throughput sequencing combined with bioinformatics have revealed that circular RNA molecules are abundant in human cells (Jeck et al., 2013; Salzman, Gawad, Wang, Lacayo, & Brown, 2012). Circular RNAs were thought to be by-products of splicing errors with no biological significance. However, recent studies have shed light on their novel role in attenuating miRNA-mediated gene silencing, referred to as "miRNA sponge," similar to what is originally designed as artificial inhibitory transcripts with tandem-repeated complementary sequences of miRNAs that sequester them from endogenous targets when transfected into cells (Ebert, Neilson, & Sharp, 2007). The studies have revealed that CDR1as/ciRS-7, an endogenous circular RNA transcript discovered in human and mouse neurons, harbors multiple binding sites for miR-7 that is a key regulator for neuronal development (Hansen et al., 2013; Memczak et al., 2013). Overexpression of CDR1as/ciRS-7 causes defect in the midbrain, which is similar to the phenotype seen in miR-7 knocking down. These results suggest that CDR1as/ciRS-7 acts as a sponge for miR-7.

The expression of the circular Sry transcripts (hereafter referred to as circSry) is spatiotemporally regulated. It is detectable in differentiated Sertoli cells at 13.5   dpc, in contrast to the linear transcripts that are transiently expressed in Sertoli cell precursors between 10.5 and 12.5   dpc (Dolci et al., 1997). At the adult stage, circSry is predominantly expressed in germ cells, particularly in round spermatids (Capel et al., 1993), although no SRY protein is present in germ cells. In the brain, circSry is highly expressed at the embryonic stages and is downregulated at the postnatal stages (Mayer et al., 2000). Interestingly, circSry carries 16 putative binding sites for miR-138 (Hansen et al., 2013). miR-138 is expressed in several cell types including neurons, cardiac cells, and osteoblasts (Eskildsen et al., 2011; Morton et al., 2008; Obernosterer, Leuschner, Alenius, & Martinez, 2006). Of particular interest is its neuronal role in the regulation of dendritic spine growth at the synapse in hippocampal neurons. miR-138 inhibits the expression of acyl protein thioesterase 1, an enzyme depalmitoylating the α13 subunits of G proteins, whose activity is required for enlargement of spines (Siegel et al., 2009).

Read full chapter

URL:

https://www.sciencedirect.com/science/article/pii/B9780128002223000073

Hormone-Behavior Relations of Clinical Importance

A.B. Dessens , ... S.L.S. Drop , in Hormones, Brain and Behavior (Second Edition), 2009

5.101.2.2 Ovarian Development: Orchestrated by Ovary-Determining Genes?

The mammalian testis-determining gene SRY is highly sex-specific. A long-standing question is whether there are any known genes which are specifically required for differentiation of the bipotential gonads to become ovaries? An interesting variety of candidate ovary-determining genes have been extensively studied, as described in several comprehensive recent reviews (Kim and Capel, 2006; Ottolenghi et al., 2007a,b; Wilhelm et al., 2007a,b; Yao, 2005). If there are any new candidate ovary-determining genes to be discovered, these might be identified by analysis of transcriptional programs, studying differential gene expression patterns of female and male gonads in the mouse, at E10.5–E11.5 of embryonic development, around the period that the Sry gene and then the Sox9 gene are first expressed (Beverdam and Koopman, 2006; Nef et al., 2005).

DAX1 (dosage-sensitive sex reversal–adrenal hypoplasia congenital–critical region of the X chromosome gene 1) was long considered to encode a promising candidate. The gene is located on the X chromosome and has no homolog on Y. Moreover, duplication of DAX1 can result in male-to-female sex reversal of XY individuals. Its possible role as an antitestis factor, antagonizing SRY action (Swain et al., 1998), was challenged by the observation that DAX1 is required for testis determination in a mouse strain that is susceptible to sex reversal because of an altered Sry gene (Meeks et al., 2003). Furthermore, other studies in mice provided evidence that the Dax1 gene is not strictly required for ovarian development (Yu et al., 1998). The encoded protein plays some quite important and dosage-dependent role(s) in early gonadal development, both ovary and testis, but it is not the elusive ovary-determining factor.

Whereas DAX1 is a nuclear transcription factor, the WNT4 gene encodes an intercellular signaling molecule that is involved in several developmental processes, including roles in sexual development. Even before gonadal sex differentiation occurs, WNT4 acts in the development of the Müllerian ducts, which later develop into the fallopian tubes, uterus, and upper vagina, in female embryos. In addition, WNT4 knockout (KO) mice show partial female-to-male sex reversal. It appears that WNT4 is involved in balancing the bipotential gonad between either ovary or testis formation (Kim and Capel, 2006). WNT4 inhibits the formation of testis-specific vascularization in the ovary, but it is not considered an ovary-determining factor.

At this point in time, FOXL2 is perhaps the most promising candidate gene to play a leading role in ovary development. The encoded protein contains a so-called forkhead box, a protein domain binding to DNA. This domain is also known as the winged helix, based on the butterfly-like appearance of the loops in its protein structure. FOXL2 is implicated in XX gonadal female-to-male sex reversal, and genetic inactivation of the FOXL2 gene in mice demonstrates a role in granulosa cell differentiation and maintenance of the ovary. This inactivation does not cause a defect in early ovary formation, so that FOXL2 cannot be named an ovary-determining factor. However, FOXL2 appears to have an ongoing role to maintain ovary function by acting on follicle formation and development, thereby counteracting sex reversal activities (Ottolenghi et al., 2005, 2007a,b; Uhlenhaut and Treier, 2006; Wilhelm et al., 2007a,b).

Ovary development from the bipotential gonads can be viewed as a pathway which unfolds in the absence of SRY action. Some genes implicated in female-to-male gonadal sex reversal normally seem to suppress testis development, rather than to exert a primary ovary-determining effect. Perhaps the situation can be best described as follows.

In XX embryos, the developing fetal gonads become populated with mitotic germline cells, forming germ cell cysts which, after some time, undergo a transition toward meiotic prophase (Pepling and Spradling, 2001). The transition to meiotic prophase is activated by retinoic acid signaling (Bowles et al., 2006; Koubova et al., 2006). When these future oocytes approach toward the end of meiotic prophase, an interaction with somatic gonadal cells becomes evident, leading to the formation of primordial follicles in which diplotene-stage prophase oocytes become arrested, enclosed by a layer of future granulosa cells. These primordial follicles form the reservoir of oocytes from which follicles are recruited for growth throughout adult reproductive life. Primordial and growing follicles can be considered as organizers of ovarian tissue structure, also recruiting somatic cells to become steroidogenic theca cells. Without germline cells, chromosomal female gonads will never develop as ovaries, but rather turn into functionless streak tissue. If the germline cells are unable to organize the surrounding cells into follicular structures, a dysgenetic gonad, in which undifferentiated gonadal tissue persists, will develop. In this process, the sex chromosome content of the germline cells is not the key factor. In mouse, XO and even XY germline cells can also form follicles, if these cells are located in a fetal gonad where they are stimulated or admitted to enter meiotic prophase (McLaren, 1995). However, there is increased early loss of such germline cells, as found for oocytes in XO mice (Burgoyne and Baker, 1985). Similarly, in human Turner syndrome, it is most likely an early loss of XO germline cells which results in the formation of a streak gonad. Since ovary development fails when no germline cells are present in the gonads, or when follicle development or maintenance is dysregulated in any other manner ( Figure 1 ), it can be suggested that perhaps we should classify several genes that are critically involved in follicle formation and maintenance as ovary-determining genes.

Ovary development fails when no germline cells are present in the gonads, or when follicle development or maintenance is dysregulated in any other manner ( Figure 1 ). Oocytes arrested in the diplotene stage of meiotic prophase, in primordial follicles, act as signaling centers. As early as day E13.5, these oocytes express the transcription factor folliculogenesis-specific basic helix–loop–helix factor in the germline alpha (FIGalpha, encoded by the gene Figla). Known target genes for this transcription factor in the oocyte are the genes encoding the zona pellucida proteins ZP1, 2, and 3 (Liang et al., 1997). Moreover, FIGalpha was shown to be essential for recruitment of pre-granulosa cells to form the primordial follicle (Soyal et al., 2000). The transcription factor FOXL2 is found in the pre-granulosa cells, where it plays a role in their differentiation associated with primordial follicle formation (Schmidt et al., 2004; Uda et al., 2004). Together, FIGalpha in the oocyte and FOXL2 in the pre-granulosa cells are critical factors in ovary development. For certain, there are many other critical factors, further downstream in the follicular and ovarian development pathway, such as growth differentiation factor 9 (GDF9) which is also originating from the oocyte acting as signaling centers (Dong et al., 1996). If any ovary-determining gene remains to be identified, it will be acting upstream of FIGalpha and FOXL2.

Nature has found a very effective way to curtail ovary formation in male mammals by expressing the SRY gene. Through its actions, the SRY protein forces the indifferent fetal gonads to embark on a tissue differentiation pathway leading to the formation of testis tubules. It turns out that retinoic acid, which signals entry into meiotic prophase in the early ovary, is metabolically inactivated in the fetal testis (Bowles et al., 2006; Koubova et al., 2006). The germline cells present in the primitive gonads will remain in a mitotic stage, not entering into the prophase of meiosis, while they are enclosed in developing testis tubules in close association with the pre-Sertoli cells.

In XX–XY chimeric mouse testis, Sertoli cells are predominantly XY (Palmer and Burgoyne, 1991). Whether human gonads in cases of 45,X/46,XY mosaicism and variants will develop as testes, ovaries, ovotestis, or streak gonads is most likely determined by the percentage of cells carrying a Y chromosome and by the germline karyotypes, although additional mechanisms may add to the complexity of this situation (see Section 101.3.2). Sertoli cells expressing SRY may exert a dominant effect, driving gonadal determination pathways toward testis due to the activation of a cascade of testis-determining genes, such as SOX9 (see above). Below a threshold of Y-positive cells, 45,X/46,XY gonads will most often develop as a streak, because continuation of ovarian development requires the continuous presence of oocytes, which in humans is incompatible with 45,X or 46,XY germline karyotypes.

Read full chapter

URL:

https://www.sciencedirect.com/science/article/pii/B9780080887838001017

Clinically Important Hormone Effects on Brain and Behavior

Martine Cools , ... Arianne B. Dessens , in Hormones, Brain and Behavior (Third Edition), 2017

4.03.2.2 Primary Ovarian Differentiation: Orchestrated by Ovary-Determining Genes?

The mammalian testis-determining gene SRY is highly sex-specific. In contrast to the testis pathway, no single ovary-determining factor has been identified; however, multiple genes, such as Foxl2, Rspo1, Ctnnb1, and Wnt4, seem to work synergistically and in parallel for proper ovary development, as reviewed in Eggers et al. (2014). An interesting variety of candidate ovary-determining genes have been extensively studied, described in several comprehensive reviews (Kim and Capel, 2006; Ottolenghi et al., 2007a,b; Wilhelm et al., 2007a,b; Yao, 2005; Eggers et al., 2014). Any new candidate ovary-determining genes to be discovered might be identified by analysis of transcriptional programs, studying differential gene expression patterns of female and male gonads in the mouse, at E10.5–E11.5 of embryonic development, around the period that the Sry gene and Sox9 genes are first expressed (Beverdam and Koopman, 2006; Nef et al., 2005).

The transcription factor DAX1 (dosage-sensitive sex reversal–adrenal hypoplasia congenital – critical region of the X chromosome gene 1, NR0B1) was long considered to be a promising candidate. The NR0B1 gene is located on the X chromosome and has no homologue on Y. Duplication of DAX1 can result in male-to-female sex reversal of XY individuals. Its possible role as an antitestis factor, antagonizing SRY action (Swain et al., 1998), was challenged by the observation that DAX1 is required for testis determination in a mouse strain that is susceptible to sex reversal because of an altered Sry gene (Meeks et al., 2003). Furthermore, other studies in mice provided evidence that the Dax1 gene is not strictly required for ovarian development (Yu et al., 1998). The encoded protein plays some important and dosage-dependent role(s) in early gonadal development, both ovary and testis, but it is not the elusive ovary-determining factor.

Forkhead box L2 (Foxl2) is a member of the forkhead box gene family, encoding an evolutionary conserved transcription factor with a forkhead or winged helix domain, based on the butterfly-like appearance of the loops in its protein structure. Foxl2 is the earliest marker known to be expressed in the developing ovary, therefore suggesting a role in early ovary differentiation. Overexpression of Foxl2 in XY mice and ablation of Foxl2 expression in XX mice result in gonadal anomalies, but not sex reversal (Ottolenghi et al., 2005, 2007a,b; Uhlenhaut and Treier, 2006). In goats with polled intersex syndrome (PIS), however, an 11.7-kb deletion upstream of FOXL2 leads to female-to-male sex reversal in XX goats, further supporting a role for this gene in female-specific development (Pailhoux et al., 2001). Furthermore, mouse studies showed a role of Foxl2 in postnatal maintenance of the ovary, where it functions to suppress genes involved in testis differentiation from early embryonic gonad differentiation throughout adult life (Uhlenhaut et al., 2009).

Furthermore, Wnt4 and Rspo1, two components of the WNT/FDZ/β-catenin (encoded by the gene Ctnnb1) signaling cascade, have been shown to play an important role in ovarian development. Wnt4 and Rspo1 are known to activate β-catenin (Ctnnb1), regulating the transcription of many genes, such as Wnt4 and Fst (Chassot et al., 2008b; Maatouk et al., 2008). Stabilization of β-catenin in somatic cells of the developing mouse XY gonad leads to male-to-female sex reversal (Maatouk et al., 2008). However, the exact mechanisms underlying sex reversal in these animals as well as the precise role of stabilized β-catenin in activation of ovary-specific genes and/or repression of testis-specific genes are not yet clear.

Ovary development from the bipotential gonads can be viewed as a pathway which unfolds in the absence of SRY action. Some genes implicated in female-to-male gonadal sex reversal normally seem to suppress testis development, rather than to exert a primary ovary-determining effect. Perhaps the situation can be best described as follows: in XX embryos, the developing fetal gonads become populated with mitotic germ line cells, forming germ cell cysts which after some time undergo a transition toward meiotic prophase (Pepling and Spradling, 2001). The transition to meiotic prophase is activated by retinoic acid signaling (Bowles et al., 2006; Koubova et al., 2006). When these future oocytes approach the end of the meiotic prophase, an interaction with somatic gonadal cells becomes evident, leading to the formation of primordial follicles in which diplotene stage prophase oocytes are arrested, enclosed by a layer of future granulosa cells. These primordial follicles form the reservoir of oocytes from which follicles are recruited for growth throughout adult reproductive life. Primordial and growing follicles can be considered as organizers of ovarian tissue structure, also recruiting somatic cells to become steroidogenic theca cells. Without germ line cells, chromosomal female gonads will never develop as ovaries but rather turn into functionless streak tissue. If the germ line cells are unable to organize the surrounding cells into follicular structures, a dysgenetic gonad in which undifferentiated gonadal tissue (UGT) persists will develop. In this process, the sex chromosome content of the germ line cells is not the key factor. In mouse, XO and even XY germ line cells can also form follicles, if these cells are located in a fetal gonad where they are stimulated or admitted to enter meiotic prophase (McLaren, 1995). However, there is increased early loss of such germ line cells, as found for oocytes in XO mice (Burgoyne and Baker, 1985). Similarly, in human Turner syndrome, it is most likely an early loss of XO germ line cells that results in the formation of a streak gonad. Since ovary development fails when no germ line cells are present in the gonads, or when follicle development or maintenance is dysregulated in any other manner, it can be suggested that perhaps we should classify several genes that are critically involved in follicle formation and maintenance as ovary-determining genes. A number of transcription factors whose expression, at least in adult tissues, appears to be restricted to germ cells or oocytes, are necessary for early folliculogenesis. As early as day E13.5, these oocytes express the transcription factor FIGalpha (folliculogenesis-specific basic helix-loop-helix factor in the germ line alpha, encoded by the gene FIGLA). Known target genes for this transcription factor in the oocyte are the genes encoding the zona pellucida proteins, ZP1, 2, and 3 (Liang et al., 1997). Moreover, FIGalpha was shown to be essential for recruitment of pregranulosa cells to form the primordial follicle (Soyal et al., 2000). Other germ line-expressed, transcription factor-encoding genes are known, including spermatogenesis and oogenesis bHLH transcription factors 1 and 2 (Sohlh1 and Sohlh2). Both Sohlh1 −/− and Sohlh2 −/− mice display postnatal oocyte loss leading to female sterility (Choi et al., 2008; Pangas et al., 2006). In addition, deletion of Nobox leads to female sterility and postnatal oocyte loss (Choi and Rajkovic, 2006; Rajkovic et al., 2004). Mice studies suggested that impairment of the human counterparts might lead to premature ovarian insufficiency. This has been corroborated by more recent human studies, in which NOBOX and FIGLA mutations have been demonstrated in women with primary ovary insufficiency (POI) (Qin et al., 2007; Zhao et al., 2008), emphasizing the importance of mouse studies for identifying candidate disease-causing genes in humans.

The transcription factor FOXL2 is found in the pregranulosa cells, where it plays a role in their differentiation associated with primordial follicle formation. Disruption of FOXL2 leads to abnormal follicle development and POI in mice (Schmidt et al., 2004; Uda et al., 2004) and humans (Meduri et al., 2010). Expression studies in early postnatal Foxl2-null ovaries indicate that FOXL2 likely affects specific basic metabolic aspects required for the proliferation and differentiation of somatic cells in the postnatal ovary in addition to its role in embryonic ovarian development (Uda et al., 2004; Crisponi et al., 2001).

Together, FIGalpha in the oocyte and FOXL2 in the pregranulosa cells are critical factors in ovary development. For certain, there are many other critical factors, further downstream in the follicular and ovarian development pathway, such as growth differentiation factor 9 (GDF9), which is also originating from the oocyte acting as a signaling center (Dong et al., 1996). If any ovary-determining gene remains to be identified, it is expected to act upstream of FIGalpha and FOXL2 (Figure 1).

Figure 1. Genes and pathways required for ovary development and differentiation. In XX mice, genes such as RSPO1, WNT4, and FOXL2 are expressed during ovary development. RSPO1−/− ovaries show reduced levels of expression of WNT4, suggesting that RSPO1 acts upstream of WNT4. However, a synergistic action of WNT4 and RSPO1 in activating β-catenin (Ctnnb1) has also been suggested. Together, RSPO1, WNT4 and FOXL2 activate Fst expression. In addition to these genes, the Gata4-Zfpm2 (Fog2) pathway has also been implicated in embryonic ovary formation. In the adult ovary, Foxl2 is required for tissue maintenance and follicle maturation.

 Reproduced with permission from Eggers, S., Ohnesorg, T., Sinclair, A., 2014. Genetic regulation of mammalian gonad development. Nat. Rev. Endocrinol. 10, 673–683.

Nature has found a very effective way to curtail ovary formation, in male mammals, by expressing the SRY gene. Through its actions, the SRY protein forces the indifferent fetal gonads to embark on a differentiation pathway leading to the formation of testis tubules. It turns out that retinoic acid, which signals entry into meiotic prophase in the early ovary, is metabolically inactivated in the fetal testis (Bowles et al., 2006; Koubova et al., 2006). The germ line cells present in the primitive gonads will remain in a mitotic stage, not entering into the prophase of meiosis, while they are enclosed in developing testis tubules in association with the pre-Sertoli cells.

In XX/XY chimeric mouse testes, Sertoli cells are predominantly XY (Palmer and Burgoyne, 1991). Whether human gonads in cases of 45,X/46,XY mosaicism and variants will develop as testes, ovaries, ovotestis, or streak gonads is most likely determined by the percentage of cells carrying a Y chromosome and by the germ line karyotypes, although additional mechanisms may add to the complexity of this situation (see Section 4.03.3.2). Sertoli cells expressing SRY may exert a dominant effect, driving gonadal determination pathways toward testis due to the activation of a cascade of testis-determining genes, such as SOX9 (see above). Below a threshold of Y-positive cells, 45,X/46,XY gonads most often will develop as a streak, because continuation of ovarian development requires the continuous presence of oocytes, which in humans is incompatible with 45,X or 46,XY germ line karyotypes.

Read full chapter

URL:

https://www.sciencedirect.com/science/article/pii/B9780128035924000687

Development of the Genital System

Hongling Du , Hugh S. Taylor , in Principles of Developmental Genetics (Second Edition), 2015

SRY

In 1991, the SRY gene was cloned from a region closely linked to ZFY, and it has been confirmed as the TDF needed from the Y chromosome to establish male development (Figure 27.1; Koopman et al., 1990; Sinclair et al., 1990). SRY/Sry is a small intronless gene that encodes a protein with a conserved DNA-binding high mobility group (HMG) box. SRY is a member of a large family of SRY-like HMG box containing genes. The presence of an SRY mutation in about 15% of human XY females supported the proposition that this gene represented the TDF. The identity of Sry as the TDF was determined in the mouse. The Sry gene is absent in a strain of XY mice that are phenotypically female. In the transgenic mouse, SRY can cause XX mice to undergo sex reversal and develop as males. This occurs despite the fact that they lack all other genes of the Y chromosome. Sry is transcribed in the genital ridges of embryos just before testis differentiation, but it is not expressed in the gonads of female mice embryos. In addition, sry was cloned from marsupials and shown to map to the Y chromosome, which indicates that it represents the common ancestral mammalian TDF. The SRY gene encodes a transcription factor that regulates the genes that are responsible for testicular development.

FIGURE 27.1. A History of the Sex-Determination Gene (Sry) Localized on the Y Chromosome.

The central role of the Y chromosome in male sex determination has been recognized for many years. In 1987, ZFY was isolated and initially equated with the TDF. In 1989, another area located near ZFY was found to play role in male sex determination. In 1991, SRY gene was isolated in this area and identified as the TDF.

 Reproduced with permission from Sultan et al. (1991).

Read full chapter

URL:

https://www.sciencedirect.com/science/article/pii/B9780124059450000272

Genetic Control of Fetal Sex Development

Rajini Sreenivasan , ... Andrew Sinclair , in Encyclopedia of Endocrine Diseases (Second Edition), 2019

SRY

SRY, the master sex determining gene, is most commonly translocated from the short arm of the Y chromosome onto the X, causing male-to-female 46,XX ovotesticular DSD. Mutations in the coding region of the gene are also responsible for a significant proportion of 46,XY DSD cases. However, there is only one known case of a mutation upstream of SRY in the promoter. This 3   bp deletion removes an important transcription factor binding site, preventing transcriptional activation of SRY in the developing gonad (Assumpcao et al., 2005). This, in turn, caused 46,XY complete gonadal dysgenesis in the patient and was inherited from the father, who carried the mutation and had severe hypospadias as a child.

Read full chapter

URL:

https://www.sciencedirect.com/science/article/pii/B9780128012383651759

Volume 1

Jennifer R. Gardiner , Amanda Swain , in Knobil and Neill's Physiology of Reproduction (Fourth Edition), 2015

SRY

Inactivating mutations in the testis-determining gene, SRY, are associated with XY,46 female sex reversal in humans. The study of DNA from XX male individuals identified small regions of the Y chromosome that had transferred onto the X chromosome by abnormal X-Y interchange during male meiosis. The SRY gene was found within the minimum region of Y-specific DNA required for male development. 4 Confirmation of its role was found by the analysis of XY female patients with mutations within this gene. 5,39

The mouse has been an important working model to study the process of mammalian sex determination. As in the case of human patients, XX mice carrying the Sxr mutation develop as males and it was shown that the X chromosome in these mice carried a small portion of the Y chromosome containing the Sry gene. 40 Conversely, a mouse mutation that produced XY females was shown to have an 11-kb deletion on the Y chromosome that included the Sry gene. 41 Direct evidence that SRY is the only gene on the Y chromosome to be necessary and sufficient for establishing male development came from transgenic experiments in the mouse. XX mice carrying a 14-kb fragment of Y chromosome DNA that contains the mouse Sry gene and no other gene developed as males 6 (Figure 7.2). These transgenic animals were infertile, however, because of a block in spermatogenesis due to the presence of two X chromosomes and the lack of other Y-linked genes. The SRY protein contains an evolutionarily conserved HMG box type of DNA-binding domain found in a number of other transcription factors. This is believed to be important for SRY function because almost all mutations found in human XY females cluster within this region. 39,42

FIGURE 7.2. XX male sex reversal with an Sry transgene.

(A) On the left is a control XY male and on the right is his littermate, an XX transgenic carrying a 14-kb genomic fragment containing the mouse Sry gene. The external and internal genitalia are indistinguishable except for the testis, which is smaller in the transgenic animal due to the lack of germ cells (see insets, in which cross-sections through the testis of each mouse are shown). (B) PCR analysis of genomic DNA from these mice as well as an XX sibling control. Bands for Sry and the DNA loading control (myogenin) are present in the XX transgenic animal but the Y chromosome marker Zfy-1 is missing. M, size marker.

 Soure: Adapted from Ref. 6.

SRY is thought to have evolved around 300   million years ago due to duplication of SOX3, resulting in the primary sex determinant present in mammals today. 43 The two proteins share a highly similar HMG box domain. 44 Despite its location on the X chromosome, Sox3 is not required for normal sex determination in mice and loss of function mutations have not been documented in DSD patients. 45 However, misexpression of Sox3 in the gonads of XX mice causes sex reversal. In vitro experiments indicate that SOX3 is capable of activating downstream targets of Sry. 46 Analysis of 16 XX male sex reversal cases revealed three patients with chromosomal rearrangements in and around the Sox3 locus. 46 Furthermore, a separate case has since been reported, 47 implying that Sox3 mutations may well be a clinically relevant cause of XX male sex reversal. Together, these data suggest that SOX3 can act in a manner analogous to that of SRY in mammals.

The master regulator role of Sry has been used in mouse models to genetically differentiate between chromosomal and gonadal regulation of sexual characteristics. In the four core genotypes (FCG) model, generation of a Y chromosome lacking Sry (Y−) 48 and a transgenic Sry expressed from an autosome 49 allows separation of gonadal and chromosomal male determination. Thus, mice can be one of four "core" genotypes: phenotypic female with XX or XY (abbreviated to XYF) chromosomes, or phenotypic male with XXSry (abbreviated to XXM) or XY-Sry (XYM) chromosomes (Figure 7.3(A) ). 50 The sexual traits controlled by gonadal sex can be established by comparison of animals of different phenotypic sexes (XXF/XYF versus XXM/XYM). Conversely, sexual traits governed by chromosome complement can be elucidated by observation of differences between XX and XY animals, regardless of Sry status (Figure 7.3(B)). The FCG model should therefore also determine traits in which both chromosomal and gonadal sex have a role to play. However, caution must be used when interpreting the data gained from these mouse models, as animals of the same phenotypic sex still have some differences; for example, XXM mice are azoospermic (lacking in sperm production), as spermatogenesis requires genes on the Y chromosome. Furthermore, these mice have different genetic imprints. For example, XXF mice have both maternal and paternal imprints, while XYF mice have only one maternally imprinted X chromosome. As genetic imprinting can determine expression of certain genes, this status needs to be taken into account when comparing phenotypically similar mice. Experiments using FCG mutants have shown that, although Sry is necessary and sufficient for male sex determination, X chromosome dosage plays a role in multiple aspects of sexual dimorphism despite X inactivation 50–52 —perhaps not surprising as the "escaper" genes on the inactive X are expected to be expressed at a higher level in XX mice than XY. 26,53

FIGURE 7.3. The four core genotypes (FCG) model separates the effects of chromosome complement from those of gonadal sex.

(A) Breeding scheme for generation of FCG mice. XX female mice are crossed with XY male mice in which Sry has been deleted from the Y chromosome (Y−) and reinserted on an autosome (Sry). Progeny of four different genotypes are generated. Their phenotype, chromosome complement, and X imprinting status are listed below. (B) A 2   ×   2 schematic demonstrates how the FCG mice can be used to determine the cause of phenotypic differences observed.

 Source: Figure reprinted with permission from Ref. 249.

FIGURE 7.4. Regulation of Sox9 expression at the TESCO enhancer.

(A) Binding of SF1 to the TESCO enhancer is sufficient for initiation of Sox9 expression. (B) Expression is subsequently upregulated by synergistic action of SF1 and SRY, which may physically interact at the enhancer. (C) Once SRY expression ceases, Sox9 expression is maintained by an autoregulatory loop in which SOX9 itself binds to TESCO and interacts with SF1 to promote expression. (D) In female embryos, FOXL2 and estrogen receptor (ER) bind to TESCO, acting synergistically to inhibit Sox9 expression. In addition, DAX1 competes with SF1 for binding sites on TESCO, preventing Sox9 expression.

 Source: Adapted from Sekido and Lovell-Badge, 2008. 251

Read full chapter

URL:

https://www.sciencedirect.com/science/article/pii/B9780123971753000077

Gametogenesis, Fertilization and Early Development

Kim L. McIntyre , ... Paul D. Waters , in Encyclopedia of Reproduction (Second Edition), 2018

Evolution of Sex Chromosomes

Recombination Repression Leads to Loss of Genes From the Y

The emergence of the testis-determining gene (SRY) on the ancestral Y chromosome provided a region of the genome that was only ever inherited in males. This allowed genes beneficial to spermatogenesis and male development to accumulate in this male-specific region, even if detrimental to females. Repression of recombination between the X and Y chromosomes across this region ensured that the cassette of male-specialized genes remained together. Recombination is critical for the purging of deleterious mutations which, arising on a chromosome, can be replaced by recombination with the paired region of the other chromosome. In the absence of recombination, selection can act only on the entire chromosome rather than individual genes (Charlesworth, 1991). Consequently, mutations accumulate because they cannot be removed. This is what happens to the Y chromosome: it accumulates deleterious mutations that cannot be removed by recombination, ultimately leading to loss of gene function.

The loss of Y-borne genes is accelerated by the elevated mutation rate of those genes, which results from a higher number of germline cell divisions in male spermatogenesis compared with female oogenesis. Loss of nonfunctional DNA from the Y chromosome has no effect on fitness, and so may result in a reduction in physical size. This is evident in the diminished size of the therian Y chromosome. The therian X chromosome is spared from the fate of the Y chromosome because it undergoes recombination in females.

The process of Y chromosome degeneration slows over time. Y chromosomes at more advanced stages of evolution have very few functional genes and so fewer mutations that are exposed to purifying selection. Accordingly, gene loss from the oldest parts of the human Y chromosome proceeded rapidly at first, and then slowed significantly as only a small number of essential genes were retained (Cortez et al., 2014).

Loss of Y chromosome genes has not proceeded randomly. Analysis of the rates of nucleotide substitution at nonsynonymous sites (where mutation to the DNA changes the protein) compared with synonymous sites (where mutation to the DNA does not change the protein) indicates that some Y-borne genes were subject to positive selection (Li et al., 2013). Many of these genes have a partner on the X chromosome that is broadly expressed and has regulatory functions, where transcription from a single gene copy may be insufficient for a normal phenotype. In these cases, conservation of the Y-borne gene partner may have resulted from selection to maintain two gene copies.

Evolutionary Strata

Recombination between the X and Y chromosomes was suppressed by inversion events on the Y chromosome, leaving successive regions of the X chromosome recombinationally isolated from the Y chromosome (Lahn and Page, 1999) (Fig. 1). These regions are called evolutionary strata and are defined by the extent of divergence between X and Y gene pairs. Divergence is measured by the rate of accumulation of nucleotide changes that do not affect the protein that is coded for (synonymous substitutions). At least four evolutionary strata are readily identifiable on the human X chromosome. On the Y chromosome, it is more difficult to distinguish evolutionary strata due to extensive rearrangements.

The sex-determining SRY gene provided the initial focal point for suppression of recombination between the X and Y chromosomes, and so is part of the first evolutionary stratum. Stratum one arose ∼   180 mya, before split of the eutherian and marsupial mammals. Stratum two arose in early eutherians, after divergence of the marsupials. At about the same time as the emergence of stratum two, an autosomal block was added to the sex chromosomes, extending the PAR. Suppression of recombination between the X and Y chromosomes within the extended PAR formed stratum three, also in early eutherians. Stratum four emerged only in the primate lineage (Cortez et al., 2014).

X Chromosome Shaped in Testis

In the testis, expression of protein-coding and noncoding genes is more prolific and complex than in other tissues, particularly during spermatogenesis. This is consistent with a hypothesis that the testis provides an environment that is more conducive than other tissues to gene expression, and to the emergence of new genes over time (Soumillon et al., 2013). Evolution of the X chromosome is particularly impacted by the emergence of new genes because of the evolutionary pressures arising from the presence of only a single X chromosome in males.

The X chromosome is enriched for recessive mutations that confer a male benefit. These recessive mutations are immediately open to positive selection on the single X in males, even if detrimental to females (i.e., sexual antagonism). Any detrimental effects of the new alternative gene form (allele) in females will initially be shielded by normal function of the dominant allele. However, as the new recessive allele occurs more frequently in the population, homozygous females (with two copies of the mutant allele) will emerge and female fitness will be affected. From this point, selection favors restricting expression of the allele to males only. The influence of sexual antagonism is highlighted by the prevalence of genes with testis-biased expression on the X chromosome (Soumillon et al., 2013). This applies particularly to genes that have evolved more recently on the X chromosome in repeat regions, where duplicated genes have redundant functions and so are free to evolve new functions (Zhang et al., 2010).

It was proposed that amplification of some male-specific genes on the X chromosome has been driven by the silencing of X- and Y-borne genes early in male meiosis. This silencing, called meiotic sex chromosome inactivation (MSCI), can silence genes on the X chromosome that have critical function both during and after meiosis. Increased copy number of these genes on the X chromosome can accommodate the silencing effects of MSCI by permitting sufficient gene expression. Consistent with this hypothesis, male-specific genes are enriched in the multicopy gene regions of the human and mouse X chromosomes (reviewed in Deng et al., 2014).

MSCI was also hypothesized to be a driving force that relocates genes required for specific stages of spermatogenesis from the X chromosome to autosomes. These genes can then be expressed unaffected by MSCI. This is supported by the observation that in mouse evolutionarily older spermatogenesis genes tend to be located on autosomes if they are expressed after MSCI initiates silencing, but are enriched on the X chromosome if expressed before MSCI (Soumillon et al., 2013; Zhang et al., 2010).

Sex Chromosome Evolution in Real-Time: Rodents Without a Y

Different sex chromosome systems have arisen many times in diverse vertebrate lineages, and are defined by different sex-determining genes. The genes involved in core testis and ovary developmental pathways are largely conserved across vertebrates, and many sex-determining switches have evolved from a mutation affecting function of one of these genes. For example, SOX3 is a transcription factor with roles in development of the central nervous system and male gonadogenesis. It has evolved into the sex-determination gene (SRY) in therian mammals and independently into a different sex-determining gene in a fish (Oryzias dancena). Similarly, the transcription factor DMRT1 has a conserved role in vertebrate male development, and has evolved into a sex-determining gene independently in chicken (Gallus gallus), a fish (Oryzias latipes), and a frog (Xenopus laevis). This suggests that the conserved gonad development pathway has an inherent plasticity that can accommodate a multitude of different sex-determining triggers (reviewed in Graves, 2013).

The plasticity of sex-determining switches is highlighted in some rodent species where the Y chromosome and Sry gene have been lost. Two species of spiny rats lost their Y chromosome and Sry gene in a common ancestor. It appears that a mutation in the Sry gene weakened its function, likely reducing selection to retain Sry and, ultimately, the Y chromosome. A conserved ovary suppressing gene (Cbx2) has been identified as the novel candidate sex-determining gene. The presence of additional copies of Cbx2 in male spiny rats suggests ovarian repression as the new (or emergent) sex determination mechanism (Murata et al., 2012), which will define a new sex chromosome system. Similarly, two species of mole vole have lost their Y chromosomes in independent events. It appears that the role of Sry may have been destabilized by mutations in another gene integral to the testis-determination pathway, rendering Sry (and consequently the Y chromosome) redundant. An alternative sex-determining gene has not yet been identified in mole voles (Mulugeta et al., 2016), but evolution of a new sex chromosome system must have commenced.

The evolutionary trajectory of sex chromosomes is unique in the genome, and begins with the emergence of a sex-determining gene. The disparities between the human X and Y chromosomes reflect a later stage of the evolutionary process. The end of this process has occurred in some rodents with the loss of their Y chromosome. This heralds the emergence of new mammalian sex chromosome systems, demonstrating the robustness of sex determination in vertebrates despite the array of novel sex determining switches.

Read full chapter

URL:

https://www.sciencedirect.com/science/article/pii/B9780128012383644598