The embryonic gonad is bipotential—that is, it will normally “give rise to one of two morphologically and functionally different organs, a testis or an ovary” (Capel et al., 2006).
In 1947, Alfred Jost demonstrated that, when female (XX) and male (XY) rabbit fetuses are gonadectomized in utero before sexual differentiation, all individuals develop female sex duct structures and female external genitals, regardless of karyotypic sex (Jost, 1947).
Researchers hypothesized that the testes triggered male development through testicular hormones. Studies in cattle showed that, when opposite-sex fetuses have placental anastomoses which allow for the exchange of hormones, XX fetuses are masculinized, but XY fetuses are not feminized (Jost et al., 1972). As a result of these and other studies, the absence of testicular hormones was expected to result in female development.
The 1905 discovery of the Y chromosome by both Nettie Stevens and Edmund Wilson led to the description of an XX/XY sex determination system, in which women were XX and men XY (Stevens, 1905; Wilson, 1905). It was not initially clear whether human sex was determined by the number of X chromosomes or by the presence or absence of the Y chromosome.
Subsequent studies of Klinefelter’s and Turner’s syndromes in the 1950s suggested that the presence of a Y chromosome determines sex in humans (Jacobs et al., 1959; Ford, 1959). If sex were determined by X-chromosome count, (47,XXY) patients with Klinefelter’s syndrome would be expected to be female, since they have the typical X count for women (two), and (45,XO) patients with Turner’s syndrome would be expected to be male, since they have the typical X count for men (one). However, Klinefelter’s syndrome patients have a male phenotype and Turner’s syndrome patients have a female phenotype.
These observations led to a search to find a sex determining gene on the Y-chromosome. In a 1990 Nature paper, Andrew Sinclair and colleagues identified a Y-chromosome gene as the Sex-Determining Region Y (SRY), while acknowledging that it is likely that many different genes are required for both male and female sex determination (Sinclair et al., 1990). Subsequent research confirmed that XX mice develop testes if injected with Sry-bearing DNA fragments during embryonic development (Koopman et al., 1991).
Studies of human patients identified (46,XX) men who had translocations of SRY onto an X-chromosome, further suggesting SRY was sufficient to trigger male development (Berkovitz et al., 1992). In subsequent years, research focused largely on the downstream targets of Sry.
In this period, research on sex determination focused on questions concerning the genetics of male testis determination (Richardson, 2013). Female sexual development, by contrast, was thought to proceed as a “default” in the absence of Sry.
The English word, “default,” means “failure to act; neglect” or “a preselected option adopted […] when no alternative is specified” (Oxford English Dictionary, 2011). In the case of sex determination, “default” became the prevailing model for female pathways—i.e., an ovary results in the absence of other action, and ovarian development was understudied.
While the majority of the research community continued to focus on genetics of testis determination as the key to mammalian sexual development, some developmental biologists protested the “default” model. In 1986, for example, Eva Eicher and Linda Washburn challenged the concept of “induction of ovarian tissue as a passive (automatic) event,” arguing that “the induction of ovarian tissues is as much an active, genetically directed developmental process as is the induction of testicular tissue or, for that matter, the induction of any cellular differentiation process.” These biologists noted “almost nothing has been written about genes involved in the induction of ovarian tissue from the undifferentiated gonad” (Eicher et al., 1986; see also Fausto-Sterling, 1989).
By the mid 1990s, developmental biologists recognized that “although factors involved in male sexual differentiation have been well studied, the pathways regulating female sexual differentiation remain incompletely defined” (see Biason-Lauber et al., 2008; Richardson, 2013).
Simultaneously, data from both animal models and human patients suggested that sex determination involved more than the presence or absence of SRY. Observations include:
Reconceptualizing the ovarian pathway as “active” yielded an important gendered innovation: Researchers began identifying specific mechanisms required to produce and maintain the ovary—during development, postnatally, and into adulthood. Several genetic candidates emerged, including WNT4 and FOXL2. Researchers came to understand that female sex differentiation requires ongoing maintenance throughout adulthood—see Method. Some genes, such as WNT4, are specifically required for female sex development but not for male sex development (Swain et al., 1998).
Current work suggests that both the male and female pathways rely on dominantly acting genes, with SRY actively promoting the male pathway by upregulating SOX9 expression, while B-catenin, Rspo1, and Foxl2 actively promote the female pathway by repressing SOX9. It is a matter of timing (and expression level) that determines which pathway prevails (Sekido et al., 2008; Veitia, 2010)—see Figure.
In addition to ovarian development, researchers sought to understand specific pathologies of the ovary. Biologists studying the genetics of blepharophimosis / ptosis / epicanthus inversus syndrome (BPES), associated with ovarian failure, identified the gene FOXL2 as necessary for ovarian maintenance (Crisponi, 2001). Later research showed that, in adults, FOXL2 is required to continuously suppress SOX9 and thereby prevent ovarian follicle cells from trandifferentiating into “testis-like” cells (Uhlenhaut et al., 2009)—see diagram below, reproduced from Uhlenhaut et al., 2009.
As occurred with FOXL2, later experiments showed “that male sex determination is not a permanent choice and that Dmrt1 is crucial for maintenance of testicular function” (Herpin et al., 2011). Similar to how loss of FOXL2 can reprogram ovarian granulosa cells into testicular Sertoli cells, loss of DMRT1 can reprogram Sertoli cells into granulosa cells. DMRT1 suppresses certain genes involved in ovarian development—see diagram below, reproduced from Matson et al., 2011.
Ovarian determination is no longer seen as a “default” process such that the absence of SRY automatically leads to the development of an ovary. Rather researchers describe both pathways as active, requiring complex cascades of gene products in proper dosages and at precise times—see Method.
Ovarian development is clearly not a default or passive pathway. Biologists, geneticists, and other researchers have recognized that understanding ovarian development is critical to understanding the genetics of sex determination. New research on the active ovarian pathway has led to changes in language used to describe sex determination—current language emphasizes the gene-driven nature of both ovarian and testis formation.
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