X, Y, SRY (loci, genes), sequence of events in development of normal male

One can view mammalian sex determination as occurring in three steps. First is the establishment of chromosomal sex. This occurs at fertilization when either an X- or Y-bearing sperm fertilizes an X-bearing oocyte giving rise to an XX or XY zygote. Second is the establishment of the primary sexual characteristics: the testes or ovaries. In XY fetuses, the fetal gonads differentiate into testes; in XX fetuses, ovaries form. Third is the establishment of the secondary sexual characteristics which is dependent upon hormones secreted by the gonads.

How does sexual differentiation occur?
A fetus is initially sexually indifferent and has the primordia for both the male and female accessory sex organs, the Wolffian and Mullerian ducts, respectively. In the 1940s, Jost demonstrated that the male phenotype is imposed on a fetus that would inherently develop into a female. Jost surgically removed the testes from fetal male rabbits at a stage when Wolffian and Mullerian ducts were present and then allowed fetal development to proceed in utero. When Jost examined the fetuses at a later date, the castrated males were phenotypic females (Fig. 1). Jost concluded that the fetus is programmed to develop into a female. However, if testes are present they secrete two factors that override the female program and masculinize the fetus. The first factor, secreted by Leydig cells, is testosterone which induces the Wolffian ducts to differentiate into the epididymides, vas deferens, and seminal vesicles. Male external genitalia form when the cells of the urogenital tubercle metabolize testosterone into dihydrotestosterone which induces the development of the penis and scrotum. The second factor, secreted by Sertoli cells, is Mullerian inhibiting substance (anti-Mullerian hormone), which induces the Mullerian ducts to regress. In the absence of these two factors the Wolffian ducts degenerate and the Mullerian ducts develop into the oviducts, uterus, cervix, and upper vagina.

What induces development of the gonads into testes or ovaries?
It was initially assumed that humans had a sex determining mechanism similar to the well studied fruitfly, Drosophila, since in both species males are XY and females are XX. In the fruitfly, sex is determined by the ratio of the number of X chromosomes to autosomal sets such that an XXY individual is female and an XO is male. However, in 1959, the identification of an XXY male patient with Klinefelter syndrome, an XO female patient with Turner syndrome, and an XO female mouse suggested that, in mammals, the Y induces testes development. Cytogeneticists have since identified individuals with varying numbers of X or Y chromosomes. All individuals who had at least one Y chromosome had testes and a male phenotype, irrespective of the number of X chromosomes. The locus on the human Y that induces testes development was designated the testes-determining factor (TDF).

How does the Y chromosome control masculinization?
By correlating deletions on the Y with the presence or absence of testes and by studying XX males which carry a tiny portion of the Y on one of their X chromosomes, investigators mapped TDF to a 35-kb region of the Y short arm. Cloning of this region resulted in the identification of a gene designated sex-determining region Y (SRY). Convincing evidence that SRY is the testis-determining gene was obtained when a 14.6-kb genomic sequence of the mouse SRY locus was shown to be capable of inducing XX fetuses to develop into males in transgenic experiments.

The hypothesis is that SRY is a master regulatory gene that initiates a cascade of gene interactions that transforms the fetal gonad into a testis (Fig. 2). SRY encodes a member of the High Mobility Group-1/-2 (HMG) protein family whose members are characterized by an 80-amino acid DNA-binding motif called the HMG domain. Several HMG proteins, including SRY, are transcription factors that recognize and bind a specific DNA target sequence and cause the bound DNA to bend into an angle. The SRY target sequence has been identified in the promoter region of genes controlling sexual differentiation such as Mullerian inhibiting substance and P450 aromatase, an enzyme that converts testosterone to estradiol. Furthermore it is present in the promoter region of SRY itself suggesting a positive feedback loop.

It took over three decades from the recognization of the Y as testis-determining to the identification of SRY as TDF. The cloning of SRY is undoubtedly a milestone in our understanding of mammalian sex determination. The difficult job of deciphering how SRY regulates transcription and identifying the genes upstream and downstream of SRY in the sex determination cascade must now be addressed.

Suggested Reading

Affara NA. Sex and the single Y. BioEssays 1991;13(9):475-478.

Koopman P, Gubbay J, Vivian N, Goodfellow P, Lovell-Badge R. Male development of chromosomally female mice transgenic for SRY. Nature 1991;351:117-121.

McLaren A. Sex determination in mammals. Trends in Genetics 1988;4(6):153-157.

McLaren A. What makes a man a man? Nature 1990;346:216-217.