Mammalian Sex Determination
Overview of mammalian embryonic development:
Mechanisms of sex determination in animals
|female||male||examples and explanation|
|XX||XY||most mammals, including humans; some amphibians, some insects -- Y determines maleness|
|ZW||ZZ||birds, reptiles, some amphibians -- W determines femaleness|
|XX||XY||some insects, including Drosophila -- X:autosome ratio greater than 0.75 is female, less is male|
|XX||XO||hemiptera (true bugs)|
|XX*||XO||C. elegans -- XX is a hermaphrodite, XO is male|
|XX, XY, YY||XX, XY, YY||some fish with environmentally determined sex or natural sex reversal in a single individual|
|XFO||XMO||Ellobius lutescens (molve-vole) -- OO and XX are lethal|
Chromosomes and Human Sex Determination
Human sex determined by sex chromosomes: 46XX is normal female, 46XY is normal male.
45XO female, Turner’s syndrome: short stature, webbing of neck, underdeveloped secondary sexual characteristics, sterile due to degeneration of gametes and ovaries after birth
Turner’s syndrome fully expressed in females lacking short arm of one X.
Lack of proximal part of long arm of X causes gonadal dysgenesis, but not somatic abnormalities.
But how does this fit in with X inactivation?
Both X’s fully functional during oogenesis, may be required for long term maintenance of oocytes in meiotic prophase.
Not all genes on X are inactivated.
47XXX fertile female, generally normal secondary sexual characteristics. However, more than 3 X’s cause severe mental retardation.
47XXY male, Klinefelter’s syndrome: sterile, no germ cells, small testes. Very tall, some are mentally retarded, but wide variation in phenotype, androgen levels.
Also occurs in XXYY, XXXY, XXXXY, XXXXXY -- more than one X incompatible with spermatogenesis? X:autosome translocations cause male sterility, but not female sterility. XXY mice phenotypically male, but sterile without germ cells.
Development of the Mammalian Reproductive System
Reproductive system composed of gonads (ovary or testes), ducts (Mullerian in females, Wolffian in males), and external genitalia (vagina or penis).
Gonad arises from intermediate mesoderm, as bipotential genital ridge. It contains 3 types of somatic tissues:
supporting cells (Sertoli cells in males, follicle cells in females),
steroidogenic cells (Leydig cells in males, theca cells in females),
connective tissue cells.
Germ cells migrate into gonad at 10.5-12 days p.c. (post-coitum) in mouse.
No visible sex differentiation until 7th week of gestation in humans (11.5-12.5 days p.c. in mouse).
Primary sex differentiation event is gonadal differentiation into ovary or testes. Germ cells not involved in primary sex determination, because mutants where germ cells do not migrate into gonads, or mutants without germ cells still differentiate female or male gonads. Germ cell differentiation depends on gonadal environment, and requires an RNA-binding protein called DAZL (Gill et al. 2011).
Female differentiation appears to be default pathway: castrated male rabbit fetuses develop into phenotypic females for both reproductive ducts and external genitals, and female fetuses with ovaries removed still develop into females. Female fetuses with testes grafts develop into males.
Testicular hormones MIS (Mullerian inhibiting substance; also known as AMH, or anti-Mullerien hormone, yet another member of the TGFbeta superfamily) and testosterone (converted to dihydroxy testosterone) cause male differentiation of the ducts and external genitalia, respectively.
Genetics of Sex Determination
Sxr mice: XX(sxr) males, XY(sxr) females
H-Y antigen and Ohno’s theory
Sxr’ mice -- no HY antigen!
Sex-reversed humans: XX males, XY females involving X:Y translocations
ZFY- zinc finger protein with 13 zinc finger motifs for DNA binding, and an acidic domain for interaction with transcription factors.
Homologue, ZFX, also present on X chromosome of mammals
Marsupials, with X-Y sex determination, have ZFY homologue on autosome!
Timing of ZFY expression is too late -- in testes, but only after testes differentiation has started.
Other sex-reversed XX males have no ZFY!
Original XY female that led to mapping of TDF had multiple deletions.
XX mice carrying 14 kb of Y chromosome containing Sry gene develop as phenotypic males -- no sperm, but testes have normal hormonal function, normal male secondary sexual diferentiation and mating behavior.
XY mice with Sry deletion develop as phenotypic females, fertile.
XX humans carrying Sry-containing fragments of Y translocated to X are phenotypically male, sterile, but with normal internal and external genitalia.
If translocation is small (35 kb), then abnormal and ambiguous genitalia.
XY humans with mutations in HMG box of Sry are sterile, have poorly developed gonads without clear testicular or ovarian histology, but with no testicular hormones, Mullerien structures persist and get female secondary sexual differentiation.
Sry encodes a member of HMG (high mobility group) family of DNA binding proteins (Lef-1/Tcf, the transcription factors regulated by Wnt signalling, also belong to this group). Induces DNA bending by 80-90 degrees. All Sry mutations in XY females located in HMG box, and alters ability to bend DNA.
Sry transcripts detectable at 10.5 days p.c. in mouse gonad, and differentiation of Sertoli cells into testes cords first detectable at 11.5-12.5 days p.c. But because of limits of detection, transcription may occur earlier, and has been reported in preimplantation mouse embryos.
MIS (Mullerian inhibiting substance), also known as AMH (anti-Mullerian hormone) may be downstream target of Sry.
WT1 (Wilm’s tumor gene): zinc-finger transcription factor, mutations associated with childhood kidney cancer. Some mutations in WT1 (Denys-Drash syndrome) causes XY sex-reversal, with disordered gonadal histology, persistent Mullerian derivatives, and feminization of external genitalia. WT1 expression primarily in common mesodermal precursor of kidney and gonad, and detectable at 9 days p.c. in mice. Mouse with homozygous WT1 knockouts fail to form kidneys or gonads from intermediate mesoderm.
SF-1 (steroidogenic factor), regulator of genes for enzymes needed for steroid biosynthesis. SF-1 knockout mice fail to make kidneys or gonads. SF-1 binding site found upstream of MIS (AMH) gene.
Sox9: mutations in this gene cause campomelic dysplasia, with severe skeletal malformation and varying degrees of feminization of XY individuals with normal Sry genes. Encodes protein 60% homologous to Sry in the HMG domain.
Wnt-4 regulates female development in mammals (Vainio et al., 1999). Wnt-4 mutant males appear normal at birth, but Wnt-4 mutant female mice are masculanized (has Wolffian duct instead of Mullerian duct). Wnt-4 required in both sexes for initial formation of Mullerian duct, suppresses development of Leydig cells in ovary. Wnt-4 mutant females activate testosterone synthesis.
Genetics of Sex Determination: Bibliography
Foster et al. (1992) Evolution of sex-determination and the Y chromosome: SRY-related sequences in marsupials. Nature 359:531-532.
Gill ME, Y-C Hu, Y Lin, DC Page (2011) Licensing of gametogenesis, dependent on RNA binding protein DAZL, as a gateway to sexual differentiation of fetal germ cells. Proc Natl Acad Sci USA 108:7443-7448 http://dx.doi.org/10.1073/pnas.1104501108
Gubbay et al. (1990) A gene mapping to the sex-determining region of the mouse Y chromosome is a member of a novel family of embryonically expressed genes. Nature 346:245-250.
Haqq et al. (1993) SRY recognizes conserved DNA sites in sex-specific promoters. Proc. Natl. Acad. Sci. USA 90:1097-1101.
Koopman et al. (1989) ZFY gene expression patterns are not compatible with a primary role in mouse sex determination. Nature 342:940-942.
Koopman et al. (1991) Male development of chromosomally female mice transgenic for SRY. Nature 351:117-121.
Mardon and Page (1989) The sex-determining region of the mouse Y chromosome encodes a protein with a highly acidic domain and 13 zinc fingers. Cell 56:765-770.
Page et al. (1987) The sex-determining region of the human Y chromosome encodes a finger protein. Cell 51:1091-1104.
Page et al. (1990) Additional deletion in sex-determining region of human Y chromosome resolves paradox of X,t(Y;22) female. Nature 346:279-281.
Palmer et al. (1989) Genetic evidence that ZFY is not the testis-determining factor. Nature 342:937-939.
Schneider et al. (1989) ZFX has a gene structure similar to ZFY, the putative human sex determinant, and escapes X inactivation. Cell 57:1247-1258.
Simpson et al. (1987) Separation of the genetic loci for the H-Y antigen and for testis determination on human Y chromosome. Nature 326:876-878.
Sinclair et al. (1990) A gene from the human sex-determining region encodes a protein with homology to a conserved DNA-binding motif. Nature 346:240-244.
Vainio, S., M. Heikkila, A. Kispert, N. Chin and A.P. McMahon (1999). Female development in mammals is regulated by Wnt-4 signalling. Nature 397:405-409.
Zwingman et al. (1993) Transcription of the sex-determining region genes Sry and Zfy in the mouse preimplantation embryo. Proc. Natl. Acad. Sci. USA 90:814-817.