Lecture 9: Sex Determination and Gametogenesis PDF

Lecture 9: Sex determination and gametogenesis Textbook Chapter 6 (pages 176 – 187) Chapter overview Mammalian sex determination How do chromosomes dictate testes or ovary formation? What gene regulatory networks are activated by the Y and X chromosomes? Primary vs. secondary s...

Lecture 9: Sex determination and gametogenesis Textbook Chapter 6 (pages 176 – 187) Chapter overview Mammalian sex determination How do chromosomes dictate testes or ovary formation? What gene regulatory networks are activated by the Y and X chromosomes? Primary vs. secondary sex determination What happens when there are mutations in key genes responsible for sex determination? Drosophila sex determination A cascade of alternative RNA splicing events dictates sex determination Mammalian gametogenesis: spermatogenesis + oogenesis “Sexual reproduction is … the masterpiece of nature” – Erasmus Darwin Different species produce offspring in different ways. Mechanisms of sex determination Chromosomal sex determination Examples: Mammals, birds, Drosophila Mammals: XY sex chromosomes; XY à formation of testes; XX à formation of ovaries Birds: males have ZZ, and females have ZW Drosophila: The Y chromosome plays no role in sex determination, but the number of X chromosomes determines sexual phenotype Environmental sex determination Example: the temperature at which the embryos incubate determines whether the embryo will develop into a male or female Mammalian sex determination X-bearing sperm + X-bearing egg à genetically female XX Y-bearing sperm + X-bearing egg à genetically male XY But how does having a Y chromosome promote testis formation? How does having two X chromosomes promote ovary development and egg production? Mammalian sex determination Mammalian embryos have a bipotential gonad. Gonad: organ of the reproductive system that produces and releases the gametes (the reproductive cell of an animal or plant) XX à ovary XY à testes The importance of the Y chromosome in mammalian sex determination Each sperm should normally have 22 autosomes + either a single X or Y chromosome. Each oocyte should have 22 autosomes + a single X chromosome. The importance of the Y chromosome in mammalian sex determination Errors in meiosis during gamete formation can lead to the generation of gametes with abnormal numbers of sex chromosomes. The importance of the Y chromosome in mammalian sex determination Gametes with an abnormal amount of sex chromosomes will lead to the creation of zygotes with extra X chromosomes (XXX or XXY), extra Y chromosomes (XYY), or missing a sex chromosome (X0). The importance of chromosomes in mammalian sex determination Individuals with only a single X chromosome develop external female genitalia but they are underdeveloped – Turner syndrome (XO) A second X chromosome is necessary to complete development of the ovaries (people with a single X chromosome begin making ovaries but ovarian follicles cannot be maintained). The presence of a Y chromosome initiates development of testis. Individuals with a Y chromosome develop testis and male genitalia, even if they have two X chromosomes – Klinefelter syndrome (XXY) Infertility in almost all affected people. Compared with XY individuals, they have lower levels of androgens (male hormones) and more estrogens (female hormones).  Mammalian sex determination Secondary sex determination Primary sex XY or XX determination (sexual phenotype chromosomes (gonads - testis or outside the gonads ovaries) – male and female duct systems and external genitalia) Gonads produce XY or XX chromosomes hormones and paracrine dictate the fate of the factors that govern bipotential cells of the secondary sex early gonad. determination.  Secondary sex determination Primary sex determination Genital ridge (mesoderm origin) The differentiation of the genital ridge into the bipotential gonad requires 4 genes: Sf1, Wt1, Lhx9, Gata4 Mice lacking any of these genes have no gonads. XX: Wnt4 + Rspo1 activate the canonical Wnt pathway à β-catenin activated. β-catenin activates transcription of ovarian gonadal cells. XY: the Y chromosome contains a gene called “Sry”. This is the testes-determining gene. Sry acts as a transcription factor to activate expression of a transcription factor called “Sox9”. Sox9 activates genes that dictate testes formation.  Germ stem cells (the precursors of the egg and sperm) develop outside the bipotential gonad and migrate into the gonadal tissue during week 6. Germ cells, like the gonad, are also bipotential. The gonad dictates whether they will become sperm or oocytes. In humans, gonadal rudiments remain sexually indifferent until week 8.  At 8 weeks, the gonadal tissue begins developing into a testes or ovary. The sperm is housed in the seminiferous tubule. The oocyte/egg is housed in follicles within ovarian follicles. Primary sex determination mechanism (XX) In the absence of a Y chromosome: Wt1, Lhx9, Gata4 and Sf1 à Wnt4 and Rspo1 are upregulated. Wnt4 and Rspo1 trigger Wnt/β-catenin pathway signaling. β-catenin activates further ovarian development and blocks the transcription of the testis-promoting transcription factor Sox9. β-catenin is an important “anti- testis/pro-ovary” molecule, and it is found in the female gonads of all vertebrates. Primary sex determination mechanism (XX) Rspo1 mutations lead to 46,XX female-to-male sex reversal. Individuals with this condition have two X chromosomes, but develop male external genitalia and small testes, and are usually infertile. Affected individuals typically need male sex hormones during puberty to induce male secondary sex characteristics. Primary sex determination mechanism (XX) A duplication of the region on chromosome 1 that contains both the Wnt4 and Rspo1 genes in XY individuals results in 46,XY male- to-female sex reversal. The overactivated β-catenin pathway overrides the effect of Sry and Sox9. Primary sex determination mechanism (XY) In the presence of a Y chromosome: Wt1,Lhx9, Gata4, and Sf1 à activate the Sry gene on the Y chromosome (sex- determining region of the Y chromosome). Primary role of Sry à activation of the Sox9 transcription factor (autosomal gene, found on chromosome 17). Sox9 is found in all vertebrates, Sry is only found in mammals. Birds, fish, frogs: Sox9 activated by Dmrt1 Primary sex determination mechanism (XY) Sox9: activates its own promoter inhibits β-catenin’s ability to induce ovary formation activates transcription of testis-promoting genes (including anti-Mullerian hormone, Dmrt1, Fgf9) Sry – the testis determining factor in mammals SRY-positive 46,XX testicular disorder: The SRY gene has been translocated onto one of the X chromosomes, so the affected individual develops male gonads despite having two X chromosomes. Female-to-male sex reversal. No Y chromosome à no SRY-negative 46,XX testicular disorder: spermatogenesis The affected individual has XX chromosomes but develops male genitalia. This is due to a duplication of the SOX9 gene on chromosome 17 which triggers testis formation. Female-to-male sex reversal. 46,XY gonadal dysgenesis or Swyer syndrome: The SRY gene on the Y chromosome has been deleted or mutated, leading to development of female phenotypes despite the presence of a Y chromosome. Male-to-female sex reversal. Gonads are underdeveloped and oocytes are not produced.  Secondary sex determination Primary sex determination Secondary sex determination Following their formation, gonads secrete hormones and factors that trigger secondary sex determination. Two phases of secondary sex determination: organogenesis in the embryo, puberty in teenagers Secondary sex determination Two undifferentiated duct systems are present in the embryo: Mullerian duct and Wolffian duct. Hormones and other factors secreted from the gonads will lead to their maintenance or destruction. Estrogen secreted by ovaries: maintenance of Mullerian ducts à oviduct, uterus, cervix, vagina Testosterone secreted by testis: maintenance of the Wolffian ducts à vans deferens, epididymis, seminal vesicle Since XX gonads do not secrete testosterone, the Wolffian duct atrophies in females (it requires testosterone for maintenance). Secondary sex determination - XY Within the developing testes, two types of cells are made: Leydig cells and Sertoli cells Leydig cells: Secrete testosterone (an androgen) which promotes maintenance of the Wolffian ducts and development of external male genitalia. Sertoli cells: Secrete anti-Mullerian hormone (AMH) which causes degeneration of the Mullerian duct. These two pathways are independent of each other. Androgen insensitivity syndrome Individuals have XY chromosomes but have a mutation in the receptor that responds to testosterone. Testes development is normal. Sertoli cells secrete AMH and Leydig cells secrete testosterone. AMH à Mullerian ducts degenerate (no internal secondary female sex organs) Testosterone à cannot respond to testosterone, no Wolffian duct maintenance or external male genitalia Androgen insensitivity syndrome Individuals can respond to estrogen made by the adrenal glands (produced normally in both XX and XY individuals) à external genitalia is female (labia, clitoris, vagina, breasts). Individuals with androgen insensitivity syndrome have external female genitalia, lack internal female genitalia (uterus, oviduct; because the Mullerian duct degenerated), and have internal testes. DHT Testosterone promotes formation of male structures that derive from the Wolffian duct but must be converted to dihydrotestosterone (DHT) to masculinize the urethra, prostate, penis and scrotum. DHT is most active prenatally and in early childhood. 5-alpha reductase deficiency Individuals lack the gene/enzyme (5-alpha reductase) that converts testosterone into DHT. XY children have functional testes, Wolffian duct development and Mullerian duct degeneration. Because of the lack of DHT, when the children are born, they have female external genitalia. At puberty, the testes produce a large amount of testosterone that overrides the lack of DHT. Scrotum descends and male external genitalia becomes apparent. Syndrome appears in ~1% of boys in Las Salinas in the Dominican Republic. About half of individuals affected with this enzyme deficiency will identify as males after puberty. Secondary sex determination - XX Estrogen (from the mother and then from the fetal ovaries) causes differentiation of the Mullerian duct into the female reproductive tract and the formation of the external sex organs. Estrogen is needed in both males and females. Male estrogen receptor knockout mice develop very few sperm – sterile Female estrogen receptor knockout mice – germ cells die, no oocyte development X chromosome inactivation (dosage compensation) Female mammals have 2 X chromosomes, which would mean double the transcription of those genes and double the gene products compared to males. To regulate dosage, one of the X chromosomes is randomly inactivated in each cell of the implanted embryo, and all descendants of those cells keep the same X chromosome inactivated. Inactivation – formation of tightly bound heterochromatin through histone modifications Inactivated X chromosome forms a Barr body. Calico cats – an illustrative example of X chromosome inactivation Calico is a description of a cat’s coat color - a calico cat is a domestic cat of any breed with a tri-color coat. Calico cats are usually always female. The gene that dictates coat color (orange or black) is located on the X chromosome. Male cats only inherit one X chromosome and are therefore only either orange or black. Female cats can be heterozygous at that gene. This leads to some groups of cells where the black allele was inactivated, and others where the orange allele was inactivated à patches of black and orange.