L9 Sex Determination Lecture Notes PDF

Lecture 9 – Sex Determination BIOL-3542- Developmental Biology 1 Key takeaways from the last lecture stem cell SELF-RENEWAL DIFFERENTIATION ‘genetic reprogramming’ (copying) (specializing)...

Lecture 9 – Sex Determination BIOL-3542- Developmental Biology 1 Key takeaways from the last lecture stem cell SELF-RENEWAL DIFFERENTIATION ‘genetic reprogramming’ (copying) (specializing) Somatic cell iPSC stem cell specialized cell Applications of Stem Cells Ø Disease modeling Ø Regenerative medicine Ø Drug discovery Ø Organoids 2 Life cycle of leopard frog Rana pipiens 3 Sex Determination Sex determination is the process that establishes an individual's sex characteristics. Primary sex determination - Regulated by chromosomes - Gonadal determination - testes and ovaries - Digital (either/or) phenomenon Secondary sex determination - Differing levels of hormones and response to hormones - Male or female phenotypes, usually coordinated with gonadal sex determination - Analogue 4 Primary sex determination in mammals In mammals, q Individuals with XX chromosomes typically develop ovaries and produce eggs. q Individuals with XY chromosomes usually have testes and produce sperm. q If an X-bearing sperm unites with an X bearing egg, the offspring will be XX. q If a Y-bearing sperm unites with an X bearing egg, the offspring will be XY. 5 The importance of Y chromosome for male sex determination XXY people (one extra X chromosome): male, develop testes structures XO people (one single X chromosome ): female, form ovaries, ovarian follicles cannot be maintained without the second X chromosome. Presence of a Y chromosome (even when multiple X chromosomes are present) initiates the development of testes. Second X chromosome is required to complete development of ovaries. But what do the X and Y chromosomes do? 6 Embryonic gonads Gonads - organ responsible for producing gametes and secreting sex hormones - testes and ovaries. The gonad in early embryonic mice and humans is bipotential. What does that mean ? Can develop into either a testis or an ovary, two organs with very different tissue architecture. 7 Early stages of embryonic development, both duct systems are present Testes: Ovaries: Wolffian duct Müllerian duct (vas deferens) (oviduct) 8 Differentiation of human gonads 9 Initiation of gonadal sex determination in mammals Lhx9, Gata4 Transcription factors in the gonad can activate Wnt and Rspo1 10 Initiation of gonadal sex determination in mammals SRY- Sex-determining Region of the Y Chromosome Lhx9, Gata4 Transcription factors in the gonad can activate SRY gene 11 Possible mechanism for the initiation of gonadal sex determination Lhx9, Gata4 Digital! 12 Design experiments to test the role of Rspo1 in primary sex determination Lose it: Lhx9, Gata4 Move it: 13 Design experiments to test the role of Rspo1 in primary sex determination Lose it: Lhx9, Gata4 XX humans born with RSPO1 mutations became phenotypic males. Move it: In XY individuals, a duplication of the region on chromosome 1 that contains both the WNT4 and RSPO1 results in a male-to- female sex reversal. Similarly, overexpression of β-catenin in XY mice leads to the formation of ovaries rather than testes. 14 Formation of different sex characteristics in mammals Internal genitalia - uterus, oviducts, cervix, upper vagina External genitalia - labia, clitoris, lower vagina Cells surrounding the oocyte o No Y chromosome - transcription factors Wt1, Lhx9, Gata4, and Sf1 activate Wnt4 protein, and a small soluble protein called R-spondin1 (Rspo1). o In the absence of testosterone Wolffian duct degenerates Formation of different sex characteristics in mammals Enclose the germ cells Secrete Dihydrotestosterone (DHT) testosterone Anti-Müllerian hormone (AMH) Internal genitalia - Epididymis, Vas deferens, Seminal vesicle o Y chromosome is present - Wt1, Lhx9, Gata4, and Sf1 in the bipotential gonad activates the Sry gene on the Y chromosome. o The testis makes anti-Müllerian hormone (AMH), which causes the Müllerian duct to regress Which statement correctly describes one of the postulated cascades involved in the differentiation of the male or female phenotype in placental mammals? A. The presence of Wnt1 leads to the production of Seroli cells in the testis. B. The presence of Wnt4 leads to the production of testosterone by the Leydig cells. C. The presence of Gata4 contributes to the initiation of the genital ridge into the female pathway. D. The presence of β-catenin contributes to the initiation of the genital ridge into the male pathway. 17 Sry gene – region of Y chromosome responsible for activating Sox9 Translocation of Sry to X chromosome in XX individual What primary sex characteristic What primary sex characteristic can you expect? can you expect? Testis formation Ovary formation 18 XX mouse transgenic for Sry Microinjection of Sry gene to fertilized egg of XX mice XY mouse XX/Sry mouse Sry is the only gene on the Y chromosome required for testis The external genitalia of the transgenic XX/Sry mouse are essentially the same determination in mammals. as those in an XY mouse. Ability of Sox9 protein to generate testes Sry (transcription factor) appears to activate autosomal gene Sox9 K - kidneys A - adrenal glands B - bladder T - testis O - ovary S - seminiferous tubule F - follicle cell XX mouse with the Sox9 transgene § Descended testes § Anti-Müllerian hormone (AMH) in 16.5-day Sertoli cells. § Seminiferous tubules lack sperm (due to the presence of two X chromosomes in the Sertoli cells). 20 Hermaphrodites: Individuals with both ovarian and testicular tissues In mammals, Ovotestes or ovary on one side and testis on the other side Ovotestes can be generated when Sry is activated later than normal (5 hours) - failure of testis development and the initiation of ovary development. Norm in many other species earthworm, nematode, clownfish 21 Secondary sex determination The development of female and male phenotypes in response to hormones secreted by the ovaries and testes; including the male and female duct systems and external genitalia. 22 Secondary sex determination - steroids Estrogens Testosterone Needed for fertility in both males and females In males: Wolffian duct differentiation In females: Mammary bud degeneration Differentiation of Müllerian duct Function of uterus, oviducts, cervix 5𝝰-Dihydrotestosterone (DHT) Fat deposition Descent of testes Genitalia development In males: Bone development In females: Concentrate sperm in testes Growth, maintenance, repair of reproductive tissues, bone mass Ø Secondary sex determination is more “analogue” Ø Differing levels of hormones and responses to hormones can create different phenotypes. 23 Secretion of Müllerian-inhibiting substance by Sertoli cells A. causes the Müllerian ducts to develop into the oviducts B. causes the Müllerian ducts to develop into Wolffian ducts C. causes the testis to produce testosterone D. causes the Müllerian ducts to regress by apoptosis in males 24 Sex chromosome dosage Females have two X chromosomes, while males have only one. If transcription were equal, females should have twice the amount of mRNA from X- Calico cat chromosome genes compared to males. How to we compensate for this? X-chromosome inactivation in mammals 25 Androgen insensitivity syndrome XY individuals Mutation in the gene encoding the A group of people with androgen insensitivity syndrome androgen receptor protein that binds testosterone (androgen). Primary sex characteristic–testes Testes present, testosterone made Secondary sex characteristic– female phenotype Testosterone not recognized by cells What is the difference between primary and secondary sex determination? A. Primary sex determination refers to the production of hormones that influence sexual characteristics and secondary sex determination refers to the actual development of those characteristics. B. Primary sex determination refers to the process by which a gonad becomes male and female whereas secondary sex determination refers to the development of other sexual characteristics. C. Primary sex determination refers to the development of characteristics based on their exposure to hormones whereas secondary sex determination refers to the effects of non-hormonal factors. D. Primary sex determination is the development of sex organ phenotypes, while secondary sex determination results in the development of the gonads into either testis or ovaries. 27 Environmental sex determination Sex ratio (% sperm producing) Temperature-dependent sex determination in three species of reptiles Genetic analysis of environmental sex determination MPT: Male-producing temperature FPT: Female-producing temperature KDM68 (Green) St. = developmental stage Activation of sex determination in the red-eared slider turtle 30 In which taxonomic group or groups is sex determined primarily by the temperature at which the embryo develops during a specific time period? A. Mammals only B. Placental mammals only C. All groups of reptiles D. Turtles and crocodilians E. Some insect classes 31 Summary Primary sex determination - Regulated by chromosomes - Gonadal determination - testes and ovaries - Digital (either/or) phenomenon Secondary sex determination - Differing levels of hormones and response to hormones - Male or female phenotypes, usually coordinated with gonadal sex determination - Analogue Environmental sex determination 32