Sex determination

1. Sex determination Dioecious: the majority of animals exist as one of two sexes, with males producing sperm and females producing eggs. Sexual dimorphism: in many species, the differences between sexes are not limited to the reproductive organs, but extend to other characteristics such as size, ornaments, and body shape. Monoecious: all individuals of a species look alike and produce eggs and sperm. Common in invertebrates. Individuals with both gonads are hermaphodites. Sex determination: the natural event by which an individual of a dioecious species becomes male or female. There are two main mechanisms for sex determination: Environmental sex determination: in some species, sex is determined after fertilization by environmental factors (temperature, population size, or sex of others). Genetic sex determination: sex is determined at fertilization by the combination of genes that the zygote receives.

2. Environmental sex determination Sex is determined by extrinsic factors after the process of fertilization. A variety of interesting methods influence environmental sex determination: Chance:Bonellia verdis: a marine worm, females are large and attach to rocks in the sea; males are small. The larvae float in the ocean. When they settle down and land on a female worm, they become male. If they land elsewhere (on the sea floor), they become female. Maximize number of offspring:M. incognita is a nematode plant parasite. If nutrients are sparse, they become males. If plentiful, the worms become females, which enhances the reproductive potential of the population. Social: coral reef fish may start out as one sex but later change to the other. The trigger may be a social change, such as the disappearance of a dominant male or female.

3. Temperature: in many reptiles (crocodiles, turtles, and some lizards) sex is determined by incubation temperature during a brief but specific stage of embryogenesis. Small differences in ambient temperature have amazing differences on sex ratio. Map turtles: the sex ratio drops from 1 (all male) to 0 (all female) when ambient temp increases from 28 to 30°C. Alligators: the pattern is reversed in alligators and lizards. Crocodiles: a third pattern exists. The males develop at intermediate temp. and females develop at the two extremes of higher of lower temp. In many other reptiles, temperature has no effect on sex determination. It is not clear why the temperature is important. Presumably, the different temp. favors either males or females.

4. Genotypic sex determination In humans, as well as most other organisms, the sex ratio always remains close to 0.5, and it is not dependent on the environment. In mammals, individuals are heterogametic or homogametic. Heterogametic: human males produce two types of sperm with different sets of chromosomes, XA and YA. Homogametic: human females produce only one type of egg, XA. Fertilization by XA sperm yields females, YA sperm produces males. Males are not always the heterogametic sex. In birds, the roles are reversed and females make two types of gametes.

5. Differences in sex chromosomes can occur in number rather than type: in the roundworm, C. elegans, females are all hermaphrodites and have XXAA eggs, and males have XOAA diploid sperm. Arrhenotoky (arrhen = male, tokos = childbirth): females arise from fertilized eggs whereas males develop from unfertilized eggs. This method is common in bees, ants, and wasps. Houseflies do not have distinct sex chromosomes, but they have a specific gene that encodes sex determination (m/m is female and m/M is male). Genotypic sex determination systems are diverse!

6. Dosage compensation occurs by X chromosome inactivation What is X chromosome inactivation? X chromosomes contain many genes that are unrelated to sex determination, but that are not present on the Y chromosome. A double dose of these genes in females presents a potential imbalance in levels of gene expression.It could be harmful. Barr body: mammals achieve balance by inactivation of one of the two X chromosomes. This occurs in almost all cells of females at the blastopore stage. Are all genes on the 2nd X chromosome inactivated? Pseudoautosomal region: a few genes in the second X chromosome escape inactivation. They belong to a region that is very similar to the Y chromosome. How does inactivation occur? X-inactive specific transcript (XIST): a product of an X-linked gene that is necessary and sufficient to inactivate one X chromosome. Initially, XIST is transcribed from both X chromosomes. When transcribed, the mRNA binds to the X chromosome, making it inactive for transcription of other genes. Once the decision is made to inactivate one of the X chromosomes, it continues to produce XIST while the active X chromosome does not.

7. What causes males and females to develop differently? Testis determining factor (TDF): maleness in mammals depends on the Y chromosome. Thus, some gene(s) encoded by this chromosome must direct development of the testis. Where and when does TDF act? Gonad development: gonads develop from two major cell types. Mesoderm forms the genital ridge on the mesonephros Primordial germ cells migrate to the future site of the gonad from the yolk sac. The mesoderm of the genital ridge proliferates and surrounds the germ cells by the 6th week in humans, forming the primitive sex cords. The gonad is undifferentiated at this point. TDF acts on the undifferentiated gonad.

8. TDF acts on supporting cells, not germ cells • The indifferent gonad has germ cells plus 3 types of somatic cells: • Supporting cells: arise from the primitive sex cords and become Sertoli cells in testes and follicle cells of the ovaries. • 2. Steroidal cells: differentiate to produce gonadal hormones. These are Leydig cells in males and thecal cells in females. • 3. Connective tissue cells: form the structural framework of the gonad. • The initial change in the gonad occurs in the supporting cells and not the germ • cells. Thus, the TDF does not act directly on germ cells to induce gonadal • differentiation. It acts on the supporting cells to set up the proper environment • for differentiation into sperm or egg.

9. Supporting cells determine development of germ cells TDF is only expressed in cells with the Y chromosome. Expression induces the supporting cells to become Sertoli cells and to to form testis cords. In this environment, meiosis of the germ cells is inhibited and they slowly develop into spermatogonia. TDF is not expressed in cells with only X chromosomes. Supporting cells develop according to the default mode as follicle cells. In this environment, germ cells start meiosis and produce oocytes very early in embryogenesis (12 weeks in humans). The oocytes further stimulate follicle development. Chimeric mice (male and female cells) can have XX sperm or XY oocytes, but they always have XX supporting cells in females and XY supporting cells in males. Supporting cells rule sex.

10. Mapping and cloning TDF The critical breakthrough for mapping TDF on the Y chromosome came from individuals whose sex did not match their genotype. Sex reversed males had 2 X chromosomes and sex reversed females had an X and Y chromosome. The origin of sex reversed individuals can be explained by rare crossover events between the X and Y chromosomes during meiosis in the male gametes. If the region encoding the TDF gene is switched, it is possible to develop an X chromosome with TDF and a Y chromosome with no TDF. Pairing a normal and mutant X chromosome creates a male with a female genotype, and pairing of mutant Y with X creates a female with a male genotype. Exactly what is the TDF gene(s)?

11. Hybridization with sex chromosome-specific DNA probes localized the DNA involved in crossover To localize and identify the gene that encoded TDF, fragments of DNA from the Y chromosome were used as probes to search for TDF in sex reversed males. DNA was isolated from tissues of normal males, normal females, and 4 different sex reversed males. P-32 labeled fragments of Y chromosomes (not pseudoautosomal) were used as probes to hybridize to each type of DNA. As expected, most probes did not cross react with either normal females or the 4 sex reversed males. However, one fragment (35 kb) did cross react specifically with each of the sex reversed males but not the normal female. This result provides strong circumstantial evidence that this DNA encodes the gene for TDF. There was only one gene encoded on this fragment of DNA, SRY. SRY was also found mutated in sex reversed females.

12. The mouse SRY gene is sufficient for testes formation To prove that the SRY gene was the key to maleness (TDF), transgenic mice were constructed that contained SRY in all cells. Fertilized eggs were injected with the SRY gene and allowed to develop in foster mothers. Of 3 XX transgenic mice that were born, 2 were females and one was a sex reversed male. This male was similar in size to the other XY males, it had normal male genitals, and it copulated with females. However, it was sterile. Thus, SRY was sufficient to induce sex reversal in female mice! The testes were normally developed, but there were no germ cells undergoing spermatogenesis. The reason why only 1/3 of mice were sex reversed is unclear (technical problems with transgenic experiments or additional genes might be important). male Sex reversed male

13. Hormonal control of sex differentiation in mammals Primary sex differentiation: mammalian sex determination is controlled by the SRY gene. SRY expression induces testes, lack of SRY results in ovaries. Secondary sex differentiation: refers to all hormonally controlled sex development beginning at embryogenesis and progressing into adulthood. This includes sex characteristics such as external genitalia, breasts, body build, and behavior. Most sex hormones are steroids derived from cholesterol. These include androgens (testosterone and DHT) and female sex hormones (estrogen and progesterone).

14. The gonads are the sites of synthesis for sex hormones Testosterone is synthesized in the Leydig cells of the testes. Estrogens are produced via the thecal and granulosa cells in the ovary. The adrenal glands also synthesize sex hormones. Testosterone and estrogen stimulate secondary sex characteristics such as larger muscles in males or breasts in females. Testosterone acts on a portion of the mesonephros called the Wolffian duct to form the epididymis, ductus deferens, and seminal vesicle. Estrogen acts on the Mullerian duct to convert it into oviduct, uterus, and upper vagina. Female external genitalia develop as the default program. If dihydrotestosterone (DHT) is present, there will be male external genitalia. If DHT is absent, female genitalia are the result.

15. Overview of somatic sexual development in mammals

16. Once the embryonic gonads develop as either testes or ovaries, all subsequent steps of sexual reproduction are controlled by sex hormones. After 6 weeks of gestation, Mullerian and Wolffian ducts exist near the genital ridge and kidney. Testosterone and AMH (anti Mullerian hormone) act on the Wolffian duct to remodel it into epididymis (where sperm mature), ductus deferens (transport sperm), and seminal vesicle (where seminal fluid is stored). The Mullerian duct degenerates.

17. In females the default program occurs. The Mullerian duct gives rise to the oviduct (connects ovary to uterus), the uterus (where the embryo develops), and the upper portion of the vagina. The Wolffian duct degenerates due to a lack of AMH and testosterone.

18. Androgen insensitivity syndrome This condition confirms the important role for sex hormones in development of secondary sex characteristics. This syndrome is caused by a mutation in the gene encoding the androgen receptor. The gene resides on the X chromosome. Males who inherit this condition produce testosterone and DHT but they are unable to respond to either hormone. Hence, they develop as phenotypic females. These individuals have normal male chromosomes. The low level of estrogen that is produced by the adrenal glands is enough to stimulate female secondary sex characteristics.

19. Male and female genitalia develop from the genital tubercle Indifferent stage: at 6 weeks of gestation the genitalia have not been determined. Urethral folds on both sides of a urogenital sinus unite to form the genital tubercle, the precursor of the genitalia. In females, it forms a clitoris and the 2 folds form the labium minora. In males, the folds fuse in the middle to form the penis. Dihydrotestosterone is the steroid that controls which pathway is followed.

20. Sex hormones control development of the brain Males and females of any species usually differ in complex behaviors such as mating, parenting, and aggression. How does this occur? For each sex hormone, there is a unique distribution of receptors throughout the brain. Androgen receptor is concentrated in areas that control aggression and mating. Estrogen receptors are concentrated in areas that control ovulation. Songbirds are a good example. Male birds attract females by singing, and the songs are learned from older males. If male birds are castrated, the amount and quality of their singing decreases. If they are subsequently treated with testosterone, singing resumes. Specific areas in the brain of male birds that are associated with singing are larger than in female birds.

21. Interesting effects of sex hormones are seen in mammals that produce litters of multiple offspring. The growing fetuses exchange sex hormones via the placental circulation. Females that develop between 2 males (2M females) are exposed to higher levels of testosterone than siblings that develop next to 1 or no males. These 2M females have masculinized genitals, have shorter reproductive cycles, and are less attractive to males. The converse effects are observed in males that develop next to 2 females. They have smaller seminal vesicles and are less aggressive than 0F males. baby boars Subtle differences in non reproductive behavior are suggested by different distributions of scores on standardized tests. Men tend to score higher on manipulating 3D images and mathematical reasoning. Women score better on tasks related to precision, verbal use, and perceptual speed.