644 3 2007

LECTURE 3Sexual Differentiation: 

LECTURE 3 Sexual Differentiation

WHY HAVE SEX?: 

WHY HAVE SEX? Sexual reproduction yields more variation. Increased variation enhances reproductive success (fitness).

SEXUAL BEHAVIOR: 

SEXUAL BEHAVIOR Sex is separateness, a division of types. Animals must locomote, gesture, posture, vocalize, display, or otherwise behave if union is to occur. Any behavior that increases the likelihood of gametic union may reasonably be called sexual behavior.

Sexual Reproduction: 

Sexual Reproduction Sex is almost ubiquitous in the animal kingdom because of the genetic variability, hence evolutionary flexibility provided by the separation of chromosome pairs into haploid cells. Animals or plants that have the option of sexual or asexual reproduction always opt for sexual reproduction when the environment becomes unstable.

WHAT IS SEX?: 

WHAT IS SEX? Chromosomal sex Gonadal sex Hormonal sex Morphological sex Behavioral sex Gender identification (humans) Legal sex (humans)

SEX DIFFERENCES: 

SEX DIFFERENCES There are differences between males and females. Sex differences may be apparent at birth; others show later in development. We want to understand how differences in behavior arise. To understand how sexually dimorphic behaviors arise, it is necessary to digress into embryology. Hopefully, digression will lead to progression.

SEX DIFFERENCES: 

SEX DIFFERENCES The idea of hormonal regulation of the process of sexual differentiation was given impetus by studies of a phenomenon known as free-martinism. Freemartins occur in cows, sheep, goats and pigs. Descriptions of free-martins include the following…

FREEMARTIN CALVES: 

FREEMARTIN CALVES The freemartin calf is a sexually abnormal chromosomal female born as a twin to an apparently normal male. ~90% of female cattle born as twins to males are sterile.

FREEMARTIN CALVES: 

FREEMARTIN CALVES External genitalia of the freemartin are of the female type, and there is mammary gland development. There is variable development of the internal genital tracts. The gonads are atrophic but resemble testes more than ovaries.

WHAT ARE THE FACTORS LEADING TO "NORMAL" SEXUAL DIFFERENTIATION?: 

WHAT ARE THE FACTORS LEADING TO 'NORMAL' SEXUAL DIFFERENTIATION? One old idea was that male babies came from right testis and females came from the left. (Hermaphrodites came from both) Primary step in the process of sex differentiation occurs at fertilization. An ovum can be fertilized by either a Y or X chromosome bearing sperm. This process, called sex determination, has consequences for the differentiation of the embryonic gonads.

Intersex (true hermaphrodite): 

Intersex (true hermaphrodite)

Mammalian Sex Determination Depends on Sperm: 

Mammalian Sex Determination Depends on Sperm

Sex Determination:Chromosomal Sex Determines Gonadal Sex: 

Sex Determination:Chromosomal Sex Determines Gonadal Sex In mammals, the ovum (X chromosome) can be fertilized by a sperm bearing either a Y or an X chromosome; this process is called sex determination. The chromosomal sex of homogametic mammals (XX) is female. The chromosomal sex of heterogametic mammals (XY) is male. Chromosomal sex determines gonadal sex. All subsequent sexual differentiation is normally the result of differential exposure to gonadal steroid hormones.

Gonadal Sex Determines Hormonal Sex Which Determines Morphological Sex.: 

Gonadal Sex Determines Hormonal Sex Which Determines Morphological Sex. Gonadal sex determines hormonal sex, which regulates morphological sex. Morphological differences in the central nervous system, as well as in some effector organs, lead to behavioral sex differences

Sexual Differentiation: There is a single anlagen for the gonads and external genitalia: 

Sexual Differentiation: There is a single anlagen for the gonads and external genitalia Each individual embryonic mammal, whether XX or XY, exhibits a thickened ridge of tissue on the ventromedial surface of each mesonephros, known as the germinal ridge. Whether the germinal ridge will develop into a testis or an ovary is determined by the cellular expression of the testis determining gene (SRY), carried on the Y chromosome. The SRY gene is expressed in the indifferent gonads.

Sexual Differentiation: Gonads: 

Sexual Differentiation: Gonads

Sexual Differentiation: 

Sexual Differentiation The SRY transcript encodes a DNA-binding protein that, in common with ligand-bound steroid receptors, binds to a specific hormone response element in the promoter region of genes that are regulated by SRY. When the protein products of these genes are produced, the middle (medulla) of the ridge develops and a testis forms. If the SRY gene is not expressed, then the outer part of the ridge (cortex) develops and an ovary is formed. (It is possible for the SRY gene to be expressed in one gonad but not in the other, which leads to unilateral differentiation).

Sexual Differentiation: 

Sexual Differentiation Mice that are chromosomal XY males, but do not have the SRY gene, develop ovaries; similarly, XX transgenic mice that have an SRY gene inserted develop testes (In reptiles and birds, SRY is not a testis determining gene). Hormonal secretions from the developing gonad determine whether the individual develops in a male or female manner.

Sexual Differentiation: 

Sexual Differentiation The mammalian embryonic testes produce androgens, as well as peptide hormones, that guide the development of the body, central nervous system, and subsequent behavior in a male direction. The embryonic ovaries of mammals are virtually quiescent and do not secrete high concentrations of hormones. In the presence of the ovaries or in the complete absence of any gonads, morphological, neural, and, later, behavioral development follow a female pathway.

Sexual Differentiation : 

Sexual Differentiation The prevailing hypothesis about the initiation of sexual differentiation indicates that the gonads are differentiated by genetic influences (SRY), and all other sexual differentiation reflects hormonal mediation. It is feasible that some sex differences in brain and behavior might be mediated more directly by gene expression in nongonadal tissue. E.g., the SRY gene is transcribed in the hypothalamus and midbrain of adult male mice. Because only genetic males possess a Y chromosome, any gene located on the Y chromosome is a candidate gene to mediate sex differences in brain and behavior directly without invoking an endocrine mediator. Absence of behavioral sex differences between XX and XY mice lacking functional Lhx9

Sexual Differentiation: Accessory Sex Organs : 

Sexual Differentiation: Accessory Sex Organs In contrast to the single, bipotential primordial gonad, there are dual anlagen for the accessory sex organs present early during ontogeny. The Wolffian duct system develops into the male accessory sex organs, that serve to connect the testes to the outside via the penis. Later-developing components of the Wolffian duct system include the seminal vesicles and vas deferens. The Müllerian duct system develops into the female accessory sex organs, which serve to connect the ovaries to the outside. The Müllerian duct system develops into the fallopian tubes, uterus, and cervix.

Sexual Differentiation: Accessory Sex Organs: 

Sexual Differentiation: Accessory Sex Organs During normal sexual differentiation, the Wolffian duct system develops in males, whereas the Müllerian duct system regresses. Conversely, in females, the Müllerian duct system develops, whereas the Wolffian duct system regresses. In the presence of ovaries or in the complete absence of gonads, normal development of the Müllerian ducts is accompanied by complete regression of the Wolffian duct system. Again, no hormones are necessary to permit normal female development among mammals.

Sexual Differentiation: Accessory Sex Organs: 

Sexual Differentiation: Accessory Sex Organs Male development of the secondary sex organs requires two products from the embryonic testes: testosterone and a peptide hormone called Müllerian inhibitory hormone (MIH). Testosterone is necessary to stimulate Wolffian duct development. Müllerian regression hormone, as its name implies, causes the regression of the Müllerian duct system. If testosterone is absent early in development, then Wolffian duct development fails to occur. If MIH is not secreted at the proper time, then the Müllerian duct system develops. Because of the twin anlagen, it is possible for both systems to develop in a single individual.

THERE IS ONE COMMON GENITAL RIDGE THAT RESULTS IN THE EXTERNAL GENITALIA (I.e., the undifferentiated state is bipotential): 

THERE IS ONE COMMON GENITAL RIDGE THAT RESULTS IN THE EXTERNAL GENITALIA (I.e., the undifferentiated state is bipotential)

There are dual anlagen for the accessory sex organs.: 

Undifferentiated Stage There are dual anlagen for the accessory sex organs.

Sexual Differentiation: Accessory Sex Organs: 

Sexual Differentiation: Accessory Sex Organs

Among Mammals, Females are the “Default Sex”: 

Among Mammals, Females are the 'Default Sex' Female organization is basic in mammals. There are more things to go wrong in males. More clinical problems with sexual differentiation of males than females.

Sex Determination in Birds: 

Sex Determination in Birds

Process of “normal” sex differentiation requires…: 

Process of 'normal' sex differentiation requires… MASCULINIZATION- active induction of male traits FEMINIZATION- active process inducing female traits DE-MASCULINIZATION- removal of potential for male traits DE-FEMINIZATION- removal of female traits

Sexual Differentiation : 

Sexual Differentiation To be male requires both masculinization and defeminization. In terms of reproductive organs, androgens from testes masculinize and MIF defeminizes. Masculization + feminization leads to ovotestis

Sexual Differentiation of Behavior?: 

Sexual Differentiation of Behavior? Masculinity-femininity scale Masculinity-demasculinity Femininity-defemininity M F + _ _ +

Sexual Differentiation of Behavior?: 

Sexual Differentiation of Behavior? Masculinity-femininity scale Masculinity-demasculinity Femininity-defemininity M F + _ _ + X

Sexual Differentiation of Behavior?: 

Sexual Differentiation of Behavior? Masculinity-femininity scale Masculinity-demasculinity Femininity-defemininity M F + _ _ + X X X X

Sexual Differentiation of Behavior?: 

Sexual Differentiation of Behavior? Masculinity-femininity scale Masculinity-demasculinity Femininity-defemininity M F + _ _ + X X X X