Sex determination of offspring in Organisms Chapter 6

Sequence of development during life cycle

Fertilization-Zygote

Cell specification (cell fate)- tissues, organ, offspring development/formation

Sex determination in offspring - Gametogenesis, Fertilization, Reproduction-New generation

Environmental sex determination

some species environment determines sex of offspring

Mammal sex determination

sex of chromosome chromosomally determined

ex. human sex determined by “Y” chromosome in (male)

Birds sex determination

two similar chromosomes ZZ (male) and ZW (females)

sex determined by “W” chromosome in (female)

Fruit Flies (Drosophila) Sex determination

the # of “X” chromosomes in a diploid cell determines the sex the ratio to autosomes

1X is male and the 2X’s is female

ex. female sex if two “X” chromosomes : three sets of autosomes

mosaic female and male

Y chromosome present for sperm development not sex determination

male is the common one (question on exam)

Sex determination Bonellia Viridis

sex determination is location-dependent

Egg lands on sea floor (female) 100 mm long

Larva lands on the female proboscis, enters the mouth, and migrates into the female’s uterus (male) lives inside and fertilizes eggs 1-3 mm long

Mammalian primary sex determination:

Determination of the gonads

ovaries- female

testes- male

Chromosomally determined

male and female gonads derive from a common precursor bipotential/ undifferentiated gonads

it a gene directed process

Significance of “Y” chromosome

crucial in determining male sex in mammals

Testis determining factor

SRY gene

SRY gene

located on the “Y” chromosome and organizes gonads into a testis (male) rather than an ovary (female)

- primary role of the protein is to activate the downstream autosomal gene SOX9 in the bipotential gonad.

Male (XXXXY)

differentiates into a male

Female (XO)

sterile female no chromosome is present, hence the “O”

female ovary formation is not completed

Secondary sexual determination/ characteristics

Involves the sexual phenotypes outside of the gonads

usually determined by hormones and paracrine factors

Female phenotype

genital tubercule - clitoris

labioscrotal folds - labia majora

mullerian ducts - uterus, cervix, oviducts, upper vagina (estrogen present)

uterus - promotes sperm movement toward oviducts (womb during birth)

Oviducts- collects oocytes released from ovary and moves them to site of fertilization

cervix- inner muscular entrance to the uterus, secretes mucus that regulates sperm entry

Male phenotype

secretion of 2 factors produced by testes

Anti-Mullerian Hormone (AMH)

Androgen

Anti-Mullerian Hormone (AMH)

is a paracrine factor

degeneration of mullerian ducts (female reproductive organs)

steriod horomone - testosterone

Androgen

- inhibits mesenchymal cells from sending signals that instruct Wolffian ducts epithelium to undergo cell death (apoptosis)

- causes Wolffian ducts to differentiate into sperm carrying tubes: epididymis and vas deferens.

Male phenotype continued

fetal testosterone causes genital tubercle to become penis

labioscrotal folds and becomes scrotum

Androgen Insensitivity Syndrome

phenotypically - female, no Mullerian ducts, no oviduct, sterile females

genotypically - male (XY)

Mutation in Androgen Receptor Gene

mutant protein binds testosterone and sends it to the nucleus

-individuals respond to estrogen and not to testosterone

Gynandromorphs

certain parts of their body are male, and other regions are female

Insects, crustaceans, and birds are examples of this

Activating Sex- lethal (Sxl) gene

-“X” chromosome dosage is critical for this activation

- RNA splicing factor regulates gonad development and its expression

- 2 promoters early: one in XX, the later one in both XX /XY

- Genes on “X” encode 4 factors early activation (based on concentration)

Results: transcription of gene in XX but not in XY! difference in RNA splicing leads to functional Sxl protein in XX

XY cells in relation to (Sxl gene)

no functional Sxl early in development

becomes activated later in development

Double sex (dsx)

is the switch gene for sex determination

-active in male and females but only expressed in cells that show sexual differences in function/anatomy

-primary transcript processed in a sex-specific manner

FEMALE SPECIFIC - produces DsxF

MALE SPECIFIC - produces DsxM

DsxF (double sex gene in females)

-allows Dsx to interact with other proteins to produce female specific gene

DsxM (double sex gene in males)

allows Dsx to interact with male specific domains to activate genes that make male specific traits

ex. mosquitos

Sex determination in Crocodiles and Alligators

depends on the temperature of the egg during week 2 and 3 of incubation

-egg incubated at 30 degrees Celsius or below hatched into female

-egg incubated at 34 degrees Celsius or above hatched into male

ex. 32 degrees Celsius: 87% will hatch into female

red eared slider turtle sex determination

eggs incubated at low temperatures produce one sex, while at high temperatures they produce the other sex

intermediate temperatures : small window for both sexes

Snapping turtles sex determination

- females dominate at cool and hot temperatures

- males dominate at intermediate temperatures

Dmrt1 gene

related to drosophila (fruit flies) double sex-dsx gene

-initiates testes determine cascade across fish, amphibians, and birds

Dmrt1 gene Slider turtles

- High levels in gonads in cooler temps (male) and low levels in warmer temps (female)

- Suppressed gonads leads to ovaries, but when added into suppressed gonads, they resume testis development

Gametogenesis in Animals

- differentiation of primordial germ cells (PGCs) into egg or sperm

-coordinated by factors of developing gonads

- proteins expressed by germ cells to suppress gene expression, conserved across the animal kingdom (Vasa, Nanos, Tudor, Piwi)

- BMP signals required for formation of PGCs in mammals and flies

PGCs (primordial germ cells)

-bipotential precursors of eggs and sperms

-Primordial germ cells form in the posterior region of the embryo and later migrate to the gonads.

Oogenesis

-Meiosis is initiated once in a finite population of cells

- 1 gamete per meiosis

- completion of meiosis delayed for months/years

- meiosis arrested at first meiotic prophase (small population of cells)

-differentiation of gamete while diploid (first meiotic prophase)

-all chromosomes exhibit equivalent transcription/recombination during meiotic prophase

the biological process by which female gametes (egg cells or ova) are produced, developing from diploid germ cells into a mature haploid egg within the ovaries. During oogenesis, the formation of three polar bodies results from the unequal division of cytoplasm and chromosomes across the two meiotic stages

Spermatogenesis

-meiosis initiated continuously mitotically dividing stem cell

- 4 gametes produced per meiosis

- meiosis completed in days or weeks

- meiosis and differentiation produced continuously without cell cycle arrest

- differentiation of gamete occurs while haploid, after meiosis ends

- sex chromosomes excluded from recombination and transcription during first meiotic prophase

the biological process by which haploid spermatozoa (sperm cells) develop from diploid germ cells in the seminiferous tubules of the testes.

sox9

is the actual driver that executes the genetic program required to transform the bipotential gonad into a testis.

How many sperm cells are contained in a pollen grain for double fertilization in flowering plants?

2 diploid

Plants can reproduce sexually as well as asexually.