chapter 5: sex determination and sex chromosomes

sexual differentiation is

crucial to many life cycles but not all

in multicellular sexually reproducing organisms, its important to distinguish between

-male/female determination

-primary sexual determination

-secondary sexual differentiation

male/female determination can be based on

specific chromosomes

primary sexual differentiation

involves only the gonads where gametes are produced

secondary sexual differentiation

involves the overall appearance of the organism

secondary sexual differentiation includes clear differentiation in

such organs as mammary glands and external genitalia

male/female determinations varies between

species

plants and animals that contain only male OR female reproductive organs are

unisexual (dioecious or gonochoric)

plants and animals that contains both male AND female reproductive organs are

bisexual (monoecious or hermaphroditic)

bisexual plants and animals can produce

both male and female gametes

in some species male/female differentiation occurs in

different tissues of the same individual

in maize (Zea mays)

-diploid sporophyte stage predominates

-both male + female structures are present on adult plant

dominance of diploid sporophyte stage in maize indicates that

sex determination must occur differently in different tissues of the same plant

sexual determination can be

genetically determined and regulated

mutant genes can cause

sex reversal

when homozygous, all mutations classified as tassel seed (ts) interfere with

tassel production and induce production of female structures instead

recessive maize mutations

-silkless (sk)

-barren (ba)

recessive mutations sickles (sk) and barren (ba) interfere with the

development of the pistil, resulting in plants with only male-functioning reproductive organs

XY systems

-mammals

-some insects

XO system

some insects

ZW system

-reptiles

-birds

-some amphibians

-insects

no special sex chromosomes

-plants

-fungi

XX/XY Lygaeus mode of sex determination

-female gametes all have an X chromosome

-male gametes have either an X or a Y chromosome

zygotes with two X chromosomes result in

female offspring

homogametous

two X chromosomes

zygotes with one X and one Y chromosome result in

male offspring

heterogamous

one X and one Y chromosome

many organisms use an X chromosome system for

sexual determination

XX/XO protenor (butterfly) mode of sex determination depends on

the random distribution of the X chromosome into half of the male gametes

in XX/XO the presence of two X chromosomes in the zygote results in

female offspring

in XX/XO the presence of only one X chromosome results in

male offspring

in ZZ/ZW system males are

not always the heterogametic sex

in some ZZ/ZW organisms, females are the

heterogametic sex

ZW

females are heterogametic sex

ZZ

males are the homogametic sex

example of Z/W system

chickens

ratio of X chromosomes to sets of autosomes determines

sex in drosophila

in drosophila, Y chromosome does not

determine sex

in drosophila Y does not determine

sex but it is needed for fertility

in drosophila sex is determined by the

ratio of X chromosomes to the haploid sets of autosomes (A)

normal drosophila female

AA and XX

-1:1

normals drosophila male

AA and XY

-1;2

in contrast to mammals, the X chromosome on male drosophila is

unregulated so that transcription levels equal that of XX female

with respect to primary sex determination, male gametes containing one of each autosome plus a Y chromosome result in

male offspring because they lack an X chromosome

genic balance theory

threshold of maleness if reached when X:A ratio is 1:2 (X:2A) but the presence of additional X (XX:2A) alters re balance + results in female differentiation

chromosome formulation

C. elegans

-model species

-has two sexual phenotypes

C elegan males with one testes have only

-one X chromosome

-no Y

-hermaphrodites

c. elegant hermaphrodites

have both testes and ovaries

-have two X chromosomes, no Y

hermaphrodite eggs are fertilized by

stored sperm-self fertilization

majority of c. elegan offspring are

hermaphrodites

less than 1% of c. elegans are

males

in some species, sex is not

genetically determined

Temperature-dependent sex determination (TSD)

Environment, specifically temperature, has profound influence on sex determination

for crocs, turtles, and lizards, sex determination is achieved according to the

incubation temperature of eggs is during a critical period of embryonic development

many reptile species do use

ZZ/WW or XX/XY, in others TSD is the norm

in reptiles, temperature can

influence sex determination

three different patterns of temperature sex determinations in reptiles

1. low temps yield 100% females, high temps yield 100% males

2. exact opposite occurs

3. low and high temps yield 100% females, intermediate temps yield various proportions of males

TSD temperature differed is believed to involve

steroid (mainly estrogens) and the enzymes involved in their synthesis

a particular enzyme aromatase

converts androgens to estrogens

androgens

male hormones such as testosterone

estrogens

females hormones such as estradiol

sex determining mechanisms involving estrogens seems to be

characteristic of nonmammalian veterbrates

human karyotype revealed that

one pair of chromosomes differs in males and females

-females: two X chromosomes

-males: one X and one Y

in humans the Y chromosome determines

maleness

during early human embryonic development, the embryo is

hermaphroditic

human hermaphrodite

gonadal phenotype is sexually indifferent

as development continues, gonadal ridges can form

either ovaries or testes (bipotential gonads)

bipotential gonads are triggered by

presence or absence of Y chromosome

in bipotential gonads the presence of Y chromosome alone

does NOT mean it determines maleness

the human Y chromosome

PAR

pseudoautosomal region

SRY

sex-determining region Y

MSY

male-specific creation of the Y

the Y chromosome has at least

75 genes

X chromosome has

900-1400 genes

pseudoautosmal regions (PARs) present on

both ends of the Y chromosome share homology with regions on the X chromosome + synapse + recombine with it during mitosis

PARs are critical to

segregation of the X and Y chromosomes during male gametogenesis

male-specific region of the Y (MSY)

non recombining region of the Y chromosome

some potions of MSY share

homology with the X chromosome, others do not

MSY has

euchromatic and heterochromatic regions

euchromatic regions

functional genes

heterochromatic regions

nonfunctioning genes

SRY (sex determine regions) is located

adjacent to the PAR of the short arm of the Y chromosome

at 6-8 weeks of development the SRY genes become

activate in XY embryos

SRY expressed

gonads develop into testes

SRY not expressed

gonads develop into ovaries

testis-determining factor (TDF) is a

protein encoded by a gene in the SRY that triggers testes formation

TDF is present in all

mammals

SRY codes for

TDF

evidence that TDF determines maleness

-deviations from normal sex determination

-transgenic mice research

in deviations from normal sex determination

-males with two X chromosomes and no Y

->SRY region attached to X chromosome

-females with one X chromosome and one Y chromosome are missing SRY gene

transgenic mice research shows that

XX normal mice eggs injected with DNA containing SRY transform the mice into males

TDF is believed to be the transcription factor that behaves as a

master switch controlling the genes involved in sexual differentiation

XX males have a

translocation from the Y to the X

XY females have a

deletion of part of the Y

human diseases with extra X chromosomes

-klinefelter syndrome

-XXX syndrome (Triplo-X)

klinefelter syndrome

-1/660 males

-tall stature with long arms and legs

-underdeveloped testes and prostate gland

-no facial hair

-normal intelligence but slow learners

-phenotypically male

-infertile

-slight breast enlargement

-hips often rounded

klinefelter syndrome has usually

XXY or a 47,XXY, 48,XXXY, 49,XXXXY karyotype

XXX syndrome (Triplo-X)

abnormal presence of three X chromosomes along with a normal set of autosomes results in female differentiation

-1/1000 live births

-47,XXX