sex determination

chromosomal sex

• females are xx so all oocytes carry the x chromosome

• in males sperm can carry an x or a y chromosome, therefore the sperm content determines the sex of the embryo

one gonad, 2 fates

• what is the early embryonic gonad

• when does sex differentiation occur

• which ducts are female and male

• which phenotype is the default

• formation of ovaries or testes is an active gene directed process

• early embryonic gonad is bi potential

• sex differentiation at week 7 of development

• wolffian- male, mullerian- female

• development of male ducts is hormone dependent

• Female duct phenotype (Mullerian) is the default

- Predominates in the absence of foetal testis

- Remove bi-potential gonads, female reproductive system develops (Jost, 1953)

making of a male

• what is male gonadal sex determined by

• where is the transcription factor

• where is it expressed start nd decreased

• what can mutations of this cause

• male gonadal sex is dtermined by inheritance of SRY

• transcription factor on Y chromosome

• starts to be expressed in the somatic cells of the genial ridge e10.5, decreases after e12.5 in mice

• mutations in SRY in humans cause xy to xx reversal

• expression of sry in xx mice result in testis formation and male phenotype

• switched on by wt-1 and sf-1 proteins

SRY expression

• where does it begin

• begins at the centre of the developing testis and spreads out along it

• expression starts in the central zona of gonad

• then spreads across long axis of gonad in a wave

• if sry not switched on in a 6 hour window the gonad will develop as an ovary

ovotestis formation

• some mouse straisn undergo partial xy to xx sex reversal resulting in ovotestis formation

• frequently has male testis tissue at centre and female ovarian tissue at poles of the gonad due to a failure of sry to spread along the gonadal axis

How do bipotential cells differentiate into sertoli cells

• what is expressed in XY genes and what does this encode

• how is the expression increased.

• what does this new gene activate and what happens

• what do they also supress

• what do they also do to bipotential signalling cells

1. in XY genes the SRY gene is expressed an encodes a transcrition factor that activates SOX9 target gene

2. SRY and SF1 bind to a SOX9 enhancer to increase expression. once SOX9 reaches a threshold the system becomes self sustaining.

3. sox9 activates FGF9 expression. this creates a positive feedback loop with FGF9

4. They also

5. this continued signalling cause bipotential supporting cells to differetiatig nto sertoli cells

what hormones does the testis produce

• testosterone: forms external male penis from urethral folds. wolffian duct becomes internal male reproduction system (epodidymis, vas deferems)

• anti mullerian hormone: degeneration of mullerian duct

female

• which gene is not present

• what is identified in XY females and what does this disrupt and downregulate

• what does this cause the formation of

• what is expressed in the bi potential gonad.

• no SRY gene

• DAX1 is identified in XY females and disrupts SRY function.

• it also downregulates testis Sf1 expresion.

• Leads to the formation of a dysfunctional testis which cannot produce testosterone and thus the mullerian ducts predominate

• WNT4 gene is expressed in the bi potential gonad. It is undetectable in the XY gonad. Maintianed in the XX gonad.

• XX Wnt4 knock out mice have abnormal ovaries.

sex determination in the bipotential gonad

• in xx and xy at 11.25dpc what transcripts and where in the gonad are they located

• how is a male pathway initiated

• how does this occur

• what happens in xy gonads

• how is the female pathway initiated

• in both xx and xy gonads at 11.25dpc, fgf9 transcrips are detected near gonad surface. wnt4 trasncrips detected near the gonad mesonephric boundary.

• genetic env switch initiates male pathway by creating an imabalance

• in females, this occurs through upregulation of sox9

• sox9 and fgf9 forms a positive feedback loop in xy gonads

• •The balance between FGF9 and WNT4 signals is shifted in favor of FGF9, and the dominance of the male pathway is established.

  In the absence of a feed-forward loop between SOX9 and FGF9 (as in XX gonads), WNT4 blocks Fgf9 initiating the female pathway

environmental influence on offspring sez

• there is athermosensitive period of development in reptiles

snapping turtle egg incubation

• below 22 or above 28- females

• intermediate temps- males

american alligator egg incubation

• above30- females

• below 25- males

• 28.5- equal number

what are advantage of a skewed sex ration

sex allocation hypothesis

• dominant males father most offspring

• subordinate males dont mate

• all females pregnant

But

Only females with high rank/bcs produce males which join dominant male

males born to nutritonlly restricted mothers are smaller as adults

• females with good condition should opt to produce male

• female with poor should opt to produce female

examples of manipulation of sex ratio bias

fig wasp

• low pop density

• in favour of male

• they dispese to fertilise females in other colonies, maximise genetic potential and minimise inbreeding

ruff

• poor food availability

• sex bias in favour of female

• so maximise offspring prod, dont waste foods on multiple male

examples in mammals

red deer

• maternal dominance correlates offpring sex ratio

• dominant females more male (dominance correlates w testoserone)

• sex bias eliminated with high popdensity (as nutrional stress, increased confrontation to maintain dominant postition)

american opossum

• maternal condition correlates offspring sex ratio

• Marsupials move to the pouch 14 days after conception

  – Mechanisms must act before conception or on very early embryo

non human primate

• – Maternal condition correlates offspring sex ratio

– Social structure is important

• Colobine monkey high ranking females have more males

• Opposite is true in baboons (matriarchal society)

in human

• maternal diet high in sat fat or glucose- male

• high in omega 6 polysunsaturated fatty acid- female

• females gaining condition produce sons, losing produce daughters

env stressor in human

• dioxin exposure of males at puperty- increased male offspring in adulthood

• dichloroethylsulpide- decreased male to female ratio, decreased testosterone:gonadotrophin ratio in males and increased sperm dna damage. but sperm dna integrity linked ot ability to progress through female tract

psychological stres

• decrease in male to female ratio

fema tract selecting speciifc sperm

female will select sperm from a particular male

heterospermic insemination cattle

sperm selection at UTJ

• can mice sperm pass through the UTJ

• what do they fail to exress

• molecular recognition systems

• mice sperm op unable to pass utj despite normal motility and morphology

• fail to expres sperm surface protein (a- disintegrin and metalloprotease3)

• implication for sperm viability assessent- these normal sperm would not reach oocyte

• evidence forsperm selection via surface proteins

sperm surface marker

• what is the name

• what is it involved in

• what does the expression correlate with

• Evidence from clinical investigation of human sperm for infertility treatment

• CD52 sperm plasma membrane glycoprotein

• CD52 involved in sperm binding to oviduct and zona pellucida

• CD52 expression correlates with sperm DNA integrity

• Female tract can select out a sperm with poor genetic integrity using CD52

difference of x and y chromosome sperm

• Nuclear content differs by 2.8-4% dependant on species

• Morphology differs subtly between X and Y sperm

– X sperm increased head, mid-piece and tail length

– Sperm morphology reflects DNA integrity

• Differentially expressed proteins

– Associated with cytoskeletal structures influencing motility

– Regulation of metabolism

• Individual sperm microRNAs

– Adopt unique 3D shapes for sperm recognition in oviduct

– Powerful modulators of gene expression

can females differentiate between x and y sperm

• Does sperm induce sex specific gene expression in the oviduct

• Inseminated X or Y sperm into contralateral oviducts of the same animal

• Sex specific oviductal responses seen by altered gene expression (microarray)

• 501 of 24,123 genes showed X & Y differential expression

• Immune response genes upregulated with Y-sperm

– Interferon regulatory factor (IRF7)

– Detects foreign DNA from pathogens via pattern recognition receptors (Toll-like receptors)