Flashcards in battle of the sexes Deck (37)
what is sex determination?
the development of characteristics which allow an individual to be identified as male or female
- reproductive system - testis vs ovaries
- external phenotype
- behaviour - hormone induced
what did Aristotle say about sex determination?
' the heat of the male partner during interocurse determines the sex of the baby'
- if the males heat overwhelms the woman coldness then a male with form
in mammals what determine the sex?
the y chromosome
since all females are XX, all oocytes carry an X chromosome.
males are XY so sperm can carry an X or a Y sex chromosome.
so the male partner determines the sex of the offspring
what gene determines the sex?
SRY gene - sex determining region Y - one found on the Y chromosome
SRY gene encodes transcription factors that activate testis formation (testis determining factor)
how does SRY gene determine sex?
several testis specific genes contain SRY-binding sites in their promoters
binding of SRY to these sites initiates testis development
what is meant by biopotential embryonic gonad?
the gonads are gene driven
early embryonic gonad is indifferent or biopotential
formation of embryos or testis is an active gene directed process (no default process)
when does sex differentiation occur?
at 7 weeks of development
what are the two stem of ducts?
wolffian - male
mullerian - female
development of ducts is hormone dependant.
which phenotype predominates if the gonads are removed?
if the biopotential gonads are removed the female reproductive system develops
female phenotype predominates
how do males develop?
in humans SRY gene stimulates the indifferent gonad to form a testis
the testis produce two hormones
- testosterone (forms male reproductive organs, wolffian duct - vas deferens, inhibits breast primordia)
- anti-mullerian hormone (degeneration of mullerian duct
testis determining factor (encoded by SRY gene) activates SOX9 - testicular development and sertoli cell proliferation
SOX9 induces steroidogenic factor 1 SF1 - up regulates AMH from sertoli cells and increases production of testosterone in leydig cells
XX humans with an extra copy of SOX9 gene develop as males despite SRY absence.
what gene appears to stop SRY
what two genes stimulate biopotential gonad to form an ovary?
DAX1 and WNT4
how does the female develop?
DAX21 and WNT4 stimulate the biopotential gonad to develop into an ovary
ovary secretes oestrogen
oestrogen acts on mullerian duct
development of uterus , oviducts and cervix.
is it only the male chromosome that influences sex of offspring?
since equal number of X?Y sperm are produced, there should be equal numbers of female/male offspring
but environment had a strong influence in some species.
how is the snapping turtle egg affected when incubated at different temperatures?
bellow 22 or above 29oC = females
intermediate temps = males
how is the american alligator egg affected when incubated at different temperatures?
above 30oC = females
below 25oC = males
at 28.5oC = equal numbers
in reptiles what determines sex?
genotypic sex determination
also a thermo-sensitive period of development
- females XX
- low temp: genotypic xx females become phenotypic males XXmales
Australian dragon lizard
- males ZZ
-high temps: genotypic ZZ males become phenotypic females ZZ females
why does temperature affect sex differentiation?
high temperatures increase aromatase
- converts androgens to oestrogen
- biased sex ratio towards females
what are the advantages of skewed sex ratio?
sex allocation hypothesis
- dominant males father most offspring
- subordinate males do not mate
- all females will get pregnant
- but only females with high rank/body condition will produce males which join the dominant reproducing males.
so to maximise reproductive output
- females with good condition should opt to produce male offspring
-females with poor condition should opt to produce female offspring
supported in many wild species including deer an mice
why may adaptive control of genes bias occur? which species may it occur in?
in many non-mammalian species
evolved as a response to changing environmental conditions e.g. food availability, temp, disease, population density, maternal condition, season
what is an adaptive control of gender bias in fig wasp?
low population density - sex bias in favour of males
- disperse to fertilise females in other colonies, maximise genetic potential and minimise inbreeding
what is an adaptive control of gender bias in ruffs?
poor food availability - sex bias in favour of females
- maximise offspring numbers, cost to mothers,
- don't waste food on multiple males - you only need one male
what affects offspring sex ratio in red deer?
maternal dominance correlates offspring sex ratio
- dominant females have more male offspring (dominance correlates with testosterone concentration
this sex bias is eliminated with high population density - nutritional stress and increased confrontation to maintain dominant position
what affects offspring sex ration in american opossum?
maternal condition correlates offspring sex ration
- increase in male offspring with high n-3 fatty acid diet
- marsupials move to the pouch 14 days after conception
mechanisms must act before conception or very early om
what affects offspring sex ration in non human primates?
– Maternal condition correlates offspring sex ratio
– Social structure is important
• Colobine monkey high ranking females have more
• Opposite is true in baboons (matriarchal society)
do humans show adaptive control of gender bias?
- socioeconomic variables (famine)
- psychological stressors
how does preconception nutrients affect sex ratio?
Maternal diet high in unsaturated fats or glucose
• Increasedmalesborn Maternal diet high in omega-6
polyunsaturated fatty acids (vegetable oils) • Increasedfemalesborn
Recent evidence: not a consistent dietary intake that determines offspring sex, but a change in nutritional status
• Females gaining condition produce sons
• Females losing condition produce daughters
environmental stressors effect offspring in humans?
Exposure of males to endocrine disruptors skews sex ratio in offspring
Dioxin exposure of males at puberty
• Bi-product of chemical, steel/iron industry
• Increased male offspring in adulthood
Dichloroethylsulphide (Mustard gas)
• Exposure of Iranian soldiers during the Iran-Iraq war
• Decreased male:female ratio in offspring
• Decreased testosterone:gonadotrophin ratio in males
• Increased sperm DNA damage
– Sperm DNA integrity linked to ability
to progress through the female tract
– Are Y chromosome sperm more susceptible to chemically induced DNA damage?
psychological stress in humans affect sex ratio?
Severe life event pre-conception or early pregnancy
– Death/cancer diagnosis in spouse or child
– Decrease in male:female sex ratio
-->Differential fertilisation – more X chromosome sperm fertilised? or Differential embryo survival–more XY embryos aborted?
Kobe earthquake and 10-day war in Slovenia – Both resulted in decrease in male:female sex ratio
– Reduced sperm motility
• Stress induced increase in prolactin and testosterone in males
• Differential effect on X or Y bearing spermatozoa?
Confounding factors with psychological stress!
– Sexual abstinence
– Hormone concentrations differ from the norm
– Poor nutrition
– All affect semen quality and embryo survival