Sex Determination in Birds Flashcards
Describe the evolutionary origin of the ZW pair
- Thought they differentiated by the same process as XY
- Derived from an autosomal pair
- Differentiation initiated when one acquires a sex-specific function
- Degeneration proceeds rapidly
- Although superficially resemble each other, not XX/XY not homologous to ZZ/ZW
- Z orthologues → human chromosomes 9 and 5
- Comparison with orothologues in fish show that genome regions human X and chicken Z were not adjacent, confirming independent origin
Describe the evolutionary origin of the W
- Degenerate relic of the Z (just as Y is with X)
- Progressive degeneration is well documented in birds as different lineages represent different stages of W degredation (ratites have little differentiation)
- Unknown why the differentiation has proceeded much further in some bird species than others → maybe the acquisition of female specific functions by other genes was delayed
- Being female specific, might be expected to accumulate female-advantage genes → indeed most genes are expressed in the ovary
- Like the human Y, has low levels of polymorphism → may be a result of hitch-hiking with a favourable allele to produce selective sweeps that homogenise the W
Describe the evolutionary origin of the Z
- Macrosome – the 6th largest
- Since the ZW pair evolved from autosomes by degradation of the W, may be expected that the Z is unchanged
- Wrong – Z is 80mb and contains around 1000 genes, well below the normal density for autosomes
- Many repetitive sequences
- Although gene density is low, number of genes has increased due to tandem amplification of 4 genes → genes on this amplicon are expressed in the testis
Describe the evidence for a SDR on the W chromosome
• Doesn’t appear to be a female specific gene – all genes have copies on the Z
• But a Z copy doesn’t disqualify a W gene from sex determining role → sequence differences could endow the W copy with an ovary determining function
− Some genes did show upregulation in the left gonad in females during the sex determining period (only left gonad forms an ovary to reduce weight for flying)
• Most promising candidate is on the short arm of the W → HINTW gene.
− Expressed in female embryonic gonads
− Structure is different to its Z homologue
− However, gene disqualified because it failed to induce ovary formation when induced in ZZ embryos
Describe the evidence for a SDR on the Z chromosome
- Theory is that threshold amount of Z gene product is required to determine testis, and an insufficient supply permits female development
- Likely dosage-dependent gene is DMRT1
- DMRT1 lies in the deletion interval on human chromosome 9 associated with male to female sex reversal in humans
- Proposed that twofold dosage of DMRT1 required for male development
Describe dosage compensation in birds
- Since W has lost nearly all of its genes, there must be major differences in the doses of Z borne genes in ZZ compared to ZW
- In mammals, X chromosome inactivation equalizes the dosage – random epigenetic inactivation of an X in somatic cells.
- Expected that Z chromosome inactivation should compensate for unequal gene dosage
- No evidence for a coordinated whole Z chromosome inactivation
- Z borne genes significantly more highly expressed in males than females in all tissues
- Female transcription from a single Z seems to be 50% higher – this seems to compensate
- There also seems to be some partial Z inactivation
- RNA FISH studies show that compensation works by decreasing the probability of expression of the Z borne genes
- Male cultures contain a mixture of cells with two Z or a single Z - stochastic inactivation
- Why is there no whole Z inactivation?
- Male bias of gens on the Z suggests they may have evolved male functions, therefore don’t need to be inactivated
- Maybe the question should be rephrased – why is there whole chromosome dosage compensation in mammals?
Describe the role of sex hormones in developing showy male plumage
Seasonal Plumage (showy plumage in the breeding season, eg Peacock):
• Showy male plumage is the default state
• Dimorphism is estrogen dependent → estrogen blocks showy plumage
− If a females ovaries cease to produce estrogen, she can start to develop male plumage
• Showy male plumage signifies lack of estrogen, not presence of testosterone
• This is how you get a seasonal moult:
− Breeding season → lack of estrogen in males gives showy plumage
− Non-breeding season → aromatization of testosterone could be providing estrogen to turn off the showy plumage
Continual Plumage (showy plumage all year round, eg, Sparrows):
• Plumage development under direct genetic control
• Consequence of a difference in chromosomal balance between the ZZ male and ZW female
Exceptions to the rule (Ruff):
• Nuptial plumage of males is testosterone dependent
− Castrate males don’t develop the nuptial plumage
− Females injected with testosterone develop rudimentary ruffs
Describe the role of sex hormones in the development of spurs
- Used as weapons during fights
- Castration causes them to regress, injection of testosterone causes them to regenerate
- Represent a typical testosterone dependent secondary sexual structure
Describe the role of sex hormones in behavioural dimorphisms in birds
• Aggression is testosterone dependent
− Castrate males are less aggressive and defensive, and less likely to sing
• Castration and testosterone injection experiments suggest that sexual displays are testosterone dependent
• Similar experiments on females suggest estrogens control nest building and receptivity
Describe how birds achieve sexual dimorphism in song structures
- Song circuit is feminine in females that have significant amounts of testicular tissue → suggests testosterone is not involved in masculinization
- Manipulations of hormones in males have not caused large changes in phenotype
- Brain is feminine females whos ovaries are blocked from developing → suggests ovarian estrogen secretions not important
However!
• Treating females with estradiol causes masculinization of the song circuit → suggesting estrogen does play a part!
Estrogen is therefore obviously important however:
• Cant be coming from the ovary, as males don’t have one
• Cant be coming from aromatic conversion of testosterone, as females with testosterone still have female brains
- In newly hatched zebrafinches, neurons project to the RA, but don’t reach their target unless exposed to estrogen
- Estrogen masculinses the song nucleus by promoting axonal growth and development of a large RA region.
- Holloway and Clayton suggests estrogen comes from locally produced neurosteroids
What are the lessons from the gynandromorph zebrafinch and song?
• Supports the role for a genetic sex of brain cells
Gynandromorphs have male plumage and teste on the right, female plumage and ovary on the left
• W linked genes present more on the left
• Song circuit more masculine on the right side, but female song circuit was larger than normal
• Circulating hormones would affect both sides equally and make them the same
• Supports the idea that locally produced neurosteroids is genetically linked → neurosteroids produced on the male side, but as hormonal they can diffuse across slightly
What are the lessons from the gynandromorph chicken?
• Male plumage and wattle on one side, female plumage on the other
• Phenotypic differences between the sides implied that somatic sexual phenotype is not hormonal, as this would effect the whole chicken
• Implies it is autonomous in ZZ and ZW somatic cells
− In mammals, sex hormones govern most somatic sexual phenotypes → SRY gene leads to turn on of sertoli cells, then leydig cells, then androgen production, then sexual phenotype
− In birds, genetics has more of an influecne
− Not surprising that birds have more cell autonomous somatic sexual dimorphism than mammals, given the Z chromosome is only partially dosage compensated.
What is the model for sex determination in mammals and birds?
• Mammals:
− SRY gives you testis, absence gives you ovary. Gonadal hormones drive the male/female phenotype
• Birds:
− Dosage effect of DMRT1 occurs in a cell autonomous manner
− Gonadal hormones have limited effects on sexual phenotype
