HB Lecture 5

A sex with two identical sex chromosomes is called ___; a sex with two different sex chromosomes is called ___.

Homogametic (two identical, e.g. XX or ZZ); heterogametic (two different, e.g. XY or ZW)

In mammals, which sex is heterogametic?

Male (XY)

In birds, which sex is heterogametic?

Female (ZW)

What are the karyotypes of male and female birds?

Male ZZ; female ZW

Are mammalian XX/XY sex chromosomes homologous to avian ZZ/ZW?

No. They evolved independently from different autosomes.

How is X-linked gene dosage compensated in mammals?

X inactivation: one X chromosome in females is randomly inactivated

How complete is Z-linked dosage compensation in birds?

Only partial for most Z genes, so Z genes have a large cell-autonomous effect

Name the master testis-determining gene in mammals and its chromosome.

SRY on the Y chromosome

Name the candidate master testis-determining gene in birds and its chromosome.

DMRT1 on the Z chromosome

How many doses of DMRT1 encode the male genotype in birds?

Two (ZZ); there is no dosage compensation for DMRT1

What happens if DMRT1 is overexpressed in female (ZW) chicken gonads?

Testis formation

Are female-determining genes in birds known?

No, not yet identified. Current candidates lie on the short arm of the W chromosome.

Which human chromosome is the bird Z largely syntenic with?

Chromosome 9 (with gene blocks from 5, 8 and 18)

Describe the W chromosome's gene content and replication.

Many repeat regions, late replicating, few bona fide genes (similar to mammalian Y)

In female chickens, which gonad forms an ovary?

Left only; the right gonad regresses

Which hormone does the regressing right gonad of the female chick produce, and what does it do in males?

AMH; in males, active AMH suppresses the Mullerian ducts

Which ducts develop in male birds, and under what influence?

Wolffian ducts, under testosterone (as in mammals)

What is the effect of castration on plumage in the 12-wired bird of paradise?

None. Showy male plumage develops from chromosomal sex, not testosterone masculinisation.

Which two male features in chickens and pheasants are testosterone-dependent?

Aggressive behaviour and fighting weapons (spurs)

What does oestrogen do to dimorphic plumage and wattle in chickens and pheasants?

Suppresses them

In a peacock, how does castration affect plumage versus plumage display?

Plumage unaffected; plumage display is affected

Which hormone drives the eclipse (dull) plumage state in seasonally dimorphic birds?

Oestrogen

What commonly happens to old female birds with diseased ovaries?

They develop male plumage

Behavioural dimorphism in zebra finch song?

Males sing; females chatter

What is the robustus archistriatum (RA)?

A song-system brain nucleus crucial for song production; larger in males

What does HVc stand for?

Higher vocal centre

Who first discovered sexual dimorphism of the song system, and in which species?

Fernando Nottebohm and PhD student Art Arnold, in canaries

What did Nottebohm's electrical recordings of neurones in canaries reveal?

Discrete brain areas for song production and for song learning

What happened when song-production areas were lesioned in adult canaries?

Song production was blocked

In newly hatched female zebra finches, what limits HVc neurones from reaching RA?

They project toward RA but do not reach it unless exposed to oestrogen

List three effects of oestrogen on the zebra finch song system.

Promotes axonal growth; promotes development of a large RA; masculinises song circuitry

Does castration of male birds block sexually dimorphic brain development?

No. This is markedly different from mammals.

How is central aromatase expression in birds described?

Very low

Who showed that the male zebra finch brain makes local oestrogen, and what term describes it?

David Clayton (Holloway and Clayton 2001); a local neurosteroid (neuro-oestrogen)

Cultured male zebra finch brain slices after 21 days: what happens to HVc-RA connectivity?

HVc innervates RA

Cultured female zebra finch brain slices after 21 days: what happens to HVc-RA connectivity?

Projection toward RA, but no innervation within RA

Cultured female slices exposed to exogenous oestrogen for 21 days: what happens?

Both projection to and innervation of RA (male-like)

Where does the oestrogen that masculinises the song circuit come from in birds?

Synthesised locally in the brain (neuronal synthesis), independent of gonadal steroids

What is a gynandromorph and why can it not occur in mammals?

An individual with male features on one side and female features on the other; the lecture says it is rare and cannot occur in mammals; two embryos fuse, giving lateralised dimorphism

Which birds were shown as examples of gynandromorphs?

Cardinal and grosbeak

In the gynandromorph zebra finch, which side has the testis and which the ovary?

Testis on right; ovary on left

Is the singing behaviour of the gynandromorph zebra finch male-like or female-like, and why?

Male-like; due to circulating testosterone acting on the whole body

In the gynandromorph zebra finch, which side is the sexually dimorphic HVC larger on?

Right side

What does lateralised brain structure under a common steroid signal indicate?

A non-hormonal primary sex mechanism regulating brain differentiation

Describe the two sides of the gynandromorph chicken (Zhao et al 2010).

Left (ZW) female: brown colour, small wattle, small leg spur; right (ZZ) male: large wattle, large leg spur, greater breast musculature

What did Zhao et al 2010 conclude about somatic sex identity in chickens?

It is cell autonomous: chromosomal dosage has a primary effect on sexual differentiation throughout the body

Compare the influence of gonads on sexual phenotype in mammals versus chickens.

Major influence in mammals; limited in chickens

Why can systemic hormones not alone determine sex in gynandromorph birds?

Hormones would act on both sides of the body; the mechanism is cell autonomous and gene dosage dependent

Are mammalian brains also sexually differentiated by steroid-dependent and steroid-independent (primary genetic) mechanisms?

Yes

How does oestrogen reach the differentiating mammalian brain?

Circulating testicular androgens are converted to local oestrogen in the CNS by aromatase

Which sex chromosome set does the platypus have?

Ten sex chromosomes (X1-X5, Y1-Y5) linked in a meiotic chain

Which chromosome does the platypus X most closely match?

Chicken Z (no homology to eutherian X)

What does the lack of SRY in the platypus indicate?

The SRY switch evolved after the split from monotremes; sex determination is likely dosage dependent

Which egg yolk gene family does the platypus have that eutherians and marsupials lack?

Vitellogenin genes

Platypus casein genes: where are they located and why does it matter?

Clustered next to tooth enamel genes, as in other mammals; reptiles lack these milk protein genes

How do platypus venom-producing pathways compare with other mammals?

They are reptilian

Behaviour of male Japanese quail?

Strut, crow and mount females

In Japanese quail, how is copulatory behaviour circuitry controlled compared with mammals?

Mammals: masculinised by oestrogen from brain testosterone. Galliformes: feminised and demasculinised by oestrogen secreted from the ovary

How does testosterone exposure during development affect adult quail behaviour?

Demasculinises adult behaviour

What is the effect of giving anti-oestrogen to female quail during development?

Induces male behaviour in adults

Which reptile groups have temperature-dependent sex determination in the lecture?

Marine turtles and crocodilians; the relevant genome regions are autosomal

[APPLICATION] A ZW chicken embryo has DMRT1 experimentally overexpressed in its gonads. Predict the gonadal outcome and the reasoning.

Testis formation; DMRT1 dosage is the critical component of the male pathway

[APPLICATION] A male 12-wired bird of paradise is castrated before the breeding season. Predict plumage and explain.

Plumage still develops normally; male plumage depends on chromosomal sex, not testosterone

[APPLICATION] A castrated peacock still has a full train. What is lost?

Plumage display behaviour; display is testosterone dependent although plumage is not

[APPLICATION] Cultured brain slices from a female juvenile zebra finch are grown 21 days with added oestrogen. What does HVc-RA connectivity look like?

Male-like: projection to and innervation of RA

[APPLICATION] Male zebra finch brain slices are cultured with an oestrogen antagonist. Predict the effect on the song circuit and why.

Song circuit development is inhibited; brain-derived oestrogen is necessary for masculinised wiring

[APPLICATION] Gonads are removed from a male zebra finch chick, yet its RA is still large. What does this suggest?

Local brain-synthesised neurosteroids (oestrogen) drive song-system dimorphism independently of the gonads

[APPLICATION] A gynandromorph chicken has a sharp midline boundary between male and female features. Why is a circulating hormone an inadequate explanation?

A circulating hormone would reach both halves; the difference reflects cell-autonomous ZZ versus ZW sex identity

[APPLICATION] An old hen with a diseased ovary grows male-type plumage and a larger wattle. Which hormonal change fits this?

Loss of ovarian oestrogen, which normally suppresses showy plumage and wattle

[APPLICATION] A newly sequenced mammal lacks any SRY gene but has milk and eggs. Which species fits, and what does it imply for sex determination?

A monotreme (platypus); sex determination is likely dosage dependent rather than an SRY switch

[APPLICATION] A bird species becomes dull-coloured after breeding. Which hormone is most likely driving this state?

Oestrogen (the eclipse state is oestrogen driven)

[APPLICATION] A cockerel is given oestrogen. Predict the change in wattle and plumage.

Showy dimorphic plumage and wattle are suppressed

[APPLICATION] Female quail receive an anti-oestrogen during development. What adult behaviour is expected?

Male-type behaviour (strutting, crowing, mounting)

[APPLICATION] Direct sex-chromosome gene effects outweigh hormone effects in which group, mammals or birds, and why?

Birds; most Z genes are only partially dosage compensated, giving large cell-autonomous effects, whereas efficient X inactivation in mammals lets hormone effects dominate

[APPLICATION] An adult canary has a lesion in a song production nucleus. Predict the behavioural outcome.

Song production is blocked

[APPLICATION] A gynandromorph zebra finch has an ovary on the left but sings like a male. Explain.

Circulating testosterone from the right testis acts on the whole body, driving male-like singing

[APPLICATION] A bird embryo is ZW. How many DMRT1 doses does it have and what gonad is expected?

One dose; ovary