Lecture 5 Lessons from Nature, sexual dimorphism, brain and behavioural sex Flashcards
birds chromosomes?
males ZZ homogametic
female ZW heterogametic
no homology to X human chromosome.
how do humans avoid getting XXX XXY?
one X chromosome suppressed in females - X inactivation.
can’t be transcribed, wrapped tightly around chromatin.
birds dont do this.
what is the ancestral role of testosterone?
Testosterone is crucial for normal maturation of male germ cells. Blockade of testosterone leads to loss of mature germ cells.
shuts down GnRH in hypothalamus
how are male specific behaviours in the brain activated?
directly via testosterone,
indirectly via local conversion to oestrogen.
what are the genetic mechanisms for sex determination in birds?
unknown silly
Current candidates are on short arm of W.
This region is involved in female determination.
In birds – 2 doses of DMRT1
encode the male genotype.
Female-determining genes not yet identified
number of testis/ovaries in birds?
2x testis
1 ovary left only
weight saving device in birds?
shrivel testis to pea in winter, grow in breeding season
how does castration effect plumage?
no effect, it is a primary genetic mechanism.
Secondary sexual characteristic of plumage develops as a consequence of chromosomal sex.
Thus, showy male plumage is
not the result of masculinization by testosterone.
what are testosterone dependent mechanisms in birds?
weapons for fighting (spurs)
aggressive behaviour.
waddle.
study by Fernando Nottebohm on canaries
certain areas strongly dimorphic.
areas for song production - HVC, RA
song learning - DLM, LMAN, X
Zebrafinch, RA larger males, song production, RA shrinks during non breeding season.
HVC?
higher vocal centre
RA?
robustus Archistriatum
Zebrafish dimorphic song system?
In newly hatched Zebrafinches, neurones
project to RA, but do not actually reach their target unless exposed to oestrogen.
Oestrogen promotes axonal growth and development of a large RA region and
masculinises song nuclei.
Exposure to neo-natal sex steroids are crucial, but these may originate from
within the brain.
Growth of the RA region may
depend on locally produced
“neurosteroids”, as removal
of gonads does not block dimporphism.
need dimorphic structures for singing to occur and to show male specific behaviour, which is driven by testosterone. but this bit oestrogen??????????
Neurones from HVc are held in a “holding pattern” over the RA in females. In males, they plunge into the RA.
David Clayton discovered that the male brain makes a local neurosteroid,
oestrogen.
evidence that male brain makes a local neurosteroid,
oestrogen.
Holloway and Clayton 2001
HVc and RA cultured:
male show projections into bod of RA
females don’t
if females + oestrogen show projections.
