Sex Chromosomes Flashcards
There is only one type of sex chromosome, that found in mammals. True or false?
False; sex chromosomes have evolved convergently multiple times
All organisms with GSD have defined sex chromosomes. True or false?
False; some have autosomes with a small region that contributes to sex, e.g. the M region in Musca (flies)
Both sex chromosomes are always functional. True or false?
False, e.g. the mammalian Y is non-functional
All organisms with sex chromosomes have two. True or false?
False; some only have one, e.g. some insects are X0
What are sexually antagonistic alleles?
Those that are beneficial in one sex but detrimental to the other.
Alleles are exposed to different selection pressures depending on which sex they are in.
How can sexually antagonistic alleles spread to fixation?
They must be linked to sex-determining loci so that they always end up in the sex they are beneficial to
What kind of chromosomal rearrangements suppress recombination and favour linked alleles?
Inversions
In XY systems, which chromosome is riddled with inversions?
The Y
What does reduced recombination in the Y lead to?
An accumulation of male-specific loci on the Y and female-specific loci on the X
So describe the process of sex chromosome differentiation.
- Sexually antagonistic alleles become linked to sex-determining loci to always end up in the correct sex
- Inversions on the Y prevented recombination with the X
- Male-specific genes accumulated on the Y as they are guaranteed to end up in a male
There are two main explanations for divergence of the sex chromosomes. What are they?
- Muller’s Ratchet
2. Hill-Robertson Effect
Explain Muller’s Ratchet in the context of sex chromosome divergence
In proto X populations, there was a stochastic loss of mutation-free chromosomes by drift. Those that remain in the population have deleterious mutations. However when these come together during sex, the mutation-free state can be restored.
In proto Y populations, the Y is unable to recombine during sex due to repeated inversions and therefore cannot escape Muller’s Ratchet.
This has led to divergence/increased mutational load on the Y.
Explain Hill-Roberton Effect in the context of sex chromosome divergence
Selective sweeps occur whereby alleles favoured by selection pull along alleles linked to it, whether they are good or bad.
As a result genes on the Y that are selected for, like male fitness genes, pull along deleterious mutations. Therefore Y shows mutation accumulation, a steady decay of functional genes and low adaptive evolution.
Who looked at mutation accumulation in D. melanogaster? What did they do/find?
Rice, 1994
Chromosome 2 and 3 prevented from recombining.
After 35 generations male fitness was reduced.
Adult male fitness measured by counting the number of males that emerged 2 days post-eclosion
Rice, 1994:
What was the point of preventing chr2 and 3 from recombining?
To assess mutation accumulation on a Y, so chr2 and 3 were made to co-segregate ‘as if they were one large, non-recombining Y chromosome’.
Therefore this experiment provides explanation as to why mutation load might accumulate on a Y
