Flashcards in Origin Of Sex Deck (43)
List 5 advantages of sex
1. Speeds up evolution
2. Generates variation
3. Repairs damaged DNA
4. Prevents Muller's Ratchet and mutational meltdown
5. Breaks up linkage disequilibrium
Sex speeds evolution. Why is there a disadvantage to this?
It is only useful at the population level.
Sex creates genetic variation, which is useful in evolutionary arms races. What is the disadvantage of this?
There must be strong selection pressure for this to occur
Sex repairs damaged DNA. Why isn't this unique?
They are other DNA repair mechanisms
Sex prevents Muller's Ratchet. Why isn't this unique?
Muller's Ratchet can also be avoided by LGT
How do we know there MUST be an advantage to sex?
Extant asexual lineages are young and many have gone extinct, whilst sexual lineages are ancient.
There are very few obvious benefits of sex, which is costly, over LGT. Except...
That LGT is irregular and asymmetric whlst sex is systematic and reciprocal.
Give 5 possible hypotheses for the origin of sex. Why aren't they compelling?
1. Oxygen-induced stress
2. Benefits of cell-cell fusion
3. Benefits of ploidy cycling
4. Driven by parasites in Red Queen dynamics
5. Small population size
Was LECA sexual?
Are there any intermediate forms of LECA?
Evolution from LECA was rapid and explosive. What does this imply?
There was an evolutionary bottleneck that LECA overcame.
LECA radiated AFTER all characteristic eukaryotic traits became established. True or false?
What did endosymbiosis allow the eukaryotic genome to do?
Expand by 4-5 orders of magnitude
Explain the process behind endosymbiosis leading to increased genome size in eukaryotes. Give 4 steps.
Mitochondria became bioenergetically specialised through reductive evolution.
Gene loss allowed for faster replication and the generation of larger quantities of ATP.
ATP is used to build actin filaments, cell grew physically.
Extra ATP allowed for growth of the nuclear genome.
Can LGT also lead to larger genomes?
What is the problem with LGT as a driver for larger genome size?
DNA acquisition is random, deleterious or selfish elements can be uptaken from the environment.
Why are large eukaryotic genomes an advantage?
There is a weak selection pressure to lose genes, so there is more chance the genome will contain genes useful in a changing environment, more chance of being adaptive.
A large eukaryotic genome has less need for LGT. Why? Give 2 reasons.
1. It harbours lots of genes anyway
2. Less effective in larger genomes; more genes in eu means that there is less chance a random gene from LGT will fix a crappy allele
Why are large eukaryotic genomes under weak selection pressure to lose genes?
Because there is no cost to the host for being large as the mitochondria are providing all the energy.
What is splicing?
When introns insert themselves into a functional gene.
What can splicing produce?
Chromosomal abnormalities like inversions, deletions, insertions and breakage.
The host genome suffered intron bombardment from...
Why was the damage of intron bombardment so extensive?
Because the nucleus had not yet evolved
The host chromosome was circular. What happened to it after intron bombardment?
It was broken into straight fragments that would have been non-homologous
Give three reasons why intron invasion would have favoured the evolution of syngamy/meiosis?
1. Intron invasion causes high mutation rate in host genome, broken genes become faulty. Cell would benefit from masking faulty copy with a healthy one (syngamy)
2. Chromosomes may have been broken into different numbers of fragments of different lengths, cell would benefit from doubling the chromosome count to make numbers even (meiosis)
3. Bacterial origin of replication is attached to membrane, chromosome breakage means DNA cannot segregate for division. Cell would benefit from co-opting plasmid machinery.
Cell-cell fusion may have initially been accidental. Why would it have been selected for?
Because syngamy masks faulty alleles that have been broken in intron invasion.
Why is the doubling of the chromosome count necessary?
So all fragments can pair up and continue in the cell cycle. This would form haploid gametes (meiosis) that must fuse to restore diploidy, leading to sex.
Were polyploidisation events important to the evolution of the eukaryote? Why, why not?
Yes, would serve as a way of doubling chromosome count
What evidence do we have for polyploidisation? Give 2 examples.
1. Drastic size increase of the eukaryotic genome
2. Existence of gene families that results from PP