Origin Of Sex

Question 1

List 5 advantages of sex

Answer 1

1. Speeds up evolution
2. Generates variation
3. Repairs damaged DNA
4. Prevents Muller’s Ratchet and mutational meltdown
5. Breaks up linkage disequilibrium

Question 2

Sex speeds evolution. Why is there a disadvantage to this?

Answer 2

It is only useful at the population level.

Question 3

Sex creates genetic variation, which is useful in evolutionary arms races. What is the disadvantage of this?

Answer 3

There must be strong selection pressure for this to occur

Question 4

Sex repairs damaged DNA. Why isn’t this unique?

Answer 4

They are other DNA repair mechanisms

Question 5

Sex prevents Muller’s Ratchet. Why isn’t this unique?

Answer 5

Muller’s Ratchet can also be avoided by LGT

Question 6

How do we know there MUST be an advantage to sex?

Answer 6

Extant asexual lineages are young and many have gone extinct, whilst sexual lineages are ancient.

Question 7

There are very few obvious benefits of sex, which is costly, over LGT. Except…

Answer 7

That LGT is irregular and asymmetric whlst sex is systematic and reciprocal.

Question 8

Give 5 possible hypotheses for the origin of sex. Why aren’t they compelling?

Answer 8

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

Question 9

Was LECA sexual?

Answer 9

Yes

Question 10

Are there any intermediate forms of LECA?

Answer 10

No

Question 11

Evolution from LECA was rapid and explosive. What does this imply?

Answer 11

There was an evolutionary bottleneck that LECA overcame.

Question 12

LECA radiated AFTER all characteristic eukaryotic traits became established. True or false?

Answer 12

True

Question 13

What did endosymbiosis allow the eukaryotic genome to do?

Answer 13

Expand by 4-5 orders of magnitude

Question 14

Explain the process behind endosymbiosis leading to increased genome size in eukaryotes. Give 4 steps.

Answer 14

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.

Question 15

Can LGT also lead to larger genomes?

Answer 15

Yes

Question 16

What is the problem with LGT as a driver for larger genome size?

Answer 16

DNA acquisition is random, deleterious or selfish elements can be uptaken from the environment.

Question 17

Why are large eukaryotic genomes an advantage?

Answer 17

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.

Question 18

A large eukaryotic genome has less need for LGT. Why? Give 2 reasons.

Answer 18

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

Question 19

Why are large eukaryotic genomes under weak selection pressure to lose genes?

Answer 19

Because there is no cost to the host for being large as the mitochondria are providing all the energy.

Question 20

What is splicing?

Answer 20

When introns insert themselves into a functional gene.

Question 21

What can splicing produce?

Answer 21

Chromosomal abnormalities like inversions, deletions, insertions and breakage.

Question 22

The host genome suffered intron bombardment from…

Answer 22

The endosymbiont

Question 23

Why was the damage of intron bombardment so extensive?

Answer 23

Because the nucleus had not yet evolved

Question 24

The host chromosome was circular. What happened to it after intron bombardment?

Answer 24

It was broken into straight fragments that would have been non-homologous

Question 25

Give three reasons why intron invasion would have favoured the evolution of syngamy/meiosis?

Answer 25

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.

Question 26

Cell-cell fusion may have initially been accidental. Why would it have been selected for?

Answer 26

Because syngamy masks faulty alleles that have been broken in intron invasion.

Question 27

Why is the doubling of the chromosome count necessary?

Answer 27

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.

Question 28

Were polyploidisation events important to the evolution of the eukaryote? Why, why not?

Answer 28

Yes, would serve as a way of doubling chromosome count

Question 29

What evidence do we have for polyploidisation? Give 2 examples.

Answer 29

1. Drastic size increase of the eukaryotic genome

2. Existence of gene families that results from PP

Question 30

Name another process that may also double the chromosome count.

Answer 30

Allopolyploidisation, where complete sets of chromosomes are derived from different species (hybridisation).

Question 31

Why can’t polyploidisation occur in every generation?

Answer 31

It can, but it would cause an exponential increase in gene content and cell would become unviable.

Question 32

What evidence is there for co-option of the plasmid machinery for DNA segregation?

Answer 32

Bacterial DNA uses actin to segregate.

Eukaryotic uses tubulin and plasmid DNA uses TubZ tubules = homologues.

Question 33

Who proposed a model for the evolution of meiosis and when?

Answer 33

Lane, 2011

Question 34

Lane, 2011:

Give 8 steps for the origin of meiosis.

Answer 34

1. Eukaryotes escaped bacterial size constraints
2. Larger genome has higher mutation rate, would benefit from syngamy (masking of faulty genes)
3. Zygote may have uneven number of chromosomes due to abnormalities caused by intron bombardment
4. One step meiosis fails as only involves a division, cells with unequal chromosomes die
5. Two-step meiosis works as division accompanied by doubling of chromosome content
6. Two-step meiosis stabilises after evolution of the nucleus as protects from intron bombardment.
7. Mutation rate falls, meaning cells that fuse more likely to have equal chromosomes
8. LECA evolves, accumulates eu traits

Question 35

Why is the origin of sex and the origin of species the same thing?

Answer 35

Because syngamy is maximised when two equivalent cells, i.e. those with same no. of chromosomes, come together. Species distinguished by numbers of chromosomes.

Question 36

Give a brief description of this view of the origin of sex.

Answer 36

Endosymbiosis lifted the genome constraints on proto-eukaryotes, allowing for larger size. This, coupled to intron invasion from the endosymbiont, allowed for the evolution of meiotic sex as a way of fixing chromosome breakage.

Question 37

Give 5 steps of the intron invasion hypothesis (to remember if you’re writing about it in the exam).

Answer 37

1. Endosymbiosis = energetic specialisation/reductive evolution of mitochondria = faster rep. = more ATP = host cell can grow
2. Intron invasion from mitochondria breaks host chromosomes
3. Syngamy masks faulty broken alleles
4. Meiosis required to double chromosome content as chromosomes in cell in a population been fragmented differently
5. Gradually eukaryotic traits accumulate (like the nucleus), leading to evolution of LECA.

Question 38

Many asexual lineages have died out, and current ones are young. Giardia was thought to be asexual. Who found evidence otherwise? What does it show?

Answer 38

Ramesh et al., 2005

Giardia shows evidence of meiotic sex genes, although may not do it anymore.
Shows sex is important for longevity.

Question 39

Many asexual lineages have died out, and current ones are young. Amoebae are considered to be asexual. Who studied this and what did they find?

Answer 39

Lahr et al., 2011

Revised the phylogenetics of amoebae lineages, found some species to be secondarily asexual

Question 40

Who described that endosymbiosis allowed for larger eukaryotic size? How? How big was the expansion in the number of genes expressed?

Answer 40

Lane and Martin, 2010

Prokaryotes restricted in size by bioenergetics
Endosymbiosis meant nuclear genome was supported by the energy powerhouse of the mitochondria
This genomic asymmetry allowed for major growth
200,000 fold expansion in the number of expressed genes

Question 41

The large size of the eukaryotic genome PLUS intron invasion caused a higher mutation rate, not just intron invasion alone. True or false?

Answer 41

True

Question 42

Who looked at the interaction between introns and LGT?

Answer 42

Owen, Lane and Pomiankowski, unpublished

Question 43

Owen, Lane and Pomiankowski, unpublished:

Explain why introns favour the evolution of meiotic sex by referring to LGT

Answer 43

In homologous recombination, when fragments are uptaken by LGT, pairing occurs at the end of fragments.

Introns insert at the end of fragments.

Fragments then lose specificity, leading to ectopic recombination.

High intron copy number increases the likelihood of ectopic recombination.

To prevent this longer fragments can be uptaken as there may be homology in the middle region.

Fragments get longer and longer to avoid introns until you end up pairing whole chromosomes as in meiosis