Lecture 34

Question 1

C-value (amount of DNA) paradox:

Answer 1

• The amount of DNA does not correlate with perceived complexity of position of the phylogenetic tree
• Complexity = number of ell types, metabolic complexity, behavioural complexity

Question 2

What explained genome size variation?

Answer 2

• Gene duplication
• Satellites (often heterochromatic
• Relative rate of insertions and deletions
• Transposable elements
• Polyploidy (whole genome duplication)

Question 3

Euploid:

Answer 3

• Having a complete complement of chromosomes

Question 4

Aneuploid:

Answer 4

• Having an incomplete complement of chromosomes

Question 5

Autopolyploidy:

Answer 5

• Whole genome is duplicated within a species
• (self duplication)
• Banana’s, peanut, potato

Question 6

Allopolyploidy:

Answer 6

• Genome duplication deriving from hybridisation between two parental species
• Eg) wheat, cotton, apples, sugar cane

Question 7

The mechanics:

Answer 7

• Somatic doubling
• Polyspermy
• Gemetic non-reduction

Question 8

Somatic doubling:

Answer 8

Mitotic double then failure of cell division

Question 9

Polyspermy:

Answer 9

Multiple sperm fertilise the same egg

Question 10

Gemetic non-reduction

Answer 10

• Failur of cell division during meiosis (2n gametes)
• Doesn’t happen much in animals, but does in plants
• Can occur from sterile hybrids

Question 11

How common is polyploidy?

Answer 11

• Very common in plants! (70% of angiosperms)
• 15% of speciation events in angiosperms
• Fairy common in fish (asexual reproduction), amphibians and insects
• Rare in mammals

Question 12

What varieties and polyploidy:

Answer 12

• AA (einkorn) crossed with BB (goat grass)
• AABB (durum wheat, used for pasta) crossed with DD (goat grass)
• Bread wheat today is AABBDD
• Happened about half a million years ago

Question 13

Red viscacha rat:

Answer 13

• 4N! it has double the amount of DNA of related species
• Genomic in situ hybridisation (label with fluorescent DNA and wash away, so only the fluorescent label is left where there is a good match)
• Showed that allopolyploidy is likely

Question 14

What evidence is there for ancient polyploidy events?

Answer 14

• Evolutionary jumps in C-value
• Chromosome pairing patterns
• Gene number
• Gene arrangement
• Gene tree topology
• Age of gene duplication events (molecular clock)

Question 15

Is yeast a peleo-polyploid?

Answer 15

• No evidence that duplicated gene blocks are duplicated again
• All blocks have the same centromere-telomere orientation (some should show an inversion if it were multiple-sub)
• Synonymous site divergence is about the same of all pairs of blocks (suggests duplication at the same time)

Question 16

Vertebrate ‘2R’ hypothesis:

Answer 16

• 2 rounds of poly-ploidy in the vertebrate lineage
• Based on evidence from C-value (amount of nuclear DNA), isozymes, and two pairs of duplicated genes on chromosome 11 and 12 delineating duplicated chromosomes
• This was confirmed

Question 17

In order to determine if duplication events were close together using the molecular clock method we need:

Answer 17

• Enough species to cover before/after putative duplication events
• Enough genes to five statistical power
• Whole genomes are ideal

Question 18

‘2R’ in the seed plants:

Answer 18

• Species tree for plants is well known, a gene tree is also well known
• There have been two whole genome duplication events, this was proved by comparing eudicots, monodictos and plotting them on a species tree

Question 19

Possible selective advantages (some are also disadvantages) for polyploidy:

Answer 19

• Increased DNA content
• Increased heterozygosity (able to retain sufficient genet diversity even with small census size, potential for subfunctionalization and differential expression, higher mutation rate)

Question 20

Possible costs of polyploidy:

Answer 20

• Chromosomal pairing errors and sex determination
• Developmental: orchestration of gene expression called a ‘genomic shock’, and timing of DNA replication, dosage compensation
• Physiological: higher DNA content requires more phosphorus
• Population: initially rare, loss due to drift. Competition with locally adapted progenitor species

Question 21

How do benefits and costs compare evolutionarily?

Answer 21

• Recently formed polyploids have a higher extinction rate, lower speciation rates and lower diversification rates
• But polyploidy seems to be able to drive longer timer evolutionary success