Overview of genome evolution

Question 1

What levels can genome evolution occurs at?

Answer 1

Cell genome: Nuclear genomes, mitochondrial genome

Chromosomes: The number of chromosomes, the structure of chromsomes

Question 2

What causes changes in chromosome structure?

Answer 2

1. Chromsome fusion (changes no. of chromosome)
2. Translocation: genetic rearrangement involving the movement of segments between non-homologous chromosomes. (lead to the formation of sex chromosomes, lead to the formation of pseudo- autosomal regions on sex chromosomes)
3. Inversions: The chromosome breaks off and is reattached in a new orientation. (This can change expression of genes and silence genes if inaccessible)
4. Segmental duplication

Question 3

Example of chromosomal evolution: Chain in Platypus

Answer 3

• Platypus have 5 X and 5 Y chromosomes due to translocation between sex chromosomes and autosomes.
• In male meiosis these chromosomes are able to form a chain and segregate successfully

Question 4

Example of chromsome evolution: Formation of sex chromosome

Answer 4

• They evolved multiple times independently

1) One autosome is a pair aqcuires a sex-determining gene creating a difference between the homologous chromsomes so this regions stops recombining.

2) The non- recombining region expands by inversions

3) When genomic (coding regions) become non-combining delerious mutations accumulate and the regions become degenerate (this leads to gene loss on the Y-chromsome)

Question 5

Gene loss examples (deletions)

Answer 5

• Loss of the ability to make Vitamin C (mutation in the L-gulano-lactone oxidase gene)
• The loss of teeth in birds and turtles

Question 6

Gene gain mechanisms

Answer 6

Gene duplication: the redundancy of the function of one of the duplicated minimises purifying selection allowing the accumulation of mutations in one copy and the divergence in function.

Exon shuffling

Novo coding regions from non-coding regions

Mutations

Gene fusion

Retroreposition: loss of introns due to the transition of RNA-> DNA and then insertion into the genome

Question 7

Gene gain example: the origin of the jinwei gene in Drosophilia

Answer 7

• Gene diplication of the ‘yellow emperor’ gene
• retrotransposition of adH into the centre of the gene
• created gene with a new function similair to adh

Question 8

The evolution of new proteins

Answer 8

• New proteins can be generated due to alternative splicing introducing new combinations of exons

Question 9

gene duplication and subfunctionalization example: the evolution of colour vision

Answer 9

• Primates used to be nocturnal and only have black and white vision
• duplication of the long wavelength gene allowed the evolution of the medium wavelength gene
• This allowed the evolution of colour vision
• Duplcuated copied evolved to have different spectral sensitivites

Question 10

What effects genome size?

Answer 10

DNA gain: genome duplication, transposable elements (e.g. accounts for >60% of wheat genome)

DNA loss: deletions (e.g. Drosophila genome has more deletions than Hawaiian cricket Laupala and also has more deletions) - Junk DNA has shorter halfe life in Drosophila

Question 11

Extreme downsizing genome examples

Answer 11

Buchnera (mutualistic intracellular symbiont)

Mitochondria

Question 12

Overview:

Answer 12

The genome can evolve at the level of the gene. This can be in the form of gene function loss and gene function gain (many mechanisms allow this).

The genome also evolves at the level of the chromosome through many mechanisms (e.g. translocation). Key examples include the evolution of the sex chromosome and the degeneracy of the Y chromosome.

The genome size also dramatically varies (larger in eukaryotes) due to:

• Gene loss (deletions, transfer into nucleus for intracellular symbionts)
• Gene gain (genome duplication, transposable elements)