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Gene evolution 2
03‐327/727 Lecture 4Fall 2013
Molecular Phylogenetics
What are the processes we are trying to reconstruct?
• Species evolution– Morphological characters
– Molecular characters
Gene evolution
Outline
• Gene family evolution• Gene trees • Multigene families
Today
Outline
• Gene family evolutionMechanisms of new gene origination– What happens after duplication?
• Gene trees • Multigene families
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Gene duplicationGene families evolve on a range of scales
• Single genes
• Chromosomal segments (a few genes)
• Partial chromosomes
• Entire chromosomes
• Whole genome duplication
Two other mechanisms of new gene origination that we will discuss later.
• Chimeric genes arise by “domain shuffling”
RL kinaseFnFnFurin like FnRL
Two other mechanisms of new gene origination that we will discuss later.
• Chimeric genes arise by “domain shuffling”
• Horizontal transfer
Gene duplicationGene families evolve on a range of scales
• Single genes
• Chromosomal segments (a few genes)
• Partial chromosomes
• Entire chromosomes
• Whole genome duplication
• Recombination• Unequal crossing over• Retrotransposition• Transposition• Non‐homologous End
Joining• ….
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L.M. Silver, Mouse Genetics http://www.informatics.jax.org/silver/
Tandem duplication and gene loss via unequal crossing over
L.M. Silver, Mouse Genetics http://www.informatics.jax.org/silver/
Non‐homologous end joining (NHEJ)
These gametes have an extra copy of 2d/8d
Gene duplicationGene families evolve on a range of scales
• Single genes
• Chromosomal segments (a few genes)
• Partial chromosomes
• Entire chromosomes
• Whole genome duplication
• Recombination• Unequal crossing over• Retrotransposition• Transposition• Non‐homologous End
Joining• ….
Retrogenes…
• … are intronless genes.• …are frequently pseudogenes unless the new copy can “recruit” a promoter.
Kaessmann, Genome Research, 2010
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Retrotransposons are similar to retroviruses
Long terminal repeats
Retrotransposition
Transposable elements are widespread in the human genome…
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… and in the Arabidopsis genome and in many other genomes.
www.nature.com/nature/journal/v408/n6814/fig_tab/408796a0_F1.html
Transposable elements: Relevance to duplication
1. Duplication by retrotransposition
2. DNA transposons can transport DNA segments from flanking regions
3. DNA transposons can insert promoters in new genomic locations
4. Repetitive DNA from “dead” transposable elements encourages duplication by recombination
EndonucleaseReverse transcriptase
Integrase
Protease
Gene duplicationGene families evolve on a range of scales
• Single genes
• Chromosomal segments (a few genes)
• Partial chromosomes
• Entire chromosomes
• Whole genome duplication
Very rare due to dosage imbalance
Gene duplicationGene families evolve on a range of scales
• Single genes
• Chromosomal segments (a few genes)
• Partial chromosomes
• Entire chromosomes
• Whole genome duplication
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Project goals
Autopolyploidy
Allopolyploidy
Paramecium
http://en.wikipedia.org/wiki/PaleopolyploidyRef: Adams & Wendel, 2005; Cui et. al, 2006; Wolfe, 2001.
Outline
• Gene family evolution– What happens after duplication?Genome rearrangements• Models of functional differentiation
• Gene trees • Multigene families
Genome rearrangements
• Transpositions• Translocations
– balanced– unbalanced
• Chromosome fusions• Inversions
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Translocation
Transposition
Cut and paste transposons move DNA fragments around the genome
Genome rearrangements
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Mouse and Human Genetic Similarities
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Duplicated regions in the human genome
McLysaght et al Nature Genetics, 2002.
Human Chr 3
Chr 17
Duplicated genes Unduplicated genes (not shown)
Human Chr 17
Duplicated regions in Arabidopsis
Outline
• Gene family evolution– What happens after duplication?
• Genome rearrangementsModels of functional differentiation
• Gene trees • Multigene families
Fates of duplicated genes
Non‐functionalization: One copy sustains deleterious mutations, loses its function, and becomes a “pseudogene”
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alpha
beta
beta
alpha
Example of degradation: hemoglobin pseudogenes
Fates of duplicated genes
Non‐functionalization: One copy sustains deleterious mutations, loses its function, and becomes a “pseudogene”
Redundancy: Both copies are retained and continue to perform the same function.
– The most common explanation for redundancy is dosage:
– Multiple copies of a gene are beneficial because the cells demand for the gene product is very high.
– The canonical example: ribosomal genes.
Both copies continue to perform the same function
Typically, both copies are retained provide more ribosomal proteins, supporting rapid, high volume protein synthesis.
“dosage”
Fates of duplicated genes
Non‐functionalization: One copy sustains deleterious mutations, loses its function, and becomes a “pseudogene”
Redundancy: Both copies are retained and continue to perform the same function
Neofunctionalization: Both copies are retained and one (or both) takes on a new function
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alpha
beta
beta
alpha
Examples of neofunctionalization in the globins
• α‐ and β‐globin play different rolls in the globin complex.
• β, γ, and ε have different oxygen binding properties
• β, γ, and ε have different oxygen binding properties
Fates of duplicated genes
Non‐functionalization: One copy sustains deleterious mutations, loses its function, and becomes a “pseudogene”
Redundancy: Both copies are retained and continue to perform the same function
Neofunctionalization: Both copies are retained and one (or both) takes on a new function
Subfunctionalization: Both copies are retained and the functions of the original gene are partitioned between them
coding region
Duplication
Degeneration
Complementation
subfunctionalization
Fates of duplicate genes The subfunctionalization model
Genome Innovation
jawed vertebrates
“A gene duplication occurred in the common ancestor of jawed vertebrates” really means…
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Genome Innovation
jawed vertebrates
A gene duplication occurred in one individual in a population…
Genome Innovation
jawed vertebrates
A gene duplication occurred in one individual in a population…
Genome Innovation
jawed vertebrates
A gene duplication occurred in one individual in a population…
… and survived
Forces governing genome innovation
Survival of a new allele depends on– genetic drift
The expected time to fixation is proportional to the effective population size, Ne
– the rate of degradation to null alleleIf the allele mutates to null before it reaches fixation, the innovation will be lost.
– the probability of becoming essentialHowever, it can be preserved through a beneficial mutation or acquisition of an essential function.
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coding region, G
Duplication
Degeneration
Complementation
subfunctionalization
Fates of duplicate genes The subfunctionalization model
If the function of G in the brain contributes to fitness, then G2 is protected by selection as soon as the expression of G1 in the brain is lost.
G1
G2
G1
G2
Similarly, G2 is protected by selection as soon as the expression of G1 in the thorax is lost.
The loss of a transcription factor binding site requires just a few mutations and can happen more easily than mutation to a new biochemical function.
nonfunctionalization
subfunctionalization
Fates of duplicate genes
redundancy
neofunctionalization
neofunctionalization
The theory predicts that sub‐functionalization is more likely than neofunctionalization immediately following duplication.
Neofunctionalization can also occur following subfunctionalization.
Outline
• Gene family evolution• Gene trees Hemoglobin: a motivating example– Properties of gene trees
• Multigene families
Vertebrate globins arose via duplication of an ancestral globin gene in a vertebrate ancestor
alpha
beta
beta
alpha
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alpha
beta
beta
alpha
Tandem duplication due to the presence of transposon repeats (not shown in figure).
Gene fates• Deletion (pseudogenes)• Subfunctionalization• Neofunctionalization
Globin family tree
alpha
Fish β Wallaby ε Human ε Human γWallaby β Human β
Duplication
Speciation
Gene trees can also show gene family evolution in several species:
Beta globin evolution in vertebrates
Duplication
Time
β ε
ε/γ
ε/β
Fish Wallaby Human
εβ βγ
Beta globin evolution in vertebrates
,
embryonic foetal adult
Globin expression
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Outline
• Gene family evolution• Gene trees • Multigene families Tuesday