Download Lecture#31 – Evolution and cis

6/28/17
BIOLOGY 207 - Dr.Locke
Lecture#31 – Evolution and cis-regulatory sequences
Required readings and problems:
Reading: Open Genetics, Chapter 12.4
Problems: Chapter 12.8
Optional
Griffiths (2008) 9th Ed. Readings: pp 679-714 especially 693-699
Campbell (2008) 8th Ed. Readings: Concept 25.5: pp. 527-528
Assigned Problems: None
Additional READING:
Carroll et al. Regulating Evolution. Sci.Am. May, 2008. pp. 60-67.
Concepts:
How can gene regulation evolve?
1. DNA mutations in intergenic, coding sequences, and regulatory regions have
different consequences for evolution
2. Mutations in regulatory sequences can cause loss or gain of enhancers
3. Mutations changing regulatory sequences circumvent the problem of pleiotropic
effects of mutations altering coding sequences.
Biol207 Dr. Locke’s section
Lecture#31 -evolution
Fall'11
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6/28/17
Genetic Variation (random mutation to DNA sequences)
Variation + Selection -> Evolution !
Mutation in:
1) Intergenic regions
a. no affect on gene expression/phenotype -> no selection for/against
b. random drift causes fixation of DNA sequence
c. useful for markers in genetic mapping /DNA finger printing
Result: Evolution occurs via random mutation and fixation by random
drift – no selection
2) Gene’s coding sequences
a. changes gene product (RNA or protein) - > alters function-> affects phenotype
b. doesn’t change gene’s transcription
c. natural selection for/against function of product
Result: Evolution occurs via random mutation and selection for/against
the function of the gene’s product
3) Gene’s regulatory region/sequences
a. same product from the gene, just its pattern of transcription changed.
b. altered time, tissue, level of expression.
c. can affect many traits/characteristics at once -> pleiotropic.
d. can create new/novel patterns of expression; gain in function – neomorph.
Result: Evolution occurs via random mutation and selection for/against
the novel expression pattern
Biol207 Dr. Locke’s section
Lecture#31 -evolution
Fall'11
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6/28/17
Examples of evolution of gene regulation.
Changes DNA sequence -> changes in physical traits
Research has focused on genes for last ~40 years –> amino acid coding sequences
Human – Drosophila comparison
Drosophila ~14K genes -> human ~35K genes
~2x change in total number, but humans are much more complex
Human – Chimp comparison
-> 99% same genes.
-> 29% of the proteins are exactly the same.
Mouse - Whale comparison
They use essential the same set of proteins to build a body
–> just instructions are different (Hox genes - page 421-426 in text)
Vertebrate on average ~20K genes
The same set of genes has been relatively stable for ~100M years
The real change is in the regulation of those genes -> altered expression
Analogy: same bricks and cement to build a doghouse and a cathedral
It’s the instructions that make the difference – this is what is evolving.
Biol207 Dr. Locke’s section
Lecture#31 -evolution
Fall'11
page 3
6/28/17
Regulatory sequences are a key to understanding evolution
Regulatory sequences
- need to be identified experimentally
- act combinatorial – multiple independent sites
-> full extent of each genes expression.
Example yellow gene in Drosophila
Loss of tissue specific enhancer
- selective –
tissue specific –
retain function in
other tissues
Biol207 Dr. Locke’s section
Lecture#31 -evolution
Fall'11
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6/28/17
Gain of enhancer
– new/novel function
– neomorphic
Gain or loss has selective advantages (facilitate survival)
- gain may –> add spot on wing to help camouflage, or strengthen wing
- loss may –> help - eg. loss of hind limb in vertebrate snakes, whales.
Biol207 Dr. Locke’s section
Lecture#31 -evolution
Fall'11
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6/28/17
Three-spine stickleback – pelvic fins.
(evolutionary fore runner of hind limbs)
Pelvic fins occur in two forms:
Deep, open water – full spiny pelvis – protect from being swallowed by large predators
Shallow water - reduced pelvis and shrunken spines – large spines grasped by
dragon fly larvae (predator)
Biol207 Dr. Locke’s section
Lecture#31 -evolution
Fall'11
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6/28/17
Evolution
Observation:
1- Same phenotypic differences have evolved repeatedly in different fish populations
over last 10K years (since last Ice Age)
2- Different fish populations are genetically close and can be inter-bred in the lab.
3- Use them to genetically map the gene(s) involved in stickleback pelvis size.
4- Found Pitx1 gene -> it has multiple functions in fish development
5- Found expression is selectively lost in tissues that give rise to the pelvic fin and
spine.
6- Found change in an enhancer -> no change in Pitx1 gene amino acid sequence
Biol207 Dr. Locke’s section
Lecture#31 -evolution
Fall'11
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6/28/17
Concept of Pleiotropy
Pleiotropy - one gene has influence over multiple traits
- the phenomenon of a single gene being responsible for a number of distinct and
seemingly unrelated phenotypic effects.
- Consequence: mutations in the gene’s protein coding sequence will have a
simultaneous affect on multiple traits -> drastic, severe (dead) -> selected against
Concept:
- Mutations in regulatory sequences circumvent the severe, pleiotropic effects of
coding sequence mutations.
- If gene product is required at one time and place already
-> can’t mutate structural gene sequence, but can modify expression via
regulation mutation.
- Common to have selective modifications of individual body parts via mutations in
regulatory sequences.
See also:
Prud’homme et al. 2007. Emerging principles of regulatory evolution. PNAS 104; 8605-8612.
Carroll, S.B. 2005. Evolution at two levels: genes and form. PloS Biology 3: 1159-1166.
Wray, G.A. 2007. The evolutionary significance of cis-regulatory mutations. Nature Reviews
Genetics 8: 206-216.
Biol207 Dr. Locke’s section
Lecture#31 -evolution
Fall'11
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