Paul E' Turner

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Evolution of mutational robustness in RNA viruses

• Paul E. Turner
• Department of Ecology and Evolutionary Biology
• and Graduate Program in Microbiology
• Yale University

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EXPERIMENTAL EVOLUTION Study, in the
laboratory, of fundamental processes of
evolutionary change Evolution-in-action
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Why use microbes to study experimental evolution?

• Rapid generation times
• Large population sizes
• Inexpensive to culture
• Indefinite freezer storage (fossil record)

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Phage f6

• virus specific to bacteria (bacteriophage)
• family Cystoviridae
• infects plant pathogenic Pseudomonads
• lipid envelope

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Natural biology of Cystoviruses
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Phage f6 genetics

• 13 kilobases
• double-stranded RNA virus (high mutation rate)
• 3 segments per particle
• no recombination (3-locus population genetics)

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VIRAL SEX
Parent viruses Host cell Progeny
Recombination (some DNA and RNA viruses)
Reassortment (ONLY in RNA viruses)
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Is reassortment important?
Webby Webster Science 2003
See also Rotavirus Plant Viruses Emerging
vector-borne viruses of humans
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SEX GOVERNED BY MULTIPLICITY OF INFECTION (MOI)
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Gauging phenotypic evolution Fitness assay
f1vs f2
R0 N1/N2
R1 N1/N2
Fitness (W) R1/R0
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Cheaters can prosper
Grimson Blanton
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Example of virus cheating
Turner ASM News 2003 See also J. J. Hollands work
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Population dynamics of helper VSVs
Morales, Ogbunugafor, Remold Turner, unpublished
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Evolution of conflicting virus strategies
Cooperate
Defect
Suckers payoff
Reward
Cooperate
Temptation to cheat
Punishment
Defect
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Evolution of conflicting virus strategies
(1-c) lt (1-s1) Mixed equilibrium
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Evolution of conflicting virus strategies
Helper
DI
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0.5
Helper
1.5
0
DI
DIs can only exist in mixed polymorphism
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(1-c) gt (1-s1) Prisoners dilemma
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Cheating in phage f6
100s of generations
Evolution of Prisoners Dilemma Turner Chao,
Nature 1999
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Virus Complementation
Nee Maynard Smith, Parasitology 1990 Turner
Chao, Am Nat 2003
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Complementation operates in f6
Froissart et al. Genetics 2004
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Consequence of co-infection is selection for
cheating
Mindich Virus Research 2004
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Molecular changes after 200 generations
Evolved Cooperator
Evolved Defectors
Turner Chao, unpublished
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Mutational Robustness
Phenotypic constancy in face of mutational
change Abundant theory, few experiments
Expected to evolve under high mutation
rates RNA viruses prime candidates But ...
most strongly selected at equilibrium Difficult
(impossible?) to achieve in lab
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Co-infection and Robustness
Co-infection allows complementation Complementati
on buffers mutational effects THEREFORE,
Complementation (hence, co-infection) should
weaken selection for robustness
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DESIGN Does infection mode impact robustness?
f6
Clonal Infection Treatment
Co-infection Treatment
300 generations
10 clones per pop (60 total)
20 bottleneck passages per lineage (mutation
accumulation)
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BOTTLENECKING EXPERIMENT
Virus
20 days of bottlenecking intense drift for
200 gens.
Bacterial lawn
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Analysis For each lineage Mean D logW
logWpost-bottleneck logWpre-bottleneck
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Prediction Var (DlogW)Clonal lineages
lt Var (DlogW)Co-infection lineages
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Prediction Var (DlogW)Clonal lineages
lt Var (DlogW)Co-infection lineages
Mean (DlogW)Clonal lineages lt Mean
(DlogW)Co-infection lineages
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Co-infecting viruses are less robust
Montville et al. 2005 PLOS Biology
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Mechanism? Genetic redundancy may be evolved
robustness mechanism
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mutation
S. Elena (after Wilke Adami Mut Res 2003)
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Conclusions Co-infection is built-in
robustness mechanism High levels of co-infection
weaken selection for robustness Mechanism
unknown, but not due to higher replication
fidelity
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Future Directions Are viruses robust,
regardless of mutagenic source? e.g., UV
irradiation, chemical mutagens Does mutational
robustness beget environmental robustness? Can
we select for de novo robustness in RNA viruses?
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Viability of phage phi-6 under UV exposure
Ogbunugafor Turner, unpublished
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Co-infection yields evolutionary consequences in
lab BUT, does co-infection readily occur in
nature?
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Phage biogeography
OKeefe Turner, in prep
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Silander et al., PNAS USA (in press)
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ACKNOWLEDGMENTS
FUNDING NSF Woodrow Wilson Foundation COLLABORAT
ORS Lin Chao, UC San Diego Remy Froissart,
Montpelier Susi Remold, U Louisville Olin
Silander, U Zurich Olivier Tenaillon, INSERM
Paris Dan Weinreich, Harvard
The Turner Lab Not pictured Brandon Ogbunugafor,
Yul Yang