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Evolution of Cooperation
Definition: Acts by one organism that benefit another(Not necessarily mutual)
Cooperation at no cost
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Cooperation without CostCross feeding – unidirectional– also called “Syntrophy”
–one organism lives off by-productsof another organism
– can lead to mutualism (bidirectional)
Important reducer of greenhouse gas emissions (90% of marine methane from marine sediments oxidized by AMO)Unable to culture in lab – could be obligate syntrophyUnclear which intermediates are exchanged
Anaerobic methane oxidation (AOM)
Sulfate Reducing Bacteria
Methanotropic Archaea
methane is oxidized with sulfate as the terminal electron acceptor:
CH4 + SO42- → HCO3- + HS- + H2O
SulfideOxidizing Bacteria
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The Importance of Cooperation
It is often more difficult to understand how cooperation evolves because it comes with a cost
“Cooperation among individuals is necessary for evolutionary transitions to higher levels of biological organization. In such transitions, groups of individuals at one level (such as single cells) cooperate to form selective units at a higher
level (such as multicellular organisms).”
(Velicer & Yu, 2003)
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Symbiosis
Mutualism Parasitism
Altruism**(*reciprocal mutualism)
Spite**
Any interaction of organisms living together
Actor
Recipient
Cos
t
Cost
Ben
efit
Benefit
(Barash, 1982; Lee, Molla, Cantor, Collins, 2010)
*Sometimes mutualism with a time delay is mistaken for altruism (you scratch my back, I’ll scratch yours later)** Not observed outside of humans?
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What happens when cooperation is disrupted?
Study cases where “cheaters” emerge
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Costly Cooperation: Wrinkley Spreaders
Cells cooperate by each producing a costly polysaccharide to form a biofilmBenefit: Increased oxygen exposure
Benefit outweighs the cost
Cheaters can emerge that stick to the biofilm, but contribute no polysaccharide!
Benfit without suffering the cost – cheaters are more fit!
Problem: Too many cheaters dilutes the polysaccharideand biofilm sinks.
Cheaters are a common problem…
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Costly Cooperation: Spore Forming Behavior
(Kuner & Kaiser, 1982)
Myxococcus xanthus: a social bacteria
When hungry, M. Xanthus cells aggregate into fruiting bodiesOnly a small fraction of bacteria become spores, the rest are structural
“Cheaters” emerge that do not contribute to fruiting body formation, but produce disproportionately large amounts of spores
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Experimental Design: Compete cheaters vs. cooperators(cheaters cannot form fruiting bodies unless mixed with cooperators)
-Create mixed cheater/cooperator populations – 3 different cheaters-Force populations to sporulate-Collect spores-Grow spores to form new population-Repeat 5-6 times
(Fiegna & Velicer, 2003)
Does cooperation ever work despite cheaters?
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(Fiegna & Velicer, 2003)
Cheaters can coexist with cooperators
What was different?
-This cheater had faster growth, but inferior sporulation
-It never rose to high enough frequency to prevent entire population from forming a fruiting body
-It could coexist with cooperators without causing population collapse
Dashed = cheaterControl, both lines are cooperators
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A “chicken game”
(Fiegna & Velicer, 2003)
Cheaters can cause population disruption
When cheaters rise in frequency, population sporulation efficiency suffersCheaters suffer more, and decrease disproportionately more than cooperators
Cooperators restore sporulation efficiency It is safe for cheaters to rise again
Dashed = cheater
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(Fiegna & Velicer, 2003)
Cheaters can cause population collapse
Cheater rises to high frequency
Sporulation produces few to zero spores
Complete extinction or cheater self extinction
Dashed = cheater
Solid = cooperator
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(Velicer & Yu, 2003)
Cheaters learn to cooperate
Experimental Design:-Plate cheaters deficient in pili production-Scrape section from edge of cheaters-Re-plate-Repeat 32 times
Results:-All 8 cheaters became more mobile-Two were even better than WT-Used fibrils to swarm across plates-Fibril production is also costly
WT cooperator
Cheater
CheaterCooperator evolved from cheater
Cooperator evolved from cheater
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When is cooperation stable?
Without considering defectors: Benefit (B) – Cost (C) > 0
With defectors (d) and cooperaters (c): Bc – Cc > Bd
What conditions could lead to cooperation?(maximize the benefits to cooperators while minimizing benefits to defectors?)
Spatial distributionKin selection
Group Selection – Chuang et. al. 2009
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Simpson’s Paradox
Band Aid Removal Open Heart Surgery Total Overall
36/90
(40%)
7/10
(70%)43/100
3/10
(30%)
54/90
(60%)57/100
Dr. Nick Dr. Nick Dr. HibbertWinner per Category
Dr. Hibbert
Dr. Nick
The winner in all sub sections may be the loser overall
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Haystack Model
Maynard SmithOrganisms live in separate haystacks
Once in a while, all leave their haystack at the same moment to mateThey then divide into equal groups and go back to a random haystack
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Haystack Model
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Cheaters emerge and do disproportionately well within each haystack(selection within haystacks leads to increase of cheaters)
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Haystack Model
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Between matings, haystacks accumulate differencesSome cheaters and cooperators are better than others
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Haystack Model
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Too many cheaters causes population decline
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Haystack Model
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When populations emerge and mate, the haystack with the most organisms “wins” by having the most individuals in each haystack
Between group selection favors cooperators Between individual selection favors cheaters
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The Prisoner’s Dilemma
Prisoner Two: your accomplice
Although cooperation leads to the best outcome for everyone, natural selection usually maximizes an individual’s benefits – not the group’s.
(Turner and Chao, 1999)
Give up your accomplice/accomplice keeps quiet: No prison timeKeep quiet while your accomplice gives you up: Twenty years in jailBoth talk: Ten years eachBoth quiet: 5 years each
Prisoner One:You
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Phage do not choose to cooperate
Phage
Ancestor = Φ6, a cooperatorproduces beneficial resource
Derived = ΦH2, a cheaterproduces less, sequesters more
Strategy: everyone should defect
(Turner and Chao, 1999)
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Escape from the Prisoner’s Dilemma
However, sometimes phage can “Escape the prisoner’s dilemma”(under clonal selection)
(Turner and Chao, 2003)
In a low density population, everyone is related
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(Sachs & Bull, 2005)
Conflict Mediation
Organisms go through cycles where either cooperation or selfishness is favored
Experimental Design:Co-infectionObligate paired vertical transmissionProduction of independent bacteriaphagex50
Compete to be better infector
But dual infection required for reproduction
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(Sachs & Bull, 2005)
Conflict Resolution
Both phage packaged together to ensure double infection
One genome shrunk to three genes