Evolution of Cooperation

Definition: Acts by one organism that benefit another(Not necessarily mutual)

Cooperation at no cost

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

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)

Symbiosis

Mutualism Parasitism

Altruism**(*reciprocal mutualism)

Spite**

Any interaction of organisms living together

Actor

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(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?

What happens when cooperation is disrupted?

Study cases where “cheaters” emerge

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…

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

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?

(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

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

(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

(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

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

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

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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Between matings, haystacks accumulate differencesSome cheaters and cooperators are better than others

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Too many cheaters causes population decline

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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

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)

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

(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

(Sachs & Bull, 2005)

Conflict Resolution

Both phage packaged together to ensure double infection

One genome shrunk to three genes