New Host – parasite coevolutiongow/biol418/pdf/8a_Host... · 2008. 3. 18. · Optimal virulence:...

Host –

parasite coevolution

Host-parasite relationship:

Host-parasite coevolutionBiol 418: Evolutionary EcologyHost-parasite coevolution

Jennifer Gow

Examples: a broad church

Diverse interactions and consequences between species:


Jennifer Gow

Biol 418: Evolutionary EcologyHost-parasite coevolution

Jennifer Gow

Host: Wood louse, Armadillidium vulgare

Parasite:Wolbachia bacterium

Examples: a broad churchIntracellular symbionts

Host-parasite coevolution

Host: Hypolimnas bolina

Overview

Parasite’s perspective: Optimal virulence

Host’s perspective: Trade-offs in resistance/defence

The host-parasite interaction: Arms races


Jennifer Gow

Overview

Parasite’s perspective: Optimal virulence•Myxoma virus

•Transmission mode

•Direct versus vector borne

•Water borne

•Horizontal versus vertical


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Optimal virulence

Virulence = the damage that a parasite does to its host

Many commensalisms and mutualisms began as host-parasite interactions

Disease outbreaks where virulence reduced

=> host-parasite interactions become commensal or mutualistic?


Jennifer Gow


Optimal virulence:Myxoma virus

Transmission mode

Direct versus vector borne

Water borne

Horizontal versus vertical

Rabbit Oryctolagus cuniculus



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Transmission mode


Water borne


Optimal virulence: myxoma virus


Jennifer Gow


Fenner (1983) Proc. Roy. Soc. B 218: 259-285.

Virulence class (fatality rate)I II III IV V

(>99% <50%)

0

50

100

0

50

100

0

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100

0

50

100

0

50

100

0

50

100

1953

1950

1956

1961

1965

1977

Relat

ive fr

eque

ncy

Australia


Transmission mode


Water borne




Jennifer Gow



Skin titre of virus increases with virulence class

I II III IV V0Mo

squit

o tra

nsmi

ssion

eff

icien

cy


Transmission mode


Water borne


50

100

0

50

100

0

50

100

France1953

1962

1968

0

50

100

0

50

100

0

50

100

0

50

100

Britain1953

1962

1975

1981



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Virulence classI II III IV V(>99% <50%) Virulence class

I II III IV V(>99% <50%)


Transmission mode


Water borne


Optimal virulence: transmission mode


Jennifer Gow


Hypothesis: for internal parasites, probability of transmission to new hosts may be increased if pathogen multiplies rapidly within an infected host even while it causes the host to become very ill.

Therefore, virulence should evolve according to mode and probability of transmission.


Transmission mode


Water borne




Jennifer Gow



Transmission mode


Water borne


Ewald (1993) Sci. Am. 268 (April): 86-93.



Jennifer Gow


Ewald (1993) Sci. Am. 268 (April): 86-93.


Transmission mode


Water borne




Jennifer Gow


Virulence should evolve according to mode and probability of transmission:

Increased opportunities for parasite transmission will permit the evolution of increased virulence.

Restricted opportunities for transmission promote less virulent parasite interactions.Optimal virulence:

Myxoma virus

Transmission mode


Water borne




Jennifer Gow


Vertical transmission:

Horizontal transmission: Optimal virulence:

Myxoma virus

Transmission mode


Water borne




Jennifer Gow


Virulence should evolve according to mode and probability of transmission:

Increased opportunities for parasite transmission will permit the evolution of increased virulence.

Restricted opportunities for transmission promote less virulent parasite interactions.


Transmission mode


Water borne




Jennifer Gow


Species-specific nematodes parasitizing fig-pollinating wasps

Species specific relationships:• Wasps enter fig, lay eggs which grow and mate before dispersal.

• Nematode attach to young wasp which they parasitize before emerging in new fig to mate. Eggs hatch synchronously with wasp eggs.

Opportunities for transmission depend on how many foundresses per fig:

In some species, wasp broods tend to be from single foundresses

…in others, from multiple foundresses

Herre (1993) Science 259: 1442-5.


Transmission mode


Water borne




Jennifer Gow


Herre (1993) Science 259: 1442-5.

Species-specific nematodes parasitizing fig-pollinating wasps

Dec

reas

ing

viru

lenc

e

Opportunity for (horizontal) transmission


Transmission mode


Water borne




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Barley and barley stripe mosaic virus

Stewart et al. (2005) Evolution 59: 730-739.


Transmission mode


Water borne


Virulence = % unviable seeds



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Transmission mode


Water borne




Jennifer Gow





Transmission mode


Water borne


Overview

Parasite’s perspective: Optimal virulence

Myxoma virus

Transmission mode


Water borne



Jennifer Gow

New Host – parasite coevolutiongow/biol418/pdf/8a_Host... · 2008. 3. 18. · Optimal virulence:...

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