Mutualism
Chapter 15
1
Introduction
• Mutualism: Interactions between individuals of
different species that benefit both partners.
– Facultative Mutualism occurs when a species can live
without its mutualistic partner.
– Obligate Mutualism occurs when a species is
dependent on a mutualistic relationship.
• Margulis and Fester amassed evidence eukaryotes
originated as mutualistic associations.
2
Plant Performance and Mycorrhizal
Fungi
• Two most common types of mycorrhizae:
1) Arbuscular mycorrhizal fungi (AMF)
Produces arbuscules - site of exchange between plants and
fungi, hyphae - fungal filaments, and vesicles - energy
storage organs.
3
Plant Performance and Mycorrhizal
Fungi
• Two most common types of mycorrhizae:
2) Ectomycorrhizae (ECM)
• Forms mantle around roots - important in increasing plant
access to phosphorus and other immobile nutrients.
4
Mycorrhizae and Plant Water Balance
• Allen and Allen studied water relations of
grass Agropyron smithii.
– Plants with mycorrhizae maintained higher leaf
water potentials.
5
Mycorrhizae and Plant Water Balance
• Hardie suggested mycorrhizal fungi improve
water relations by providing more extensive
contact with moisture in rooting zone and
providing extra area for water absorption.
6
Nutrient Availability and Mutualistic
Balance Sheet
• Johnson investigated whether fertilization can
select for less mutualistic mycorrhizal fungi.
– Fungal partner received an equal or greater quantity
of photosynthetic product in trade for low quantity
of nutrients.
• In nutrient poor environments, many plants
invest disproportionately in roots.
7
Nutrient Availability
• Results suggested mycorrhizal fungi from
unfertilized soils supplied plants with more
nutrients.
– Plants able to invest more energy in above-ground
photosynthetic material… principle of ?
8
 The Private Life of Plants - ?, Bull’s horn acacia clip
9
Ants and Bull’s horn Acacia
• Herbivores attempting to forage on accacia
plants occupied by accacia ants are met by a
large number of fast, agile, highly-aggressive
defenders.
• Ant Benefits:
– Thorns provide living space.
– Folliar nectaries provide sugar.
– Beltian bodies are a source of oils and protein.
10
Ants and Bull’s horn Acacia
11
Experimental Evidence For Mutualism
• Janzen demonstrated ants significantly
improve plant performance.
– Acacias without ants had more herbivorous
insects.
– Suckers growing from accacia stumps occupied by
ants lengthened at seven times rate of suckers
without ants.
• Suckers with ants survived at twice the rate of suckers
without ants.
12
 Planet Earth - Shallow Seas, corals clip
13
Coral Mutualisms
• Zooxanthallae and Corals
– Zooxzanthallae live within coral tissues.
• Receive nutrient from coral. In return, coral receives
organic compounds synthesized by zooxanthallae
during photosynthesis.
– Corals induce release of organic compounds with “signal
compounds” that alter permeability of zooxanthallae cell
membrane.
14
Coral Mutualisms
• Corals also control rate of zooxanthallae
population growth and density by influencing
organic matter secretion.
• Main zooxanthallae benefit appears to be
access to higher nutrient levels, especially N
and P.
– Uptakes ammonium excreted by coral.
15
Coral Protection Mutualism
• Glynn found 13 coral species protected by
crustacean mutualists.
– Crustacean mutualists substantially improved
chances coral will avoid attack by sea stars.
16
Coral Protection Mutualism
• Also found crab activity promotes coral health
and integrity.
– Pocillopora coral increases production of fat
bodies in the presence of crabs.
• Digestive tract of crabs inhabiting corals contained large
quantities of lipids.
17
Evolution of Mutualism
• Theory predicts mutualism will evolve where
the benefits of mutualism exceed the costs.
– Keeler developed models to represent relative
costs and benefits of several types of mutualistic
interactions.
• Successful mutualists
– Give and receive benefits.
• Unsuccessful mutualists
– Give, but do not receive benefit.
18
Evolution of Mutualism
• Non-mutualists
– Neither give nor receive benefit.
• For a population to be mutualistic, fitness of
successful mutualists must be greater than
unsuccessful or non-mutualists.
– If not, natural selection will eventually eliminate
the interaction.
19
Facultative Ant-Plant Mutualisms
• Keeler proposed for a facultative ant-plant
mutualism to evolve and persist…
• proportion of plant’s energy budget ants save
from destruction by herbivores…
• must exceed proportion of the plant’s energy
budget invested in extrafloral nectaries and
nectar.
20
Facultative Ant-Plant Mutualisms
• Conditions that may produce higher benefits
than costs:
– Low proportion of plant’s energy budget invested
in extrafloral nectaries.
– High probability of attracting ants.
– Low effectiveness of alternate defenses.
– Highly effective ant defense.
21
Facultative Ant-Plant Mutualisms
p [H ( 1 – D ) A ] > IA
Probability
of attracting
mutualist
Rate of
herbivory
Effectiveness
of plant
defenses
Effectiveness
of ant
defenses
Plant’s
energy
budget
devoted to
“bribes”
22
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