Mutualism

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Mutualism

• Chapter 12

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Outline

• Introduction
• Plant Mutualisms
• Mycorrhizae
• Ants
• Coral Mutualisms
• Evolution of Mutualisms

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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.

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Plant Performance and Mycorrhizal Fungi

• Two most common types of mycorrhizae
• Arbuscular mycorrhizal fungi (AMF)
• Produces arbuscules - site of exchange between
  plants and fungi, hyphae - fungal filaments, and
  vesicles - energy storage organs.
• Ectomycorrhizae (ECM)
• Forms mantle around roots - important in
  increasing plant access to phosphorus and other
  immobile nutrients.

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Mycorrhizae and Plant Water Balance

• Allen and Allen studied water relations of grass
  Agropyron smithii.
• Plants with mycorrhizae maintained higher leaf
  water potentials.
• Plants with greater access to phosphorus may
  develop roots that are more efficient at
  extracting and conducting water.

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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.

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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.
• Found higher root investment in low N soils.

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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.
• Found Andropogon produced inflorescens only in
  nitrogen-supplemented treatments.

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Ants and Bullshorn 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.

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Ants and Bullshorn Acacia
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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.

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Temperate Plant Protection Mutualism

• Ants are attracted to Aspen sunflowers because
  they produce nectar at extrafloral nectaries.
• Rich in sucrose and amino acids.
• Inouye and Taylor never observed several species
  of ants collecting nectar from aspen sunflower
  blossoms nor tending aphids.
• Extrafloral nectaries sufficient attractant.

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Temperate Plant Protection Mutualism

• Proposed ants may protect sunflowers developing
  seeds from seed predators.
• Damaged over 90 of seeds produced by some
  flowers.
• Flowers without ants suffered 2-4 times more seed
  predation.
• Relationship remains facultative.
• Every few years flower heads of aspen sunflowers
  are killed by late frost.

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Temperate Plant Protection Mutualism
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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.

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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.

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Coral Protection Mutualism

• Glynn found 13 coral species protected by
  crustacean mutualists.
• Crustacean mutualists substantially improved
  chances coral will avoid attack by sea stars.

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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.

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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.

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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.

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Facultative Ant-Plant Mutualisms

• Keeler proposed for a facultative ant-plant
  mutualism to evolve and persist, proportion of
  plants energy budget ants save from destruction
  by herbivores must exceed proportion of the
  pants energy budget invested in extrafloral
  nectaries and nectar.

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Facultative Ant-Plant Mutualisms

• Conditions that may produce higher benefits than
  costs
• Low proportion of plants energy budget invested
  in extrafloral nectaries.
• High probability of attracting ants.
• Low effectiveness of alternate defenses.
• Highly effective ant defense.

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