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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 sunflower’s 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 plant’s energy budget ants save from destruction by herbivores must exceed proportion of the pant’s 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 plant’s energy budget

invested in extrafloral nectaries. High probability of attracting ants. Low effectiveness of alternate defenses. Highly effective ant defense.

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