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Mutualism: Chapter 15

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Introduction

• Mutualism: Interactions between individuals of different species that benefit both partners. Obligate Mutualism occurs when a species

is dependent on a mutualistic relationship. Margulis and Fester amassed evidence

eukaryotes originated as mutualistic associations. (Also called Symbiosis)

Facultative Mutualism occurs when a species can live without its mutualistic partner. (Also called proto-cooperation)

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Mycorrhizae

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Plant Performance and Mycorrhizal Fungi

• Two most common types of mycorrhizae: Vesicular-Arbuscular mycorrhizal fungi

(VAM) 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.

Plants are benefiting from a VAM fungi to increase the supply of water directly.

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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

• Results suggested mycorrhizal fungi from unfertilized soils supplied plants with more N. Plants able to invest more energy in above-

ground photosynthetic material with VAM. Andropogon produced inflorescens only in

nitrogen-supplemented treatments, and greatest with VAM inoculum from unfertilized soils.

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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 (swollen & pithy) are living spaces. Folliar nectaries on stem provide sugar. Beltian bodies (yellow leaflets) are a

source of oils and protein.

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Figure 53.9 Mutualism between acacia trees and ants

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Ants and Bullshorn Acacia

Also, demonstrated fewer herbivorous insects with ants.

What about the pollination paradox?

Janzen demonstrated improve plant performance (growth and survival)

Acacia tree stump suckers studied.

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Coral Mutualisms

• Zooxanthallae and Corals Zooxzanthallae live within coral tissues.

Receive nutrients from coral. In return, coral receives organic compounds synthesized by zooxanthallae during photosynthesis.

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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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Human-Avian Mutualism

Honey-guides

http://www.beekeeping.com/articles/us/honey_war.htm

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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.