1. Mutualisms - Chap. 15 I. Terms and concepts A. Symbiosis vs. Mutualism
B. Degree of dependence: Obligate vs. facultative
C. Degree of specialization: Specialists vs. generalists
D. What types of "currency“?
E. Coevolution
F. Effects on realized niche
II. Examples
A. Mycorrhizae - fungi and plants
Ectomycorrhizae
Endomycorrhizae (Arbuscular)
B. Nitrogen fixation
C. Corals- zooxanthellae, anthozoans

2. Questions to focus on
What is the difference between symbiosis and mutualism?
2. What do the terms “obligate” and “facultative” mean with respect to mutualisms? How do the mutualisms discussed in the book and powerpoint differ in their degree of dependence?
3. What are specialists and generalists? If one species in a mutualism is a specialist, does that mean the other must be as well?
4. What are the different types of “currency” exchanged in mutualisms? Which ones are important for the different examples in the book and powerpoint?
5. What is co-evolution and how does it differ from convergent evolution? How might mutualisms affect the realized niche of the species involved?
6. For the different examples of mutualisms in the book and powerpoint, know what the different currencies are, the relative degrees of dependence and specificity, and how those mutualisms are important in community and/or ecosystem processes.

3. Symbiosis vs. Mutualism
Symbiosis: “sym” = together, “biosis”= living; close physical association (e.g., host and internal symbiont). Could be beneficial or parasitic.
Mutualism: +/+ interaction, both species benefit. Could be symbiotic or free-living

4. B. Degree of dependence
Obligate: at least on species could not grow and reproduce without the other
Facultative: both organisms do better with their mutualist, but can survive and reproduce without it.

5. Ant-Acacia Mutualism

6. Ant-Acacia Mutualisms

7. Facultative or Obligate?
The book says that this is an obligate mutualism, which would be so only if no acacias survived without ants.
15.8
15.9

8. Leafcutter ants: a complex mutualism
Central and south american leafcutter ants – don’t eat the leaves themselves, but rather they bring it back to the nest and use it to grow a particular fungus that they use for food. Now if you’re a farmer, you might wonder how these ants keeps parasites from taking over their fungus patches. Turns out there’s another fungus, this in the genus Escovopsis, that is a parasite on the ant grown fungus. Specific groups of ants host specific groups of fungi which in turn have specific microfungal pathogens. Pathogenic fungi can completely overgrow ant “crop” fungus if not held in check. How is it held in check?
Ants cultivate a particular bacteria (Pseudonocardia, Actinomycetes) that makes an antibiotic effective at suppressing the pathogenic fungus.
Closeup – Cyphomermex costatus showing white outlets of crypts all over the worker ant’s body (fig E), and crypts with bacteria embedded in the cuticle (blue), which are fed by specialized exocrine glands that nurture the bacteria. Different species of ants farm different species of fungi and have different strains of bacteria protecting their fungus gardens.
Ok, so what’s the point? Here we have 4 species interacting in a highly evolved mutualism (five if you count the tree from which the leaves were originally cut, though that’s not as species specific). So, rather than just counting species and thinking about this as one species of ant, we need to think about the other species it’s interacting with as well – species interactions count.
These relationships have evolved over an estimated 50 million years! As a biologist, I can think of many interesting questions related to that. And I just also have to stand back and say “wow”! That is really cool! Rudyard Kipling’s “Just-so-stories” pale by comparison. Stepping out of my biologist hat for a moment, and just commenting as a human being, I see that relationship as a thing of beauty. Allowing it’s destruction as a by product of our everyday activities or misplaced policies for rainforest conversion is to me both an ethical and aesthetic affront.
But beyond that, I don’t think it’s practically very wise. In an age where more and more diseases are becoming resistant to more and more antibiotics, it would seem prudent to keep as many of these pieces around not only for their beauty, but also for their practical medical potential. That is, in the terminology of economics, there’s potentially an opportunity cost to losing these species – though that’s just speculation at this point.
Influence of Nests of Leaf-Cutting Ants on Plant Species Diversity in Road Verges of Northern Patagonia A. G. Farji-Brener, L. Ghermandi Journal of Vegetation Science, Vol. 11, No. 3 (Jun., 2000), pp doi: /
Currie et al Science
Leaf cutter ants

9. C. Degree of specialization
Specialists: reliant on only one other species
Generalists: multiple species can perform the function

10. Degree of specialization? Pollination
Some pollination mutualisms are highly specialized, with particular adaptations for each species.
Hawk moth pollinated
Hummingbird pollinated

12. Hawk moth

13. Generalist pollinators
Other pollination arrangements are more generalized – honey bees pollinate a wide variety of species, many of which are visited by other species of bees, flies, and other insects as well.

14. D. The “currency” Nutrition – energy and nutrients
Protection – biotic and abiotic
Fertilization/gamete dispersal (e.g., pollination)
Seed dispersal

15. E. Co-evolution
Co-evolution occurs when two species interact so strongly with one another that they are dominant evolutionary forces on one another.
Examples:
- Obligate, specialist mutualisms
- Specialist predator/prey interactions (the term was first coined in describing the “evolutionary arms race” between plant chemical defenses and insect herbivores that evolve resistance to those defenses).

16. F. Effects on realized niche
Mutalisms can increase the size of the realized niche compared to the fundamental niche.
fundamental niche
realized niche
Competition, exploitation often reduce the realized niche of an organism compared to it’s fundamental niche.

17. II. Examples of important mutualisms

18. A. Mycorrhizae “mycor” = fungi, “rhizae” = roots
- In almost all families of vascular plants

20. Ectomycorrhizae

21. Endomycorrhizae

22. B. N-fixation 4 2 1 3 Importance to N-cycle
Root nodules on a soybean plant. The nodules are made by the plant root in response to contact with bacteria in the soil, as those bacteria colonize the root.
Soybean field.
cross-section of a root nodule. Dark stained portions contain symbiotic, mutualistic bacteria.
Bacteroids inside plant cell are separated from other cell contents by a membrane.
Bacteria have the nitrogenase enzyme allowing them to fix atmospheric nitrogen (N2) into biotically-available amino acids. But this is an energy intensive process. Plants provide the bacteria with fixed carbon from photosynthesis that helps fuel the fixation process, and in turn plants receive available nitrogen. 3
Importance to N-cycle

23. C. Corals

24. Zooxanthellae + Anthozoan1
Closeup of coral showing colonial nature – many individual polyps, each with it’s own anthozoan animal.
An individual anthozoan showing tentacles for food gathering (floating particles, phytoplankton), all of which are also lined with a layer of dinoflagellates – algae, which, when in association with corals, are known as zooxanthellae. 2

25. Anthozoans provide protection and nutrients to the zooxanthellae, in exchange for fixed carbon from algal photosynthesis