Lecture 13 Mutualism and Commensalism
The First Farmers
The fungusgrowing ants started cultivating fungi for food at least 50 million years before the first
14.1 – Carry leaf segments to their colony, where the leaves will be fed to the fungi the ants
cultivate for food. The ants riding on the leaves are of the same species as the leaf carries and are
Fungi produce specialized structures called gongylidia, on which the ants feed on.
When a queen leaves the nest to mate and begin a new colony, she carries in her mouth some of
the fungi from her birth cology.
The fungi are cultivated in subterranean gardens. A colony may contain hundreds of gardens, each
size of a football; they provide enough food to support 28million ants.
Figure 14.2 A Representation of a large Atta leafcutter ant colony the fungus is cultured in
garden chambers, each about the size of a football. The mound above ground is made up of soil
excavated by the ants. The dump chambers contain refuse from the fungal gardens.
Positive interactions occur when neither species is harmed nor the benefits for the interaction are
greater than the cost for at least one species.
Mutualism – mutually beneficial interaction between individuals of two species (+/+ relationship)
Commensalism – individuals of one species benefit, while individuals of one species benefit,
while individuals of the other species do no benefit and are not harmed (+/0 relationship)
Symbiosis – a relationship in which the two species live in close physiological contact with each
other, such as corals and algae. Or humans and bacteria.
Parasites can also form symbiotic relationships. Symbiosis can include parasitism (+/),
commensalism (+/0) and mutualism (+/+)
Mutualistic associations are everywhere – land surfaces of Earth
o Most plants form mycorrhizae, symbiotic associations
between plant roots and various types of fungi.
o The fungi increase the surface area over which the plants can
extract soil nutrients (over 3m of fungal hyphae may extend
from 1cm of plant root). Typically benefit the fungi by
supplying them with carbohydrates.
Two categories of mycorrhizae
o Ectomycorrhizae – the fungus grows between root cells and
forms a mantle around the exterior of the root.
o Figure 14.4 part 1 – fungal hyphae form a mantle or sheath
around the root, and some of the hyphae glow between the root cells.
o Arbuscular mycorrhizae – the fungus grows into the soil, extending some distance
away from the root; and also penetrates into some of the plant cells.
o Figure 14.4 part 2 – mycorrhizae fungal hyphae can extend far into the
soil. Hyphae also grow between some root cells and penetrate others.
Corals form a mutualism with symbiotic algae. The corals provides the alga with
home, nutrients (N and P) and access to sunlight. The alga provides the coral with
carbohydrates produced by photosynthesis.
o All the numerous invertebrate and vertebrate species that live in and on coral
depend directly or indirectly on the corralalga mutualism.
Lichens o Lichens are unique, unclassifiable: composite organism of fungi and algae. There are
14,000 species. Huge variety in size, form and color. Found everywhere on Earth. Dual
nature only described in 1868. Great example of symbiosis (or parasitism).
o Mostly green algae: many/most lichenized algae can live alone
o Morphed of fungus alga than fungus again.
o Are they symbiosis? Or parasitism?
First contact: fungus recognized “correct” alga & penetrates into algal cell ▯
many algae cells die
Algal cell walls become permeable ▯sugars leak to fungus.
Mammalian herbivores such as cattle and sheep depend on bacteria ad protists that live in their gut
and help them metabolize otherwise indigestible plant material like cellulose
o Figure 14.5: protest gut mutualist – this wood eating cockroach would stave if gut
mutualist such as the protest did not help it to digest wood.
Commensalism is also everywhere. Millions of species form +/0 relationships with organisms that
o Like lichens that grow on trees, ot the harmless bacteria that grow on the surface of your
o In kelp forests, many species depend on the kelp for habitat and do no harm or benefit the
o Countless insects and understory plant species live in tropical rainforests and depend on
the forests for habitat, yet many have little or no effect on the trees that tower above
How can a mutualism evolve?
o This was observed in a strain of Amoeba proteus that was infected by bacterium.
o Initially the bacteria caused the hosts to be smaller, grow slowly, and often killed the
o Five years later the bacterium had evolved to be harmless to the amoeba. And the amoeba
had evolved to be dependent on the bacterium for metabolic functions.
o Neither species could survive without the other. Thus, what began as a parasitic
relationship evolved into mutualism in which each species provided the other with a clear
benefit (in this case it’s the ability to survive) .
Some positive interactions are highly speciesspecific,, and obligate (not optional for either
o Ex: the leaf cutter ants and fungus cannot survive without each other, and the interaction
has led both to evolve unique features that benefit the other species.
Their interaction has lef each partner to evolve unique features that benefit the
o 14.6 Fig and wasp relationship where neither species can reproduce without the other.
Also show clear signs of coevolution. The receptacle contains both male and female
flowers, but the female flowers mature 34weeks earlier. Some of the female flowers
have short styles, while others have long styles,
Example of specialized behavior that provides a benefit to another species.
Many mutualisms and commensalisms are facultative (not obligate) and some show few signs of
o In deserts, the shade of adult plants creates cooler, moister conditions. Seeds of many
plants can only germinate in this shade. The adult is called a nurse plant.
o One species of nurse plant may protect the seedlings of many other species.
o Ex: desert ironwood serves as a nurse plant for 165 different species, most of which also
germinate and grow under other plant species. Recap: a species that requires nursing may be found under a variety of nurse
plant species, and the nurse plant and the beneficiary species may evolve little in
response to one another.
Paramecium bursaria contains a photosynthetic algae P.bursaria gets extra energy, the algae are
protected and in a more constant environment. When Grown in the dark P.bursaria loses the algae,
but survives with minimal impact. If P. bursaria dies the algae live freely in the water, but quickly
recolonize an empty P.bursaria.
Interactions between two species can be categorized by the outcome for each species
o Postive (benefits > costs)
o Negative (costs > benefit)
o Neutral (=)