Class Notes (808,045)
Canada (493,031)
Biology (6,677)
Hugh Henry (242)
Lecture 14

Lecture 14 - Mutualism and Commensalism

5 Pages
Unlock Document

Western University
Biology 2483A
Hugh Henry

LECTURE 14: MUTUALISM AND COMMENSALISM  The fungus-growing ants started cultivating fungi for food at least 50 million years before the first human farmers  The ant farmers nourish, protect, and eat the fungal species they grow, forming a relationship that benefits both – in balance, the fungus does better with the ants than without  The ants cannot survive without their fungi; many of the fungi cannot survive without the ants  The fungi are cultivated in underground gardens. A colony may contain hundreds of gardens, each the size of a football; they can feed 2–8 million ants  Leaf-cutter ants cut bits of leaves from plants and feed them to the fungi  The ants chew the leaves to a pulp, fertilize them with their own droppings, and “weed” the fungal gardens to help control bacterial and fungal invaders  The fungi produce specialized structures called gongylidia, on which the ants feed  Both ants and fungi benefit from the relationship – work together to overcome defenses of the plant o The ants scrape a waxy covering from the leaves that the fungi have difficulty penetrating o The fungus digests and detoxifies the chemicals that plants use to deter insect herbivores  Nonresident fungi, pathogens, and parasites can sometimes invade the colonies. What prevents invaders from destroying the gardens? o In the 1990s, a parasitic fungus (Escovopsis) was discovered that attacks the fungal gardens of leaf-cutter ants o The parasite can be transmitted from one garden to another, and rapidly destroys the gardens, leading to death of the ant colony o Ants respond to Escovopsis by increasing garden weeding rate  They also enlist the help of other species: o The ants carry a bacterium that makes chemicals that inhibit Escovopsis o The bacteria also secrete compounds that promote the growth of the cultivated fungi.  The bacteria also benefit: They get a place to live (in specialized structures called crypts on the ant’s exoskeleton), and a source of food (glandular secretions) from the ants o Thus, the bacterium is a third mutualist Positive Interactions  Positive interactions occur when neither species is harmed and the benefits of the interaction are greater than the costs for at least one species  Facilitation – synonym for positive interactions  Mutualism – mutually beneficial interaction between individuals of two species (+/+)  Commensalism – individuals of one species benefit, while individuals of the other species do not benefit and are not harmed (+/0)  Symbiosis – a relationship in which the two species live in close physiological contact with each other, such as corals and algae o Symbioses can include parasitism (+/-), commensalism (+/0), and mutualism (+/+) o Mutualism aren’t always symbioses, are not the same thing  The benefits of positive interactions can take many forms  Sometimes there is a cost to one or both partners, but the net effect is positive o For each species, the benefits are greater than the costs  Mutualistic associations are everywhere  Most plants form mycorrhizae – symbiotic associations between the roots and various fungi  The fungi increase the surface area for the plant to take up water and soil nutrients (over 3 m of fungal hyphae may extend from 1 cm of plant root)  Ectomycorrhizae – the fungus grows between root cells and forms a mantle around the root o Helps increase the surface area for absorption  Arbuscular mycorrhizae – the fungus grows into the soil, extending away from the root; and also penetrates into some of the plant root cells  Coral form a mutualism with symbiotic algae  The coral provides the alga with a home, nutrients (nitrogen and phosphorus), and access to sunlight  The alga provides the coral with carbohydrates produced by photosynthesis  Herbivores such as cattle and sheep depend on bacteria and protists that live in their guts to help metabolize cellulose  Wood-eating insects also have gut protists that can digest cellulose  Commensalism is also everywhere  Millions of species form +/0 relationships with organisms that provide habitat  Examples: lichens that grow on trees or bacteria on your skin  In forests, many species depend on the trees for habitat, and do no harm to the trees  Different types of ecological interactions can evolve into commensalism or mutualism  Mutualism can arise from a host – parasite interaction o In a strain of Amoeba proteus that was infected by a bacterium, the bacterium initially caused the host to be smaller, grow slowly, and often killed it o Five years later, the bacterium had evolved to be harmless to the amoeba; the amoeba had evolved to be dependent on the bacterium for metabolic functions  Various tests showed that the two species could no longer exist alone (Jeon 1972)  Some positive interactions are highly species-specific, and obligate – not optional for either species)  The leaf cutter ants and fungus cannot survive without each other, and both have evolved unique features that benefit the other species  Tropical figs are pollinated by fig wasps. Neither species can reproduce without the other o The wasps and the figs have coevolved o The wasps have complex reproductive behaviors in the fig receptacle that ensures pollination. Wasp larvae develop by eating some of the fig seeds  Many mutualisms and commensalisms are facultative – not obligate – and show few signs of coevolution  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  One species of nurse plant may protect the seedlings of many other species  Desert ironwood serves as a nurse plant for 165 different species  The nurse plant and the beneficiary species may evolve little in response to one another  Large herbivores such as deer or moose consume seeds of herbaceous plants  Many seeds pass through unharmed, and are deposited with feces. Thus, it becomes a dispersal mechanism  Such interactions are sporadic and facultative; there is little evidence to suggest that the species have coevolved  Interactions between two species can be categorized by the outcome for each species: o Positive (benefits > costs) o Negative (costs > benefits) o Neutral (benefits = costs)  But costs and benefits can vary  Soil temperature determines whether a pair of wetland plants are commensals or competitors: o Wetland soils can be anoxic. Some plants such as cattails aerate soils by passively transporting oxygen through continuous air spaces in the leaves, stems, and roots. o Some of this oxygen becomes available to other plants.  In an experiment, cattails (Typha) and a forget-me-not (Myosotis), that lacks continuous air spaces, were grown at different temperatures  At low temperatures, soil oxygen increased when cattails were present,
More Less

Related notes for Biology 2483A

Log In


Don't have an account?

Join OneClass

Access over 10 million pages of study
documents for 1.3 million courses.

Sign up

Join to view


By registering, I agree to the Terms and Privacy Policies
Already have an account?
Just a few more details

So we can recommend you notes for your school.

Reset Password

Please enter below the email address you registered with and we will send you a link to reset your password.

Add your courses

Get notes from the top students in your class.