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Cheaters in nature – are mutualisms always beneficial?
Data provided by and written by REU Iniyan Ganesan
Background Information:
Mutualisms are a special type of relationship in nature where two species work together
and both benefit. Each partner “trades” a resource to the other species, while receiving a
resource of their own. This cooperation should lead to each partner species doing better when
the other is around – without their mutualist partner, the species will have a harder time getting
a resource on their own.
But what happens when one partner cheats and takes more than it gives? Scientists at
the Kellogg Biological Station study a mutualism between clover (a type of plant) and rhizobia (a
bacteria). The rhizobia live in small root bumps, called nodules, and receive protection and food
from the plant. In return, the rhizobia give nitrogen to the plant, which plants need to
photosynthesize and make new DNA. This mutualism functions well when nitrogen is rare in the
soil – rhizobia benefit the plant because it is hard for plant roots to find nitrogen on their own.
However, what happens when humans fertilize the soil, such as in gardens and farm fields?
Now nitrogen is freely available in the soil and plants have all they need. With high nitrogen,
rhizobia no longer benefit the plant, and so the plant may start to trade less with rhizobia. In
response, rhizobia from high nitrogen areas will try harder to get food from the plant and both
species will no longer be acting as mutualists.
To test whether high nitrogen can eliminate this mutualism, a lab at KBS grew two
species of clover with two types of rhizobia – one from a high nitrogen environment, and one
from low nitrogen. Some of the clovers were not inoculated with any rhizobia as a control.
Researchers collected data on leaf number for each clover plant. Plants with fewer leaves are
considered less healthy.
Clover plant in the field growing with
rhizobia mutualist
Rhizobia growing on a Petri
Dish in the lab
DATA (A)
Look at the
_____________________________________________________________________________
Developed by NSF-funded Kellogg Biological Station GK-12 Fellows at Michigan State University
Updated 2/18/2016 5:55:00 AM by EHS
Graph Types A-E
Background/Content Level 4
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data below. See if you identify some potential patterns. Then, proceed to (B).
Leaf number Data
Clover T. hybridum
species
T. pratense
Rhizobia type
Rhizobia from
Rhizobia from
low nitrogen
high nitrogen
environment
environment
No
rhizobia
control
10.2
6.8
9.5
4.6
4.6
5.5
(B) Use the data given above collected by MSU scientists to answer the following
question:
Scientific Question: Do rhizobia from high nitrogen environments act as mutualists with
clover?
Evidence: What scientific evidence supports your claim? Refer to specific data, and use
graphs to illustrate your point.
Use the data given above collected by MSU scientists to draw a graph of the
relationship between rhizobia type and leaf number for both of the clover species
(Draw your graphs below).
_____________________________________________________________________________
Developed by NSF-funded Kellogg Biological Station GK-12 Fellows at Michigan State University
Updated 2/18/2016 5:55:00 AM by EHS
Graph Types A-E
Background/Content Level 4
Page 3
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Date______________________
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Claim: Write a conclusion that answers the above question.
Rhizobia from high nitrogen environments decrease clover leaf number compared
to rhizobia from low nitrogen environments in T. hybridum, and therefore
affect the clover-rhizobia mutualism. This pattern is not seen in the clover
species T. pratense.
Reasoning: How does this evidence support your claim? Draw arrows and/or circle the
specific parts of the graph that you used as evidence.
Leaf number is an indicator of how much nitrogen the clover gets from the
rhizobia per amount of food given to the rhizobia. As such, higher leaf
numbers indicate a more cooperative mutualism between the two partners. T.
hybridum inoculated with rhizobia from low nitrogen environments had more
leaves that those inoculated with rhizobia from high nitrogen environments
(10.2 and 6.8 leaves respectively). T. pratense inoculated with the different
rhizobia types did not show a significant difference in leaf number. Both
types seem to be less mutualistic because T. pratense that were not
inoculated had more leaves.
You can also discuss the patterns as they relate to the control. In T.
pratense, the clover growing with rhizobia do worse than the controls without
rhizobia. This could indicate that rhizobia are not mutualists with this
plant species and always take more food from the plant than they give
nitrogen back.
_____________________________________________________________________________
Developed by NSF-funded Kellogg Biological Station GK-12 Fellows at Michigan State University
Updated 2/18/2016 5:55:00 AM by EHS