Mutualism and Symiosis

Question 1

Symbiosis

Answer 1

“living together”

Question 2

Mutualism

Answer 2

beneficial
interaction for both species

Question 3

Mutualisms typically involve
reciprocal exchange of goods or
services between species

Answer 3

Nutritional mutualisms:
* Legumes & rhizobia: exchange fixed C for fixed N
* Plants & mycorrhizal fungi: exchange C for P
* Defensive mutualisms:
* Ants & plants: exchange protection for food (e.g.,
extrafloral nectar) or housing
* Cleaner fish & client fish: exchange parasite removal
for food
* Dispersal mutualisms:
* Plants & animal seed dispersers: exchange seed
dispersal for food
* Plants & animal pollinators: exchange gamete
dispersal for food

Question 4

Lotka Volterra and Mutualism

Answer 4

Just change the sign to be addition instead of subtraction.

Question 5

What limits the population growth of
mutualists?

Answer 5

Strong intra-specific competition
* A third species such as a predator or a competitor
* Diminishing returns to mutualism as the
population grows

Question 6

Invasional meltdown

Answer 6

Positive feedback between mutualists tends
to generate runaway population growth
* What if two invasive species interact as
mutualists?
* Simberloff and Von Holle (1999) coined the
term “invasional meltown” for the process by
which two non-native species facilitate one
another’s spread

Question 7

coevolution

Answer 7

Reciprocal adaptation
(coevolution) between
flowers and insects

Question 8

Bacterial
endosymbionts in
aphids

Answer 8

Aphids feed on phloem
sap that is rich in sugars,
but poor in essential
amino acids
* Aphids have intracellular
bacteria (Buchnera) that
provide their hosts with
essential amino acids
* Buchnera are vertically
transmitted; they are
passed in aphid eggs
from mothers to offspring

Question 9

Vertically transmitted endosymbionts
often have tiny genomes

Answer 9

• Buchnera has a much smaller genome than free-
  living bacteria (e.g., E. coli); other endosymbiotic
  bacteria also have tiny genomes
• In humans: mitochondrial genome is ~17000 base
  pairs (and encodes just 37 genes); nuclear
  genome is > 3 billion base pairs
• Endosymbiotic bacteria lose genes that they no
  longer need
• Some functions unnecessary because bacteria are no
  longer free-living; bacteria protected inside host cells
• Other functions ‘outsourced’ to host genome

Question 10

Are mutualisms often highly
specialized?

Answer 10

Most aphid species have their own species of Buchnera
bacteria
* But most mutualisms are NOT tightly coevolved,
species-specific interactions
* Most mutualisms are horizontally transmitted; partners
are acquired anew each generation
* Mutualisms are rarely one-to-one interactions; usually
many-to-many interactions
* Current ‘hot’ areas of mutualism research include:
* Understanding networks of interactions among large numbers
of species
* Microbiomes: this term refers to either all the microbes living
together in a community (often, a host) or their collective
genomes

Question 11

Characterizing microbial diversity
in a host (or environmental sample)

Answer 11

Sequencing-based
methods
* Sequence a highly
conserved (i.e., slowly
evolving) gene, usually the
bacterial 16S rRNA gene
* Use DNA sequence data to
identify microbes
* Frees us from having to
culture microbes in order to
study them