Lecture 21: Mutualism and Symbiosis

Question 1

Symbiosis

Answer 1

living together

Question 2

Mutualism

Answer 2

each species benefits from the other in the mutualistic interaction

Question 3

Nutritional mutualisms

Answer 3

• legumes and rhizobia: exchange fixed C for fixed N

- plants and mycorrhizal fungi: exchange C for P

Question 4

Defensive mutualisms

Answer 4

• ants and plants: exchange protection for food or housing

- cleaner fish and client fish: exchange parasite removal for food

Question 5

Dispersal mutualisms

Answer 5

• plants and animal seed dispersers: exchange seed dispersal for food
• plants and animal pollinators: exchange gamete dispersal for food

Question 6

Yao people of Mozambique

Answer 6

mutualistic relationship between them and honeyguide bird

Question 7

Lotka-Volterra models of mutualism predict what?

Answer 7

Both populations undergo unbounded exponential growth in an orgy of mutual benefaction.

Question 8

What actually limits the population growth of mutualists

Answer 8

• strong intra-specific competition
• a third species (predator or competitor)
• diminishing returns to mutualism as the population grows

Question 9

Invasional meltdown

Answer 9

Two invasive species can interact as mutualists and just EXPLODE
(positive feedback between mutualists tends to generate runaway population growth)

Question 10

Spring ephemerals

Answer 10

perennial plants (more than two years) which produce aerial parts of the plant and flower and seed right after the snow melts (the roots remain even after the aerial parts have withered)
- strategy is common in deciduous forests where small plants can take advantage of the sunlight before a leaf canopy has formed

Question 11

Weedy ephemerals

Answer 11

also called ruderals, fast growth after soil is disturbed by human plowing

Question 12

Ant-plant experiments

Answer 12

Invasive ants spread invasive plants

Question 13

Cleaner fish

Answer 13

feed off ectoparasites on bodies of client fish. Cleaners often have territories that clients visit. Presence of cleaner fish affects biodiversity of other reef fish.

Question 14

What happens when cleaner fish are removed from a reef?

Answer 14

Reduced biodiversity

Question 15

Darwin’s Orchid

Answer 15

he predicted that the orchid (which had a very long nectar spur) would be pollinated by an insect with a very long proboscis
- nectar spur must be slightly longer than the proboscis of the animal so that the animal actually has to come in contact with the pollen

Question 16

Bacterial endosymbionts in Aphids

Answer 16

• aphids feed on phloem sap that is rich in sugars but poor in essential amino acids
• they have developed a mutualism with bacteria

Question 17

What do we see when we look at the phylogenies of Buchnera bacteria and aphids?

Answer 17

Every time an aphid lineage branches, so does it’s bacteria, as they are vertically transmitted (passed from mothers to offspring in aphid eggs) (allopatric speciation)

Question 18

Vertically transmitted endosymbionts often have…

Answer 18

tiny genomes, much smaller than free-living bacteria
(human mitochondrial genome is only around 17 000 base pairs and encodes 37 genes, while the nuclear genome has more than 3 billion base pairs)
- lose genes they no longer need, other functions outsource to host genome

Question 19

Most mutualisms are..

Answer 19

• NOT tightly coevolved, species-specific interactions
• transmitted horizontally (partners are acquired new each generation)
• rarely one-to-one interactions (most plants have multiple pollinators)

Question 20

What are we studying about mutualisms right now?

Answer 20

• understanding networks of interactions among large numbers of species
• microbiomes

Question 21

Microbiome

Answer 21

term refers to either all the microbes living together in a community (often a host) or their collective genomes

Question 22

Antonie van Leeuwenhoek

Answer 22

Amateur scientist from 17th century, father of microbiology

- discovered that diverse microbes live on plants and animals

Question 23

How are sequencing-based methods of characterizing microbial diversity useful?

Answer 23

Frees us to study microbes that won’t grow in lab conditions; we don’t have to culture them in order to study them.

Question 24

How do we characterize microbial diversity

Answer 24

Sequence a highly conserved gene (slowly evolving), usually bacterial 16s rRNA gene, then cluster the sequences into taxonomic units based on similarity

Question 25

The closer the dots….

Answer 25

the more similar the microbiomes (the dots reflect diet, phylogeny, and morphology)

Question 26

A host’s microbiome affects its…

Answer 26

metabolism, immune system

Question 27

Mirrored phylogenies suggest

Answer 27

…extremely specific mutualisms (vertical transmission)