Lecture 21

Question 1

Mutualism vs Symbiosis

Answer 1

symbiosis = living together
mutualism = beneficial interaction for both species

Question 2

what does mutualism typically involve?

Answer 2

reciprocal exchange of goods or services between species

Question 3

3 types of mutualisms

Answer 3

• nutritional
• defensive
• dispersal

Question 4

nutritional mutualisms

Answer 4

• legumes and rhizobia: exchange fixed C for fixed N
• plants and mycorrhizal fungi: exchange C for P

Question 5

defensive mutualisms

Answer 5

• ants and plants: exchange protection for food (eg extrafloral nectar) or housing
• cleaner fish and client fish: exchange parasite removal for food

Question 6

dispersal mutualisms

Answer 6

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

Question 7

mutualism between humans and free-living wild animals

Answer 7

• Yao people in Mozambique harvest wild honey, but can’t find bees’ nests easily
• honeyguides (Indicator indicator) eat bees wax and know where nests are, but can’t access them easily
• honeyguides recognise the specific sound that Yao honey-hunters make to attract them

Question 8

how would you change Lotka-volterra equations to model mutualism?

Question 9

constraints of the Lotka-Volterra models of mutualism

Answer 9

both populations undergo unbounded exponential growth in an orgy of mutual benefaction

Question 10

what limits the population growth of mutualists?

Answer 10

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

Question 11

invasions meltdown

Answer 11

• positive feedback between mutualists tends to generate runaway population growth
• if two invasive species interact as mutualists, they may facilitate each other’s spread

Question 12

Spring ephemerals

Answer 12

• perennial understory herbs that flower right after the snow melts, producing a short-lived ‘carpet of flowers’
• have seeds dispersed by ants

Question 13

elaiosome

Answer 13

fleshy appendage on seeds of spring ephemerals that attracts ants - rich in fats

Question 14

experiment on seed dispersal by ants

Answer 14

Prof. F wanted to compare seed dispersal by a native seed-dispersing ant (Aphaenogaster rudis) and an invasive seed-dispersing ant, (Myrmica rubra)
- she placed one type of plant in each mesocosm, then added a bunch of native and invasive seeds for the ants to disperse
- with native ant, mostly native plants were dispersed
- with invasive ant, mostly invasive plants were dispersed

Question 15

Myrmica rubra

Answer 15

native to Europe, where it disperses many seeds

Question 16

Cleaner fish (Labroides dimidiatus)

Answer 16

• Cleaner fish feed on ectoparasites on the bodies of client fish; cleaners gain food, clients benefit from fewer parasites
• Cleaners often have “cleaning stations” (territories) that clients visit
• Experimentally removing cleaner fish increases parasite (gnathiid) abundance on client fish

Question 17

effect of the presence of cleaner fish in reefs

Answer 17

affects species diversity of other reef fish
- when cleaners were removed, the number of fish species went down
- when cleaners were added, number of fish species went up
- presence of cleaner fish attracts client fish

Question 18

Darwin’s orchid, Angraecum sesquipedale

Answer 18

• epiphytic orchid from Madagascar with a very long nectar spur
• Darwin famously predicted that it must be pollinated by an insect with a very long proboscis (sucking mouthpart)
• Alfred Russel Wallace drew a moth like this
• Xanthopan morganii praedicta was discovered in 1903

Question 19

Reciprocal adaptation (coevolution) between flowers and insects

Answer 19

how is long proboscis/floral tube beneficial?
- when nectar spur of the plant is longer than the mouthpart of the insect, this forces the insect to get deep into the flower in order to drink all the nectar
- this forces the insect to contact the pollen-receiving parts of the flower, improving pollination
- pollinators with longer mouthparts are able to get more food

Question 20

Bacterial endosymbionts in aphids

Answer 20

• 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
• Buchner are vertically transmitted; they are passed in aphid eggs from mothers to offspring

Question 21

what is a feature of vertically transmitted endosymbionts?

Answer 21

• Buchnera has a much smaller genome than free-living bacteria (eg E.Coli); other endosymbiotic bacteria also have tiny genomes
• in humans; mitochondrial genome is ~17000 base pairs (and encodes just 27 genes); nuclear genome is >3 billion base pairs
• endosymbiotic bacteria lose genes that they no longer need

Question 22

why do endosymbiotic bacteria lose genes that they no longer need

Answer 22

• some functions unnecessary because bacteria are no longer free-living; bacteria protected inside host cells
• other functions outsourced to host genome

Question 23

are mutualisms often highly specialised?

Answer 23

• 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

Question 24

current areas of mutualism research

Answer 24

• 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 25

do plants only have one pollinator species?

Answer 25

no, most plants have many pollinator species, and most pollinators visit many plant species

Question 26

how can we characterise microbial diversity in a host (or environmental sample)?

Answer 26

• culture-based methods
• sequencing based methods

Question 27

sequencing-based methods

Answer 27

• 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

Question 28

mammal gut microbiomes reflect

Answer 28

diet, phylogeny, and morphology