5. Mutualism

Question 1

What are symbioses? 7

Answer 1

1. species interactions involving close physical association of participants
2. at least 1 partner benefits, so competing species are not in symbioses
3. exploitative or cooperative
4. frequent evidence of coevolution
5. cooperative interactions thought to have evolved from exploitative reactions, when benefits outweigh costs for both partners
6. spectrum of parasite vs mutual - changes based on environment
7. commensal - benefits one and neutral for the other

Question 2

describe mutualism. 5

Answer 2

1. extremely widespread in nature
2. benefits in terms of growth, survival or reproduction
3. examples of benefits include food, environment, protection, pollination, dispersal and cleaning
4. farming is one example
5. another is coral (animals plus algae) and lichen

Question 3

what are the types of mutualism? 6

Answer 3

1. obligate - both partners depend on each other for survival
2. obligate-facultative - one depends on the other
3. facultative - neither depends on the other for survival
4. 2 organisms with surface - surface contact eg. pollination
5. one organism living in extracellular spaces - between cells
6. one organism living in intracellular spaces - within cells

Question 4

Describe the legume-rhicobium association. 7

Answer 4

1. the major component of n2 fixation
2. form nodules in root where fixation occurs, n from atmosphere
3. more n than input from artificial fertiliser and wet and dry atmospheric deposition combined
4. facultative between cells mutualism
5. plant gets access to fixed nitrogen from the atmosphere
6. bacteria have competition-free habitats and steady supply of carbon form photosynthate
7. legumes are n rich, esp in leaves and nodules

Question 5

What is the influence of the environment on legume-rhizobium association? 5

Answer 5

1. encouraged by higher pH (more alkaline)
2. sensitive to soil aluminium levels, which increase with increasing acidity
3. Influence rhizobia - water stress
4. soil type
5. legume health

Question 6

Describe mycorrhizal associations.

Answer 6

1. the majority of vascular plants have mycorrhizal associations
2. there are four major types
3. vesicular arbuscular mycorrhizas - form in root
4. ectomycorrhizas grow on root surface
5. ericoid mycorrhizas - grow on heather and rhododendrons
6. orchid mycorrhizas
7. important for plant growth and evolution - drove colonisation of land by plants

Question 7

What are arbuscular mycorrhizal (AM) fungi? 7

Answer 7

1. hyphae penetrate intracellular spaces of host cortical cells
2. form highly branched arbuscles within root cortex - thought to be nutrient exchange site
3. some AM fungi also form vesicles - lipid filled storage structures
4. hyphal structures extend into soil and provide plants with phosphorus in exchange for carbon
5. infect a wide variety of plants, incl agricultural species
6. obligate-facultative symbiosis, formed by Glomeramycota
7. fungus needs plant eg. strawberries

Question 8

What is ectomycorrhizal (EM) fungi? 6

Answer 8

1. hyphae penetrate intercellular spaces of root outer cortex to form hartig net
2. some form hard sheath over root surface
3. hyphal strands extend into soil so plants can access nutrients that are further away and access nutrients better
4. hyphae provide plants with phosphorous in exchange for carbon
5. most abundant in temperate and boreal forests, infecting the roots of trees and shrubs
6. primarily Basidiomycota - toadstools are fruiting bodies

Question 9

Describe ericoid mycorrhizal fungi. 6

Answer 9

1. infects ericaceous plants ie. heathers
2. penetrates root cortex xells but no arbuscles formed
3. loose hyphal covering on surface but no mantel or hard sheath
4. found in acidic, nutrient poor, high organic matter soil
5. provide plants with more nutrients than other associations, incl n and phosphorous in exchange for carbon
6. formed by ascomycota

Question 10

Describe orchid mycorrhizas. 4

Answer 10

1. very close association of plants and fungus
2. orchid seeds have obligate requirement for mycorrhizal fungi to germinate in nature
3. orchid seeds are small with very little reserves
4. little evidence of plant derived benefits to fungus so may not be mutualistic

Question 11

What are the advantages of plants being involved in mycorrhizal associations? 5

Answer 11

1. increased nutrient uptake as plant can’t break down all alone
2. access to organic nutrients
3. increased rootlet size and longevity
4. protection from pathogens-fungus takes up space
5. improved water relations and better drought tolerance

Question 12

What are the advantages to mycorrhizal fungi being in association with plants? 2

Answer 12

1. competition free habitat

2. steady supply of carbon from photosynthate (excl orchid mycorrhiza)

Question 13

How does mycorrhizal symbioses increase nutrient uptake? 5

Answer 13

1. access to increased soil volume through mycelium network - away from own depleted root zone
2. absorption of nutrients by fungi at lower concentrations
3. access to organic nutrients
4. potential for nutrient transfer between plants linked through hyphal network - evidence inconclusive
5. linkage between plants via fungi

Question 14

what are the environmental influences on mycorrhizal interactions? 5

Answer 14

1. development depends on soil nutrient status (especially for AM)
2. growth of AMs restricted at low pH - ericoid niche
3. water availability restricts development
4. high soil water content - anaerobic conditions all soil fungi obligate aerobes

Question 15

What are the types of lichen and why are they important? 5

Answer 15

1. Crustose lichens
2. fruticose lichens
3. foloise lichens
4. live on surfaces, not in soil
5. important in soil formation as they weather substrate

Question 16

describe the lichen symbiosis. 4

Answer 16

1. fungal (mycobiont) - algal/cyanobacterial (photobiont)
2. facultative symbiosis
3. 35 genera of algae/cyanobacteria found in symbiosis
4. fungus usually ascomycota

Question 17

Describe the lichen structure. 4

Answer 17

1. fungal cortex
2. photobiont relatively close to surface of lichen
3. medulla - fungal anchor to substrate
4. different types have different structures

Question 18

What are the advantages to a mycobiont being in a lichen symbiosis? 3

Answer 18

1. carbon from photobiont
2. higher dessication tolerance
3. protection from oxidative stress

Question 19

What are the advantages to a photobiont of being in a lichen symbiosis? 3

Answer 19

1. tolerance of higher light levels
2. higher dessication tolerance
3. protection from oxidative stress

Question 20

How do mutualisms evolve? 4

Answer 20

1. joint benefits to fitness must outweigh joint costs to fitness for both partners
2. cheating must be kept to a minimum
3. allows long term, stable mutualism
4. there is a threshold for how much cheating can be tolerated

Question 21

give some examples of cheating in mutualistic relationships. 5

Answer 21

1. bees and flowers - flowers have long parts to limit pollination to those with longer tongues
2. bee cheats by biting petals without pollinating
3. bees and orchids: bee orchid mimics female bee
4. pollinated by male bees who think they’re mating
   5, bee moves from flower to flower

Question 22

how is cheating reduced in mutualisms by vertical symbiont transmission through cells? 5

Answer 22

1. partners can tie themselves extremely closely to each other eg. eukaryotic cells their organelles
2. each loses some function so relationship is strengthened
3. organelles lost free living habitat
4. reduction of symbiont genetic diversity
5. vertical transmission from parent maintains relationship

Question 23

How is cheating reduced in mutualisms by vertical symbiont transmission through ant colonies? 6

Answer 23

1. fungus farming ants - leafcutters - collect leaves from tropical forests
2. leaves feed fungus, fungus feeds ants
3. different colonies maintain different genetic strains of fungus
4. fidelity maintained through parent offspring transmission
5. maintenance of mutualism requires control of pathogenic fungi either by cultivation of antibiotic producing bacteria or antibiotic secretions from ants
6. new colonies take a piece of old fungus with them

Question 24

Describe an example in which partner fidelity reduces cheating in mutualisms. 5

Answer 24

1. lifestyles and life cycles are coupled together
2. ants provide protection from herbivores and reduce plant competition by cutting them
3. acacias provides food (nectar cup and protein secretions) and shelter
4. ants take 2 years to successfully produce offspring
5. trees can’t survive for 1 year without ants

Question 25

Describe an example of partner choice designed to reduce cheating in mutualisms.

Answer 25

1. bioluminescent bacteria enable Hawaiian bobtail squid to camouflage self more effectively 2. the squid provides shelter 3. bacteria needs to pass certain tests before squid accepts them as a symbiont 4. bacteria must swim up tentacles/tubes 5. must then survive inhospitable environments eg. high NO 6. accepted bacteria are given better conditions 7. bacteria must be resistant to antibacterial substances as squid secretes antibiotics 8. 95% bacteria expelled each day and process repeats

Question 26

how do partner sanctions between legumes and rhizobium reduce cheating in their mutualisms? 7

Answer 26

1. plant root exudates attract bacteria 2. if proteins on bacterial cell surface match plant lectins, bacteria attach to root 3. bacteria release nodulation factors in response plant compounds 4. nod factors cause root hairs to branch and curl, creating a pocket in the cell wall 5. plant forms a tube through which bacteria travel into the nodules 6. non-ammonia producing bacteria are starved of o2 by the plant 7. plant ensures it receives benefit from the bacteria