5. Mutualism Flashcards
1
Q
What are symbioses? 7
A
- species interactions involving close physical association of participants
- at least 1 partner benefits, so competing species are not in symbioses
- exploitative or cooperative
- frequent evidence of coevolution
- cooperative interactions thought to have evolved from exploitative reactions, when benefits outweigh costs for both partners
- spectrum of parasite vs mutual - changes based on environment
- commensal - benefits one and neutral for the other
2
Q
describe mutualism. 5
A
- extremely widespread in nature
- benefits in terms of growth, survival or reproduction
- examples of benefits include food, environment, protection, pollination, dispersal and cleaning
- farming is one example
- another is coral (animals plus algae) and lichen
3
Q
what are the types of mutualism? 6
A
- obligate - both partners depend on each other for survival
- obligate-facultative - one depends on the other
- facultative - neither depends on the other for survival
- 2 organisms with surface - surface contact eg. pollination
- one organism living in extracellular spaces - between cells
- one organism living in intracellular spaces - within cells
4
Q
Describe the legume-rhicobium association. 7
A
- the major component of n2 fixation
- form nodules in root where fixation occurs, n from atmosphere
- more n than input from artificial fertiliser and wet and dry atmospheric deposition combined
- facultative between cells mutualism
- plant gets access to fixed nitrogen from the atmosphere
- bacteria have competition-free habitats and steady supply of carbon form photosynthate
- legumes are n rich, esp in leaves and nodules
5
Q
What is the influence of the environment on legume-rhizobium association? 5
A
- encouraged by higher pH (more alkaline)
- sensitive to soil aluminium levels, which increase with increasing acidity
- Influence rhizobia - water stress
- soil type
- legume health
6
Q
Describe mycorrhizal associations.
A
- the majority of vascular plants have mycorrhizal associations
- there are four major types
- vesicular arbuscular mycorrhizas - form in root
- ectomycorrhizas grow on root surface
- ericoid mycorrhizas - grow on heather and rhododendrons
- orchid mycorrhizas
- important for plant growth and evolution - drove colonisation of land by plants
7
Q
What are arbuscular mycorrhizal (AM) fungi? 7
A
- hyphae penetrate intracellular spaces of host cortical cells
- form highly branched arbuscles within root cortex - thought to be nutrient exchange site
- some AM fungi also form vesicles - lipid filled storage structures
- hyphal structures extend into soil and provide plants with phosphorus in exchange for carbon
- infect a wide variety of plants, incl agricultural species
- obligate-facultative symbiosis, formed by Glomeramycota
- fungus needs plant eg. strawberries
8
Q
What is ectomycorrhizal (EM) fungi? 6
A
- hyphae penetrate intercellular spaces of root outer cortex to form hartig net
- some form hard sheath over root surface
- hyphal strands extend into soil so plants can access nutrients that are further away and access nutrients better
- hyphae provide plants with phosphorous in exchange for carbon
- most abundant in temperate and boreal forests, infecting the roots of trees and shrubs
- primarily Basidiomycota - toadstools are fruiting bodies
9
Q
Describe ericoid mycorrhizal fungi. 6
A
- infects ericaceous plants ie. heathers
- penetrates root cortex xells but no arbuscles formed
- loose hyphal covering on surface but no mantel or hard sheath
- found in acidic, nutrient poor, high organic matter soil
- provide plants with more nutrients than other associations, incl n and phosphorous in exchange for carbon
- formed by ascomycota
10
Q
Describe orchid mycorrhizas. 4
A
- very close association of plants and fungus
- orchid seeds have obligate requirement for mycorrhizal fungi to germinate in nature
- orchid seeds are small with very little reserves
- little evidence of plant derived benefits to fungus so may not be mutualistic
11
Q
What are the advantages of plants being involved in mycorrhizal associations? 5
A
- increased nutrient uptake as plant can’t break down all alone
- access to organic nutrients
- increased rootlet size and longevity
- protection from pathogens-fungus takes up space
- improved water relations and better drought tolerance
12
Q
What are the advantages to mycorrhizal fungi being in association with plants? 2
A
- competition free habitat
2. steady supply of carbon from photosynthate (excl orchid mycorrhiza)
13
Q
How does mycorrhizal symbioses increase nutrient uptake? 5
A
- access to increased soil volume through mycelium network - away from own depleted root zone
- absorption of nutrients by fungi at lower concentrations
- access to organic nutrients
- potential for nutrient transfer between plants linked through hyphal network - evidence inconclusive
- linkage between plants via fungi
14
Q
what are the environmental influences on mycorrhizal interactions? 5
A
- development depends on soil nutrient status (especially for AM)
- growth of AMs restricted at low pH - ericoid niche
- water availability restricts development
- high soil water content - anaerobic conditions all soil fungi obligate aerobes
15
Q
What are the types of lichen and why are they important? 5
A
- Crustose lichens
- fruticose lichens
- foloise lichens
- live on surfaces, not in soil
- important in soil formation as they weather substrate
16
Q
describe the lichen symbiosis. 4
A
- fungal (mycobiont) - algal/cyanobacterial (photobiont)
- facultative symbiosis
- 35 genera of algae/cyanobacteria found in symbiosis
- fungus usually ascomycota
17
Q
Describe the lichen structure. 4
A
- fungal cortex
- photobiont relatively close to surface of lichen
- medulla - fungal anchor to substrate
- different types have different structures
18
Q
What are the advantages to a mycobiont being in a lichen symbiosis? 3
A
- carbon from photobiont
- higher dessication tolerance
- protection from oxidative stress
19
Q
What are the advantages to a photobiont of being in a lichen symbiosis? 3
A
- tolerance of higher light levels
- higher dessication tolerance
- protection from oxidative stress
20
Q
How do mutualisms evolve? 4
A
- joint benefits to fitness must outweigh joint costs to fitness for both partners
- cheating must be kept to a minimum
- allows long term, stable mutualism
- there is a threshold for how much cheating can be tolerated
21
Q
give some examples of cheating in mutualistic relationships. 5
A
- bees and flowers - flowers have long parts to limit pollination to those with longer tongues
- bee cheats by biting petals without pollinating
- bees and orchids: bee orchid mimics female bee
- pollinated by male bees who think they’re mating
5, bee moves from flower to flower
22
Q
how is cheating reduced in mutualisms by vertical symbiont transmission through cells? 5
A
- partners can tie themselves extremely closely to each other eg. eukaryotic cells their organelles
- each loses some function so relationship is strengthened
- organelles lost free living habitat
- reduction of symbiont genetic diversity
- vertical transmission from parent maintains relationship
23
Q
How is cheating reduced in mutualisms by vertical symbiont transmission through ant colonies? 6
A
- fungus farming ants - leafcutters - collect leaves from tropical forests
- leaves feed fungus, fungus feeds ants
- different colonies maintain different genetic strains of fungus
- fidelity maintained through parent offspring transmission
- maintenance of mutualism requires control of pathogenic fungi either by cultivation of antibiotic producing bacteria or antibiotic secretions from ants
- new colonies take a piece of old fungus with them
24
Q
Describe an example in which partner fidelity reduces cheating in mutualisms. 5
A
- lifestyles and life cycles are coupled together
- ants provide protection from herbivores and reduce plant competition by cutting them
- acacias provides food (nectar cup and protein secretions) and shelter
- ants take 2 years to successfully produce offspring
- trees can’t survive for 1 year without ants
25
Q
Describe an example of partner choice designed to reduce cheating in mutualisms.
A
- bioluminescent bacteria enable Hawaiian bobtail squid to camouflage self more effectively
- the squid provides shelter
- bacteria needs to pass certain tests before squid accepts them as a symbiont
- bacteria must swim up tentacles/tubes
- must then survive inhospitable environments eg. high NO
- accepted bacteria are given better conditions
- bacteria must be resistant to antibacterial substances as squid secretes antibiotics
- 95% bacteria expelled each day and process repeats
26
Q
how do partner sanctions between legumes and rhizobium reduce cheating in their mutualisms? 7
A
- plant root exudates attract bacteria
- if proteins on bacterial cell surface match plant lectins, bacteria attach to root
- bacteria release nodulation factors in response plant compounds
- nod factors cause root hairs to branch and curl, creating a pocket in the cell wall
- plant forms a tube through which bacteria travel into the nodules
- non-ammonia producing bacteria are starved of o2 by the plant
- plant ensures it receives benefit from the bacteria
