mutualistic interactions

Question 1

types of symbiosis

Answer 1

• mutualism
• neutralism
• parasitism
• the outcome of a symbiotic relationship is variable/hard to measure, cost/benefit analysis

Question 2

example of variable symbiosis

Answer 2

• american cuckoos
• parasitism to mutualism
• host birds have increased breeding success with cuckoos as cuckoo chicks release smelly compounds that reduce predation

Question 3

fungi/bacteria inhabiting a plant root

Answer 3

• type of relationship often unclear
• either mycorrhizas or pathogens
• > 75% plants are mycorrhizal
• link between the plant and decomposer subsystem

Question 4

mycorrhizas

Answer 4

• increases root SA:V with hyphae
• release exoenzymes that solubilise insoluble compounds, diversifying plants’ nutrient sources
• plant growth is rarely limited by carbon (usually N or P), so has enough to transfer to fungus
• nutrient exchange is driven by mass flow and uses haustoria ( measured through use of radiolabelled C and P)
• fungus transfers N, P and water to plant
• plant transfers C to fungus in the form of sucrose to the fungus, immediately converted into trehalose which plants can’t metabolise, maintaining gradient for mass flow

Question 5

mycorrhizal types

Answer 5

• arbuscular
• ericoid
• ectomycorrhizae
• example of convergent evolution
• arbuscular is oldest, ectomycorrhizae evolved the latest

Question 6

ectomycorrhizas

Answer 6

• sheaths root, only colonises outer root tissues
• not intracellular, no haustoria
• not as efficient nutrient transfer
• changes root morphology, plant doesn’t need to grow roots as long
• main nutrients transferred are N and P
• tree hosts
• Basidiomycota fungal symbiont e.g. Fly Agaric (white mycorrhizas, colour is species specific)

Question 7

arbuscular mycorrhizae

Answer 7

• large spores found in soil, 0.25mm
• appressorium entry
• haustoria distinct in shape, form arbuscles
• mainly transports P
• evolved with earliest land plants, has many hosts
• Glomeromycota phyla, diverged from zygomycetes 400mya, ancient and highly specialised
• sometimes forms vesicles and oil stores inside plant
• obligate biotrophs, lost genes for fatty acid metabolism

Question 8

ericaceous/ericoid mycorrhizae

Answer 8

• coils of hyphae inside host
• mainly transports N
• main hosts are shrubs on acidic, peaty soils
• ascomycota fungi
• coils are short lived (3-4 weeks), plant suppresses defence response but eventually attacks and kills fungal coils

Question 9

mutualism with herbivores

Answer 9

• herbivores need to access lignocellulose but don’t have the genes to make cellulases
• have cellulolytic gut microbes
• can be bacteria, protozoans, rumen fungi

Question 10

fungus farming termites

Answer 10

• Old world termites create fungus gardens with Termitomyces fungus (agaric), different termite species have a relationship with different termitomyces species (new world termites have protist and bacteria gut symbionts, larger body size, large fermenting gut)
• ‘inside out guts’
• major agent of wood decay in the tropics (25%)
• create tall ventilated earth nests that act as a fermenter

Question 11

fungus farming termites, method

Answer 11

• termites collect wood pulp and chew it (communation adds chitinases and antifungals from saliva, sterilising the pulp so there is no competition)
• add it to store to give time for antifungals to act
• mix wood pulp with the fungal comb containing termitomyces, termitomyces breaks down lignocellulose
• termites feed on the swollen hyphae formed from below, fungal enzymes continue to act in the termite gut
• termite faeces also feeds the fungus

Question 12

fungus farming termites, dispersal of termitomyces

Answer 12

• occasional fruiting bodies
• new queens transport fungus on maiden flight to new nest in pocket

Question 13

termite nests

Answer 13

• convective cooling to maintain 30C optimum temperature (+/-2C)
• also elongated, largest area pointed East to speed up fermentation in the morning, smallest area pointing South so nest doesn’t get too hot at midday

Question 14

insect mutualisms, convergent evolution of ‘tasty’ swollen hyphae

Answer 14

• ants, attamyces fungus
• termites, termitomyces fungus
• ambrosia/ bark beetle, ambrosia fungus

Question 15

fungus farming ants

Answer 15

• Attine (New world) ants (leaf cutter ants) and Attamyces fungus (basidiomycete, white rot)
• ants feed fungus with pieces of lead
• ants carry streptomyces bacteria as a biocontrol agent
• Streptomyces produces a specific antifungal that kills Escovopsis pathogen (Ascomycete), a parasite of the attamyces fungus garden
• specialised dump chambers for the leaves
• queen on maiden voyage carries fungus and streptomyces to new nest in pockets