Lecture 12: Fungi as parasites and pathogens Flashcards
Parasite:
an organisms which lives in or on another organism (its host) and benefits by deriving nutrients at the other’s expense (negative effect but can be very small)
pathogen:
causes disease in its host (definite negative effect on host)
what makes fungi such good parasites?
Asexual reproduction
how does asexual reproduction aid parasites?
- allows more rapid dispersal than sexual reproduction, great when conditions are ideal
- Adverse environmental conditions often cause sexual reproduction in fungi
- Asexual achieved through different methods
Asexual reproduction methods in fungi
- asexual spores
- hyphal fragmentation
- cell vision/budding (unicellular fungi)
asexual reproduction via vegetative spores:
Asexual spores are genetically identical to the parent and may be released either outside or within sporangium
asexual reproduction via cell fission or budding (unicellular fungi)
1) cell replicates its DNA
2) The cytoplasmic membrane elongates, separating DNA molecules
3) Cross wall forms; membrane invaginate
4) cross wall forms completely
5) daughter cells
Asexual reproduction via hypha fragmentation
hypha simply fragments into single cells
2 types of fungal plant parasites:
- Necrotrophic
- Biotrophic
necrotrophic:
- host plants cells are destroyed, and the fungal pathogen feeds off this material
- sometimes the damage is discrete, sometimes it causes whole plant death
biotrophic:
-tissues are invaded but are not killed
necrotrophic/biotrophic or both?
some fungal species undergo both a biotrophic stage and a necrotrophic
airborne Biographic infection (Uromyces fabae)
1) spore arrives on plant surface
2) spore germination
3) spore swells and germ-tube emerges
4) Appressorium develops if a leaf/stem infecting fungus
5) infection peg exerts pressure against cell wall
6) cell wall penetration and a haustorium is formed
haustorium:
hyphae specialised for extracting nutrients from living cells
- 2 parts: haustorial mother cell and large SA bit inside the cell.
- transport of H+ and metabolites
haustorium membrane and wall
- haustorial plasma membrane
- haustorial wall
- extrahaustorial matrix
- extrahaustorial membrane
what links haustorial mother cell to main bit
neckband
sugar deposition in healthy leaf
- sugars are produced in the leaf via photosynthesis (SOURCE)
- sugares are transported to the centres of growth or storage (SINK)
sugar deposition in unhealthy leaf (biotrophic infection e.g Rhytisma acerinum)
- MOST sugar transported towards the fungal mycelium
- LITTLE sugar transported to the centres of growth or storage (SINK)
2 examples of biotroophic infections:
-stem rust –>(Basidiomycete: Puccinia graminis)
-Powdery mildew –> (Ascomycete:
Podosphaera fuliginea)
Necrotrophic infection example
- (Phytophthora infestans)
- The Irish Famine of 1846-50
- potato blight causes black rot on potatoes
- peasants who ate the rotten potatoes became unwell and many got cholera and typhus. Wages to landlords couldn’t be paid
- many died (1mill), or emigrated (2mill). Population went from 8 to 5 million
sporangia
encolure in which spored are formed
Zoospore:
spore with flagella
potato blight lifecycle:
- sporangia formed on diseased seedling and leaves are dispersed to healthy leaves
-zoospores are produced and released form sporgania
-Zoospores infect leaves
-THEN EITHER
– lesions and fungus develop on leaves
OR
–zoospores infect tubers
-seedlings produced by infected tubers come diseased
BACK TO BEGINNING
New P. infestans strains
- new very virulent strain called P. infestans US-8
- no known fungicide can harm it
- rapidly mutates to form resistance against new potato resistance genes or fungicides
Dutch elm disease:
- caused by an Ascomycete fungi species and spread bu the Elm bark beetle
- Ophiostoma ilmi(1930’s attack) pathogen has newer, very aggressive subgroup Ophiostoma Novo-ulmi (1970’s attack)
- ~90% of UK’s Elms have died (>25 million trees), and is still spreading northwards
Dutch Elm disease cycle:
-Adult beetles emerge in spring & summer from bark of dead/decaying elms carrying fungal spores
-They feed on the twig crotches of healthy elms and introduces spores into wood
-Infected parts wilt and diseased twigs show dark streaks/ spots
-Trees weakened by disease become breeding sites for beetles
-Larvae develop galleries with in the bark
-Fungus fruits in the pupal chambers
AND REPEAT
The actual cause of death for dutch elm disease is:
DEHYDRATION/STARVATION
- in an attempt to block the fungus from spreading farther, the tree reacts by plugging its own xylem tissue with gum
- as the xylem delivers water and nutrients to the rest of the plant, these plugs prevent them from travelling up the trunk of the tree, eventually killing it
fungi can also be the solution ( to other fungi)
Biocontrol: plant pathogenic gun can be infected by mycoparasitic fungi
e.g. ~30 genera of fungi can be hyperparasitic on rust fungi
fungi parasitic on insects:
- Ability to attack and invade insects has evolved repeatedly; most fungi taxonomic groups have insect parasites
- Most species specialised on one stage (larval/pupal/adult)
cordyceps:
genus of ascomycete fungi. all are endoparasitoids, parasitic mainly on insects and other arthropods
Anti-zombie-fungus fungus: Hyperparasitic control of ant parasite
-Ophiocordyceps infection in ants
-Only ~ 6.5% of all fruiting bodies
are viable spore-producers.
-This is caused by the castration of the fungus by an unknown hyperparasite, which may cause the limiting of the viable infectious spores.
-Ant colonies might not be in as much danger as previously thought
Nematophagous fungus
- loops and snares (can be inflated in 1/10th second)
- sticky hyphae
Chytridiomycosis
- infectious disease affects amphibians worldwide.
- caused by chytrid fungus: Batrachochytrium dendrobatidis
- the fungus invades the surface layers of the frogs skin. Infected skin cannot breathe, hydrate, osmorefulate, or thermoregulate correctly
Chytridiomycosis in frogs example cycle
-Motile zoospores: swim in water, penetrate skin & form zoosporangium
-growth in diameter and complexity
-dischage papilla forms
-mortaliy ~2 weeks
-cap lost, zoospores escape through skin
(23 degrees critical temp)
Chytrid fungus associated with how many species
- Associated with >200 species extinctions of amphibians; 30% of the amphibian species of the world affected
- -the mechanism behind the ra[id global emergence of the disease is poorly understood
- no control method has proved to be feasible on a large scale
enhanced call effort in Japanese tree frogs infects by amphibian chytrid fungus paper:
- infected male Japanese tree frogs called more rapidly and produces longer calls than uninfected males
- this enhanced call effort may reflect pathogen manipulation of host behaviour to foster disease transmission
white nose syndrome:
- emerging disease in North American bats caused by the fungus Pseudogymnoascus destructans, a psychrophilis (cold-loving) ascomycete fungus
- disease is characterised by invasive growth of P.destructans on the muzzle and wings of hibernating bates
- Death from expending the fat reserves for winter survival? Associated with unusual winter behavior like abnormally frequent or long arousal from hibernation
when was white nose syndrome first identified
- 2006
- By 2012 was associated with at least 5.7 million bat deaths; some species have declined >90% within five years of the disease reaching a site
- No obvious treatment or means of preventing transmission is known
examples of human fungal diseases:
- dandruff
- ring worm, barbers itch, athletes foot
- mucorosis
- aspergillosis
