6. Parasitism Flashcards
1
Q
What are the two main challenges for parasites? 2
A
- transmission - movement of progeny/self to new hosts 2. virulence - overcoming host defences
2
Q
What are the methods of parasite transmission? 4
A
- vertical - host parent to offspring 2. horizontal - among hosts withing a population 3. direct transmission 4. vector transmission
3
Q
What is parasite virulence? 3
A
- measure of negative impact on the host 2. can evolve quickly, especially for microparasites due to short generation time 3. join phenotype - depends on traits of parasite and host
4
Q
Describe the incidence of the rabbit and the myxoma virus. 7
A
- european settlers introduced rabbits to australia, they spread rapidly due to lack of predators 2. disrupted ecosystem by eating vegetation 3. myxoma virus (S. american) introduced to control them 4. can use this to test host resistance and parasite virulence 5. wild rabbits collected from australia at succession points and measured against original myxoma strain 6. wild parasites measured against lab rabbits 7. resistance and virulence increased, but virulence never very high
5
Q
Describe the evolution of virulence/the red/green hypothesis. 4
A
- highly virulent strains that kill hosts won’t proliferate - reduces transmission 2. resistance/fast recovery in hosts encourages evolution of virulence in parasites 3. host must be able to reproduce or meet other susceptible individuals to transmit, which selects for less virulent strains 4. prevents virulence from getting too high
6
Q
Give some examples of soil pathogens/parasites as plants. 2
A
- Rhinanthus spp/yellow rattle - increases plant diversity in grasslands so used as a conservation tool 2. euphrasia spp. - eyebright
7
Q
Describe the impact of phytophthora infestans. 6
A
- late blight 2. oomycota - water mold 3. caused the irish potato famine of 1846 4. irish potato farming had few varieties of potato 5. whole crop gone in one week 6. mass mortality and migration
8
Q
Describe some plant responses to infection. 5
A
- no mobile defender cells 2. lack of somatic adaptive immune system, no antibodies so no defense against future infection 3. rely on innate immunity of cells 4. and response to systematic signals from infection sites 5. isolate and sacrafice infected areas, grow replacements
9
Q
How can we control soil-borne disease in agriculture? 8
A
- breed resistant plants and use cultivars 2. mixed cultivar planting - reduces effect of infection 3. chemical controls eg. fungicides and bactericides 4. crop rotation- about 7yrs for agriculture, 4 for horticulture 5. increase drainage as aerated, well drained soils discourage diseases 6. control soil pH - club root exists in acidic, potato scab reduced in acidic 7. high nitrogen levels encourage tender growth so more susceptible to disease 8. poor nutrition also increases susceptibility
10
Q
How do we model disease? 3
A
- epidemiology - mathematically modelling of host-pathogen/parasite interactions 2. simplest models are compartment models 3. these are box and arrow models with simplified population dynamics
11
Q
What assumptions are followed when looking at disease models? 7
A
- constant host population size 2. pathogen directly transmitted between hosts 3. pathogen reproduces within host 4. pathogen has short generation time and high reproductive rate compared to host 5. infection duration is short compared to host lifespan 6. hosts acquire some immunity 7. don’t really work for agricultural disease!!
12
Q
what is the SIR model of disease? 9
A
- useful for predictions of whether epidemics will occur
- determined by pathorgen reproductive rate and avergae number of secondary infected individuals produced by each infected individual
13
Q
Explain the SIR internet trajectory. 3
A
- Vaccination disrupts SIR model and controls spread of disease
14
Q
describe the different types of parasite. 4
A
- obligate - the parasite depends on the host for survival
- facultative - the parasite can survive without the host, but benefits from the interaction eg. better growth and reproduction
- macroparasites (parasites) are larger organisms eg. tape worm, flukes, nematodes, plants
- microparasites (pathogen) cause disease eg. bacteria, fungi, viruses, prions
15
Q
what do parasites and pathogens do? 5
A
- infect animals and plants
- sub-lethal (more common) and lethal effects
- sub-lethal effects more ecologically important as they allow parasite to continue to survive
- cause reduction in growth, development and reproduction in host
- direct or vector transmission
16
Q
Describe soil pathogens. 3
A
- large potential impact on ecnomic crops
- cause most soil borne diseases of vegetables
- fungi can persist in the soil for a long time by production of resilient spores and resting spores
17
Q
Describe bacterial soil pathogens. 4
A
- cause fewer diseases in soil than fungi
- persistance usually short lived
- don’t usually produce resistant spores
- eg. E. carotovora/soft rot
18
Q
Describe viral soil pathogens. 3
A
- usually only survive in living tissue-plants or vectors eg. gunfi and nematodes
- eg. O. brassicae (soil fungus) transmits Mirafiori lettuce virus causing lettuce big vein disease
- the veins expand and lettuce not fit for consumption
19
Q
describe nematode soil pathogens. 4
A
- nematodes reduce growth and fruit production
- can persist in soil as free living worms, eggs or long-lived cysts
- eg. root knot nematodes cause root distortions and swellings on tomatoes
- macro/micro border
20
Q
give some examples of parasitic plants. 3
A
- orobanche spp. - tomatoes
- striga spp. - grass crops
- 1% angiosperms are parasitic
21
Q
what are the direct vs indirect affects of parasites? 3
A
- yellow rattle infects grass - direct
- loss of grass reduces competition so more species present - indirect
- grass may pass infection to legumes, or attempt to pass to resistant forbs - indirect
22
Q
What are the effects of parasitic plants on the soil? 4
A
- hemiparasites (facultative) accumulate nutrients in their leaves, which fall more quickly than the host leaves
- this increases decomposition rate of all plant litter
- facilitates return of nutrient to soil
- B. alpina (arctic parasite) increased N input from litter to soil by 42%
