Communities and ecosystems: Co-evolution and parasitism

Question 1

Parasitism and co-evolution

Answer 1

Parasitism can lead to co-evolution through escalation or cyclical dynamics

Escalation: The hosts evolve defensive mechanisms to resist the parasites, and in response, parasites evolve more effective means of overcoming host defenses. ‘arms race’.

Example: barassica chemical compounds and butterfly resistance

Example: phage and bacteria study

Cyclical dynamics (red queen): As defense mechanism become more common in the host population, the parasite evolves to evade them, and they become less effective and rare mechanisms are more effective.

Example of cyclical dynamic: Pasteuria and drosophila
- Time shift assay is used to compare ancestral/ current/ future host with ancestral/ current/ future pathogen
- If there is esclation then future host is the most resistant to ancestral pathogen and futre pathogen is most effective to ancestral host.
- If there is cyclical dynamics current host is most effective against current pathogen and vice versa.
- Cyclical dynamics are seen

Question 1

Complex interactions in communities

Answer 1

Interactions become more complex in communities as they are no longer pairwise.

Hosts can have multiple parasites and parasites can have multiple hosts.

Example: Bacteria with multiple phage

• Study infected bacteria with a varying number of phage and then tested the resistance/ infectivity of past, present and future.
• Bacteria which where mixed with the most phage became the most resistance due to the increased selective pressure speeding up evolution.
• The phage became less infective with greater diversity
• suggests positive covariants with selection pulling in the same direction.

Escalation example

However in some scenarios greater diversity of parasite may reduce evo response of host as selection pressure pulling in different directions or negative covariance. Cannot become optimally adapted to every parasite.

Example:
- negative genetic covariance between genes involved in plant resistance slave bugs and other herbivorous insects
- Co infection would restrain evolution

Question 2

The effect of parasitism/ pests on food security and the importance of co-evolutionary theory

Answer 2

Pests, disease and parasitism by fungi and bacteria has lead to 20% of global crop production being lost. This has major effects on the economy as it costs billions and whole farms and businesses are lost.

Example: Irish Potato famine
- In 1845-1849 there was >80% loss of yield
- 1 million deaths, > 1 million emigrated
- Bacteria

Rice blast
- 30% loss of rice
- Fungi

Example: also in live stock
- avian influenza virus
- genetic modification lead to expression of TNA to act as decoy to inhibit and block influenza virus polymerase
- huge economic impact -> push price of eggs to $4.82

Understanding co-evolutionary theory and the dynamics of pathogen/ pest communities is important when trying to control the pathogens because:
- They can rapidly co-evolve
- eradication of one pathogen can lead to the emergence of another
- virulence and resistance can jump between pathogens

Question 3

Solutions against crop pests and disease

Answer 3

Basic solutions:
- New resistant crop varients (plant breeding)
- Pesticides
- Crop heigine and rotations

More specific considerations:

1. Reduce strength of selection, e.g. plant mosaic of crops so harder for pathogen to speciliase
2. Target mechanisms for rapid genetic changes, e.g. block transposable elements
3. Limit opportunity for horizontal gene transfer, e.g. manage co-cropping, limit moving organisms around
4. Allow crop plant to evolve and acquire wild resistance genes (c.f. land races) -> introgression lines (e.g. frequently done with Solanum pennelii)
5. Biocontrol – use a coevolving antagonist of your pest

Example: release of parasitic wasps to control caterpillar populations which destroy vegetation

6. Promote plant micro biome (cultivation practised and soil)

Understanding co-evolutionary theory and the dynamics of pathogen communities is important when trying to control the pathogens.

Question 4

Plant breeding

Answer 4

Plant breeding aims to enhance resistance of the plant to pests and pathogens

However, this creates a selection pressure on the pathogen and they co-evolve to overcome the resistance.

This leads to ‘boom-bust cycles’ as new resistant strains emerge then pesticides overcome resistance so new resistant strains are formed.

Example:
- cycle of oat strains

Question 5

What promotes fast evolution of the plant pathogens

Answer 5

Strong selection pressure
- this is created by resistant plants or pesticides

High genetic variability
- Mutation/ recombination rate
- population size

Question 6

Complications and considerations when trying to eradicate a plant pathogen

Answer 6

Must consider every aspect of pathogen life cycle

The eradication of the threat of one pathogen may provide an opportunity for another pathogen
- Example: the eradication of stripe rust by fungicide provides opportunities for black head mold.

The genetic interaction between species: virulence can jump between species through horizontal gene transfer
- Example: Tan spot aqcuired a toxA gene from wheat blotch fungus allowing infection of resistant wheat
- Example: Coffee wilt outbreaks in the 1990s due to HGT from banana pathogens

Question 7

Example of investigating the origin of pathogen virulence: Coffee Wilt

Answer 7

Coffe Wilt affects coffee plantations
- There was an outbreak in the 1940s
- There were two more outbreaks in the 1990s (robusta and Arabica)
- Were these outbreaks caused by pathogens from the origonal outbreak or did they pick up virulenec genes.

Conclusion
- The pathogens causing the second two outbreaks picked up virulence genes as they were being grown next to bananas allowing HGT between pathogens.
- The two strains picked up different virulence genes from two independent HGT events

Question 8

Overview

Answer 8

The two interesting examples of co-evolution occuring due to interactions:
- Competition
- Parasitism

Mechanisms of parasite/ host co-evolution
- Arms race
- Cyclical

Threats of parasites to food secuirty
- Causes major crop decline
- Threatens the economy

Solution
- Make plants that are resistant to parasites/ pests
- Use pesticides

Problems
- the parasites co-evolve with the plants to overcome the resistance.
- Pesticides and resistant plants create strong selection pressures intensifying this dynamic.

There are also other complication for eradicating pathogens
- Eradication of one pathogen provides opportunities to others
- Virulence genes can be transferred to pathogens by HGT

Question 9

Cool example of co-adaptation

Answer 9

Caterpillar fall onto ground

Release phereomone like ant eggs

Caterpillar taken back to nest

Wasp enters nest that it knows contain caterpillar larvae (unsure how it does this)

Release pheromone to make ants fight

Wasp injects caterpillar with eggs

Ants continue to care for eggs as they form crysalis

Butterfly emerges out of some and wasps hatch out of others

Exploitation and deception