W7 Coevolution

Question 1

What is coevolution?

Answer 1

• reciprocal evolutionary interactions between pops belonging to two entities- leading to the change in allele frequencies in both pops (host pathogen)

Question 2

Types of coevolution

Answer 2

1. Mutualistic= evolved mutually beneficial relationships
2. Antagonistic= adaptations in 1 species matched by counter adaptations in the other- aim to overcome
3. Red queen dynamic= running to stay in same place- pathogen and host constantly evolving to maintain advantage

Question 3

Would a pathogen potentially evolve faster or slower than a host?

Answer 3

faster, due to short generation times and faster mutation rates

Question 4

Which type of coevolutionary dynamics are best suited to simple versus
complex pathogens?

Answer 4

simple pathogens more likely to experience selective sweeps due to higher mutation rate

Question 5

What are the requirements for co-evolution to occur?

Answer 5

• genetic variation exists for relevant host and pathogen traits
• traits of both interacting species are detrimental to each others fitness
• outcome of pathogen interaction depends on combination of genes involved

Question 6

What are the two main GENETIC MODELS

of host-pathogen interactions?

Answer 6

1. Gene for gene model

2. Matching alleles

Question 7

How do the genetic models RELATE to

coevolutionary dynamics?

Answer 7

1. Gene for gene model- The model predicts a universally virulent parasite, associated with arms race dymamics
2. Matching alleles- Leads to Fluctuating Selection Dynamics, resulting continued genetic polymorphism

Question 8

How can we study coevolution

EXPERIMENTALLY?

Answer 8

time shift experiment

Question 9

Why are the predictions of a TIME-SHIFT
experiment different for each type of
coevolution?

Answer 9

If coevolutionary changes were driven by selective sweeps;
- pathogens from past less infective, from future more infective
- larger time shift between the parasite and its host allows the parasite to have accumulated a larger number of adaptive mutations
If coevolution were dependent on fluctuating selection
- on average there should be no net increases in pathogen infectivity, or decreases in host resistance, over time

Question 10

Explain the gene for gene model of host-pathogen interaction

Answer 10

-The parasite has avirulence (AB) and virulence genes (ab)- The host has susceptibility (ab) and resistance alleles (AB)
- Resistance occurs when at least one parasite ‘avirulence’ gene is recognised by a host ‘resistance’ gene
The model predicts a universally virulent parasite
- associated with arms race dymamics

Question 11

Explain the Matching alleles model for host-pathogen interaction

Answer 11

• A parasite must exactly match a host’s genotype to elude the host’s immune system and successfully infect the host
• All parasites genotypes infect an equal number of host genotypes
• Leads to Fluctuating Selection Dynamics, resulting continued genetic polymorphism

Question 12

What is the arms race dynamic (selective sweeps)?

Answer 12

A Coevolutionary dynamic

• Selective sweeps of novel host resistance and pathogen infectivity alleles
• Repeated fixation of new alleles for parasite infectivity or host resistance
• relies on mutation to introduce new alleles- occur very rare

Question 13

What is fluctuating selection dynamics?

Answer 13

A coevolutionary dynamic

• Host and pathogen allele frequencies oscillate over time because of negative frequency dependent selection
• Rare alleles in host/pathogen have advantage
• Genetic variants are maintained in the population of a long period