W10L2 Host parasite co-evolution Flashcards
1
Q
The host parasite evolutionary race history
A
- Humans have created many new evolutionary pressures (antimicrobials, pesticides) and modified some existing ones
- Nonetheless, competition between and within species for resources remains a nearly ubiquitous evolutionary driver
2
Q
What is paratism? The effect of selection on host and parasite
A
- Parasitism is characterised by an ongoing association between two organisms which is detrimental to host fitness
- Natural selection on host favours genotypes suppressing parasite replication or limiting damage
- Natural selection on parasites favours more efficient reproduction and spread
3
Q
Is parasitism stable
A
- It is difficult to know how long most parasitic relationships have lasted, but some can be inferred to be ancient
- Relationships among primate body lice indicate descent from a louse living on primate common ancestors
- Body lice transmit numerous pathogens - typhus, trench fever
4
Q
Parasite pressure on host
A
- As well as vectoring disease, parasites can also impose various direct costs on their hosts:
- Consuming nutrients
- Blocking airways/blood vessels
- Tissue damage
- Some parasites can also provide benefits, e.g. Wolbachia
- Parasite resistance mechanisms can also be costly (e.g. reduced development time)
5
Q
possible cause of host parasite co evolution
A
Two main modes of evolution:
* Variants affecting host-parasite interaction regardless of the genotype of the other; these will be the subject of selective sweeps (‘directional selection’)
* Variants improving performance relative to host/parasite when they are rare (‘balancing selection’)
6
Q
Selective sweeps in host-parasite interactions
A
- An ‘arms race’ of selective sweeps would be expected to reduce genetic diversity in both host and parasite
- Particularly easy to detect in bacterial chromosomes
- Xanthomonas axonopodis causative agent of bacterial spot disease in capsicum and tomato –resistance gene Bs2 introduced to relevant crops in 1990s
- Genetic diversity in X. axonopodis extremely low, suggestive of selection on avrBs2 (prevents detection)
7
Q
The Plasmodium vivax arms race
A
- Plasmodium vivax is a common cause of malaria in Asia and the Americas, but rare in Africa
- High levels of resistance in African populations due to spread of alleles removing the Duffy receptor from blood cells
- In recent years, multiple reports of P. vivax infection in Duffy-negative individuals - possibly related to P. vivax DBP duplication assisting alternate receptor binding
8
Q
Detecting selective sweeps
A
- In species which undergo recombination, selective sweeps can be identified by lack of genetic diversity in surrounding regions
- Drosophila melanogaster sigma virus –vertically transmitted, reduces egg viability and overwintering success
- Polymorphism in D. melanogaster ref(2)P makes extremely strong contribution to resistance by reducing viral replication
- A viral strain less susceptible to ref(2)P spread across Europe in the 1990s with low genetic diversity
9
Q
Detecting frequency-dependent selection
A
- In contrast to the reduced diversity around adaptive loci expected with positive selection, frequency-dependent balancing selection will result in preservation of variation lost under neutral evolution
- Various statistical measurements of diversity can be used, e.g.Wright’s FST, depending on how long balancing selection has existed
10
Q
Experimental co-evolution
A
- Field studies of co-evolution are technically challenging - time-shift approaches common in experimental evolutionary studies
- Co-evolve e.g. bacteria and phage for many generations, isolating individuals at numerous time points and comparing performance on earlier/later generations
- Variants which allow infection of a broader range of hosts are often costly
11
Q
Direct evidence of co-evolution
A
- Where specimens are preserved adequately, time-shift analyses can be conducted in field contexts
- Daphnia magna (waterbug) hatched from dormant eggs preserved over forty years, tested against endosymbionts from various sediment layers (various time period)
- Average trend supporting balancing selection, but clear variation across D. magna isolates
12
Q
Field co-evolution and genomic mapping
A
- result from analysis on Bacillus thuringiensis infection in Caenorhabditis elegans over 23 generations indicates balancing selection drives co-adaptation
- No clear evidence of genomic changes consistent with this pattern, e.g. changes in direction of selection in two halves of study
- B. thuringiensis underwent oscillation in Cry6B levels due to change in copy number
13
Q
Co-adaptive side effect
A
- A variety of genetic changes which increase parasite resistance are deleterious in the absence of the parasite, e.g. mucoidy in bacteria
- Reversion of these changes can occur experimentally, but field data is limited
- Costliness of resistance likely to vary with environment
14
Q
Parasite adaptation in human evolution
A
- Selective sweeps in human evolution appear to have been largely ‘soft’, i.e. arising from novel selective pressure on existing variation
- Adaptive immune genes over-represented among both genes having undergone soft sweeps and genes under balancing selection in the human lineage
- Identifying specific drivers of immune evolution is often challenging
15
Q
Sexual reproduction and parasitism
A
- The benefits of genetic diversity in parasite adaptation are commonly proposed as an explanation of sexual reproduction
- Some clear advantages of asexuality -no need to find a mate, transmission of entire genome to all offspring
- However, asexual reproduction is rare among multicellular organisms, and most asexual lineages prove short-lived
