12. Host-Symbiont Relationships III - Host Adaptation and Divergence

Question 1

Structure of this lecture

Answer 1

Host ADAPTATION
1. Genetic and Adaptive Plasticity
2. Adaptive plasticity via beneficial microbes
3. Adaptive plasticity shapes genetic plasticity via the Baldwin effect
4. This creates adaptation, which has long term impacts

Host DIVERSIFICATION
5. Microbial symbionts allow hosts to access new niches
6. Microbial symbionts contribute to pre-zygotic isolation
7. Microbial symbionts contribute to post-zygotic isolation

Question 2

3 major pieces of evidence that microbes contribute to host adaptation

1.1

Answer 2

1. Obligate symbioses require genetic basis for symbionts to fit the environment (e.g., aphids and their Buchnera)
2. Cross-species inoculations show that host species is adapted to its own microbiome, and survival/fitness is significantly higher when hosts have their own microbiomes
3. Microbiome convergence show that microbiomes drive host adaptation to particular divergence (e.g., shown by PCO plotting of microbiomes vs., prey)

Question 3

What are the two components required for host adaptation?

1.2

Answer 3

Genetic plasticity, adaptive (phenotypic) plasticity

Question 4

How did Carmody et al., 2015 show that microbes provide hosts with adaptive phenotypic plasticity?

2.1

Answer 4

New environment alters microbial composition by selecting for some taxa over others

e.g., testing high-fat and low-fat diets in mice showed changes in gut microbiome composition over a matter of hours

Question 5

How did Sonnenburg et al., 2005 show that microbes provide hosts with adaptive phenotypic plasticity?

2.2

Answer 5

New environment is able to alter microbial metabolism due to microbial phenotypic plasticity

e.g., change in glycoside hydrolase expression in gut bacteria with different poly-saccharide molecules available

Question 6

How did Barroso-Batista et al., 2014 show that microbes provide hosts with adaptive phenotypic plasticity?

2.4

Answer 6

New evolution drives the evolution of symbiotic microbes

e.g., two strains of fluorescentally-labelled E. coli inoculated into the gut microbiome, and antibiotics applied as a selective pressure. Both strains showed rapid, short term evolution of antibiotic resistance within 24 days

Question 7

How did Hehemann et al., 2012 show that microbes provide hosts with adaptive phenotypic plasticity?

2.4

Answer 7

New environment can drive the evolution of symbiotic microbes via HGT

e.g., Bacteroides have developed a Polysaccharide-Utilisation-Locus (PUL) via HGT in individuals in Japan who eat a lot of seaweed

Question 8

How did Kikuchi et al., 2012 show that microbes provide hosts with adaptive phenotypic plasticity?

2.5

Answer 8

Host acquires pre-adapted microbes from the environment

e.g., selection of insecticide resistance strains of Burkholderia from the environment, which allowed for Beanbug resistance to insecticides via pre-adapted plasticity

Question 9

What is the Baldwin effect?

3.1

Answer 9

The phenotypic challenge of accomodating to a new challenge, and eventual genetic adaptation to it.

Plasticity turns to adaptation.

Question 10

How does the Baldwin effect explain adaptive plasticity?

3.2

Answer 10

The Baldwin effect shows that the microbiome provides phenotypic plasticity during host adaptation, ensuring host survival and persistence in a new environment

Question 11

How does the Baldwin effect lead to eventual genetic adaptation?

3.3

Answer 11

The Baldwin effect shows that selection acts on the host to ‘cement’ beneficial relationships by maintaining new relationships and supporting successful colonisation

Question 12

What is an example of the Baldwin effect?

3.4

Answer 12

Rafaluk-Mohr et al., 2018

Proof of principle study in C. elegans

Two evolutionary treatments applied to population (food pathogen, and S. aureus parasite).

Showed that host evolves with repeated exposure to pathogen colonisation

Evidence shows initial plasticity that turns into genetic fixation over time

Question 13

What are the three long-term evolutionary outcomes of host adaptation via microbes?

4.1

Answer 13

1. Host adaptation may take over microbial genes (e.g., Lactase persistence in humans taken from Bifidobacterium)
2. Symbiotic microbes change the pace, or prevent adaptation
3. Loss of host genetic adaptation, and increasing out-sourcing to microbes

Question 14

Essay plan for ‘How do microbes contribute to host adaptation?’

Answer 14

1. Host adaptation requires genetic and adaptive plasticity, which microbes contribute to (1.1/1.2)
2. Microbes contribute to host adaptive plasticity (2)
3. Host adaptive plasticity is fixed via Baldwin effect (3)
4. Long-term evolutionary outcomes (4)

Question 15

How can microbial symbionts provide access to new niches?

5.1

Answer 15

Symbionts can promote speciation by allowing access to previously inaccessible niches (i.e., new habitats, nutrition etc.,)

Question 16

Example of how symbionts allow for access to new niches?

5.2

Answer 16

Comparative analysis between obligate symbioses and insect diversification showed that obligate symbioses have evolved in insects 13 times in various forms of niche, and has allowed for invvasion of nutrient-deficient feeding niches

Question 17

Evidence against symbionts providing access to new niches

5.3

Answer 17

Bennet and Moran, 2015

Symbiotic diversification could hinder diversification.

There may be an initial ‘burst’ of diversification that is then slowed by the narrow parameters of symbionts

Hosts could become ‘trapped’ by narrow constraints of symbiont

Question 18

Two examples of how symbionts could contribute to pre-zygotic isolation

6.1

Answer 18

1. D. melanogaster and reproductive isolation. Highly controversial evidence (argued against by Leftwich et al., 2017) as to whether starch concentrations change reproduction
2. Microbial metabolites may be used in scent communications (e.g., meerkats, badgers, hygenas)

Question 19

Example of how microbial symbionts could lead to post-zygotic reproductive isolation?

7.1

Answer 19

Hybrid lethality - i.e., in Nasonia spp. in mosquitos

Question 20

Essay plan for ‘How do microbes contribute to host diversification’

Answer 20

1. Allows access to new niches (5)
2. Pre-zygotic isolation (6)
3. Post-zygotic isolation (7)