L9 host symbiont 3 Flashcards
What are the two main evolutionary processes examined in host-symbiont relationships?
Host adaptation via genetic change and host diversification over evolutionary time through speciation.
How is the lecture structured?
Part 1 covers how microbial symbionts shape host adaptation; Part 2 explores their role in host diversification (niche alteration and reproductive isolation).
Define genetic adaptation of a host.
An evolutionary process driven by natural selection causing heritable genetic changes across generations that improve host fit to its environment.
How does adaptive plasticity differ from genetic adaptation?
Adaptive plasticity is a single genotype producing multiple phenotypes in response to the environment, whereas genetic adaptation involves changes in allele frequencies over generations.
Give an example of an obligate symbiosis in tube worms.
Tube worms house chemosynthetic bacteria in trophosomes; these symbionts provide essential nutrients, and worms cannot survive without them.
How do aphids demonstrate obligate symbiosis?
Aphids contain bacteria in specialized cells (bacteriocytes), and both host and symbiont depend on each other for vital nutrients.
What role do microbial symbionts play in weevil cuticle formation?
Symbionts produce compounds that harden the cuticle; removal of these microbes yields a brown, crumbly cuticle.
What did microbiome‐swap experiments in Nasonia wasps show?
Wasps receiving non‐native microbiomes had lower survival, while those with their native microbiome showed highest fitness, indicating genetic adaptation to species-specific microbiomes.
What does correlational evidence in mammals reveal about diet and microbiomes?
Mammals with similar feeding strategies cluster together by microbiome composition (e.g., aardwolves cluster with termite-eaters), suggesting microbiomes aid niche adaptation.
What is the “extended phenotype” mechanism for host adaptation?
If microbiome traits are heritable, they extend the host phenotype and undergo selection alongside host genes.
How can microbes contribute to adaptive phenotypic plasticity in hosts?
Through vast gene reservoirs, rapid gene‐expression shifts, horizontal gene transfer, short generation times, and transmissibility, providing flexible host responses.
Describe the diet‐oscillation experiment in mice.
Mice alternated low- and high-fat diets; their microbiomes flipped correspondingly, demonstrating rapid microbial compositional plasticity tied to host metabolism.
What did the polysaccharide utilization loci study reveal about microbial plasticity?
A gut bacterium altered glycoside hydrolase expression when grown on glucose, in a mouse caecum, or in maltotriose, showing context-dependent enzyme regulation.
What is an obligate symbiosis?
A relationship where host and symbiont are mutually dependent for survival or reproduction.
Define a trophosome.
A specialized organ in some hosts (e.g., tube worms) that houses symbiotic microbes for nutrient exchange.
What are polysaccharide utilization loci (PULs)?
Gene clusters in microbes encoding enzymes (like glycoside hydrolases) to break down complex carbohydrates.
What is a glycoside hydrolase?
An enzyme that catalyzes hydrolysis of glycosidic bonds in carbohydrates, enabling microbes to digest polysaccharides.
How do microbial symbionts integrate with host genetics to drive adaptation?
They act as an extended phenotype, adding both genetic functions and plastic responses that selection can act upon.
What did the antibiotic‐selection experiment with fluorescent E. coli strains in mice demonstrate?
Under antibiotic pressure, two fluorescent E. coli strains evolved triple‐antibiotic resistance within 24 days, showing rapid microbial evolution in hosts.
How did horizontal gene transfer enable Bacteroides to adapt to a seaweed‐rich diet?
Human‐gut Bacteroides acquired a porphyran polysaccharide‐utilisation locus from marine bacteria, allowing digestion of seaweed porphyran.
What is direct environmental uptake of adapted microbes?
Hosts acquire microbes pre‐adapted to local conditions by ingesting or contacting environmental substrates (e.g., soil, food).
How did bean bugs benefit from insecticide‐resistant Burkholderia symbionts?
Bugs picking up resistant Burkholderia from soil survived significantly better on insecticide‐coated soybeans than those with susceptible strains.
What is the Baldwin effect in host–symbiont relationships?
Phenotypic plasticity via microbiomes allows initial survival in new environments, followed by genetic accommodation of the beneficial association.
What are the steps of the microbiome‐mediated Baldwin effect?
(1) Microbiome provides plasticity for survival; (2) selection favors hosts that house, maintain, or acquire beneficial microbes; (3) genetic accommodation cements the association.
What did the C. elegans–E. faecalis study reveal about symbiont‐mediated protection?
Worms with ancestral E. faecalis showed immediate survival against S. aureus, then evolved increased protection and gut colonisation over time, supporting the Baldwin effect.
What is a polysaccharide utilisation locus (PUL)?
A microbial gene cluster encoding enzymes to degrade specific complex carbohydrates.
Define genetic accommodation.
The process by which a phenotype initially induced by environmental factors becomes genetically encoded through selection.
What is porphyran?
A seaweed‐derived polysaccharide that certain gut microbes can digest after acquiring the appropriate utilisation locus.
Define horizontal gene transfer.
The movement of genetic material between organisms without descent, enabling rapid acquisition of new traits.
What are symbiotic microbes?
Microorganisms living in close, often mutualistic, association with a host organism.
Define antibiotic resistance.
The ability of microbes to survive antibiotic exposure through genetic mutations or acquired resistance genes.
What is insecticide degradation by microbes?
The enzymatic breakdown of insecticide compounds by symbionts, allowing hosts to tolerate insecticide exposure.
Why use fluorescent labelling in microbial evolution experiments?
To visually track and quantify competing microbial strains and monitor emergence of adaptive mutants within hosts.
What is adaptive phenotypic plasticity?
The capacity of an organism (or its microbiome) to alter phenotype in response to environmental changes to improve fitness.
What is microbiome-mediated genetic assimilation?
The process where initial adaptive benefits from microbes become encoded in the host genome over evolutionary time, so hosts take over functions once provided by symbionts.
What factors favor microbiome-mediated genetic assimilation?
High energy or fitness benefits when hosts perform the function themselves, and situations requiring only regulatory changes rather than novel innovations.
Give two examples of functions hosts may assimilate from their microbiome.
Colonisation resistance (immune functions) and nutrient degradation (digestive enzymes).
How does lactase persistence in humans illustrate genetic assimilation?
Gut bacteria aided lactose digestion ancestrally; selection for adult milk consumption led to host LCT-locus mutations enabling self-digestion.
What evidence links lactase persistence genotype to microbial change?
Western lactase-persistent populations show reduced abundance of lactose-digesting Bifidobacterium correlated with milk consumption.
What is a regulatory change in the context of genetic assimilation?
A mutation affecting gene expression timing or level (e.g., LCT promoter mutations maintaining lactase into adulthood).
How can adaptive plasticity inhibit further genetic change?
Plastic responses can place hosts near a fitness peak, reducing selection pressure for additional genetic adaptation.
What are the two schools of thought on plasticity’s role in adaptation?
One view holds plasticity reduces the need for genetic change; the other sees plasticity as a precursor enabling genetic adaptation.
What happens when hosts fully outsource functions to their microbiome?
Host selective pressure relaxes, potentially leading to loss of the host trait and increasing dependency on symbionts.
How can symbiotic microbes open new niche spaces for hosts?
By providing obligate nutritional capabilities, hosts can exploit previously inaccessible diets or habitats.
What insect evidence shows niche expansion via symbiosis?
Many insects with obligate symbionts survive on specialized diets (e.g., wood, phloem) that are unusable without them.
What constraints of obligate symbiosis can raise extinction risk?
Vertical transmission leading to small symbiont populations, restricted genetic exchange, and accumulation of deleterious mutations.
What is the “evolutionary rabbit hole” in host-symbiont systems?
A scenario where tightly co-evolved genomes incur fragility from reduced genetic exchange and specialized dependencies.
How can microbial symbiosis both promote and hinder host diversification?
It promotes diversification by enabling new niches, but can hinder it via increased dependency, genetic incompatibility, and extinction risk.
Define vertical transmission in symbiotic relationships.
The passage of symbionts directly from parent to offspring, often through eggs or reproductive tissues.
What is reproductive incompatibility in the speciation context?
A situation where divergences (possibly including symbiont-driven changes) prevent successful interbreeding between lineages.
