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.
