L7 host symbiont 1 Flashcards
How is the lecture series structured?
Three lectures: (1) Intro to host–symbiont relationships; (2) Ecology of microbial community assembly; (3) Evolutionary impacts of symbioses.
What is the modern definition of symbiosis?
A close, prolonged association between two or more different species, including both long-term and transient interactions.
In symbiosis, what roles do ‘host’ and ‘symbiont’ denote?
The host is the larger partner providing habitat; the symbiont is the smaller partner living on or in the host.
What is ectosymbiosis?
Symbionts living on the host’s surface, e.g., bacteria on frog skin.
What is endosymbiosis?
Symbionts living inside the host, either extracellularly or intracellularly (e.g., Buchnera in aphids, Wolbachia in insects).
How are symbiotic interactions classified by fitness outcome?
Mutualism (both benefit), parasitism (one benefits at expense of the other), commensalism (one benefits with little/no effect on the other).
What defines mutualism?
Both partners derive a benefit (e.g., bees and flowers).
What defines parasitism?
One partner benefits at the host’s expense (e.g., tongue lice feeding on fish tissue).
What defines commensalism?
One partner benefits while the other is unaffected (e.g., cleaner fish removing parasites from rays).
Can symbiotic relationships change over time?
Yes—partners may shift between parasitism and mutualism over ecological and evolutionary timescales.
How does microbial diversity vary among hosts?
Examples: bobtail squid harbors one specific microbe; honeybee gut has ~8 species; human gut hosts hundreds of species.
What term describes the collective community of host-associated microbes?
The microbiome (or host-associated microbiota).
What is facultative symbiosis?
Symbionts provide benefits but are not essential for host survival (e.g., mammalian gut microbes, legumes and rhizobia).
What is obligate symbiosis?
Partners depend entirely on each other for survival and often show co-evolved specialization and genome reduction (e.g., leafhopper and Baumannia).
What is horizontal transmission of symbionts?
Acquisition from the environment or social contacts (e.g., corals take up dinoflagellates from seawater; humans pick up microbes from family and pets).
What is vertical transmission of symbionts?
Direct mother-to-offspring transfer (e.g., Asian citrus psyllid females pass key symbionts into oocytes, which form a specialized adult organ).
What are the implications of strict vertical transmission?
It limits symbiont genetic diversity, aligns host–symbiont fitness, drives deeper dependence, metabolic integration, and symbiont genome reduction.
What experimental evidence links transmission mode to host dependence?
A meta-analysis of 89 species found hosts more dependent on vertically transmitted symbionts (via antibiotic removal studies) and an inverse correlation between symbiont genome size and host reliance.
What hypotheses explain the emergence of symbiotic benefits?
(1) De novo hypothesis: symbionts provided entirely new functions (e.g., novel digestion, defense). (2) Co-opted hypothesis: hosts evolved to depend on microbial signals for normal physiology, so loss disrupts development and behavior.
What three broad categories describe how microbial symbionts benefit hosts?
(1) Nutritional, (2) Defensive, (3) Alternate benefits (e.g., disguise, homeostasis regulation).
How do chemosynthetic bacteria provide new energy sources to hosts?
They convert inorganic chemicals (e.g., H₂S, CO₂) into organic carbon and amino acids, fueling deep-sea animals via chemoautotrophy.
Describe the giant tube worm–chemosymbiont partnership.
Tube worms absorb O₂, CO₂, H₂S through plumes; blood carries these to the trophosome, where intracellular bacteria use sulfide oxidation and the Calvin cycle to make amino acids and vitamins.
How do gut microbes break down resistant starch in mammals?
Bacteria with polysaccharide utilization loci (PULs) ferment resistant starch into short-chain fatty acids (SCFAs) in the large intestine.
What experimental evidence shows gut microbes affect host fat storage?
Germ-free mice conventionalized with microbiota gained ~33% more body fat in 10 days despite eating less, demonstrating microbial contribution to energy harvest.
How much of the host’s energy needs are met by microbial fermentation in cows vs. humans?
In cows, fermentation supplies ~70% of energy; in humans, about 10%.
How do symbionts biosynthesize essential nutrients for hosts?
They produce vitamins and amino acids absent or scarce in the diet—for example, Buchnera in aphids synthesizes 13 essential amino acids and B-vitamins via a reduced genome.
What experiment demonstrates microbial detoxification in desert wood rats?
Antibiotic-treated rats lost mass on a creosote diet; transferring microbiota from experienced rats restored tolerance, proving microbes detoxify dietary toxins.
What is ‘defensive symbiosis’?
Symbionts protect hosts by inhibiting or outcompeting pathogens, acting as ‘the enemy of my enemy is my friend.’
Summarize the mushroom-feeding fly–Spiroplasma–Howardula case study.
Flies infected with nematode Howardula suffer low fecundity; co-infection with Spiroplasma bacteria restores egg production and drives symbiont-bearing fly population growth.
What mechanisms underlie gut microbiome–mediated colonization resistance?
Production of bacteriocins (direct inhibition), exploitative competition for nutrients/attachment sites, immune stimulation, and altering local conditions.
How does Lactobacillus defend the vaginal environment?
It ferments sugars to lactic acid, lowering pH and creating hostile conditions for pathogens.
What are ‘alternate benefits’ of symbionts beyond nutrition and defense?
They can mediate host disguise (e.g., chemical camouflage) and regulate physiological homeostasis and allostasis via microbial signals.
What was the design and key finding of the Citrobacter–Bacteroides colonisation resistance study?
Conventional mice with a normal microbiome cleared Citrobacter over time, whereas germ-free mice had persistent infections, demonstrating that an established microbiome protects against pathogen colonisation.
How did substrate preferences differ between Citrobacter pathogen and Bacteroides in the study?
Both favor monosaccharides, but Bacteroides can also hydrolyze complex polysaccharides, showing greater metabolic flexibility than the pathogen.
What did the diet-switch experiment reveal about nutrient competition and colonisation resistance?
On a simple-sugar diet, Bacteroides and Citrobacter competed for the same monosaccharides, reducing pathogen colonisation; isolated growth controls confirmed this effect was due to competition.
How does the Hawaiian bobtail squid use Vibrio fischeri for camouflage?
The squid houses V. fischeri in a light organ and emits ventral bioluminescence that matches moonlight (counter illumination), hiding its silhouette from predators and prey.
What role does quorum sensing play in squid bioluminescence?
V. fischeri release autoinducers; once the concentration threshold is reached, they activate light-production genes, synchronizing bioluminescence.
What experimental evidence supports dynamic bioluminescence adjustment in squid?
Manipulating external light intensity caused corresponding changes in ventral luminescence, confirming real-time symbiont-mediated camouflage.
How does the gut microbiome influence host cold-temperature responses?
It increases insulin sensitivity and induces browning of white adipose tissue into heat-producing brown fat in cold conditions.
What did cold-microbiome transplant experiments demonstrate?
Transferring microbiota from cold-acclimated mice to germ-free mice at room temperature raised body temperature, promoted fat browning, and improved metabolic responses.
Why might community-level microbiome interactions be described as ‘beneficial’ rather than strict mutualism?
Because terms like mutualism apply to pairwise relationships, whereas complex microbiomes comprise many taxa with context-dependent, dynamic roles.
How can symbiotic relationships shift along a continuum?
Environmental factors (e.g., diet, light) can cause associations to range from parasitism to mutualism within a host’s lifetime or across generations.
What defines the aphid–Buchnera symbiosis as obligate mutualism?
Both partners are entirely dependent on each other—Buchnera provides 13 essential amino acids and vitamins; the aphid supplies housing and nutrients.
What did light-intensity experiments reveal about the Paramecium–Chlorella symbiosis?
Low light yielded no benefit; high light benefited the host more than the algae; only medium light gave equal fitness, showing context-dependent interaction.
How does the Paramecium–Chlorella case illustrate facultative exploitation?
Because the net fitness benefit varies with light, the algae sometimes benefit more, indicating exploitation rather than consistent mutualism.
