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.
