Lecture 21 Flashcards
Mutualism vs Symbiosis
symbiosis = living together
mutualism = beneficial interaction for both species
what does mutualism typically involve?
reciprocal exchange of goods or services between species
3 types of mutualisms
- nutritional
- defensive
- dispersal
nutritional mutualisms
- legumes and rhizobia: exchange fixed C for fixed N
- plants and mycorrhizal fungi: exchange C for P
defensive mutualisms
- ants and plants: exchange protection for food (eg extrafloral nectar) or housing
- cleaner fish and client fish: exchange parasite removal for food
dispersal mutualisms
- plants and animal seed dispersers: exchange seed dispersal for food
- plants and animal pollinators: exchange gamete dispersal for food
mutualism between humans and free-living wild animals
- Yao people in Mozambique harvest wild honey, but can’t find bees’ nests easily
- honeyguides (Indicator indicator) eat bees wax and know where nests are, but can’t access them easily
- honeyguides recognise the specific sound that Yao honey-hunters make to attract them
how would you change Lotka-volterra equations to model mutualism?
constraints of the Lotka-Volterra models of mutualism
both populations undergo unbounded exponential growth in an orgy of mutual benefaction
what limits the population growth of mutualists?
- strong intra-specific competition
- a third species such as a predator or a competitor
- diminishing returns to mutualism as the population grows
invasions meltdown
- positive feedback between mutualists tends to generate runaway population growth
- if two invasive species interact as mutualists, they may facilitate each other’s spread
Spring ephemerals
- perennial understory herbs that flower right after the snow melts, producing a short-lived ‘carpet of flowers’
- have seeds dispersed by ants
elaiosome
fleshy appendage on seeds of spring ephemerals that attracts ants - rich in fats
experiment on seed dispersal by ants
Prof. F wanted to compare seed dispersal by a native seed-dispersing ant (Aphaenogaster rudis) and an invasive seed-dispersing ant, (Myrmica rubra)
- she placed one type of plant in each mesocosm, then added a bunch of native and invasive seeds for the ants to disperse
- with native ant, mostly native plants were dispersed
- with invasive ant, mostly invasive plants were dispersed
Myrmica rubra
native to Europe, where it disperses many seeds
Cleaner fish (Labroides dimidiatus)
- Cleaner fish feed on ectoparasites on the bodies of client fish; cleaners gain food, clients benefit from fewer parasites
- Cleaners often have “cleaning stations” (territories) that clients visit
- Experimentally removing cleaner fish increases parasite (gnathiid) abundance on client fish
effect of the presence of cleaner fish in reefs
affects species diversity of other reef fish
- when cleaners were removed, the number of fish species went down
- when cleaners were added, number of fish species went up
- presence of cleaner fish attracts client fish
Darwin’s orchid, Angraecum sesquipedale
- epiphytic orchid from Madagascar with a very long nectar spur
- Darwin famously predicted that it must be pollinated by an insect with a very long proboscis (sucking mouthpart)
- Alfred Russel Wallace drew a moth like this
- Xanthopan morganii praedicta was discovered in 1903
Reciprocal adaptation (coevolution) between flowers and insects
how is long proboscis/floral tube beneficial?
- when nectar spur of the plant is longer than the mouthpart of the insect, this forces the insect to get deep into the flower in order to drink all the nectar
- this forces the insect to contact the pollen-receiving parts of the flower, improving pollination
- pollinators with longer mouthparts are able to get more food
Bacterial endosymbionts in aphids
- aphids feed on phloem sap that is rich in sugars, but poor in essential amino acids
- aphids have intracellular bacteria (Buchnera) that provide their hosts with essential amino acids
- Buchner are vertically transmitted; they are passed in aphid eggs from mothers to offspring
what is a feature of vertically transmitted endosymbionts?
- Buchnera has a much smaller genome than free-living bacteria (eg E.Coli); other endosymbiotic bacteria also have tiny genomes
- in humans; mitochondrial genome is ~17000 base pairs (and encodes just 27 genes); nuclear genome is >3 billion base pairs
- endosymbiotic bacteria lose genes that they no longer need
why do endosymbiotic bacteria lose genes that they no longer need
- some functions unnecessary because bacteria are no longer free-living; bacteria protected inside host cells
- other functions outsourced to host genome
are mutualisms often highly specialised?
- Most aphid species have their own species of Buchnera bacteria
- But most mutualisms are NOT tightly coevolved, species-specific interactions
- Most mutualisms are horizontally transmitted; partners are acquired anew each generation
- Mutualisms are rarely one-to-one interactions; usually many-to-many interactions
current areas of mutualism research
- Understanding networks of interactions among large numbers of species
- Microbiomes: this term refers to either all the microbes living together in a community (often, a host) or their collective genomes
do plants only have one pollinator species?
no, most plants have many pollinator species, and most pollinators visit many plant species
how can we characterise microbial diversity in a host (or environmental sample)?
- culture-based methods
- sequencing based methods
sequencing-based methods
- Sequence a highly conserved (i.e., slowly
evolving) gene, usually the bacterial 16S rRNA gene - Use DNA sequence data to identify microbes
- Frees us from having to culture microbes in order to study them
mammal gut microbiomes reflect
diet, phylogeny, and morphology
