Lecture 21: Mutualism and Symbiosis Flashcards
Symbiosis
living together
Mutualism
each species benefits from the other in the mutualistic interaction
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 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
Yao people of Mozambique
mutualistic relationship between them and honeyguide bird
Lotka-Volterra models of mutualism predict what?
Both populations undergo unbounded exponential growth in an orgy of mutual benefaction.
What actually limits the population growth of mutualists
- strong intra-specific competition
- a third species (predator or competitor)
- diminishing returns to mutualism as the population grows
Invasional meltdown
Two invasive species can interact as mutualists and just EXPLODE
(positive feedback between mutualists tends to generate runaway population growth)
Spring ephemerals
perennial plants (more than two years) which produce aerial parts of the plant and flower and seed right after the snow melts (the roots remain even after the aerial parts have withered) - strategy is common in deciduous forests where small plants can take advantage of the sunlight before a leaf canopy has formed
Weedy ephemerals
also called ruderals, fast growth after soil is disturbed by human plowing
Ant-plant experiments
Invasive ants spread invasive plants
Cleaner fish
feed off ectoparasites on bodies of client fish. Cleaners often have territories that clients visit. Presence of cleaner fish affects biodiversity of other reef fish.
What happens when cleaner fish are removed from a reef?
Reduced biodiversity
Darwin’s Orchid
he predicted that the orchid (which had a very long nectar spur) would be pollinated by an insect with a very long proboscis
- nectar spur must be slightly longer than the proboscis of the animal so that the animal actually has to come in contact with the pollen
Bacterial endosymbionts in Aphids
- aphids feed on phloem sap that is rich in sugars but poor in essential amino acids
- they have developed a mutualism with bacteria
What do we see when we look at the phylogenies of Buchnera bacteria and aphids?
Every time an aphid lineage branches, so does it’s bacteria, as they are vertically transmitted (passed from mothers to offspring in aphid eggs) (allopatric speciation)
Vertically transmitted endosymbionts often have…
tiny genomes, much smaller than free-living bacteria
(human mitochondrial genome is only around 17 000 base pairs and encodes 37 genes, while the nuclear genome has more than 3 billion base pairs)
- lose genes they no longer need, other functions outsource to host genome
Most mutualisms are..
- NOT tightly coevolved, species-specific interactions
- transmitted horizontally (partners are acquired new each generation)
- rarely one-to-one interactions (most plants have multiple pollinators)
What are we studying about mutualisms right now?
- understanding networks of interactions among large numbers of species
- microbiomes
Microbiome
term refers to either all the microbes living together in a community (often a host) or their collective genomes
Antonie van Leeuwenhoek
Amateur scientist from 17th century, father of microbiology
- discovered that diverse microbes live on plants and animals
How are sequencing-based methods of characterizing microbial diversity useful?
Frees us to study microbes that won’t grow in lab conditions; we don’t have to culture them in order to study them.
How do we characterize microbial diversity
Sequence a highly conserved gene (slowly evolving), usually bacterial 16s rRNA gene, then cluster the sequences into taxonomic units based on similarity
The closer the dots….
the more similar the microbiomes (the dots reflect diet, phylogeny, and morphology)
A host’s microbiome affects its…
metabolism, immune system
Mirrored phylogenies suggest
…extremely specific mutualisms (vertical transmission)
