10. Host-Symbiont Relationships - Symbiotic Relationships Flashcards
Structure of the lecture
- Defining symbiosis
- Different types of symbiotic relationship
- Parasite-Mutualist continuum
- Variance within communities
- Evolutionary explanations for symbiosis
- Benefits of symbiosis
- Resource-exchange vs., mutualism
How do we define symbiosis?
1.1
A prolonged, physical association between organisms of two species
What are some caveats with defining symbiosis?
1.2
There are blurry lines between what constitutes a symbiont, and how ‘close’ or ‘prolonged’ relationships are defined, particularly when relationships can be dynamic or transient
What is the difference between endo- and ecto- symbionts?
1.3
An ectosymbiont lives on the surface of the host. An endosymbiont lives within the host (can be intra- or extra- cellular)
Define facilitation
2.1
Facilitative relationships occur when the symbiosis is at least partially beneficial for both parties. It includes commensalism and mutualism
Define commensalism
2.2
A facilitative relationship in which one species (the commensal) has a fitness benefit, and the other experiences no net benefit
+/0
e.g., epiphytes on living trees
Define mutualism
2.3
A type of facilitative relationship, in which there is a net positive relationship for both species involved
+/+
e.g., pollinators on flowers
Define parasitism
2.4
Another class of symbiotic relationship in which the parasite benefits at the expense of the fitness of the host
+/-
e.g., tongue-eating louse in fish
What is some issues with defining symbiotic relationships?
3.1
- We cannot use ‘facultative’ or ‘parasitic’ etc., to describe communities. These terms only refer to pairwise relationships
- These terms describe dynamic, fluctuating and sometimes transient relationships that exist along the parasite-mutualist continuum and change across ecological space and evolutionary time
- Symbiotic relationships can often be context dependent and will appear differently depending on the unique environment of the organisms involved
Give an example of a symbiotic relationship that is highly context dependent
3.2
Squirrels infected with trypanosomes will often have a greater mass than un-infected squirrels in a Vitamin B6 deficient environment.
However, the opposite is seen in environments with excess B6.
This is because trypanosomes provide squirrels with B6 under deficient conditions, allowing squirrels to maintain their mass.
Thus, a benefit is only conferred under certain conditions
List four ways in which symbiotic communities can vary
4.1
- Composition
- Complexity
- Host-dependence
- Transmission
How can symbiotic relationships vary in terms of composition?
4.2
Variation within the number of organisms. Communities are the ‘norm’ and symbioses are rarely 1-1. Communities depend on the number of organisms and the subsequent ecological relationships/dynamics
How can symbiotic relationships vary in terms of diversity?
4.3
Relationships can be relatively simple and be pairwise (e.g., Hawaiian bobtail squid and Vibrio fischeri)
Relationships can have a low-level of complexity (e.g., Honeybee has ~8 core species)
Relationships can have a high-level of complexity (e.g., human gut microbiome has around ~200 species that are in a constant state of flux with each other)
How can symbiotic relationships vary in terms of host dependence?
4.4
Relationships can be facultative (e.g., legume-rhizobia)
Relationships can be obligate (e.g., Aphids and their Buchnera)
How can relationships vary in terms in terms of transmission?
4.5
Transmission can be horizontal, and can come from the environment
Transmission can be vertical, and come from the mother.
Fisher et al., 2017 performed a meta-analysis and argued that host-dependence may be linked to mode of transmission, with more dependence linked to vertical transmission. Higher dependence is also more linked to symbionts involved in nutrition
What are the four major benefits of symbiosis?
5.1
- Nutrition
- Defense
- Disguise
- Homeostasis/allometry
How can symbiosis benefit nutrition?
5.2
- Biosynthesis of previously inaccessible nutrients (e.g., Legume-Rhizobia provision of nitrogen)
- Neutralisation of dietary toxins (e.g., desert woodrat feeds on the toxic creosote bush, and has a creosote-experienced microbiome)
- Providing a new form of energy capture (e.g., Chemoautotrophic symbioses in the giant tube worm)
- Breakdown of previously inaccessible compounds (e.g., bacterial breakdown of starch in the mammalian gut microbiome)
How can symbioses benefit defence?
5.3
- Protection from pathogens (e.g., Spiroplasma fungi protects D. neotestaecea flies from pathogenic nematodes)
- Colonisation resistance. This is useful for 4 main purposes, including stimulation of host immunity (e.g., via producing AMPs), direct inhibition of pathogens (e.g., via production of bacteriocins), exploitative competition, and altering local conditions (e.g., pH)
How can symbioses benefit disguise?
5.4
e.g., like in Vibrio fischeri bacteria, which act as symbionts in the light organ of Hawaiian bobtail squid
How can symbioses benegift homeostasis?
5.5
e.g., symbionts in the mammalian gut microbiome are important for protection against cold temperatures
How can we untie resource-exchange and mutualism?
6.1
It is important to understand that resource exchange does not necessarily imply mutualism
For instance, in Chlorella alga in their Paramecium host, photo-symbiosis is occurring, which is leading to resource exchange. However, the Chlorella does better alone. Therefore, this does not imply mutualism, but instead facultative resource exploitation
