Chapter 17 Beinart Oral Exam Guide

• Go through citations and summarize each one with 1 sentence
  
• consequences of symbiont transmission mode on the evolution of the partners
• ecological and evolutionary trade-offs of symbiont transmission mode
• cooperation/communication and interactions with multicellular organisms topics from class

Notes based on Hmelo 2017 and Smith and Schuster 2019.

Symbiont transmission mode and evolution

Symbiont transmission type: vertical vs. horizontal modes in addition to mixed and ‘pseudo-vertical.’

In vertical transmission, beneficial evolution would depend on parental experiences. Could go either way - beneficial or detrimental..

Horizontal transmission relies on the availability of symbionts nearby. If limited numbers, symbionts can be housed by several types of animals not just coral.. so if the coral has competition, they may not get the best symbionts.

Trade-offs:
- Horizontal transmission: ability to sort through symbionts/filtering,
- Vertical: potential for adaptive inheritance and more precise/adapted partnership

Cooperation and communication

Cooperation = collective, multi-cellular behaviors (5):
- Biofilm formation
- Quorum sensing: signaling molecules coordinate the behavior of the population
- Nutrient acquisition
- Dispersal
- Hunting

Public Goods: Compounds or functions that provide a collective benefit, usually through extracellular release:
- Benefit generally increases with density; sometimes only effective above a certain threshold
- large proteins; small metabolites or light produced by bioluminescent bacteria
- passively or actively released

Cheaters:
- individuals in the population that benefit from the public goods produced by the cooperators
- but do not share the cost of producing those goods

Prisoner’s dilemma: cheating rather than cooperating is a more evolutionarily stable strategy.

Tragedy of the commons: As cheaters invade, the fraction of public good producers in the population decreases to the point of population collapse.

Hamilton’s Rule: social trait will spread in a population if
- m+rn>0
- m is the direct fitness gained by the actor performing the behavior
- n is the indirect fitness gained by the recipient of the behavior weighted by the relatedness r between actor and recipient

Within a population (kin selection / inclusive fitness):
- kin selection: allelic frequency increases when genetic relatedness of a recipient of an actor multiplied by benefit to the recipient is greater than the reproductive cost to the actor; evolutionary strategy that favors reproductive success of an organism’s relatives even at own cost
- inclusive fitness: ability of an organism to pass on its genes to the next generation, taking into account the shared genes passed on by the organism’s close relatives (think shrimp protecting young example)

Cooperation + (m) / + (n)
- m > 0 : mutualism, m+rn always > 0

Altruism - (m) /+ (n)
- m < 0: whether m+rn > 0 scales with relatedness among individuals

How to increase cooperation and altruism (increase m+rn > 0):
1. Increasing direct fitness = increasing m
2. Promoting relatedness = increasing r

How to increase direct fitness:
1. Preferential access to the public good: through either limited diffusion or partial privatization. Limited access
2. Nutrient dependent fitness costs: Public goods only produced when costs are low (when nutrients are not low)

How to promote relatedness:
1. Spatial structuring and limited mixing: isolated groups keeping public goods inside bubble
2. Kin discrimination by club goods: public good needs a recognition ability; keep cheaters out who don’t have that ability
3. Pleiotropy and policing: The joint regulation of public goods along with toxin/immunity systems allows cooperators to punish regulation-deficient cheaters that produce neither.
4. Facultative cooperation and kin discrimination through quorum sensing: produce a lot when cooperators are around but produce little when cheaters are around

Non-social adaptation: Mutations in cooperator cells allow increased uptake and processing of nutrients made available by public goods.
- In Pseudomonas bacteria and yeast, for example, mutations improve the intracellular uptake of cooperatively acquired nutrients

Autoinducer: a signal molecule that induces a response in the same bacterium that produces it

luxl luxR genes

Quorum sensing vs. quorum quenching

Maintaining public goods/cooperation with cheating:
Because selection acts on the levels of individuals and genes, the benefit that cooperation provides at the population level cannot explain its evolution… kin selection explains why cooperation is selected for over cheating

Black Queen Hypothesis: explains selective gene loss in microbial species (essentially a form of cheating) as an adaptation to inadvertently leaky cellular products provided by other community members.

Quorum sensing

Most metabolically active microbes are on the surface: reside in communities called biofilms

QS: coordinate gene expression and cooperate with one another; use small signaling molecules to gauge density of clonal population and initiate group-beneficial behaviors at high densities

QS system:
1. a gene that encodes a signal synthase protein
2. a chemical signal produced by the signal synthase
3. a gene encoding a response regulator that can be activated by the signal in a density-dependent way and in its active form initiates transcription of QS-regulated genes – allows for synchronous transcription

The more signaling molecules, the bigger the population and upon a threshold (a quorum)

Common classes of QS molecules:
- acylated homoserine lactones (AHLs): luxl and luxR
- furanosyl-borate diesters [autoinducer-2 (AI-2) molecules]: luxS and luxP
- α-hydroxyketones

AHL

Can be used in production of antibiotics
AHL-producing bacteria have been isolated from corals

Settlement of invertebrate larvae
Maintenance of coral reef ecosystems

Degradation of AHLs is faster when the pH is higher (base-catalyzed hydrolysis); susceptible to spontaneous condensation reactions and products of this lose signaling fxn

AGLs can be biologically degraded by AHL acylases or AHL lactonases in a process referred to as quorum quenching (QQ)

Changes in pH directly affect the half-life of AHLs;
- In turn, the half-life of AHLs affects both the calling distance (the maximum distance over which QS is effective) of the signals and the quorum size required to initiate QS behaviors.
- At reduced pH, calling distance may be expected to increase because the molecules degrade more slowly, which may impact the expression of QS-regulated traits.
- therefore the quorum required to induce transcription may be reduced

AI2

AI-2 biosynthesis is commonly exhibited by Vibrio species isolated from coral
antibiotic production and biofilm formation
may be involved in regulation of interspecies interactions in complex microbial communities

Environment on QS in Corals

In 3 places: surface mucus layer (SML), the coral tissue, and the calcium carbonate skeleton

SML = QS - great env. for this b/c mildly acidic, biofilms:
- limits diffusion of metabolites
- protects from desiccation and UV light
- source of nutrition for associated heterotrophs
- Vibrios = vast majority of this

In diseased states, Vibrios increase in abundance and populations shift towards pathogenic strains

the addition of exogenous AHLs to a healthy coral microbial community induced disease and coral death, lending support to a role for QS in the progression of white band disease

pH changes

pH changes can induce shifts in microbial community composition in ways that may affect the abundance and diversity of the QS community: QS associated with Proteobacteria and if population shifts away from that

AHL serve as recruitment signals so if less AHL b/c of pH shift induced community shift, then less recruitment

Temp changes

1. rate of AHL degradation is accelerated as temp increases
   
2. temperature-induced instability is expected to affect the efficacy of AHLs.

expression of AHL-dependent regulons appears to be sensitive to temperature fluctuations in the range of natural inter-seasonal variability

prolonged periods of peak temperatures or more frequent temperature anomalies provide an opportunity for greater expression of QS-regulated disease factors.