Chapter 10 Mechanisms of Plastic Rescue

Notes may not be my own language, as some sections were directly taken from the manuscripts I studied and are cited in the main header or in a parenthesis.

Snell-Rood et al 2018

Mechanisms (i.e. developmental switches) can move an organism towards a new adaptive peak or could result in a maladaptive response. Ability to be plastic and plastic responses come with high energetic demand (i.e. delays in reproduction, increased individual investment, reduced fecundity).

Plasticity: the ability of a genotype to develop different phenotypes in response to cues like environmental conditions. Includes morphological development, regulation of gene expression, and behavior.

This process of individual plasticity can be immediate and within a single generation, thus providing a potential first stage of rescue that comes before adaptation and evolutionary rescue of a population. BUT, this is not always beneficial. Immediate plastic responses can be maladaptive because little to no selection has occured on this response to completely new conditions.

Developmental mechanisms (causes) of plasticity (in order of least likely to most likely to produce an adaptive response):
1. Evolved developmental switches
2. Generalized physiological responses to stressors
3. Development selection

The mechanism may influence proportion of population that survives a novel condition, fecundity, time between generations, rate of population growth, and how evolution plays out.

The mechanisms that are likely to produce a highly adaptive response to a novel environment are also the most costly, shifting a slower life history and less pronounced evolutionary response at the population level.

1.) Evolved developmental switches are least likely to result in an adaptive response because they are specific to historically relevant environments. 2.) Generalized physiological responses are relatively more likely because these are general to a range of environments. 3.) Developmental selection are most likely because individuals sample phenotypes over developmental time, adopting those that specifically match the new condition.

Most interested in the learning and acquired immunity section of this.. potential of environmental memory.. but this means it is highly costly. High risk, high reward.

3 categories of developmental mechanisms > be able to explain this figure without the caption for exams.

fig1

Big question: can this be optimized within an individual’s lifespan?

Evolved developmental switches: Least likely

Polyphenism: where one of several discrete alternate phenotypes is induced in different conditions.
- nutrition-dependent mating tactics in beetles (Moczek & Emlen 2000)
- smaller males are hornless, large males that exceed a critical body size develop a pair of long, curved horns
- “Thus, horn possession confers a clear advantage to males using fighting behaviours to access females, whereas hornlessness may be favoured in males that rely primarily on sneaking behaviours. Combined, the two alternative reproductive tactics used by male O. taurus appear to favour opposite horn phenotypes, which may explain the paucity of intermediate morphologies in natural populations of O. taurus.”

These are in response to specific, predictive environmental cues.

A polyphenic trait is a trait for which multiple, discrete phenotypes can arise from a single genotype as a result of differing environmental conditions. It is therefore a special case of phenotypic plasticity (Wikipedia). Polyphenisms are discrete compared to continuous scale (i.e. height and weight is continuous and the ability to curl your tongue is discrete).

Come back to thinking about if corals have any polyphenisms? There are some corals of the same species that develop in plate morphologies or pillar morphologies… then the cue would be sunlight exposure?

Examples of cues: predator chemicals, photoperiod, temperature induced developmental regulatory program, UV light (can induce melanization in some species)

How adaptive this is would depend on the context of the novel environment:
- If the novel condition was on a continuum then a preexisting plastic response could be adaptive in the new condition (i.e. rising temperatures)
- If the environment shifts to an extreme or in a discrete manner (i.e. toxins), the tailored switches may not be adaptive in this scenario. Also in this scenario, previously cryptic variation could be exposed and lead to adaptive responses. This could be more up to chance and only a subset of the population to survive.

Responses of evolved developmental switches in novel environments.

be able to explain this figure without the caption for exams.

fig2

Generalized physiological responses to stressors: More likely

Stress response pathways, toxin metabolism, innate immunity have been optimized over evolutionary time, but can be general to a variety of environments.
- i.e. higher expression of heat shock proteins confers resistance to not only temperature stress but oxidative damage, toxin exposure, and osmotic stress (Feder & Hoffman 1999, Kregel 2002, Wang et al 2004).

Hormesis: sublethal stress can have beneficial effects on performance
- upregulating general responses when challenged with low levels of stress
- Examples found in other markdown sheets about environmental history

Innate immunity can be “generalized”:
- see another markdown sheet for details.
- a fever: heat stressor against pathogens, but also coordinates other immune processes and is effective against cancers and many pathogens

Physiological mechanisms that are generalized or cross-reactive can be extremely powerful survival method. But, sometimes generalized methods aren’t specific or good enough to a particular stressor. Constant upregulation is also highly costly. These costs could favor a specialized process.

Developmental selection: Most likely

Developmental selection: forms of plasticity that are reinforced over developmental time in response to feedback from the environment. This is powerful b/c it requires no prior evolutionary history with an environment for a potential phenotype match.
- allows adaptive responses to a wide range of conditions without suffering costs of relaxed selection on developmental programs (Snell-Rood et al 2010, Whitlock 1996)

Selection can occur within or b/w cells, over space and time, or on the basis of the location/ID of the trait, and across a lifetime.

Tissue Architecture:

Selection on location of cells
- i.e. jaw and skull morphology based on diet in fish and mammals; locomotor abilities based on experience in athletes; and *animals reared in different conditions**
- Corals branching arrangement can be altered by light levels and water flow (Bruno & Edmunds 1997)
- The above is similar to one of my questions above..

what is the selection pressure from thick-tissued corals in the same environment and/or adjacent to each other as thin-tissued corals?

Learning and acquired immunity:

Individuals can “sample” phenotypes over time and reinforce the traits that match local conditions and atrophying those that do not (leading to specialized local adaptation). With behavior (i.e. motor response to sensory input) this can be done by attending to a range of cues or varying motor patterns in response to a cue (basically this can happen at the input and the output level).

In corals, maybe this could look like extending polyps for feeding - after sampling several times of the day, they eventually learn what time is best to feed. Come back to adding to this….

Microbiome and patterns of gene expression:

Shuffling of different strains or species of symbionts may allow corals to cope with increasing temperature b/c harboring a more diverse microbiome can allow for a greater range of acclimation responses (Baker 2003, Berkelmans & van Oppen 2006).

Could this also be a “learned” behavior, sampling various microbiomes and keeping the “strongest” that match the new condition?

The microbiome can be selected on via the interaction with the environment and the active mechanisms of the host.

For gene expression, stochastic expression can be beneficial in a variable environment. (Miller-Jensen et al 2011). And epigenetic patterning could focus this expression to lead to high performance. Selection could act on the higher performing patterning of gene expression (analogous to a learning process).
- transcriptional memory of past developmental experiences; like priming effects

Plastic responses are likely a combination of evolved developmental switches that match temperature cues with physiological and behavioral responses and of learning as individuals gain information on whether new climate conditions match the best breeding conditions.

Costs of plasticity

Global costs of plasticity explains the co-existence of specialists and generalists. Costs are only felt in an environment where plasticity is beneficial.

Costs of plasticity Figure 3
> explain figure without the caption for exams.

fig3

Big picture is that this is costly regardless of mechanism.
- Process (vs. the ability) of learning has trade-offs with fecundity (Mery & Kawecki 2004)
- Induction of chemical and physical defenses comes with reproductive trade-offs (Agrawal et al 1999, Baldwin 1998)

Costs can be higher in stressful conditions, reinforcing the general observation that costs are specific to certain induced phenotypes. Costs can be amplified in nutrient and resource poor environments.
- In environments where nutrients are increasing, this nutritional demand of plasticity could be ameliorated (Snell-Rood et al 2015a).

For DNA methylation, in nutrient rich area - the cost of plasticity could be taken out. If hypomethylation is consistent with phenotypic plasticity, then could higher nutrient rich areas be lower in methylation? Because the organism has learned that resource will be there to use?

Costs to plasticity can lead to lower life histories.
- Upregulation of general stress mechs like antioxidant pathways and heat shock proteins is linked to longer life spans but with trade-offs in fecundity (Gruber et al 2007, Marshall & Sinclair 2010)
- come back to outlining the above….

Developmental selection is particularly costly
- species that rely on learning have a delay in performance; the cost of being naive or exploration-exploitation trade-off
- Therefore, benefits of exploration has more likely to pay off with longer lifespans (Eliassen et al 2007, Kaplan et al 2000)

Critical variables of plasticity:
- costs of plasticity
- generation time
- population growth rate
- degree the population is shifted toward a new optimal phenotype

Applied to corals

The coral holobiont provides a fascinating study system: the coral host, endosymbiont, and associated microbiome. Each partner has the ability for developmental selection and generalized physiological mechanisms. Overall survival in a climate change context is dependent on all three partners working in unison (or unison enough..) to produce an adaptive response.

Examples of learning and acquired immunity:

Microbiome, gene expression, and epigenetics:

Come back to ending of the above paper and the following questions:
- Pros and cons of each strategy?
- Is plasticity always adaptive?
- How is plasticity selected upon?
- Integrating plasticity and genetic variation?
- Interaction of trans-generational plasticity
- epiphenotype hypothesis