Box 2: What’s up with the microbes?
Microbe can affect host phenotypes: 1. by affecting important plant
functional traits(Friesen et al. 2011), 2. acting as an extended
phenotype (i.e. acquiring resources otherwise not available, Koskella &
Bergelson 2020) or 3. by serving as an environmental cue (Metcalfet al. 2019). However, understanding the characteristics/identity
of the microbes underlying these effects can be complicated.
Demonstrating reciprocal evolutionary change in microbes is complicated
when in many cases fitness is hard to quantify and relevant traits are
unknown, although new methods are making this seem possible(Burghardtet al. 2018). Documenting differences in community composition is
more tractable given the explosion of sequencing-based technologies
(Hugerth & Andersson 2017). However, detecting changes in microbial
richness or shifts in community composition does not exclude the
possibility of simultaneous (and relevant) genetic change to microbial
communities or microbial plasticity. Additionally, microbial diversity
may not be a relevant reflection of microbial function. At this point,
understanding and differentiating ecological from evolutionary effects
on microbial communities is probably not possible given current
technologies but will be an important research goal of the future. Given
the issues with understanding the microbial end of the interaction,
researchers more commonly explore the population biology of the plants.
For example, in one study(Johnson et al. 2010) highlighted in the
main text, reciprocal transplants of Andropogon gerardiigenotypes collected from areas that either had limiting phosphorus or
limiting nitrogen demonstrated that local mycorrhizal communities were
locally adapted to local nutrient conditions, such that home communities
of AMF provided optimal benefits to plants when transplanted into home
environments. AMF also had higher fitness when matched with their
sympatric plant genotype. What is unclear from this study is which
microbial players were responsible for the observed effect. Some options
include: 1) AMF genotype(s) that were particularly good at accessing
nutrients locally, 2) AMF species that were particularly good at
accessing nutrients locally, or 3) synergistic effects of multiple AMF
mutualists, such that the effects on plants could not be predicted based
on the presence or abundance of a particular species or genotype.