Microbe-mediated adaptation and the spatial scale of adaptation
Selection can vary over exceptionally small spatial scales (Turkington
& Harper 1979; Kalisz 1986), posing challenges to the evolution of
local adaption by plants if the scale of gene flow through pollen and/or
seeds exceeds the strength of selection (Richardson et al. 2014).
However, microbe-mediated local adaptation and adaptive plasticity can
affect the spatial scale of adaptation. Microbes can facilitate
adaptation via microbe-mediated local adaptation by acting as a
selective barrier, where the fitness of migrants (relative to resident
populations) is reduced before they are incorporated into the gene pool
(Richardson et al. 2014). An example of this is microbe-mediated
germination or survival (Petipas et al. 2020b), where local
microbes facilitate germination and survival of local plant genotypes
potentially allowing them to outcompete or exclude foreign plant
genotypes.
Microbe-mediated adaptive plasticity may be another hidden solution to
dealing with heterogenous environments. Small-scale adaptive responses
could be facilitated by microbes if plants exhibit a high degree of
plasticity for traits related to interactions with microorganism, if
microbes differentiate across fine-scale spatial variation (Nackeet al. 2016) and elicit adaptive phenotypic changes in host
plants, or even if microbial communities vary little spatially but their
effects on plant phenotypes are highly context-dependent and influenced
by other aspects of the abiotic or biotic environment. For example,
microbe-mediated adaptive plasticity may affect the drought phenotype ofThemeda triandra, a native Kenyan grass, over small spatial
scales. The presence of termite mounds dramatically increases nutrient
availability, but reduces variability in water availability over small
spatial scales (>50m), posing an adaptive challenge for
wind pollinated Themeda triandra , which is unlikely to
genetically differentiate in the on versus off-mound environments.
However, microbes from on vs. off termite mounds differentially affectT. triandra’s response to drought. Plants inoculated with
arbuscular mycorrhizal communities collected off mounds closed stomata
quickly when exposed to drought and halted biomass accumulation, whereas
plants inoculated with on-mound fungi kept stomata open longer under
drought conditions and continued to acquire biomass (Petipas et
al. 2017). In this case, fine-scale variation in microbial community
composition on vs. off termite mounds led to the production of plant
phenotypes that are potentially adaptive in those two different
environments. Future work, should include looking at the fitness
consequences of these plant trait responses especially over a more
realistic time scale for a long-lived perennial grass.