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.