Genotypic controls on mycorrhizal-host relationships in Pinus radiata

Background/Question/Methods

Coevolution describes evolutionary change in which two or more interacting species reciprocally drive each other’s evolution. The strength of this selection process may vary spatially and temporally due to abiotic and biotic contextual factors. Interactions among plant hosts and their microbes may provide an ecologically unique arena in which to examine the nature of selection in multispecies interactions. In particular, interactions between coniferous plants and their microbes provide a good system for experiments exploring the relative importance of biotic versus abiotic sources of selection, as conifers interact with a suite of belowground microorganisms including mutualistic ectomycorrhizal fungi (ECM), and these interactions vary along environmental gradients. Using populations of Monterey pine (Pinus radiata) from geographically separated sites along the west coast of California (USA) and Baja California (Mexico), I performed a common garden experiment at a site that contains native stands of Monterey pine on mainland California (Cambria) to investigate the relationship between abiotic and biotic sources of selection on pine traits, including candidate coevolving traits. In each garden, we planted seedling genotypes from three populations, as well as crosses to represent intermediate phenotypes/genotypes. We then measured seedling traits, ECM fungal traits, and soil ECM composition.

Results/Conclusions

Seedling biomass was significantly affected by host genotype as pines from the island Cedros were extremely small, but when these genotypes were crossed with mainland pines from Cambria (where the common garden was located), such hybrids were larger than any of the other genotypes. These differences in seedling biomass may be driven by differences in ECM traits. Specifically, mainland pines had more mycorrhizal root tips than island pines, but when island pines were crossed with Cambria pines, the average number of colonized tips tended to increase. Despite these genetic differences in ECM absolute abundance, host genotypes did not differ in average number of colonized tips per cm root length. Variation in overall mycorrhizal abundance may be an example of how a trait of the symbiosis may be evolving in response to geographic variation in climate. This work represents the first field-based, community-level approach towards investigating selection in mycorrhizal relationships.