Exploring the Relative Importance of Biotic and Abiotic Sources of Selection for Pine-fungal Interactions

Coevolution describes evolutionary change in which two or more interacting species reciprocally drive each other’s evolution, potentially driving trait diversification and ecological speciation. Yet, we still understand very little about how coevolution works in multi-species interactions or about the relative importance of biotic and abiotic sources of selection. Interactions between conifers and their microbes are ideal for such studies as conifers form obligate associations with mutualistic ectomycorrhizal fungi, and these interactions vary along abiotic gradients. Using genotypes of Monterey pine (Pinus radiata) from geographically separated sites along the coast of California and Baja California, as well as crosses to represent intermediate phenotypes/genotypes, I performed a common garden experiment examining the relationship between sources of selection on candidate coevolving traits. We found direct linear selection on all traits, but the strength of selection differed for fungal and plant traits. The strength of selection was greatest for fungal traits (foraging strategy) while plant traits (relative growth rate, diameter, root:shoot, specific root length) were all undergoing moderate to no selection. Variation in these traits of the symbiosis indicates an important step in establishing evolution in response to abiotic variation. This work represents the first field-based, community-level approach towards investigating selection in mycorrhizal relationships