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Abstract:
To address the role of insect herbivores in adaptation of plant populations and the persistence of selection through succession, we
manipulated herbivory in a long-term field experiment. We suppressed insects in half of 16 plots over nine years and examined
the genotypic structure and chemical defense of common dandelion (Taraxacum officinale), a naturally colonizing perennial
apomictic plant. Insect suppression doubled dandelion abundance in the first few years, but had negligible effects thereafter.
Using microsatellite DNA markers, we genotyped >2500 plants and demonstrate that insect suppression altered the genotypic
composition of plots in both sampling years. Phenotypic and genotypic estimates of defensive terpenes and phenolics from the field
plots allowed us to infer phenotypic plasticity and the response of dandelion populations to insect-mediated natural selection. The
effects of insect suppression on plant chemistry were, indeed, driven both by plasticity and plant genotypic identity. In particular,
di-phenolic inositol esters were more abundant in plots exposed to herbivory (due to the genotypic composition of the plots) and
were also induced in response to herbivory. This field experiment thus demonstrates evolutionary sorting of plant genotypes in
response to insect herbivores that was in same direction as the plastic defensive response within genotypes.
