Unraveling the roles of genotype and environment in the expression of plant defense phenotypes

1. Phenotypic variability results from interactions between genotype and
environment and is a major driver of ecological and evolutionary
interactions. Measuring the relative contributions of genetic variation,
the environment, and their interaction to phenotypic variation remains a
fundamental goal of evolutionary ecology. 2. In this study, we assess the
question: How do genetic variation and local environmental conditions
interact to influence phenotype within a single population? We explored
this question using seed from a single population of common milkweed,
Asclepias syriaca, in northern Michigan. We first measured resistance and
resistance traits of 14 maternal lines in two common garden experiments
(field and greenhouse) to detect genetic variation within the population.
We carried out a reciprocal transplant experiment with three of these
maternal lines to assess effects of local environment on phenotype.
Finally, we compared the phenotypic traits measured in our experiments
with the phenotypic traits of the naturally-growing maternal genets to be
able to compare relative effect of genetic and environmental variation on
naturally-occurring phenotypic variation. We measured defoliation levels,
arthropod abundances, foliar cardenolide concentrations, foliar latex
exudation, foliar carbon and nitrogen concentrations, and plant growth. 3.
We found a striking lack of correlation in trait expression of the
maternal lines between the common gardens, or between the common gardens
and the naturally-growing maternal genets, suggesting that environment
plays a larger role in phenotypic trait variation of this population. We
found evidence of significant genotype-by-environment interactions for all
traits except foliar concentrations of nitrogen and cardenolide. Milkweed
resistance to chewing herbivores was associated more strongly with the
growing environment. We observed no variation in foliar cardenolide
concentrations among maternal lines but did observe variation among
maternal lines in foliar latex exudation. 4. Overall, our data reveal
powerful genotype-by-environment interactions on the expression of most
resistance traits in milkweed.