Eco-evolutionary feedbacks or general-purpose genotype? An assessment of plastic and genetic contributions to glucosinolate production and competitive interactions in an invasive plant.

Background/Question/Methods

The introduction and spread of invasive species may often involve novel biotic interactions, which can produce eco-evolutionary feedbacks during invasion if natural selection is variable and genetic constraints are not limiting. Phenotypic plasticity can also facilitate invasion and may impede adaptation across unpredictable, heterogeneous environments like those that involve biotic interactions. The biennial herbaceous herb Alliaria petiolata (garlic mustard) is widespread across North America where it can form dense stands and produce secondary chemicals with inhibitory effects on native plants. To asses the relative contribution of plasticity and adaptive eco-evolutionary feedbacks during the invasion of A. petiolata, we studied 22 naturally inbred genetic lines collected from Georgia to Connecticut in the northeastern United States. After establishing genetic lines in a common garden to limit maternal effects, we quantified plasticity, broad-sense genetic variation, and natural selection via lifetime fitness when replicated genetic lines were reared alone, under intraspecific competition, and interspecific competition with naïve Acer saccharum (sugar maple). Importantly, soil collected directly from the field was used to ensure interactions with naturally-occurring biota in both glasshouse (year 1) and field (year2) environments.

Results/Conclusions

Total glucosinolate production was strongly correlated with the production of Chlorophyll A and the first principal component (PC1) of these correlated traits was highly plastic across growing environments, with limited broad-sense heritability. Thus, PC1 captures phenotypic plasticity in these two traits. In contrast, investment into glucosinolates relative to Chlorophyll A (PC2) was significantly heritable with no statistically significant effect of plasticity across competition treatments. Thus, PC2 captures genetic variation for these two traits. Path analysis revealed contradictory effects on the growth of A. saccharum, which was negatively associated with plastic variation in (PC1) but positively associated with heritable variation in (PC2). These results suggest a limited role for soil-mediated eco-evolutionary feedbacks as A. petiolata spread across North America. Instead, plastic responses to competitive environments demonstrate that A. petiolata is more akin to Herbert Baker's ‘general-purpose genotype’.