Direct and indirect fitness effects of plant metabolites and genetic constraints limit evolution of allelopathy in an invading plant

Invading species encounter novel communities of consumers, pathogens, and competitors. Both phenotypic plasticity and rapid evolution can facilitate invasion across these heterogenous communities. However, the rate and extent of adaptive evolution on contemporary timescales can be constrained by adaptive phenotypic plasticity and the genetic architecture of traits under selection. We measured phenotypic plasticity and quantified genetic variation for growth, leaf chlorophyll a (Chl a ) and glucosinolates, and lifetime fitness among 23 naturally inbred seed families of Alliaria petiolata (garlic mustard) collected across its invasive range in eastern North America. After growing a self-pollinated generation in a uniform common garden to reduce maternal effects, we reared second-generation plants in a two-year greenhouse and field experiment with naïve soil from an uninvaded habitat. We estimated selection gradients and causal factors affecting lifetime fitness when reared alone, with an intraspecific competitor, and under interspecific competition with naïve Acer saccharum (sugar maple) saplings. We defined Total Metabolite Production (TMP) as the first principal component of Chl a and glucosinolate concentrations, accounting for 84% of variation in these two traits. TMP was significantly plastic across growing environments ( p < 0.001) with limited broad-sense heritability ( H 2 = 2.91; p = 0.08). Path analysis revealed that plastic phenotypes with higher TMP had an indirect positive effect on A. petiolata fitness via a direct, negative effect on performance of A. saccharum competitors. In contrast, the second principal component defined Relative Glucosinolate Investment (RGI), which was significantly heritable ( H 2 = 16.91, p < 0.001) with no detectable plasticity across treatments. Variation in RGI among A. petiolata genotypes had a direct, positive effect on A. saccharum performance and an indirect negative effect on A. petiolata fitness. Synthesis. Adaptive evolution of allelopathy during invasion has been constrained by (i) a lack of heritable genetic variation for allelopathy, (ii) high plasticity for TMP across competition treatments, and (iii) selection for lower RGI under interspecific and intraspecific competition. As an alternative to eco-evolutionary feedbacks, plasticity in TMP may be an overlooked explanation for variable performance of A. petiolata across its introduced range.