Founder effects, post-introduction evolution and phenotypic plasticity contribute to invasion success of a genetically impoverished invader.

Multiple mechanisms may act synergistically to promote success of invasive plants. Here, we tested the roles of three non-mutually exclusive mechanisms-founder effects, post-introduction evolution and phenotypic plasticity-in promoting invasion of Chromolaena odorata. We performed a common garden experiment to investigate phenotypic diversification and phenotypic plasticity of the genetically impoverished invader in response to two rainfall treatments (ambient and 50% rainfall). We used ancestor-descendant comparisons to determine post-introduction evolution and the Q-F approach to estimate past selection on phenotypic traits. We found that eight traits differed significantly between plants from the invasive versus native ranges, for two of which founder effects can be inferred and for six of which post-introduction evolution can be inferred. The invader experienced strong diversifying selection in the invasive range and showed clinal variations in six traits along water and/or temperature gradients. These clinal variations are likely attributed to post-introduction evolution rather than multiple introductions of pre-adapted genotypes, as most of the clinal variations were absent or in opposite directions from those for native populations. Compared with populations, rainfall treatments explained only small proportions of total variations in all studied traits for plants from both ranges, highlighting the importance of heritable phenotypic differentiation. In addition, phenotypic plasticity was similar for plants from both ranges although neutral genetic diversity was much lower for plants from the invasive range. Our results showed that founder effects, post-introduction evolution and phenotypic plasticity may function synergistically in promoting invasion success of C. odorata.