Cross-continental variation of herbivore resistance in a global plant invader

O_LISuccessful plant invasions are often explained with adaptation to novel environments. However, invasive species often occupy broad niches within their native and introduced ranges, and a true understanding of microevolution during invasion therefore requires broad sampling of ranges, ideally with a knowledge of introduction history. C_LIO_LIWe tested for genetic differentiation in herbivore resistance among 128 introduced (Europe, North America) and native (China, Japan) populations of the invasive Japanese knotweed (Reynoutria japonica) in two common gardens in the native range. C_LIO_LIIn both common gardens we found that resistance traits of introduced populations differed from most Chinese native populations, but not from populations in Japan, the putative sources of introduction. Compared to Chinese populations, introduced European populations had thicker leaves with a lower C:N ratio but higher flavonoids contents. In the native range, variation in herbivore resistance was much more strongly associated with climate of origin than in introduced populations. C_LIO_LIOur results support the idea that founder effects played a key role in the invasion of knotweed into Europe and North America, with introduction of particular resistance phenotypes from Japan. Our study also demonstrates how knowledge of introduction history can avoid drawing wrong conclusions from observed biogeographic divergence. C_LI