Ambrosia artemisiifolia (common ragweed) is an herbaceous pioneer weed native to North America. Palynological records from sediment cores suggest that the species was relatively uncommon until colonial times, whereupon ragweed populations grew dramatically. The species has become an aggressive global invasive, reaching Europe, Australia, China, Japan, Russia, Israel, and beyond. It is likely that its enormous post-colonial population expansion has been coupled with admixture of previously isolated populations, and it may be that admixture primed this species for invasion upon its introduction to novel landscapes. We report preliminary findings of an ongoing study of phylogeography and genetic structure revealed through an analysis of polymorphisms in eight intergenic chloroplast (cpDNA) spacer regions and several nuclear microsatellite loci.
Results/Conclusions
The species is composed of divergent haplotypes that imply a long period of genetic isolation. However, haplotype frequencies in extant populations sampled throughout eastern United States and Canada suggest admixture in the intervening time. Our initial attempts to amplify and sequence cpDNA from historical herbarium specimens collected up to 110 years ago have been successful. Ongoing work with herbarium collections will be used to estimate changes in population genetic structure in the time since anthropogenic disturbance. Finally, our data show that diversity evident among extant North American populations should make it possible to trace invasion routes by assigning worldwide invasive populations to their geographic origins in the native range.
