Patterns of genetic variation reflect multiple introductions and pre-admixture sources of common ragweed (Ambrosia artemisiifolia) in China

Ambrosia artemisiifolia is native to North America but has become a worldwide invasive weed. It was introduced to China more than 80 years ago and has spread into 20 provinces since then. To assess the population structure of A. artemisiifolia in China and whether this invasion involved a single event or multiple events, we investigated patterns of genetic variation for three chloroplast DNA intergenic spacer regions, a nrITS region and five microsatellite loci. Our dataset consists of 370 individuals from 19 sites throughout China. We compared their cpDNA-haplotypes to those published for native North American populations. The distribution of cpDNA-haplotypes indicates that A. artemisiifolia was introduced to China multiple times from different source regions. The numbers of alleles in Chinese populations were not significantly lower than in native populations. Both nrITS-haplotypes and microsatellite alleles showed that there was no evidence for a genetic bottleneck. Four populations were genetically well separated from the other 15 populations. However, the absence of isolation by distance, and the low levels of genetic differentiation and gene flow among the other 15 population suggest that most populations in China come from pre-admixed populations. To find the exact source regions of the Chinese populations, more samples from the native region and other invaded regions will be necessary. Nevertheless, our study provides important insights into the genetic background of A. artemisiifolia invasion in China.