Abstract：Ageratina adenophora (Crofton weed) is one of the most widespread invasive species in China. Its genetic diversity and population structure in China was analyzed by amplified fragment length polymorphism (AFLP) in this study. Three primer pairs were selected for the analysis and 490 bands were produced from 62 A. adenophora populations of 5 major geographic areas in China, of which 328 bands showed polymorphic. Diversity levels within populations were relatively high (mean expectedheterozygosity = 0.171, mean Shannon index = 0.268), and that between populations was low (Fst = 0.287). At the regional level, the AMOVA indicated that about 70% variations in the data set were from genotypic variations within populations, 8.04% variations due to regional differences, and the remaining 21.71% due to differences among populations within the provincial regions. Cluster analysis based on the UPGMA method grouped the majority of the 62 A. adenophora populations into four main clusters, corresponding with their geographic regions. Elevation was approved by Sigmaplot 2000 analysis to be the main environmental factor affecting the genetic diversity of A. adenophora. Mantel test indicated that there was a low correlation between geographic and genetic distances among populations (r =0.34). It is concluded that A. adenophora spread mainly by wind or water and its genetic diversity level in newly invaded area was lower than that in former invaded areas.
