Population genetic insights into biogeography and clonality of the perennial Ambrosia psilostachya DC. (Asteraceae) in the United States of America

• Population genetics enables analysis of postglacial range expansion of western ragweed in USA • Prairie populations show high gene flow and highly variable genetics due to high rates of sexual reproduction. • Northeastern and any young populations are genetically less variable and reproduce rather clonally. Western ragweed ( Ambrosia psilostachya DC.) is native to North America and invaded all other continents in the late 19th century. It persists and spreads via both seeds and root suckers. The combined sexual and clonal reproduction and population genetics in the native range are still unexplored, although they are relevant for the invasion success abroad. 20 populations were sampled throughout North America and genotyped using 15 Simple Sequence Repeats (SSR). AMOVA indicated that most genetic variation is partitioned within populations (76%). Allelic richness (Na = 2.88 ± 0.77), expected heterozygosity (He = 0.49 ± 0.09), observed heterozygosity (Ho = 0.56 ± 0.55), and inbreeding coefficient (Fis = −0.171 ± 0.26) were not associated with population size and age. All populations deviated from Hardy-Weinberg Equilibrium due to clonality. Populations from roadsides differed significantly, indicating that habitat perturbation affected the degree of clonality and genetic diversity. Bayesian clustering indicated that populations from the Great Lakes area were genetically distinct, while other (western) populations showed high degrees of admixture − specifically in the prairie region, where sexual reproduction is important. Ambrosia psilostachya populations from the core native range are likely in a state of equilibrium resulting from both sexual and clonal reproduction as well as gene flow. Populations from anthropogenic habitats are younger and genetically similar to their neighboring populations. Compared to Europe, clonality acts a smaller part in the establishment and growth of populations in their native range, except for very young populations.