Résumé
The common ragweed (Ambrosia artemisiifolia L.; Asteraceae) is a North American native that is invading Eurasia. Besides its economic impact on crop yield, it presents a major health problem because of its highly allergenic pollen. The plant was imported inadvertently to Europe in the eighteenth century and has become invasive in several countries. By analyzing French and North American populations, it was previously shown that French populations were best described as a mixture of native sources and that range expansion in France probably involved sequential bottlenecks. Here, our aim was to determine whether Eastern European populations of A. artemisiifolia originated from the previously established French populations or from independent trans-Atlantic colonization events. We used nuclear microsatellite markers to elucidate the relationships among populations from Eastern and Western Europe in relation to populations from a broad survey across the native North American range. We found that A. artemisiifolia from Eastern Europe did not originate from the earlier established French populations but rather represents multiple independent introductions from other sources, or introductions from a not yet identified highly diverse native population. Eastern European populations show comparable amounts of genetic variability as do previously characterized French and North American populations, but analyses of population structure clearly distinguish the two European groups. This suggests separate introductions in Eastern and Western Europe as well as divergent sources for these two invasions, possibly as a result of distinct rules for trade and exchange for Eastern Europe during most of the twentieth century.