Reconstructing the invasion history of an exotic species is useful for understanding the ecological and evolutionary processes that underlie the success of its invasion.

Cardamine hirsuta (Brassicaceae) invaded Japan several decades ago and has expanded its distribution recently. To infer the detail invasion history, we detected the changes in the population genetic structure in Japan from the past to the present using 9 SSR markers. We used plant leaf samples collected around Japan in 2009 and 2010 and herbarium specimens during 1988-2007 for the analyses.

The present populations could be divided into three genetic groups, which were distributed in north, east, and west regions of Japan. This suggests that there are three invasive lineages (North, East, and West) and multiple invasions have occurred in Japan. As compared between the past and the present genetic structure, the West lineage distributed more widely in the past and the lineages distributed in the east region have changed to the East lineages for the last 10-20 years. These results suggest that multiple introductions contributed to the success of the rapid invasion in Japan and population genetic structure of C. hirsuta has changed widely for only a few decades.