Neutral genetic variation among wild North American populations of the weedy plant Arabidopsis thaliana is not geographically structured

We investigated neutral genetic variation within and among 53 wild-collected populations of the weedy annual plant, Arabidopsis thaliana, in North America, using amplified fragment length polymorphism (AFLP) markers. A. thaliana is thought to have been introduced to North America from Eurasia by humans; such an introduction might be expected to leave a clear geographical signal in the genetic data. To detect such patterns, we sampled populations at several hierarchical geographical levels. We collected individuals from populations in two areas of the Southeast and one in the Midwest, as well as individuals from populations in the Pacific Northwest and Northeast. To estimate within-population variation, we sampled eight individuals from each of six populations in the Southeast and Midwest. Among all 95 individuals analysed, we detected 131 polymorphic AFLP fragments. We found no evidence for continental or regional diversification. Individuals sampled from Midwestern and Southeastern populations intermingled in a neighbour-joining tree, and Mantel tests conducted within the Midwestern and Southeastern regions as well as the full data set failed to detect any significant relationship between geographical and genetic distance. These results mirror those found for most global surveys of neutral genetic variability in A. thaliana. Surprisingly, we detected substantial amounts of neutral genetic variability within populations. The levels of genetic variation within populations, coupled with the nongeographical nature of divergence among populations, are consistent with contemporary gene flow and point to a complex and dynamic population history of A. thaliana in North America.