Genetic Variation and Molecular Biogeography of a North American Invasive Plant Species (Alliaria petiolata, Brassicaceae)

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Abstract
A study was conducted using molecular‐based population genetic data to interpret biogeographic relationships and survey genetic similarity within and among populations of Alliaria petiolata from its native and introduced ranges. Three of the populations examined were from Europe, the native range of A. petiolata, whereas eight populations were from North America, where A. petiolata was introduced over 125 yr ago and where it has since become an invasive pest. Inter–simple sequence repeat (ISSR) analysis using two different primers revealed 56 unique fragments. Genetic variation was greater in some native populations (Scottish and Dutch) compared with introduced populations. Estimates of the Shannon phenotypic diversity index among populations ranged from 0.917 to 0.996. Analysis of molecular variance indicated that there was strong population structuring, with the greatest variance among populations (61.0%) and with much less variance both between continents (16.3%) and within populations (22.7%). Significant differences were detected within and among populations and between ranges (native and introduced). Unweighted pair‐group mean analysis and principal‐coordinates analysis separated individuals into two large groups comprising two European populations (Belgium and The Netherlands), on the one hand, and the remaining nine populations, including Scotland, on the other. The data indicate that several North American populations, including those from Ohio, West Virginia, New York, and Kentucky, may have originated from plants from the British Isles, although it is possible that multiple introductions of A. petiolata from Europe have occurred.