GBS sequencing of native and invasive Medicago polymorpha

To establish and spread in a new location, invasive plant species must rapidly adapt to novel environmental conditions. A key trait underlying fitness is the shift from vegetative to reproductive growth through floral development. In this study, we used a common garden experiment and genotyping-by-sequencing to test whether the latitudinal flowering cline of the North American invasive plant Medicago polymorpha was translocated from M. polymorpha's European native range through multiple introductions, or whether the cline re-established due to evolution following a genetic bottleneck. Analysis of flowering time in common garden plants showed a latitudinal flowering time cline in both the native and invaded ranges where genotypes from lower latitudes flowered earlier. Genotyping-by-sequencing of 9,658 SNPs in 446 individuals revealed two major subpopulations of M. polymorpha present in the native range, with only one of these present in the invaded range. Additionally, native range populations have higher genetic diversity than invaded range populations, suggesting that a genetic bottleneck occurred during invasion. All invaded range individuals are closely related to plants collected from native range populations in Portugal and southern Spain, and population assignment tests assigned invaded range individuals to this same narrow source region. Despite the bottleneck, our results suggest that latitudinal clinal variation in flowering time has rapidly re-established across the invaded range following introduction.