High but variable outcrossing rates in the invasive Geranium carolinianum (Geraniaceae).

• Premise of the study: Mating system plays an important role in population establishment and persistence, maintenance of genetic variation, and adaptive ability, especially for invasive species that colonize new environments to which they may be poorly adapted. In mixed-mating species, population differences in self-fertilization rates often arise due to variation in local ecological conditions (e.g., pollinator or mate availability) or genetic variation in traits promoting selfing or outcrossing. Knowledge of how and why selfing rates vary can help us understand how populations respond to different environments, how this affects patterns of genetic variation, and the role of mating systems in biological invasions.• Methods: We determined outcrossing rates in invasive (China) and native (US) populations of the weedy annual Geranium carolinianum in natural populations and an open-pollinated common garden to answer the following questions: To what extent do populations vary in mating system? Do invasive populations differ from native populations? Is interpopulation variation in mating system under genetic control?• Key results: Despite having many characteristics of selfing species, we found high variation in outcrossing rates (t from 0.1 to 1.0) resulting from both environmental and genetic variation. Outcrossing rates were generally high and plastic in the Chinese populations.• Conclusions: A delayed selfing mechanism provides reproductive assurance while allowing facultative outcrossing when mates are not limiting. In invasive populations, high outcrossing rates were facilitated by large admixed founding populations, promoting local adaptation and the maintenance of genetic diversity.