A continent-wide clone: Population genetic variation of the invasive plant hieracium aurantiacum (orange hawkweed; Asteraceae) in North America

We investigated the population genetic structure of the invasive plant Hieracium aurantiacum (Asteraceae), a facultative apomict. We generated amplified fragment length polymorphism fingerprints for H. aurantiacum samples from across its invasive range in North America (N = 226) and from six other North American native and invasive Hieracium species (N = 60). Almost no genetic variability was found in the North American H. aurantiacum across locations from Alaska and Oregon to Pennsylvania and Ontario (clonal diversity = 0:035). In contrast, other Hieracium species showed a range of clonal diversities (range = 0:154-1:0). The single H. aurantiacum genotype that dominated the North American invaded range was identical to a sample from the native range (Czech Republic), where low genetic diversity has also been reported. However, we did find evidence of hybridization between H. aurantiacum and at least one other nonnative Hieracium species in North America, indicating that the generation of novel hybrid genetic combinations may be an important factor in this invasive group of Hieracium taxa. Our findings suggest that sexual recombination and genetic diversity are not essential for successful plant invasion and that phenotypic plasticity alone may provide the flexibility necessary for the establishment of H. aurantiacum in diverse habitats.