Genetic analyses reveal complex introduction histories for the invasive tree Acacia dealbata Link around the world

Aim: To compare genetic diversity and structure between Acacia dealbata populations sampled across the species’ native range in Australia and from its non-native ranges in Chile, Madagascar, New Zealand, Portugal, La Réunion island, South Africa and the United States, and to investigate the most likely introduction scenarios to non-native ranges. Location: Global. Taxon: Acacia dealbata, Fabaceae. Methods: Our dataset comprised 1615 samples representing 92 populations sampled in the species’ native and non-native ranges. We employed a combination of genetic fingerprinting (microsatellite markers) and genetic modelling approaches. We calculated genetic diversity for each population and tested for genetic isolation by distance within each range. A combination of Bayesian assignment tests and multivariate ordination was applied to identify genetic structure among populations. Approximate Bayesian Computation (ABC) analyses were conducted to test different competing introduction scenarios for each non-native range. Results: The majority of the species’ non-native ranges was characterized by high genetic diversity and low levels of genetic structure. With regard to introduction histories, however, our results supported different introduction scenarios for different non-native ranges. We did not find strong support for any of tested introduction scenarios for populations in Chile and Madagascar, but these likely originated from multiple introductions followed by admixture. Populations in New Zealand and La Réunion most likely originated directly from Tasmania, possibly through multiple introductions. Similar to previous findings for South African populations, no clear introduction history could be identified for populations in Portugal and the United States. Main conclusions: Our study shows that global introductions of A. dealbata were complex and one scenario does not fit the invasion history of the species in different regions. We discuss how this complexity needs to be considered when formulating strategies for the effective management of the species. Future research needs to help bridge persisting knowledge gaps are discussed