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Abstract. Acacia longifolia is an aggressive invader worldwide. This species was brought to Portugal by the forestry services and is now found throughout the country with very strong ecological and social impacts. Although several ecological and physiological studies have been carried out, molecular studies in this species are sparse. Particularly, genetic variability evaluation in invasive ranges clearly deserves more attention. The aim of this study was to evaluate the genetic diversity and the genetic differentiation among populations of the alien invasive A. longifolia under different Mediterranean conditions. For that we studied three sandy dune populations along the Portuguese coast: Osso da Baleia (mesomediterranean), Pinheiro da Cruz and Vila Nova de Milfontes (termomediterranean). All sampled acacia plants were located underneath a pine forest, with the exception of the latter case study, where we also sampled in a diversified habitat conditions associated with the margins of agriculture fields. According to the historical records, it was also possible to compare acacia samples from original plantations and recently invaded areas. We have used two molecular markers -inter-simple sequence repeats (ISSR) and microsatellites (SSR). ISSR analysis showed that A. longifolia populations at all sampled locations have similar levels of diversity and a relatively low differentiation ( PT = 0.135). However, samples tended to cluster according to the regional (macro-scale) collection site. Microsatellites confirmed this low differentiation pattern. Our results indicate that differences in phenology and reproductive success observed during a previous study cannot be accounted for by genetic diversity and differentiation alone. Furthermore, our results taken together with historical information of A. longifolia suggest a single original introduction and a subsequent acclimation process.

Background and Aims Invasive species may undergo rapid evolution despite very limited standing genetic diversity. This so-called genetic paradox of biological invasions assumes that an invasive species has experienced (and survived) a genetic bottleneck and then underwent local adaptation in the new range. In this study, we test how often Australian acacias (genus Acacia), one of the world’s worst invasive tree groups, have experienced genetic bottlenecks and inbreeding. Methods We collated genetic data from 51 different genetic studies on Acacia species to compare genetic diversity between native and invasive populations. These studies analysed 37 different Acacia species, with genetic data from the invasive ranges of 11 species, and data from the native range for 36 species (14 of these 36 species are known to be invasive somewhere in the world, and the other 22 are not known to be invasive). Key Results Levels of genetic diversity are similar in native and invasive populations, and there is little evidence of invasive acacia populations being extensively inbred. Levels of genetic diversity in native range populations also did not differ significantly between species that have and that do not have invasive populations. Conclusion We attribute our findings to the impressive movement, introduction effort, and human usage of Australian acacias around the world.

Acacia longifolia (Sydney golden wattle) is considered one of the most problematic plant invaders in Mediterranean-type ecosystems. In this study, we investigate the species’ invasion history by comparing the genetic diversity and structure of native (Australia) and several invasive range (Brazil, Portugal, South Africa, Spain, and Uruguay) populations and by modelling different introduction scenarios using these data. We sampled 272 A. longifolia individuals – 126 from different invasive ranges and 146 from the native range – from 41 populations. We genotyped all individuals at four chloroplast and 12 nuclear microsatellite markers. From these data we calculated diversity metrics, identified chloroplast haplotypes, and estimated population genetic structure based on Bayesian assignment tests. We used Approximate Bayesian Computation (ABC) models to infer the likely introduction history into each invaded country. In Australia, population genetic structure of A. longifolia appears to be strongly shaped by the Bass Strait and we identified two genetic clusters largely corresponding to mainland Australian and Tasmanian populations. We found invasive populations to represent a mixture of these clusters. Similar levels of genetic diversity were present in native and invasive ranges, indicating that invasive populations did not go through a genetic bottleneck. Bayesian assignment tests and chloroplast haplotype frequencies further suggested a secondary introduction event between South Africa and Portugal. However, ABC analyses could not confidently identify the native source(s) of invasive populations in these two countries, probably due to the known high propagule pressure that accompanied these introductions. ABC analyses identified Tasmania as the likely source of invasive populations in Brazil and Uruguay. A definitive native source for Spanish populations could also not be identified. This study shows that tracing the introduction history of A. longifolia is difficult, most likely because of the complexity associated with the extensive movement of the species around the world. Our findings should be considered when planning management and control efforts, such as biological control, in some invaded regions.