Aim: Biological invasions at the intracontinental scale are poorly studied, and intracontinental invasions often remain cryptic. Here, we investigate the recent range expansion of scotch broom (Cytisus scoparius) into Norway and clarify whether the genetic patterns indicate natural spread or human introduction. Furthermore, we investigate whether plants were moved within the native range and how this influences invasion success. We also infer the level and structuring of genetic diversity within and between the putative native and introduced range. Location: Europe.Methods: We analysed the chloroplast sequence variation in 267 scotch broom samples from its northern expansion front and from its native range across Europe, including herbarium samples dating back to 1835. For 37 populations, we analysed variation in nuclear single-nucleotide polymorphic markers to study gene flow and genetic diversity. Results:We identified 20 different haplotypes, which lacked spatial and temporal distribution patterns in the recent expansion range in Norway. They also mostly lacked patterns across the native European range of scotch broom. The genetic diversity of nuclear genomic SNP markers across populations in the introduced range was similar to that of populations in the native range, with limited differentiation among populations. Main conclusions: Scotch broom is alien to Norway and was introduced by humans on multiple occasions from diverse origins over a long period of time. High propagule pressure has probably maintained the high genetic diversity in the novel range through a combination of genetically diverse source populations and high gene flow among them. Within the native European range, our results suggest the presence of cryptic intraspecific admixture, most likely mediated by humans moving genotypes among the regions occupied by distinct native genotypes. Intracontinental invasions may easily go unnoticed and revealing these invasions and the factors driving them may be of great importance for the management of alien species.
Cytisus scoparius is a global invasive species that affects local flora and fauna at the intercontinental level. Its natural distribution spans across Europe, but seeds have also been moved among countries, mixing plants of native and non-native genetic origins. Hybridization between the introduced and native gene pool is likely to threaten both the native gene pool and the local flora. In this study, we address the potential threat of invasive C. scoparius to local gene pools in vulnerable heathlands. We used nuclear single nucleotide polymorphic (SNP) and simple sequence repeat (SSR) markers together with plastid SSR and indel markers to investigate the level and direction of gene flow between invasive and native heathland C. scoparius. Analyses of population structures confirmed the presence of two gene pools: one native and the other invasive. The nuclear genome of the native types was highly introgressed with the invasive genome, and we observed advanced-generation hybrids, suggesting that hybridization has been occurring for several generations. There is asymmetrical gene flow from the invasive to the native gene pool, which can be attributed to higher fecundity in the invasive individuals, measured by the number of flowers and seed pods. Strong spatial genetic structure in plastid markers and weaker structure in nuclear markers suggest that seeds spread over relatively short distances and that gene flow over longer distances is mainly facilitated by pollen dispersal. We further show that the growth habits of heathland plants become more vigorous with increased introgression from the invaders. Implications of the findings are discussed in relation to future management of invading C. scoparius.
The cover image is based on the Original Article Introduction history and population genetics of intracontinental scotch broom (Cytisus scoparius) invasion by Siri Fjellheim et al., https://doi.org/10.1111/ddi.12979.
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