Invasiveness and the impacts of introduced plants are known to be mediated by plant-microbe interactions. Yet, the microbial communities associated with invasive plants are generally poorly understood. Here we report on the first comprehensive investigation of the bacterial and fungal communities inhabiting the rhizosphere and the surrounding bulk soil of a widespread invasive tree, Acacia dealbata. Amplicon sequencing data indicated that rhizospheric microbial communities differed significantly in structure and composition from those of the bulk soil. Two bacterial (Alphaproteobacteria and Gammaproteobacteria) and two fungal (Pezizomycetes and Agaricomycetes) classes were enriched in the rhizosphere compared with bulk soils. Changes in nutritional status, possibly induced by A. dealbata, primarily shaped rhizosphere soil communities. Despite a high degree of geographic variability in the diversity and composition of microbial communities, invasive A. dealbata populations shared a core of bacterial and fungal taxa, some of which are known to be involved in N and P cycling, while others are regarded as plant pathogens. Shotgun metagenomic analysis also showed that several functional genes related to plant growth promotion were overrepresented in the rhizospheres of A. dealbata. Overall, results suggest that rhizosphere microbes may contribute to the widespread success of this invader in novel environments.

Members of the balloon vine genus, Cardiospermum, have been extensively moved around the globe as medicinal and horticultural species, two of which are now widespread invasive species; C. grandiflorum and C. halicacabum. A third species, C. corindum, may also have significant invasion potential. However, in some regions the native status of these species is not clear, hampering management. For example, in South Africa it is unknown whether C. halicacabum and C. corindum are native, and this is a major constraint to ongoing biological control programmes against invasive C. grandiflorum. We review the geography, biology and ecology of selected members of the genus with an emphasis on the two most widespread invaders, C. halicacabum and C. grandiflorum. Specifically, we use molecular data to reconstruct a phylogeny of the group in order to shed light on the native ranges of C. halicacabum and C. corindum in southern Africa. Phylogenetic analyses indicate that southern African accessions of these species are closely related to South American taxa indicating human-mediated introduction and/or natural long distance dispersal. Then, on a global scale we use species distribution modelling to predict potential suitable climate regions where these species are currently absent. Native range data were used to test the accuracy with which bioclimatic modelling can identify the known invasive ranges of these species. Results show that Cardiospermum species have potential to spread further in already invaded or introduced regions in Australia, Africa and Asia, underlining the importance of resolving taxonomic uncertainties for future management efforts. Bioclimatic modelling predicts Australia to have highly favourable environmental conditions for C. corindum and therefore vigilance against this species should be high. Species distribution

Accurate identification of weedy species is critical to the success of biological control programs seeking host-specific control agents. Phylogenetic relationships based on internal transcribed spacer region (ITS1, ITS2) DNA sequence data were used to elucidate the most likely origin and taxonomic placement of Senecio madagascariensis Poir. (fireweed; Asteraceae) in the Hawaiian archipelago. Putative S. madagascariensis populations from Madagascar, South Africa, Swaziland, and Hawaii were included in the analysis. Different phylogenetic models (maximum parsimony and maximum likelihood) were congruent in suggesting that Hawaiian fireweed is most closely related to populations from the KwaZulu-Natal region in South Africa. Phylogenetic divergence and morphological data (achene characteristics) suggest that the S. madagascariensis complex is in need of revised alpha-level taxonomy. Taxonomic identity of invasive fireweed in Hawaii is important for finding effective biological control agents as native range populations constitute different biotypic variants across a wide geographical area. Based on our phylogenetic results, research directed at biological control of Hawaiian infestations should focus on areas in the KwaZulu-Natal region in South Africa where host-specific natural enemies are most likely to be found. Our results show that phylogeographical analysis is a potential powerful and efficient tool to address questions relevant to invasion biology of plants.

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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.

A longstanding question in evolutionary biology is how natural selection and environmental pressures shape the mitochondrial genomic architectures of organisms. Mitochondria play a pivotal role in cellular respiration and aerobic metabolism, making their genomes functionally highly constrained. Evaluating selective pressures on mitochondrial genes can provide functional and ecological insights into the evolution of organisms. Collembola (springtails) are an ancient hexapod group that includes the oldest terrestrial arthropods in the fossil record, and that are closely associated with soil environments. Of interest is the diversity of habitat stratification preferences (life forms) exhibited by different species within the group. To understand whether signals of positive selection are linked to the evolution of life forms, we analysed 32 published Collembola mitogenomes in a phylomitogenomic framework. We found no evidence that signatures of selection are correlated with the evolution of novel life forms, but rather that mutations have accumulated as a function of time. Our results highlight the importance of nuclear-mitochondrial interactions in the evolution of collembolan life forms and that mitochondrial genomic data should be interpreted with caution, as complex selection signals may complicate evolutionary inferences.