Biological invasions in remote areas that experience low human activity provide unique opportunities to elucidate processes responsible for invasion success. Here we study the most widespread invasive plant species across the isolated islands of the Southern Ocean, the annual bluegrass, Poa annua. To analyse geographical variation in genome size, genetic diversity and reproductive strategies, we sampled all major sub-Antarctic archipelagos in this region and generated microsatellite data for 470 individual plants representing 31 populations. We also estimated genome sizes for a subset of individuals using flow cytometry. Occasional events of island colonization are expected to result in high genetic structure among islands, overall low genetic diversity and increased self-fertilization, but we show that this is not the case for P. annua. Microsatellite data indicated low population genetic structure and lack of isolation by distance among the sub-Antarctic archipelagos we sampled, but high population structure within each archipelago. We identified high levels of genetic diversity, low clonality and low selfing rates in sub-Antarctic P. annua populations (contrary to rates typical of continental populations). In turn, estimates of selfing declined in populations as genetic diversity increased. Additionally, we found that most P. annua individuals are probably tetraploid and that only slight variation exists in genome size across the Southern Ocean. Our findings suggest multiple independent introductions of P. annua into the sub-Antarctic, which promoted the establishment of genetically diverse populations. Despite multiple introductions, the adoption of convergent reproductive strategies (outcrossing) happened independently in each major archipelago. The combination of polyploidy and a mixed reproductive strategy probably benefited P. annua in the Southern Ocean by increasing genetic diversity and its ability to cope with the novel environmental conditions.

Australian Antarctic Division Tasmania Kingston Australia

Biodiversity Informatics Unit African Institute for Mathematical Sciences Cape Town South Africa

Departamento de Biodiversidad Ecología y Evolución Universidad Complutense de Madrid Madrid Spain

Departamento de Biología Universitat de les Illes Balears Institut Mediterrani d'Estudis Avançats Mallorca Spain

Departamento de Botánica Facultad de Ciencias Universidad de Granada Granada Spain

Department of Botany and Zoology Stellenbosch University Stellenbosch South Africa

Department of Mathematical Sciences Centre for Invasion Biology Stellenbosch University Stellenbosch South Africa

Department of Zoology Centre for Ecological Genomics and Wildlife Conservation University of Johannesburg Auckland Park South Africa

Faculty of Science Department of Botany Charles University Prague Czech Republic

Institute of Botany Czech Academy of Science Průhonice Czech Republic

School of Natural Sciences Macquarie University New South Wales Sydney Australia

Securing Antarctica's Environmental Future School of Biological Sciences Monash University Melbourne Victoria Australia

Securing Antarctica's Environmental Future School of Biology and Environmental Sciences Queensland University of Technology Brisbane Queensland Australia

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