Multiple introductions, polyploidy and mixed reproductive strategies are linked to genetic diversity and structure in the most widespread invasive plant across Southern Ocean archipelagos

This work received support from the Swiss Polar Institute and Ferring Pharmaceuticals through the Antarctic Circumnavigation Expedition (“ACE”). Additional fi- nancial and logistical support was provided by the South African National Research Foundation (NRF) and by the South African National Antarctic Programme (SANAP). MM and CH were also supported by the National Research Foundation (grant 89967). We thank Tasmanian Parks and Wildlife Service for granting ac- cess and collection permits for Macquarie Island and the Australian Antarctic Program for logistical support. We thank Institut polaire français Paul- Émile Victor for logistical support for sampling on Iles Kerguelen and Ile de la Possession. We thank wintering staff for sam- ple collection on Ile de la Possession. We thank the Department of Conservation for granting access and collection permits for the New Zealand islands. We thank Heritage Expeditions for providing logis- tics and voyage support. Collections were undertaken on the ACE expedition under permits granted to the expedition and its research- ers. Collections at the Prince Edward Islands, and at the Tristan da Cunha and Gough islands were permitted through the South African National Antarctic Program, notably via the Prince Edward Islands Management Committee for the former and the Tristan da Cunha Conservation Department for the latter.

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.

National Research Foundation 89967

Antarctic Circumnavigation Expedition “ACE”

South African National Research Foundation (NRF)

South African National Antarctic Programme (SANAP)