LAUSR

BACKGROUND AND AIMS Bristol rock cress is among the few plant species on the British Isles considered to have Mediterranean-montane element. Spatiotemporal patterns of colonization of the British Isles since the last interglacial and after Last Glacial Maximum (LGM) from mainland Europe are underexplored and have not yet included such floristic elements. Herewith we shed light on the evolutionary history of a relic and outpost metapopulation of Bristol rock cress in the southwestern United Kingdom. METHODS Amplified fragment length polymorphisms (AFLPs) were used to identify distinct genepools. Plastome assembly and respective phylogenetic analysis revealed the temporal context. Herbarium material was largely used to exemplify the value of collections to obtain a representative sampling covering the entire distribution range. KEY RESULTS AFLPs recognized two distinct genepools with the Iberian Peninsula as the primary centre of genetic diversity and the origin of lineages expanding before and after LGM towards mountain areas in France and Switzerland. No present day lineages are older than 51 ky, which is in sharp contrast to the species stem group age of nearly 2 My, indicating severe extinction and bottlenecks throughout the Pleistocene. The British Isles were colonized after the LGM and feature a high genetic diversity. CONCLUSIONS The short lived perennial herb Arabis scabra, which is restricted to limestone, has expanded its distribution range after LGM following corridors within an open landscape and may have reached the British Isles via the desiccated Celtic Sea at about 16 kya. This study may shed light on the origin of other rare and peculiar species co-occurring in limestone regions in the southwestern British Isles.