Ghosts of Giants: high levels of clonality may represent massive remnant clones in European aspen (Populus tremula)

Clonal reproduction is an important strategy for the establishment, expansion and persistence of many plant species. It is well-documented in Populus spp. Populus tremula (European Aspen) is a native species in Ireland, a pioneer which has persisted from the early Holocene to the present. Our main aim was to determine whether clonal and genetic diversity across Irish populations of P. tremula, are lower compared to populations from Britain and continental Europe. Twenty geographically discrete populations of P. tremula from Ireland, and 14 populations from mainland Britain, Sweden and Italy, were genotyped for eight nuclear microsatellites and five chloroplast microsatellites. Levels of genetic diversity were extremely low in Ireland compared to those from Britain, Sweden and Italy. While such low levels of variation can be linked to founder effects during postglacial colonisation, all but two of the Irish sites were monoclonal with extensive fragmentation of the Irish clones across large areas. Collectively, these results suggest that the present-day distribution of P. tremula in Ireland comprises remnants of once massive clones that have persisted by asexual reproduction. This is consistent with Baker’s Law that self-compatible organisms are more likely to colonize the extremities of a species range than self-incompatible organisms. Clonality in P. tremula across the Irish landscape may be an adaptation enabling establishment and persistence of isolated populations during climate change associated with glacial advances and retreat. Extreme clonality, however, is also associated with loss of genetic diversity which may increase vulnerability to recent, more rapid, anthropogenic climate change, novel plant pathogens and hybridisation.