Molecular biogeography of prickly lettuce (Lactuca serriola L.) shows traces of recent range expansion.

Prickly lettuce (Lactuca serriola L., Asteraceae), a wild relative of cultivated lettuce, is an autogamous species which greatly expanded throughout Western and Northern Europe during the last two centuries.Here we present a large-scale biogeographic genetic analysis performed on a dataset represented by 2622 individuals from 110 wild European populations. Thirty-two maternally inherited chloroplast RFLP-markers and ten nuclear microsatellite loci were used. Microsatellites revealed low genetic variation and high inbreeding coefficients within populations, as well as strong genetic differentiation between populations, which was in accordance with the autogamous breeding system.AMOVA-based clustering indicated the presence of three populations clusters, which showed strong geographical patterns. One cluster occupied United Kingdom and part of Northern Europe, and characterized populations with a single predominant genotype. The second mostly combined populations from Northern Europe, while the third cluster grouped populations particularly from Southern Europe. Kriging of gene diversity for L. serriola corroborated northwards and westwards spread from Central (Eastern) Europe.Significant lower genetic diversity characterized the newly colonised parts of the range compared to the historical ones, confirming the importance of founder effects. Stronger pattern of isolation by distance was assessed in the newly colonised areas than in the historical areas (Mantel's r=0.20). In the newly colonised areas, populations at short geographic distances were genetically more similar than those in the historical areas.Our results corroborate the species' recent and rapid northward and westward colonisation from Eastern Europe, as well as a decrease of genetic diversity in recently established populations.