A unique A. palustris lineage across the Otranto strait: botanical evidence for a past land-bridge?

Phylogeography allows to reconstruct the history of genealogic lineages at the intra- and interspecific level. Previous studies on the distribution of minisatellite and microsatellite plastid haplotypes in populations of the Mediterranean marsh orchid Anacamptis palustris revealed that populations from the Salento peninsula (Apulia region, west coast of Otranto strait) formed a distinct and isolated lineage. To further explore the peculiarity of this biogeographic region, in the present study we examined both fresh samples and herbarium specimens of the rare orchid Anacamptis palustris from populations located on the eastern coast of the Otranto strait (i.e. on the Greek-Albanian side). All the investigated samples exhibited a unique combination of chloroplast microsatellites, which was previously found exclusively in populations from the Salento peninsula. Samples from Corfu, Igoumenitsa and Durazzo showed a novel minisatellite repeat type, while those of Zakynthos possessed the widespread ancestral repeat type. Interestingly, the novel repeat type and those previously found in Salento populations clustered together on a well-differentiated evolutionary branch in a minisatellite repeat type network of the A. palustris lineages. This finding indicates a close relationship between the populations from both sides of the Otranto strait and supports the geological evidence for the occurrence of an ancient land-bridge connection between the southern most part of Italy and Greece during the Messinian desiccation and/or the "Lago-Mare" period.