LAUSR

The Mediterranean region is one of the 25 biodiversity hotspots, with several species distributed on both sides of the Adriatic Sea, i.e. on the Apennine and the Balkan Peninsulas. One of these is Euphorbia spinosa, distributed in the coastal regions of the Ligurian, Tyrrhenian, Ionian and Adriatic Seas, whereas its closest relative, E. glabriflora, occurs in continental regions of the western and southern Balkan Peninsula, mostly on serpentine substrates. They differ morphologically, but transitional forms resembling E. glabriflora can be found within the range of E. spinosa and in the overlapping areas of both species, and phylogenetic relationships between them remain unclear. Using nuclear ribosomal ITS sequences, we here show that these two species form a monophyletic group closely related to the E. acanthothamnos alliance, but the relationships between them remain unresolved. Also, amplified fragment length polymorphism (AFLP) fingerprints are weakly structured, but indicate a north-west to south-east trend in genetic differentiation and two main genetic groups; however, these are not entirely congruent with morphological species boundaries. In addition, morphometric analyses indicate only weak differentiation between the taxa, mostly due to occurrence of morphologically transitional forms in contact areas between the two genetic groups. Based on our results we propose to treat both taxa as subspecies of E. spinosa and provide a taxonomic treatment with descriptions. Finally, based on the high genetic variability on the Balkan Peninsula, we hypothesize a trans-Adriatic dispersal from the Balkans to the Apennine Peninsula, probably during the Pleistocene. This is in line with previous studies of different plants with amphi-Adriatic distributions, with the exception that in E. spinosa the distributions on both peninsulas are of similar size, whereas in most other cases the distribution area on the Balkan Peninsula is larger than on the Apennine Peninsula.