Genetic and morphological differentiation within Euphorbia japygica (Euphorbiaceae) suggests divergence of populations from the south-eastern Apennine Peninsula

The Mediterranean Basin is a hotspot of animal and plant diversity. Contrary to the Balkan and Iberian Peninsulas that were subject of many phylogeographic studies in past decades, the Apennine Peninsula and, in particular, the diversification of its flora has been neglected in contemporary phylogenetic studies. The few available studies showed a complex pattern of north–south differentiation of genetic diversity in Italy, both among the Alps, the Apuan Alps, and the Apennines, as well as within the Apennines. Here, we explore phylogeographic patterns within recently described Euphorbia adriatica, distributed in the central and northern Apennine Peninsula, the southern margin of the Eastern Alps and the north-western Balkan Peninsula, and its relationship to the southern Italian E. japygica. Our integrative approach using nuclear ITS sequences, amplified fragment length polymorphisms, relative genome size estimation combined with chromosome counting, as well as multivariate morphometrics inferred a weak genetic differentiation that only partly corresponds to the morphological differentiation. Whereas all southern populations have hairy capsules characteristic for E. japygica, only those in south-eastern Italy (Puglia and Basilicata) are genetically divergent. There are, however, additional morphological characters that differentiate them from other populations. Our data are thus in favour of recognizing a single species, E. japygica, which includes E. adriatica that should be treated as a subspecies. We provide a revised taxonomic treatment for E. japygica, including the typification. Our study shows the necessity of further in-depth investigations of diversification of Italian biota.