Disentangling relationships in <i>Euphorbia agraria</i> s.l. (Euphorbiaceae) in south‐east Europe: One or two species?

Abstract We explored the diversification of the south‐eastern and eastern European Euphorbia agraria s.l. applying molecular (amplified fragment length polymorphism [AFLP] fingerprinting and sequencing of the nuclear ribosomal internal transcribed spacer [ITS]), cytogenetic (relative genome size [RGS] estimations) and morphometric methods. The AFLP data, and to a lesser extent ITS sequences, inferred two main phylogenetic lineages corresponding to eastern Balkan‐Pontic E. agraria and central Balkan E. subhastata ; their closest relatives are E. tommasiniana endemic to the north‐westernmost Balkan Peninsula and E. salicifolia , which is more widespread in the Balkan Peninsula. Genetic divergence is reflected in morphological and ecological differentiation, rendering recognition of E. subhastata – previously segregated as a variety – at the species level, resulting in a revised taxonomic treatment. Euphorbia agraria thrives in grasslands and ruderal places that are widespread in the lowlands of the eastern Balkan Peninsula and the adjacent Pontic region; the continuity of the habitat probably confers weak genetic differentiation within this species. On the other hand, E. subhastata grows in screes and open forests of river gorges separated by mountain ridges, leading to more pronounced inter‐population differentiation. The RGS data revealed di‐ and tetraploid populations within both species and in combination with phylogenetic results suggest recurrent autopolyploidisation. Our results support the Balkan Peninsula as a hotspot of genetic and species diversity and indicate that future biodiversity research in this part of Europe should focus on the central, eastern and southern Balkan Peninsula that were largely neglected in previous phylogenetic studies.