Polyploidisation and niche differentiation drive the diversification of the Euphorbia epithymoides group (Euphorbiaceae) in southeastern Europe

Species groups spanning large areas of southeastern Europe gained little attention in phylogenetic studies. We here inferred relationships within the Euphorbia epithymoides group, distributed across the Balkan Peninsula, the southeastern margin of the Alps, the Pannonian Basin, and the Carpathians, and explored how polyploidisation is related to environmental niche differentiation. We used an integrative approach, including phylogenetic analyses (AFLP fingerprinting, ITS sequencing), estimates of ploidy level based on relative genome size, multivariate morphometrics, environmental niche analyses and species distribution modelling. Dating analyses inferred the origin of the E. epithymoides group at the Miocene/Pliocene boundary. Environmental changes facilitated the main divergence within the group, leading to the polyploid origin of E. fragifera and its niche contraction to a warmer Mediterranean climate. Within the more widespread and mesophyllous E. epithymoides s.l., the main genetic divergence, although with considerable geneflow, occurred in the central Balkan Peninsula. It was not accompanied by pronounced morphological differentiation, but rather by significant divergence of genome sizes and environmental niches. Our results corroborate the niche shift hypothesis as one of the powerful mechanisms for overcoming the minority cytotype exclusion after polyploidisation. Distribution modelling highlights the importance of the southern and eastern Balkan Peninsula and the adjacent Carpathians for the large-scale long-term survival and persistence of mesophilous (forest) vegetation throughout the Pleistocene. With exception of distinct E. fragifera, all other taxa (E. epithymoides, E. gregersenii, E. lingulata) should rather be considered subspecies of widespread, morphologically and ecologically variable E. epithymoides.