Multiple long-distance dispersals accompanied by reduction of body size shaped the evolutionary history of four spurges (Euphorbia) endemic to Mediterranean islands.

Long-distance dispersal (LDD) followed by successful colonization influences many life-history traits and can lead to drastic changes in morphology. Although still controversial, a radical change in body size following colonization of islands ("island rule") has been observed in many animals, but also in plants. Here, we test this rule in a group of Euphorbia species endemic to four Mediterranean islands and compare them to their mainland ancestors. We challenge the previously proposed hypothesis of a vicariant origin of these endemics from a widespread common ancestor and suggest more recent origins via LDDs from adjacent mainland areas. We applied amplified fragment length polymorphism [AFLP] fingerprinting and phylogenetic analyses of nuclear ITS and plastid trnT-trnF sequences to infer the origin and spatiotemporal diversification of four single-island endemic Euphorbia species from the Mediterranean Basin, namely E. corsica, E. fontqueriana, E. rechingeri and E. veneris. Further, we applied multivariate morphometrics to explore the island rule. Phylogenetic analyses suggest a recent (Late Pliocene-Early Pleistocene) origin of the four island endemics via LDD, and an east-west phylogeographic divergence within the Mediterranean Basin. While Corsica and Mallorca were probably colonized from the Iberian Peninsula, the species from Crete and Cyprus originated from Asia Minor. The colonization of the islands was associated with a reduction in plant and leaf size; distribution ranges remained extremely narrow. This study highlights the importance of recent LDDs for the colonization of Mediterranean islands and their contribution to high insular endemism. Colonization of the islands was followed by a drastic reduction in plant and leaf size, consistent with the island rule. However, it remains unclear whether the tendency to dwarfism is related to island colonization per se, or simply a consequence of genetic drift and adaptation to extreme rocky habitats.