Biogeographic connections between north-west Africa, southern Italy, and the Balkan Peninsula in the <i>Euphorbia amygdaloides</i> alliance (Euphorbiaceae)

Abstract The Mediterranean Basin is an important biodiversity hotspot with high levels of endemism. It is a biogeographically complex region in which narrow sea straits, such as those between Sicily and north Africa, acted as migratory links for biota, including a number of mesophilous Euro-Siberian floristic elements, which extend their ranges to north-western Africa. One such species is Euphorbia amygdaloides, a widespread European forest understory species. North African and south Italian populations were sometimes recognized as distinct taxa, E. durandoi and E. meuselii, respectively, but were more commonly considered conspecific with E. amygdaloides. Furthermore, their relationship to the recently rediscovered Balkan endemic E. orjeni remain unknown. In this study, we use an integrative approach to disentangle the relationships among the aforementioned taxa. Phylogenetic analyses of ITS sequences reveal a close relatedness between E. durandoi, E. meuselii, and E. orjeni, which are clearly divergent from E. amygdaloides. They share a monopodial growth form with branching of the main stem at different heights above the ground vs. the sympodial growth form with underground branching in E. amygdaloides. Despite the presence of numerous shared morphological traits, these species are well-differentiated and also differ in their ecological preferences. We provide a revised taxonomic treatment for the group and numerous novel distribution records for E. meuselii and E. orjeni. Our study emphasizes the importance of previously little-studied biogeographic connections between northern Africa and Europe, as well as those between the Balkan and Apennine peninsulas. It demonstrates the substantial potential of extensive fieldwork and citizen science platforms to enhance our understanding of plant distributions, even in regions that have been extensively studied, such as Europe.