Polyploidization was not involved in the origin of five endemic species from southern Europe but is otherwise frequent in Euphorbia section Esula (Euphorbiaceae)

Abstract

Polyploidization is a key factor in the diversification of flowering plants, but appears to be of only minor importance in the evolution of Euphorbia subgenus Esula. An exception is Euphorbia section Esula, including c. 100 species, many of which are widespread, but several have restricted distribution areas. We used ITS sequencing and relative genome size estimation to explore the evolutionary origin of E. gayi, E. graminifolia, E. tommasiniana, E. valliniana and E. variabilis, which are endemic to disjunct areas along the southern and western margins of the Alps, Corsica and Sardinia. In addition, we explored the incidence of polyploidy across different lineages of E. section Esula. Our phylogenetic results revealed several lineages with unresolved relationships that probably diverged in the Late Miocene, but only one (the Eurasian Group) underwent considerable diversification, probably due to numerous polyploidization events. Several polyploid species or mixed ploidy species in this group are distributed across large areas of Eurasia. All other lineages remained mostly diploid and species-poor, but dispersed to various continents, where most of the species have restricted distribution areas. The five focal species are diploid, but they do not share a common ancestor. Euphorbia graminifolia and E. tommasiniana belong to the Eurasian Group, whereas E. gayi, E. valliniana and E. variabilis belong to the Cosmopolitan Group and are thus not closely related to other European members of the section. Phylogenetic analyses recovered E. valliniana and E. variabilis as genetically distinct, although closely related. This, in combination with a clear morphological differentiation as revealed by morphometric analyses, supports their recognition as independent species. Our study highlights the important role polyploidization had for plant diversification as well as the effects of other factors such as vicariance in disjunct refugia, which probably led to the origin of the five endemic species from Euphorbia section Esula.