Out of Liguria: How polyploidy affected diversification of the sweet spurge (Euphorbia dulcis, Euphorbiaceae), European widespread forest species

Polyploidy is an important evolutionary mechanism in flowering plants that can strongly affect their morphological and distributional traits. In this study, we investigated the differences in these traits among di-, tri-, and tetraploid populations of sweet spurge (Euphorbia dulcis), a common understory species in deciduous and mixed forests across Europe. We inferred the ploidy of 188 populations of E. dulcis by estimating relative genome size and chromosome counting. The data indicate that tri- and tetraploids are more widespread compared to the ancestral diploid populations, which are restricted to Liguria (north-western Italy) and adjacent regions. We suggest that polyploidisation was crucial for range expansion and the colonisation of higher latitudes, but not for the colonisation of higher elevations, where diploids appear to be more successful. Similarly, morphological differentiation after polyploidisation is only partly consistent with the classical hypothesis that polyploids are larger and have larger organs. Using nuclear ITS and plastid ndhF–trnL sequences, we inferred the spatio-temporal diversification of E. dulcis. It diverged from its diploid sister species E. duvalii, an endemic of southwestern France and adjacent Spain, in the mid-Pliocene. This divergence was likely due to vicariant speciation accompanied by adaptation to forest and grassland environments in E. dulcis and E. duvallii, respectively. Whereas the diploid populations of both taxa have restricted ranges today, polyploidisation within E. dulcis likely triggered by the Pleistocene climatic oscillations contributed to its significant range expansion and diversification. The species exhibits the highest genetic diversity in the south-western Alps, where all three ploidies co-occur. Based on the ploidy differentiation and the less pronounced genetic and morphological differentiation, we propose treating di- and triploids as well as two geographically and genetically divergent groups of tetraploids (eastern and western) as four subspecies. This challenges various taxonomic treatments previously proposed for this species. Our study highlights the importance of polyploidisation for diversification and range expansion, and indicates the necessity of further research to test hypotheses related to the morphological and distributional characteristics of polyploid organisms.