Going west - A subtropical lineage (Vincetoxicum, Apocynaceae: Asclepiadoideae) expanding into Europe

Vincetoxicum sensu lato is a tropical lineage comprising two clades that have reached high northern latitudes. Of the temperate clades, one is restricted to the Far East, the other one (Vincetoxicum s. str. Clade) extends into Europe, but their ranges overlap in Central China and Japan. Three species invasive in North America, V. hirundinaria, V. nigrum and V. rossicum, are members of the Vincetoxicum s. str. Clade. We explore the prerequisites for the range expansion in the Vincetoxicum s. str. Clade performing Bayesian and Maximum Likelihood phylogenetic analyses on sequences of the nuclear internal transcribed spacer (ITS) region, the nuclear external transcribed spacer region (ETS), and five plastid markers. The resulting phylogeny is used to conduct biogeographic analysis using BioGeoBEARS to reconstruct ancestral species ranges. Moreover, we map the known occurrences of two rare characters in Asclepiadoideae, the possession of phenanthroindolizidine alkaloids and reported cases of autogamy onto our phylogeny. Finally, we have conducted ecological niche modelling using Maxent on a total of 220 spatially unique occurrences of nine Vincetoxicum s. str. species spanning more than 4,000 km along the east-west gradient to learn about the climatic conditions along the presumed migration route. Our results indicate a north-westward migration in Vincetoxicum s. str. along the Asian mountain chains to Europe. Climatic preferences of the nine species sampled are dissimilar, except for the common exposure to at least one month of subfreezing temperatures, indicating a rather wide climatic tolerance for the clade as a whole. The three species invasive in North America belong to the northern Eurasian subclade and show the rare combination of phenanthroindolizidine alkaloids and autogamy.