Allopatric hybrids as evidence for past range dynamics in Sempervivum (Crassulaceae), a western Eurasian high mountain oreophyte

Abstract

Fossil evidence, phylogeographic analyses, species distribution modelling and ancient DNA analyses have all shown that plant distributions have been highly dynamic through time. We use the geographical distribution of intra- and interspecific hybrids in Sempervivum, a western Eurasian high mountain oreophyte, as evidence for the past range dynamics of their parents. Sequences of the nuclear ribosomal internal transcribed spacer (ITS), parts of the nuclear ribosomal intergenic spacer region (IGS), and the three chloroplast markers atpI–atpH, rps16-intron and trnQ–rps16 were generated for 101 individuals of Sempervivum from across its entire distribution range. Hybrid individuals were identified by the presence of double base calls in direct sequencing of polymerase chain reaction products of ITS and IGS. Parentage was inferred from comparison with sequences without double base calls and with cpDNA sequences. We identified 27 hybrid individuals, which were found outside the current distribution range of one parent in 10 cases and with both (or all three) parents in eight cases. Geographical distance of hybrids and allopatric parents ranged from 25 to 2100 km. The distribution of hybrid individuals in relation to their parents provides evidence for past range dynamics and migration over sometimes large geographical distances. As all taxa involved had been postulated to be of Quaternary origin in an earlier study, we hypothesise that hybridisation took place in glacial refugia where the parental species came into contact.