Colonization and diversification in the African 'sky islands' by Eurasian Lychnis L. (Caryophyllaceae)

Many plants occurring on the isolated mountain peaks of Africa have their closest relatives in very remote areas, even in temperate Europe and Asia. Their biogeographical history is poorly understood. The Afro-montane element of the primarily Eurasian genus Lychnis is a typical example of such a disjunction. Here, we aim to reconstruct the phylogeny of the Afro-montane endemics of Lychnis and to infer the history of immigration into Africa and of the subsequent dispersal and speciation. The Afro-montane representatives of Lychnis occur in Ethiopia, Kenya, Tanzania, Uganda, Rwanda, Burundi, the Democratic Republic of Congo, Cameroon, and Nigeria. Silica-dried plant materials collected in Ethiopia in 2004 as well as herbarium material were used for DNA extractions. We used parsimony analysis of nucleotide data from the rps16 intron and psbE-petL region in plastids, and the internal transcribed spacer (ITS) and a region spanning exon 18-24 in the second largest subunit of RNA polymerase II (RPB2) from the nucleus. DAPI-flow cytometry was used to determine relative genome size and to infer polyploidization events. Monophyly of the Afro-montane Lychnis with the Eurasian diploid L. flos-cuculi resolved as sister was strongly supported by the phylogenetic analyses. The Afro-montane group was further divided into two strongly supported groups correlated with DNA ploidy levels. The relative genome size was species-specific except for L. abyssinica, which had two genome-size variants. Our results demonstrate that the biogeographical history of the Afro-montane Lychnis has been highly dynamic, including polyploidization and both old and recent long-distance dispersal events, even between Eastern and West Africa. The ancestor is inferred to have immigrated once from Eurasia via the Arabian Peninsula to the Ethiopian highlands. Divergence in the lineage containing diploid taxa was followed by several dispersals from Ethiopia to the Eastern and Western Rift Mountains, and recently to West Africa. Divergence of the lineage including the tetraploid L. crassifolia may have taken place in Ethiopia, from where it dispersed to the Eastern and Western Rift Mountains; alternatively, it may have taken place in the Eastern Rift Mountains, followed by dispersal from there to the Western Rift Mountains and then to Ethiopia.