Molecular phylogeny of Isodon (Lamiaceae) in Japan using chloroplast DNA sequences: recent rapid radiations or ancient introgressive hybridization?

Phylogenetic relationships between 14 taxa of Isodon (Lamiaceae) occurring in Japan and South Korea were examined using sequence variations in 11 chloroplast DNA regions. The phylogenetic tree based on the sequence of 6478 bp did not reflect a previous taxonomic treatment of the genus at the section level: the three main clades contained species in different sections. Furthermore, individuals from different populations in the same species were rarely monophyletic in the tree. In addition, individuals forming monophyletic groups did not show geographical proximity. Lineage sorting of chloroplast DNA variations following rapid divergence of the taxa is likely to have caused complex phylogeny of the taxa, which was reconstructed in the present study. Another possible explanation could be ancient introgressions and resultant chloroplast capture.