New insights into evolutionary relationships within the subfamily Lamioideae (Lamiaceae) based on pentatricopeptide repeat (PPR) nuclear DNA sequences

Premise of the study: Lamioideae, one of the most species-rich subfamilies within Lamiaceae, exhibits a remarkable diversity in morphology and habit and is found in many temperate to subtropical regions across the globe. Previous studies based on chloroplast DNA (cpDNA) sequence data produced a tribal classification of Lamioideae, but so far this has not been confirmed with nuclear DNA loci.

Methods: We investigated sequence variation in a low-copy nuclear pentatricopeptide repeat (PPR) region and compared the phylogenetic results with previously published sequence data from a concatenated data set comprising four cpDNA loci. We incorporated representatives of all 10 lamioid tribes and some unclassified taxa, analyzed the data using phylogenetic inference, and estimated divergence times and ancestral areas for major nodes.

Key results: Our results showed overall topological similarities between the cpDNA and PPR phylogenies with strong support for most tribes. However, we also observed incongruence in the circumscription of some tribes, including Gomphostemmateae and Pogostemoneae and in the relationships among tribes. Our results suggest an Oligocene-Miocene origin of the Lamioideae crown group. Asia and the Mediterranean region appear to have been centers of diversity and place of origin for many lamioid tribes.

Conclusions: This study represents the first phylogeny of subfamily Lamioideae inferred from low-copy nuclear DNA data. We show that most lamioid tribes are corroborated, although the exact circumscription of two tribes is questioned. We have shed further light on the evolutionary relationships within Lamioideae, and this study demonstrates the utility of the PPR region for such subfamilial-level phylogenetic studies.