Overall phylogenetic relationships of Scutellaria (Lamiaceae) shed light on the origin of the predominantly Caucasian and Irano-Turanian S. orientalis group

Abstract

Scutellaria is one of the largest genera in the Lamiaceae with an estimated 400–500 species with a nearly worldwide distribution. Most species occur in the N hemisphere, with the Caucasus and the wider Irano-Turanian region housing a large number of taxa, many of them considered endemic. We present an overall phylogeny of the monophyletic genus Scutellaria based on rapidly evolving plastid regions (matK-trnK, rpl16, trnL-F). Three well-supported clades are evident, which render the currently accepted S. subg. Scutellaria paraphyletic to S. subg. Apeltanthus, which appears nested in “clade A”, in which the African S. schweinfurthii is sister to all remaining taxa, followed by other lineages of S. subg. Scutellaria. Ancestral states of 12 morphological characters frequently used as diagnostic from subgenus to species level were reconstructed with BayesTraits. The S. orientalis group appears as a major radiation in the Caucasus area and the Irano-Turanian region that may comprise up to a quarter of the species in the genus. This radiation corresponds to a monophyletically defined S. sect. Lupulinaria, characterized by decussate inflorescences and specialized (e.g. cucullate) bracts. Our phylogenetic data present significant resolution at the species level within the S. orientalis group, indicating complex geographically centred patterns of speciation in adaptation to steppe and high mountain habitats, including multiple evolution of pinnate and tomentose leaves. The detailed infrageneric classification of Juzepczuk (1951, 1954) mostly does not reflect natural groups.