Molecular phylogeny of Conradina and other scrub mints (Lamiaceae) from the southeastern USA: Evidence for hybridization in Pleistocene refugia?

Conradina (Lamiaceae) consists of six allopatric species endemic to the southeastern United States, four of which are federally endangered or threatened. The limits and status of several taxa have been contested based on morphological grounds, and clarification of these limits is necessary for the design and implementation of effective and fiscally responsible protection and management plans. The objectives of this study were to investigate the monophyly of Conradina and its relationship to other endemic mints of the southeastern United States, to understand the patterns of diversification in Conradina, and to clarify species relationships. A molecular phylogeny was inferred by sequencing ITS and plastid regions from multiple accessions of each species of Conradina (except for a single accession of C. verticillata) and multiple individuals from species of Clinopodium, Dicerandra, Piloblephis, Stachydeoma, Monarda, Pycnanthemum, and Mentha. ITS sequence data strongly support the monophyly of Conradina, in agreement with evidence from morphology. In contrast, plastid sequence data do not support a monophyletic Conradina and place the genus as paraphyletic to Clinopodium, Stachydeoma, and Piloblephis. Similar plastid haplotypes are shared by different genera, perhaps due to shared ancestral polymorphisms, or more likely, introgression that occurred recently or during the Pleistocene. Within Conradina, ITS sequence data do not resolve species relationships, while plastid sequence data do not support the monophyly of most traditionally defined species of Conradina that are distinguishable morphologically. Species relationships in the plastid data set may also be obscured by introgression or ancestral polymorphism. More rapidly evolving sequence data from nuclear markers will be necessary to clarify relationships in Conradina and related mints from the southeastern United States.