Phylogeny of Conradina and related southeastern scrub mints (Lamiaceae) based on GapC gene sequences

Phylogeny reconstruction at the species level, especially using organellar markers, is often complicated by problems such as incomplete lineage sorting and interspecific hybridization. Single-copy nuclear genes may be useful for these cases because they have higher mutation rates and are biparentally inherited. One plant group in which hybridization and incomplete lineage sorting have been proposed based on analyses of internal transcribed spacer (ITS) and plastid data is a clade of mints from the southeastern United States: Conradina and the related genera Dicerandra, Piloblephis, Stachydeoma, and Clinopodium (Lamiaceae). To clarify the phylogeny in this clade and investigate the possibility of incomplete lineage sorting and interspecific hybridization, we isolated three members of the nuclear GapC gene family and used two to reconstruct phylogeny. Separate phylogenetic analyses of the two GapC loci did not resolve species relationships. We then used two approaches to concatenate the two heterozygous GapC loci with ITS and plastid data sets from a previous study and carried out combined analyses. Trees resulting from the two concatenation approaches were similar in the resolution and support of generic relationships, but they differed drastically in resolution and support for relationships within Conradina. Conradina species are probably very recently derived, and it may be unreasonable to reconstruct species relationships in Conradina using DNA sequence data due to widespread hybridization or lack of coalescence. Rapidly evolving microsatellite data may be more useful for detecting hybridization and clarifying species boundaries in Conradina.