Naranjo, A. A., C. E. Edwards, M. A. Gitzendanner, D. E. Soltis, and P. S. Soltis. 2024. Abundant incongruence in a clade endemic to a biodiversity hotspot: Phylogenetics of the scrub mint clade (Lamiaceae). Molecular Phylogenetics and Evolution 192:108014.
https://doi.org/10.1016/j.ympev.2024.108014

Highlights

  • General relationships in the group were confirmed, while the large amounts of gene tree discordance are likely due to ILS, reticulation, or both, depending on the subclade.
  • Of all the major phenomena that may lead to conflicting phylogenetic hypotheses, reticulation and ILS are plausible causes for the incongruence that we detected across perennial Dicerandra.
  • In parts of the Conradina + allies phylogeny where reticulation indices were lower, both hard incongruence (via ILS and hybridization) and soft incongruence (via gene tree estimation error) likely contributed to poor gene tree concordance.

Abstract

The Scrub Mint clade (Lamiaceae) provides a unique system for investigating the evolutionary processes driving diversification in the North American Coastal Plain from both a systematic and biogeographic context. The clade comprises Dicerandra, Conradina, Piloblephis, Stachydeoma, and four species of the broadly defined genus Clinopodium (Mentheae; Lamiaceae), almost all of which are endemic to the North American Eastern Coastal Plain. Most species of this clade are threatened or endangered and restricted to sandhill or a mosaic of scrub habitats. We analyzed relationships in this clade to understand the evolution of the group and identify evolutionary mechanisms acting on the clade, with important implications for conservation. We used a target-capture method to sequence and analyze 238 nuclear loci across all species of scrub mints, reconstructed the phylogeny, and calculated gene tree concordance, gene tree estimation error, and reticulation indices for every node in the tree using ML methods. Phylogenetic networks were used to determine reticulation events. Our nuclear phylogenetic estimates were consistent with previous results, while greatly increasing the robustness of taxon sampling. The phylogeny resolved the full relationship between Dicerandra and Conradina and the less-studied members of the clade (PiloblephisStachydeomaClinopodium spp.). We found hotspots of gene tree discordance and reticulation throughout the tree, especially in perennial Dicerandra. Several instances of reticulation events were uncovered between annual and perennial Dicerandra, and within the Conradina + allies clade. Incomplete lineage sorting also likely contributed to phylogenetic discordance. These results clarify phylogenetic relationships in the clade and provide insight on important evolutionary drivers in the clade, such as hybridization. General relationships in the group were confirmed, while the large amount of gene tree discordance is likely due to reticulation across the phylogeny.