Revisiting the backbone phylogeny and inferring the evolutionary trends in inflorescence of Elsholtzieae (Lamiaceae): new insights from orthologous nuclear genes

Abstract

The angiosperm tribe of Elsholtzieae (Lamiaceae) is characterized by complex inflorescences and has notable medicinal and economic significance. Relationships within Elsholtzieae, including the monophyly of Elsholtzia and Keiskea, and relationships among Mosla, Keiskea and Perilla, remain uncertain, hindering insights into inflorescence evolution within the tribe. Using hybridization capture sequencing and deep genome skimming data analysis, we reconstruct a phylogeny of Elsholtzieae using 279 orthologous nuclear loci from 56 species. We evaluated uncertainty among relationships using concatenation, coalescent and network approaches. Using a time-calibrated phylogeny, we reconstructed ancestral inflorescence traits to elucidate the patterns in their evolution within the tribe. Our analyses consistently support the paraphyly of the genus Elsholtzia. Phylogenetic network analyses, confirmed by PhyloNetworks and SplitsTree, showed reticulation events among the major lineages of Elsholtzieae. The unstable polyphyly of Keiskea observed in ASTRAL (accurate species tree algorithm), ML (maximum likelihood) and MP (maximum parsimony) analyses may be related to introgression from Perilla and Mosla. Based on the analyses of phylogenetic trees within Elsholtzieae, the evolutionary trajectory of inflorescences demonstrates a pattern of diversification, with specialization as one aspect of this process. Elsholtzieae support the hypothesis that compressed inflorescences evolved from larger and more complex ancestral forms through successive compressions of the inflorescence axis. Additionally, certain lineages within the tribe display a trend towards simplified inflorescences, characterized by a reduction in the number of florets. This highlights both the specialization and the diversity in the evolution of inflorescence structures within the tribe.