Phylogenomics recovers monophyly and early Tertiary diversification of Dipteronia (Sapindaceae)

Dipteronia (Sapindaceae) is an ancient relict woody genus, and contains just two extant species endemic to Southwestern and Central China. As sharing numerous morphological characters, Dipteronia and Acer have long been considered as sister groups forming the traditional family Aceraceae. However, molecular phylogenetics has generally not resolved the phylogenetic placement of Dipteronia, especially not in its expected position as sister to Acer. In this study, we present large-scale, phylogenomic data sets, incorporating complete chloroplast (cp) genome sequences and transcriptome data from 13 Sapindaceae species, to resolve the phylogenetic relationship between Dipteronia and Acer. Moreover, the impact of long-branch attraction (LBA) artefacts and robustness of inferred topologies are assessed by long-branch excluding and coalescent-based methods. Corroborating classical morphology-based classifications, both cp genome and nuclear datasets (2466 co-orthologous genes and 273 co-SCNGs) recover Dipteronia and Acer as mutually monophyletic groups. In addition, our fossil-calibrated molecular phylogenies date the crown of the two extant Dipteronia species to the Paleocene/Eocene boundary, implying that these morphologically highly similar taxa are amongst the oldest 'living fossils' of the East Asian Flora.