Resolution, conflict and rate shifts: insights from a densely sampled plastome phylogeny for Rhododendron (Ericaceae)

• <strong>Background and Aims</strong> <em>Rhododendron </em>is a species-rich and taxonomically challenging genus due to recent adaptive radiation and frequent hybridization. A well-resolved phylogenetic tree would help to understand the diverse history of <em>Rhododendron </em>in the Himalaya–Hengduan Mountains where the genus is most diverse. • <strong>Methods </strong>We reconstructed the phylogeny based on plastid genomes with broad taxon sampling, covering 161 species representing all eight subgenera and all 12 sections, including ~45 % of the <em>Rhododendron </em>species native to the Himalaya–Hengduan Mountains. We compared this phylogeny with nuclear phylogenies to elucidate reticulate evolutionary events and clarify relationships at all levels within the genus. We also estimated the timing and diversification history of <em>Rhododendron</em>, especially the two species-rich subgenera <em>Rhododendron </em>and <em>Hymenanthes </em>that comprise &gt;90 % of <em>Rhododendron </em>species in the Himalaya–Hengduan Mountains. • <strong>Key Results </strong>The full plastid dataset produced a well-resolved and supported phylogeny of <em>Rhododendron</em>. We identified 13 clades that were almost always monophyletic across all published phylogenies. The conflicts between nuclear and plastid phylogenies suggested strongly that reticulation events may have occurred in the deep lineage history of the genus. Within <em>Rhododendron</em>, subgenus <em>Therorhodion </em>diverged first at 56 Mya, then a burst of diversification occurred from 23.8 to 17.6 Mya, generating ten lineages among the component 12 clades of core <em>Rhododendron</em>. Diversification in subgenus <em>Rhododendron </em>accelerated c. 16.6 Mya and then became fairly continuous. Conversely, <em>Hymenanthes </em>diversification was slow at first, then accelerated very rapidly around 5 Mya. In the Himalaya–Hengduan Mountains, subgenus <em>Rhododendron </em>contained one major clade adapted to high altitudes and another to low altitudes, whereas most clades in <em>Hymenanthes </em>contained both low- and high-altitude species, indicating greater ecological plasticity during its diversification. • <strong>Conclusions </strong>The 13 clades proposed here may help to identify specific ancient hybridization events. This study will help to establish a stable and reliable taxonomic framework for <em>Rhododendron</em>, and provides insight into what drove its diversification and ecological adaption. Denser sampling of taxa, examining both organelle and nuclear genomes, is needed to better understand the divergence and diversification history of <em>Rhododendron</em>.