Multiple Dataset-Based Insights into the Phylogeny and Phylogeography of the Genus Exbucklandia (Hamamelidaceae): Additional Evidence on the Evolutionary History of Tropical Plants

Southeast Asia's biodiversity refugia, shaped by Neogene-Quaternary climatic shifts and the Tibetan Plateau uplift, preserve relict lineages like <i>Exbucklandia</i> (Hamamelidaceae). Once widespread across ancient continents, this genus now survives in Asian montane forests, offering insights into angiosperm diversification. Chloroplast haplotypes formed three clades-Clade I (<i>E. tricuspis</i>), Clade II (<i>E. populnea</i>), and Clade III (<i>E. tonkinensis</i>)-with <i>E. longipetala</i> haplotypes nested within II/III. Nuclear microsatellites (SSRs) identified two ancestral gene pools: <i>E. populnea</i> and <i>E. tricuspis</i> showed predominant ancestry in Pool A, while <i>E. tonkinensis</i> and <i>E. longipetala</i> were primarily assigned to Pool B. All taxa exhibited localized genetic admixture, particularly in sympatric zones. Divergence dating traced the genus' origin to tropical Asia, with northward colonization of subtropical China ~7 Ma yielding <i>E. populnea</i> and <i>E. tonkinensis</i>. Quaternary Glacial Cycles triggered southward expansions, chloroplast capture, and localized hybridization. Morphological, nuclear, and plastid molecular evidence supports reclassifying <i>E. longipetala</i> as <i>E. populnea</i> × <i>E. tonkinensis</i> hybrids lacking genetic cohesion and <i>E. tricuspis</i> as a distinct species with a mixed nuclear composition. This study highlights how paleoclimate-driven gene flow shaped the phylogeography of relict taxa in Southeast Asia and the urgency of habitat restoration to conserve <i>Exbucklandia</i>.