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

TL;DR

This study explores the evolutionary history of the Exbucklandia genus in Southeast Asia, revealing how ancient climate changes shaped its genetic diversity and distribution.

Contribution

The study provides new phylogenetic and phylogeographic insights using multiple datasets to clarify the evolutionary relationships and hybridization patterns in Exbucklandia.

Findings

Chloroplast haplotypes grouped into three clades, with E. longipetala nested within Clades II and III.

Nuclear SSRs identified two ancestral gene pools, with distinct ancestry patterns among species.

Quaternary Glacial Cycles influenced southward expansions and hybridization in Exbucklandia.

Abstract

Southeast Asia’s biodiversity refugia, shaped by Neogene–Quaternary climatic shifts and the Tibetan Plateau uplift, preserve relict lineages like Exbucklandia (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 (E. tricuspis), Clade II (E. populnea), and Clade III (E. tonkinensis)—with E. longipetala haplotypes nested within II/III. Nuclear microsatellites (SSRs) identified two ancestral gene pools: E. populnea and E. tricuspis showed predominant ancestry in Pool A, while E. tonkinensis and E. longipetala 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 E. populnea and E. tonkinensis. Quaternary Glacial Cycles triggered southward expansions, chloroplast capture, and localized hybridization. Morphological, nuclear, and plastid molecular evidence supports reclassifying E. longipetala as E. populnea × E. tonkinensis hybrids lacking genetic cohesion and E. tricuspis 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 Exbucklandia.

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