Abstract
BACKGROUND: Viburnum (Adoxaceae) is a species-rich woody genus whose taxonomy is complicated by morphological convergence and hybridization. METHODS: We assembled complete plastomes of eight species representing five sections and analyzed their structural variation, sequence divergence, and molecular evolution. RESULTS: All plastomes displayed the conserved quadripartite structure typical of angiosperms, with limited size variation attributable primarily to intergenic spacer-length polymorphisms. Sequence divergence was unevenly distributed, with single-copy regions exhibiting substantially higher nucleotide diversity than inverted repeat regions. We identified multiple hypervariable intergenic spacers such as the region trnK-UUU-rps16, suitable as molecular markers for population genetics and species identification. Selection pressure analysis revealed that while most protein-coding genes evolved under strong purifying selection, three genes involved in fatty acid biosynthesis and protein import-accD, ycf1, and ycf2-showed significantly relaxed constraints, suggesting ongoing functional divergence. Phylogenetic analysis recovered well-supported relationships consistent with previous classifications, while clarifying the positions of Viburnum amplificatum and Viburnum tinus. CONCLUSIONS: These findings provide molecular resources for Viburnum systematics and offer insights into the evolutionary dynamics of plastome genes with non-photosynthetic functions.