Plastome Evolution in Viburnum (Adoxaceae): Comparative Genomics Reveals Hypervariable Markers and Relaxed Selection on Protein Import Genes

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.