Comparative Plastome Analysis of Two Viburnum Species from East Asian Islands: Genome Conservation, Localized Divergence, and Plastome-Based Phylogenetic Placement

The genus Viburnum represents an evolutionarily informative lineage of woody angiosperms, but plastome variation in the two focal species remains insufficiently characterized. In this study, we assembled and compared one complete plastome from each of V. suspensum and V. taitoense to characterize genome conservation, sequence divergence, codon-usage patterns, and plastome-based phylogenetic placement. The complete plastomes of V. suspensum and V. taitoense were 158,577 bp and 158,544 bp in length, respectively, and both contained 133 annotated genes with an identical quadripartite architecture and the same GC content (38.1%). Comparative analysis with 52 additional publicly available Viburnum plastome records revealed highly conserved genome organization across the sampled records, whereas sequence divergence was unevenly distributed among genomic regions. The inverted-repeat regions showed higher conservation than the single-copy regions, and sliding-window analysis of 98 plastomes identified the ndhF–rpl32 intergenic region as the only major variable region exceeding the predefined threshold (maximum Pi = 0.06171). Both focal plastomes exhibited highly similar codon-usage patterns, with 29 preferred codons, 28 ending in A or U and UUG as the only preferred codon ending in G, and weak overall codon bias (ENC = 50.26 and 50.24). Complete-plastome phylogenetic analysis recovered the sampled plastomes of V. suspensum and V. taitoense as a sister pair, although this node received only moderate bootstrap support (BS = 72). These findings provide plastome-level evidence for the sampled accessions of V. suspensum and V. taitoense and establish a basis for broader population-level and phylogenomic investigations.