Styrax japonicus is a medicinal and ornamental shrub belonging to the Styracaceae family. To explore the diversity and characteristics of the chloroplast genome of S. japonicus, we conducted sequencing and comparison of the chloroplast genomes of four naturally distributed S. japonicus. The results demonstrated that the four chloroplast genomes (157,914–157,962 bp) exhibited a typical quadripartite structure consisting of a large single copy (LSC) region, a small single copy (SSC) region, and a pair of reverse repeats (IRa and IRb), and the structure was highly conserved. DNA polymorphism analysis revealed that three coding genes (infA, psbK, and rpl33) and five intergene regions (petA-psbJ, trnC-petN, trnD-trnY, trnE-trnT, and trnY-trnE) were identified as mutation hotspots. These genetic fragments have the potential to be utilized as DNA barcodes for future identification purposes. When comparing the boundary genes, a small contraction was observed in the IR region of four S. japonicus. Selection pressure analysis indicated positive selection for ycf1 and ndhD. These findings collectively suggest the adaptive evolution of S. japonicus. The phylogenetic structure revealed conflicting relationships among several S. japonicus, indicating divergent evolutionary paths within this species. Our study concludes by uncovering the genetic traits of the chloroplast genome in the differentiation of S. japonicus variety, offering fresh perspectives on the evolutionary lineage of this species.

Melliodendron xylocarpum is a member of the Styracaceae family, which is well-known for its remarkable ornamental and medicinal properties. In this research, we conducted comparative analysis of the chloroplast genomes from four samples of M. xylocarpum, representing Melliodendron. The results demonstrated that the chloroplast genome of four M. xylocarpum samples ranging from 157,103 bp to 158,357 bp exhibited a typical quadripartite structure, including one large single-copy (LSC) region (90,131 bp to 90,342 bp), one small single-copy (SSC) region (18,467 bp to 18,785 bp), and two inverted repeat regions (IRs) (24,115 bp to 24,261 bp). Different levels of expansion and contraction were observed in the IR region of four M. xylocarpum samples. Besides, accD and ycf1 have been identified under positive selection, potentially linked to the adaptive response of Melliodendron to various environmental changes. Conflicting phylogenetic relationships were identified among various genera within the Styracaceae family in the phylogenetic tree constructed using CDS sequences and complete chloroplast genomes. Furthermore, the significance of a large sample size was also highlighted in this study for enhancing the accuracy of findings from phylogenetic analyses. The findings of this research will provide significant insights for future investigations into the evolutionary trends and conservation of the Melliodendron species.