Intraspecific Differentiation of Styrax japonicus (Styracaceae) as Revealed by Comparative Chloroplast and Evolutionary Analyses

Abstract

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.