Complete Chloroplast Genome of Clethra fargesii Franch., an Original Sympetalous Plant from Central China: Comparative Analysis, Adaptive Evolution, and Phylogenetic Relationships

Clethra fargesii, an essential ecological and endemic woody plant of the genus Clethra in Clethraceae, is widely distributed in Central China. So far, there have been a paucity of studies on its chloroplast genome. In the present study, we sequenced and assembled the complete chloroplast genome of C. fargesii. We also analyzed the chloroplast genome features and compared them to Clethra delavayi and other closely related species in Ericales. The complete chloroplast genome is 157,486 bp in length, including a large single-copy (LSC) region of 87,034 bp and a small single-copy (SSC) region of 18,492 bp, separated by a pair of inverted repeat (IR) regions of 25,980 bp. The GC content of the whole genome is 37.3%, while those in LSC, SSC, and IR regions are 35.4%, 30.7%, and 43.0%, respectively. The chloroplast genome of C. fargesii encodes 132 genes in total, including 87 protein-coding genes (PCGs), 37 tRNA genes, and eight rRNA genes. A total of 26,407 codons and 73 SSRs were identified in C. fargesii chloroplast genome. Additionally, we postulated and demonstrated that the structure of the chloroplast genome in Clethra species may present evolutionary conservation based on the comparative analysis of genome features and genome alignment among eight Ericales species. The low Pi values revealed evolutionary conservation based on the nucleotide diversity analysis of chloroplast genome in two Clethra species. The low selection pressure was shown by a few positively selected genes by adaptive evolution analysis using 80 coding sequences (CDSs) of the chloroplast genomes of two Clethra species. The phylogenetic tree showed that Clethraceae and Ericaceae are sister clades, which reconfirm the previous hypothesis that Clethra is highly conserved in the chloroplast genome using 75 CDSs of chloroplast genome among 40 species. The genome information and analysis results presented in this study are valuable for further study on the intraspecies identification, biogeographic analysis, and phylogenetic relationship in Clethraceae.