RESUMO
italic>Cynanchum wallichii and Cynanchum otophyllum belong to the genus Cynanchum in the family Apocynaceae, and are important medicinal plants. In this study, we sequenced and assembled the chloroplast genomes of C. wallichii and C. otophyllum, and performed a phylogenetic analysis of the structural characteristics of their chloroplast genomes and their phylogenetic positions. The results showed that the chloroplast genomes of both C. wallichii and C. otophyllum had a typical tetrad structure, with 133 genes annotated, and the total GC contents of both were similar. Codon preference analysis showed that the relative synonymous codon usage in the chloroplast genomes of C. wallichii and C. otophyllum differed slightly, but the differences were not significant, and there was a strong A or U preference at the third codon position. In both chloroplast genomes, 91 and 103 simple sequence repeats were detected respectively, and the largest proportion of A/T type repeats. Nucleotide polymorphism analysis showed that the nucleotide diversity of the intergenic sequences in the chloroplast genome of genus Cynanchum were generally higher than those of the common gene sequences. A pair of primers was designed based on the high variation region of the chloroplast genome to identify C. wallichii and C. otophyllum. The phylogenetic analysis showed that the C. wallichii and Cynanchum thesioides were the closest relatives, while the C. otophyllum, Cynanchum bungei and Cynanchum wilfordii formed a stable monophyletic clade within the genus Cynanchum, and the three species were closely related. The comparative analysis of the chloroplast genomic characteristics and phylogeny of C. wallichii and C. otophyllum will provide a theoretical basis for the species identification of the two plants and for the study of genetic diversity and phylogeny of the genus Cynanchum.