Comprehensive Analysis of the Complete Mitochondrial Genomes of Dendrobium nobile Lindl. and Dendrobium denneanum Kerr., Two Precious Traditional Chinese Medicinal Herbs

The plant mitochondrial genome has become a current research hotspot as an independent genetic model. Nevertheless, mitochondrial genome information for most Dendrobium species remains unknown. In this study, the assembly of mitochondrial genome of Dendrobium nobile Lindl.,1830 and Dendrobium denneanum Kerr., 1933 was conducted through the application of second- and third-generation sequencing technologies, with the mitochondrial genome of D. denneanum Kerr. being reported first. The results revealed that the mitochondrial genomes of the two species possessed a multi-chromosome circular structure. Their total lengths were 641,414 bp and 558,760 bp, consisting of 21 and 19 contigs, respectively. A total of 67 and 72 genes, 993 and 1491 repeat sequences, and 549 and 553 RNA editing sites were identified. Gene loss was observed. A total of 26 and 36 homologous fragments were detected between the mitochondrial and the chloroplast genome, accounting for 5.09% and 4.93% of the total lengths, respectively, indicating intracellular gene transfer. Synteny and phylogenetic analyses revealed that the two species shared extensive collinear regions and clustered together in a distinct clade of the phylogenetic tree, indicating a close sister relationship. These findings enrich the mitochondrial genome database and provide valuable insights to guide future research on species identification and molecular evolution of the genus Dendrobium.