Assembly and analysis of the complete mitochondrial genome of an endemic Camellia species of China, Camellia tachangensis.

Background: Camellia tachangensis F. C. Zhang is an endemic Camellia species of the junction of Yunnan, Guizhou and Guangxi Provinces in China. It is characterized by a primitive five-chambered ovary morphology and serves as the botanical source of the renowned "Pu'an Red Tea". Unfortunately, the populations of the species have declined due to the destruction of their habitats by human activities. The lack of mitochondrial genomic resources has hindered research into molecular breeding and phylogenetic evolution of C. tachangensis.

Result: In this study, we had sequenced, assembled, and annotated the mitochondrial genome of C. tachangensis to reveal its genetic characteristics and phylogenetic relation with other Camellia species. The assembly result indicated that the mitochondrial genome sequence of C. tachangensis was 746,931 bp (GC content = 45.86%). It consisted of one multibranched sequence (Chr1) and one circular sequence (Chr2), with Chr1 capable of producing 7 substructures. The comparative analysis of the mitochondrial and chloroplast DNA of C. tachangensis revealed 23 pairs of chloroplast homologous fragments, with 10 fully preserved tRNA genes within them. Interspecies comparison of Ka/Ks ratios revealed that mutations in mitochondrial protein-coding genes (PCGs) of C. tachangensis were predominantly shaped by purifying selection throughout its evolution (Ka/Ks < 1). The mitochondrial CDS-based phylogenetic tree indicated that within the Camellia lineage, C. tachangensis was phylogenetically independent of the species of sections Oleifera, Camellia, Heterogenea, and Chrysantha. However, it also did not support the clustering of C. tachangensis with certain variants of C. sinensis, due to the extremely low support (BS = 22, PP = 0.41). Meanwhile, the chloroplast PCG-based phylogenetic analysis revealed that C. tachangensis formed a strongly supported basal clade (BS = 100, PP = 1.00), alongside C. makuanica (NC_087766), C. taliensis (NC_022264), and C. gymnogyna (NC_039626).

Conclusions: Our study deciphered the mitochondrial genome and its multibranched structure of C. tachangensis. These findings not only enhanced our comprehension of the complexity and diversity of mitochondrial genome structures in Camellia species, but also established a foundational genetic data framework for future research on molecular breeding programs and phylogenetic relationship involving C. tachangensis and its related species.