Complete Sequencing of the Mitochondrial Genome of Tea Plant Camellia sinensis cv. 'Baihaozao': Multichromosomal Structure, Phylogenetic Relationships, and Adaptive Evolutionary Analysis

Zhiyin Chen^*Zixu WangWei ZhouSijie LiuYuxin XiaoYihui Gong^*

• Hunan University of Humanities, Science and Technology, Loudi, China

This study represents the first comprehensive report on the full sequence characteristics of the mitochondrial genome of Camellia sinensis cv. 'Baihaozao' systematically revealing its multichromosomal structure, ribonucleic acid (RNA) editing patterns, and adaptive evolution mechanisms. The mitochondrial genome was found to consist of 11 linear chromosomes (total length: 909,843 bp; guanine-cytosine (GC) content: 46%), with 73 annotated functional genes. Notably, 14 variable genes (e.g., ribosomal protein-coding genes) retained complete functionality without pseudogenization, which is rare in Theaceae. RNA editing site predictions revealed significant spatial heterogeneity, with the cox1 gene identified as a hotspot containing 19 editing sites. Of these, 58% of the editing events occurred at the second codon position, and 49% of the sites led to hydrophilic-to-hydrophobic amino acid transitions, suggesting a regulatory role in mitochondrial protein function. The analysis of codon usage bias reveals a significant enrichment of high-frequency codons UUU (phenylalanine), AUU (isoleucine), and UUC (phenylalanine). Such preferences may be attributed to mutational biases driven by the high Adenine-Thymine (AT) content of the genome or reflect adaptive selection pressures for translational efficiency. The genome's repetitive sequences were predominantly dispersed repeats (accounting for 71%), with forward and palindromic repeats in the 30-40 bp range being dominant.Tandem repeats exhibited significant distribution heterogeneity among chromosomes.Adaptive evolutionary analysis indicates that the Non-synonymous substitution rate/Synonymous substitution rate (Ka/Ks) values of the vast majority of protein-coding genes (PCGs) are less than 1 (ranging from 0.07 to 0.78), with the atp9 gene exhibiting the lowest ratio (0.07), consistent with the prevailing trend of purifying selection in mitochondrial genomes. Notably, the mttB gene displays a Ka/Ks value of 3.48, significantly higher than that of other genes, suggesting that it may have undergone positive selection to adapt to environmental pressures. Notably, 3% of the mitochondrial genome sequences were homologous to the chloroplast genome, carrying 26 complete functional genes, including 15 transfer ribonucleic acids (tRNAs) and 2 ribosomal ribonucleic acids (rRNAs), providing evidence of inter-organellar gene transfer. This study provides critical theoretical insights into the structural complexity and adaptive evolution mechanisms of the tea mitochondrial genome.