Polymeric structure of the Cannabis sativa L. mitochondrial genome identified with an assembly graph model

Cannabis sativa L. belongs to the family Cannabaceae in Rosales. It has been widely used as medicines, building materials, and textiles. Elucidating its genome is critical for molecular breeding and synthetic biology study. Many studies have shown that the mitochondrial genomes (mitogenomes) and even chloroplast genomes (plastomes) had complex polymeric structures. Using the Nanopore sequencing platform, we sequenced, assembled, and analyzed its mitogenome and plastome. The resulting unitig graph suggested that the mitogenome had a complex polymeric structure. However, a gap-free, circular sequence was further assembled from the unitig graph. In contrast, a circular sequence representing the plastome was obtained. The mitogenome major conformation was 415,837 bp long, and the plastome was 153,927 bp long. To test if the repeat sequences promote recombination, which corresponds to the branch points in the structure, we tested the sequences around repeats by long-read mapping. Among 208 pairs of predicted repeats, the mapping results supported the presence of cross-over around 25 pairs of repeats. Subsequent PCR amplification confirmed the presence of cross-over around 15 of the 25 repeats. By comparing the mitogenome and plastome sequences, we identified 19 mitochondria plastid DNAs, including seven complete genes (trnW-CCA, trnP-UGG, psbJ, trnN-GUU, trnD-GUC, trnH-GUG, trnM-CAU) and nine gene fragments. Furthermore, the selective pressure analysis results showed that five genes (atp1, ccmB, ccmC, cox1, nad7) had 19 positively selected sites. Lastly, we predicted 28 RNA editing sites. A total of 8 RNA editing sites located in the coding regions were successfully validated by PCR amplification and Sanger sequencing, of which four were synonymous, and four were nonsynonymous. In particular, the RNA editing events appeared to be tissue-specific in C. sativa mitogenome. In summary, we have confirmed the major confirmation of C. sativa mitogenome and characterized its structural features in detail. These results provide critical information for future variety breeding and resource development for C. sativa.