Comparative analysis of the complete mitochondrial genomes of Firmiana danxiaensis and F. kwangsiensis (Malvaceae), two endangered Firmiana species in China.

We reported the mitogenomes of F. danxiaensis and F. kwangsiensis for the first time. Mitogenome size and structure differ significantly between them. Firmiana danxiaensis and F. kwangsiensis belong to the Firmiana genus and are distributed in the Danxia and Karst regions of southern China. Both species have been designated as endangered. Currently, the chloroplast genomes of F. danxiaensis and F. kwangsiensis have been sequenced, but the mitochondrial genome (mitogenome) of these two species has not been reported. To further understand the mitogenome characteristics, evolution, and phylogeny of F. danxiaensis and F. kwangsiensis, we assembled the mitogenomes of these two species based on a combination of Illumina and Nanopore sequencing methods. The mitogenome of F. danxiaensis exhibits a branching structure consisting of nine circular molecules with a total length of 938,890 bp, while the F. kwangsiensis has a circular structure with a length of 736,334 bp. Compared to F. kwangsiensis, F. danxiaensis has more tRNA genes, SSRs, tandem repeats, and dispersed repeats, while the codon use patterns are similar in these two species. There were 24 and 23 homologous sequences between mitogenome and chloroplast genome of F. danxiaensis and F. kwangsiensis, accounting for 0.37% and 0.49% of the mitogenome, respectively. In addition, the Ka/Ks ratio and the nucleic acid diversity analysis revealed that most of the mitochondria protein-coding genes in F. danxiaensis and F. kwangsiensis are highly conserved and may have undergone purifying selection. Furthermore, the collinear and comparative analysis showed that extensive genomic rearrangement events existed among the Malvaceae species. Lastly, a phylogenetic tree based on shared mitochondrial PCGs of 29 species revealed that F. danxiaensis and F. kwangsiensis form a sister group with high support values. Overall, the current study reports two mitogenomes (F. danxiaensis and F. kwangsiensis) in the Firmiana genus for the first time, which will help enhance comprehension of the mitogenome evolutionary patterns within Firmiana and promote the evolutionary and comparative genomic analyses within Malvaceae species.