Comparative Analyses of the Complete Mitogenomes of Two Oxyria Species (Polygonaceae) Provide Insights into Understanding the Mitogenome Evolution Within the Family

<i>Oxyria</i> (Polygonaceae) is a small genus only comprising two species, <i>Oxyria digyna</i> and <i>O. sinensis</i>. Both species have well-documented usage in Chinese herbal medicine. We sequenced and assembled the complete mitogenomes of these two species and conducted a comparative analysis of the mitogenomes within Polygonaceae. Both <i>O. digyna</i> and <i>O. sinensis</i> displayed distinctive multi-branched conformations, consisting of one linear and one circular molecule. These two species shared similar gene compositions and exhibited distinct codon preferences, with mononucleotides as the most abundant type of simple sequence repeats. In the mitogenome of <i>O. sinensis</i>, a pair of long forward repeat sequences can mediate the division of molecule 1 into two sub-genomic circular molecules. Homologous sequence analysis revealed the occurrence of gene transfer between the chloroplast and mitochondrial genomes within <i>Oxyria</i> species. Additionally, a substantial number of homologous collinear blocks with varied arrangements were observed across different Polygonaceae species. Phylogenetic analysis suggested that mitogenome genes can serve as reliable markers for constructing phylogenetic relationships within Polygonaceae. Comparative analysis of eight species revealed Polygonaceae mitogenomes exhibited variability in gene presence, and most protein-coding genes (PCGs) have undergone negative selection. Overall, our study provided a comprehensive overview of the structural, functional, and evolutionary characteristics of the Polygonaceae mitogenomes.