Assembly and comparative analysis of the complete mitochondrial genome of Ilex metabaptista (Aquifoliaceae), a Chinese endemic species with a narrow distribution.

backgroundIlex metabaptista is a woody tree species with strong waterlogging tolerance and is also admired as a landscape plant with high development prospects and scientific research value. Unfortunately, populations of this species have declined due to habitat loss. Thus, it is a great challenge for us to efficiently protect I. metabaptista resources from extinction. Molecular biology research can provide the scientific basis for the conservation of species. However, the study of I. metabaptista genetics is still in its infancy. To date, no mitochondrial genome (mitogenome) in the genus Ilex has been analysed in detail.

resultsThe mitogenome of I. metabaptista was assembled based on the reads from Illumina and Nanopore sequencing platforms; it was a typical circular DNA molecule of 529,560 bp with a GC content of 45.61% and contained 67 genes, including 42 protein-coding genes, 22 tRNA genes, and 3 rRNA genes. Repeat sequence analysis and prediction of RNA editing sites revealed a total of 286 dispersed repeats, 140 simple repeats, 18 tandem repeats, and 543 RNA editing sites. Analysis of codon usage showed that codons ending in A/T were preferred. Gene migration was observed to occur between the mitogenome and chloroplast genome via the detection of homologous fragments. In addition, Ka/Ks analysis revealed that most of the protein-coding genes in the mitogenome had undergone negative selection, and only the ccmB gene had undergone potential positive selection in most asterids. Nucleotide polymorphism analysis revealed the variation in each gene, with atp9 being the most notable. Furthermore, comparative analysis showed that the GC contents were conserved, but the sizes and structure of mitogenomes varied greatly among asterids. Phylogenetic analysis based on the mitogenomes reflected the exact evolutionary and taxonomic status of I. metabaptista.

conclusionIn this study, we sequenced and annotated the mitogenome of I. metabaptista and compared it with the mitogenomes of other asterids, which provided essential background information for further understanding of the genetics of this plant and helped lay the foundation for future studies on molecular breeding of I. metabaptista.