An Improved Assembly Strategy Helps Parsing the Cryptic Mitochondrial Genome Evolution in Plants

Although plant mitochondrial genomes (mitogenomes) are small, they exhibit considerable complexity not seen in other eukaryotic mitogenomes. Assembly and analysis of plant mitogenomes is hampered by their large variations in structure and size, and the mitogenome remains the last genome to be deciphered in many plant species. As a result, very few plant mitogenomes have been assembled and little is known regarding their evolution. In this study, a strategy was devised for assembly of mitogenomes from existing short reads from whole-genome sequencing projects. The strategy combined current tools and manual steps to resolve the two main challenges to mitogenome assembly: repeat and plastid insertion sequences. High-quality complete mitogenomes were assembled for 23 species from five families of the Fagales. Mitogenomes varied 2.4 times in size. The largest, Carpinus cordata, did not contain large amounts of unique sequences, but instead contained a high proportion of sequences homologous to other Fagales. Further analysis of the Fagales mitogenomes revealed highly mosaic characteristics, with horizonal transfer (HGT)-like sequences identified from almost all seed plant taxa. Independent and unequal transfers of third-party DNA may partially account for the HGT-like fragments and unbalanced size expansions observed in Fagales mitogenomes. Supporting this, a mitochondrial plasmid of nuclear origin was found in Carpinus, and this may represent an intermediate stage prior to incorporation into the mitogenome. The approaches used in this study are widely applicable and provide new insights into the mechanisms of mitogenome evolution in plants.