Conservative and Dynamic Evolution of Mitochondrial Genomes in Early Land Plants

Abstract

Early land plant mitochondrial genomes (chondromes) might have captured important changes of mitochondrial genome evolution when photosynthetic eukaryotes colonized land in a unprecedented scale, and thus deserve special attention in investigation of plant mitochondrial genomes. The chondromes of land plants that are well adapted to the terrestrial environment, namely seed plants, show many derived characteristics, including large genome size variation, frequent occurrence of intra-genomic rearrangements, abundant introns and high levels of RNA editing. In contrast, the chondromes of charophytes, the closest algal relatives of land plants, are still largely ancestral in these aspects, resembling chondromes of early eukaryotes. Several recently sequenced chondromes from basal land plants including liverworts, mosses, hornworts and lycophytes have provided fresh insights into mitochondrial genome evolution of early land plants. Comparative analyses of these genomes have identified lycophytes, which represent the most ancient extant vascular plants, as the major point of change in plant mitochondrial genome evolution, with long conserved mitochondrial gene synteny largely disrupted. The chondromes of bryophytes are conservative in gene order, but rather dynamic in intron content. The gene contents and RNA editing levels also show wide variation from lineage to lineage. Overall, the mitochondrial genomes experienced dynamic evolutionary changes during the origin and early evolution of land plants when the major lineages of bryophytes and vascular plants appeared, but have remained relatively conservative afterwards except in vascular plants.