Abstract
Angiosperm mitochondrial genomes (mitogenomes) are notable for their extreme diversity in both size and structure. However, our current understanding of this diversity is limited, and the underlying mechanism contributing to this diversity remains unclear. Here, we completely assembled and compared the mitogenomes of three kiwifruit (Actinidia) species, which represent an early divergent lineage in asterids. We found conserved gene content and fewer genomic repeats, particularly large repeats (>1 kb), in the three mitogenomes. However, sequence transfers such as intracellular events are variable and dynamic, in which both ancestral shared and recently species-specific events as well as complicated transfers of two plastid-derived sequences into the nucleus through the mitogenomic bridge were detected. We identified extensive whole-genome rearrangements among kiwifruit mitogenomes and found a highly variable V region in which fragmentation and frequent mosaic loss of intergenic sequences occurred, resulting in greatly interspecific variations. One example is the fragmentation of the V region into two regions, V1 and V2, giving rise to the two mitochondrial chromosomes of Actinidia chinensis. Finally, we compared the kiwifruit mitogenomes with those of other asterids to characterize their overall mitogenomic diversity, which identified frequent gain/loss of genes/introns across lineages. In addition to repeat-mediated recombination and import-driven hypothesis of genome size expansion reported in previous studies, our results highlight a pattern of dynamic structural variation in plant mitogenomes through global genomic rearrangements and species-specific fragmentation and mosaic loss of intergenic sequences in highly variable regions on the basis of a relatively large ancestral mitogenome.