Evolution of extensively fragmented mitochondrial genomes in bilateral animals

Bilateral animals are featured by an extremely compact mitochondrial (mt) genome that consists of a single circular chromosome. In the human body louse, Pediculus humanus, however, the typical mt genome has fragmented into 20 minichromosomes. I sequenced the mt genomes of two other human lice: the head louse, Pediculus capitis, and the pubic louse, Pthirus pubis. Comparison among the three human lice provided unprecedented insights into the evolution of fragmented mt genomes in bilateral animals. First, all three human lice have fragmented mt genomes, indicating this novel genome organization was present in the most recent common ancestor of the three human lice, which lived ~7 million years ago. Second, the head louse has exactly the same set of mt minichromosomes found in the body louse. Thus, the number of mt minichromosomes, and the gene content and gene arrangement on each minichromosome have remained unchanged since the body louse evolved from the head louse ~107,000 years ago. Third, the pubic louse has the same pattern as the body louse and the head louse for the distribution of mt protein-coding and rRNA genes. This pattern, thus, has been stable for at least 7 million years. Most tRNA genes of the pubic louse, however, are on different minichromosomes when compared with their counterparts in the body and head louse. There is evidence that rearrangement of four tRNA genes (for leucine, arginine and glycine) was due to gene-identity switch by point mutation at third anti-codon positions, whereas rearrangement of 10 other tRNA genes was due to gene translocation between minichromosomes