Mitochondrial DNA in metazoa: degree of freedom in a frozen event

The mitochondrial genome (mtDNA), due to its peculiar features such as exclusive presence of orthologous genes, uniparental inheritance, lack of recombination, small size and constant gene content, certainly represents a major model system in studies on evolutionary genomics in metazoan. In 800 million years of evolution the gene content of metazoan mitochondrial genomes has remained practically frozen but several evolutionary processes have taken place. These processes, reviewed here, include rearrangements of gene order, changes in base composition and arising of compositional asymmetry between the two strands, variations in the genetic code and evolution of codon usage, lineage-specific nucleotide substitution rates and evolutionary patterns of mtDNA control regions.