Organization and Evolution of Metazoan Mitochondrial Genomes

Although almost certainly derived from the same ancestral progenitor, mitochondrial genomes have followed multiple and different evolutionary pathways in various taxa; it shows great variability in terms of structure, gene content, organization and mode of expression in different organisms. Nevertheless, the evolutionary process has been frozen, in several senses, in the Metazoa, particularly with respect to genome size and gene content. As mitochondrial (mt) DNA is a relatively small, abundant and easy to isolate molecule, it has been particularly favoured as a molecular evolutionary marker and the nucleotide sequence from a large number of species has now been determined(http://www.ncbi.nlm.nih.gov/genomes/ORGANELLES/mztax_short.html). Metazoan mtDNA is, in general, a single circular molecule, although some cnidarians (Medusozoa) are known to possess atypical linear mt DNA. It is characterized by a compact arrangement, the constancy of gene content and the presence of a single noncoding region. With the exception of the replication origin region(s), the genome is saturated with intron‑less genes, lacking intronic sequence, which are often contiguous and sometimes slightly overlapping or separated by only a few nucleotides.