Evolution of the mitochondrial genetic system: an overview

Mitochondria, semi-autonomous organelles possessing their own genetic syste m, are commonly accepted to descend from free-living eubacteria, namely hyd rogen-producing alpha-proteobacteria. The progressive loss of genes from th e primitive eubacterium to the nucleus of the eukaryotic cell is strongly j ustified by the Muller rachet principle, which postulates that asexual geno mes, like mitochondrial ones, accumulate deleterious and sublethal mutation s faster than sexual genomes, like the nucleus. According to this principle , the mitochondrial genome would be doomed to death; instead, we observe th at the mitochondrial genome has a variable size and structure in the differ ent organisms, though it contains more or less the same set of genes. This is an example of genetic conservation versus structural diversity. From an evolutionary point of view the genetic system of organelles is clearly unde r strong selective pressure and for its survival it needs to utilize strate gies to slow down or halt the ratchet. Anyway, the mitochondrial genome cha nges with time, and the rate of evolution is different for both diverse reg ions of the mtDNA and between lineages, as demonstrated in the case of mamm alian mt genomes. We report here our data on the evolution of the mitochond rial DNA in mammals which demonstrate the suitability of mtDNA as a molecul ar tool for evolutionary analyses. (C) 2000 Elsevier Science B.V. All right s reserved.