Animal mitochondrial DNA recombination revisited

Exchange of homologous sequences between mitochondrial DNA (mtDNA) molecules is thought to be absent in animals, primarily because of a failure to observe clear cases of recombinant haplotypes in natural populations. However, whether mtDNA recombination occurs is a different issue from whether it produces new haplotypes. A requirement for the latter is heteroplasmy - the presence of more than one type of mtDNA in an individual, which is rare in animals. In male mussels, in which heteroplasmy is the rule, recombination is common, arguing against an innate impediment to mtDNA recombination in animals. In addition, recent biochemical studies suggest that recombination is an indispensable part of the mtDNA replication and repair machinery and that most animal genomes have the necessary enzymes for mtDNA recombination. When strict maternal mtDNA transmission is compromised, recombinant haplotypes can be generated and eventually become fixed. Although the pervasiveness of mtDNA recombination in animals is unknown, its presence could have important consequences for phylogenetic studies of closely related taxa (e.g. leading to incorrect phylogenetic inferences and incorrect rejection of the molecular clock) and human mtDNA-associated diseases.