Detection of recombinant mitochondrial genomes: Implications for the mechanism of mtDNA inheritance in mussel Mytilus

Recombination plays a fundamental role in the creation of biodiversity. It is the mechanism inducing formation of rearrangements within the genomes which, beside mutations, are the major source of genetic variation. In the process of recombination a single or double DNA strand is broken and rejoined with unassociated DNA fragments. There are several types of recombination: homologous recombination, sitespecific recombination and transposition. Within mitochondrial genomes, inter- and intra-molecular recombination can occur. Except for intramolecular recombination of mtDNA, the other types of recombination always result in the creation of mosaic genomes. However, in the natural populations mtDNA recombination is detected extremely rarely. It is caused by the clonal inheritance of mitochondrial genomes and consequential lack of sufficient divergence between parental mitochondrial molecules. Mussels of the genus Mytilus possess two types of mitochondrial genomes inherited from males and females, respectively, and their mode of mtDNA inheritance is called doubly uniparental inheritance (DUI). The presence of two highly diverged parental molecules gives the opportunity for detection of recombinant variants. This feature of Mytilus mtDNA can be broadly exploited in the search for and characteristics of recombinant sequences. Apart from the high level of sequence divergence, fusion of mitochondria and appropriate enzymatic toolkit are principal requirements for the occurrence of recombination. The majority of phylogenetic and demographic analysis based on mtDNA assumes the lack of recombination. If this assumption turned out to be erroneous, previous analyses would be weakened. Recombination is associated with DUI abnormalities, e.g. masculinization of mitochondrial genomes. It may even lead to the breakdown of DUI system resulting in the new, unidentified mode of mtDNA inheritance in mussel Mytilus that might be regulated by stochastic events.