A survey of mitochondrial evolution in Metazoa : large variability in little genomes

The animal mitochondrial genome (mtDNA) has been traditionally used as molecular marker for phylogenetic reconstructions but the study of the evolutionary history of this small genome can also help to unravel the hidden link between structural and functional genomic features. In this respect, the animal mtDNA can be regarded as a model in comparative genomics, whose use is also enhanced by the availability of complete sequences for about 2000 species. In spite of the biased taxon sampling, the analysis of the present mtDNA dataset is doubtless promising to reconstruct the evolutionary history of this entire genome in a wide phylogenetic range and to identify possible differences in its evolutionary trend among different lineages. In order to carefully analyze the plethora of available mt sequences of Metazoa, we have developed a specialized mtDNA database, MitoZoa, collecting (nearly) complete mtDNA genomes whose annotations have been significantly corrected/improved using a semi-automatic reannotation pipeline. MitoZoa has been designed both to address comparative analyses of genomic features, such as gene order, non-coding regions and gene content, and to help comparisons at short evolutionary distances, such as in congeneric species. Here, we will briefly present the MitoZoa database and the preliminary results of our investigation on the mtDNA evolutionary dynamics in Metazoa: we have got insight into the variability of basic mitogenomic features, such as gene content, gene compactness, and base composition, and into the trend of gene order rearrangements in the main metazoan lineages. Especially in fast-evolving lineages, the same features have also been investigated in congeneric species, i.e. in closely-related species where saturation of evolutionary changes is unlikely to occur. Our data show that the mtDNA plasticity of Metazoa is higher that previously thought and that the mt evolutionary trend has changed several times, and sometimes dramatically, in the metazoan phylogenetic tree.