Alphaproteobacteria species as a source and target of lateral sequence transfers.

Alphaproteobacterial genomes show a remarkable genome plasticity linked with different lifestyles (intracellular, facultative, and free-living). They represent the major source of the genome repertoire of mitochondria, and their genes (specifically those of Wolbachia) have been massively transferred into their modern eukaryotic hosts, such as arthropods and nematodes. Conversely, other organisms (bacteria, viruses, archaea, and eukaryotes) and selfish DNA have contributed to their genomes. This bidirectional lateral sequence transfer explains the mosaic nature of their genomes. In contrast to those living in allopatry, alphaproteobacteria living in sympatry (in protist cells such as in the environment) favor lateral sequence transfer. Evidence shows that intracellular transfer of the type IV secretion system might have played a critical role in the evolution of these alphaproteobacteria.