Bacterial endosymbionts in animals.

Molecular phylogenetic studies reveal that many endosymbioses between bacteria and invertebrate hosts result from ancient infections followed by strict vertical transmission within host lineages. Endosymbionts display a distinctive constellation of genetic properties including AT-biased base composition, accelerated sequence evolution, and, at least sometimes, small genome size; these features suggest increased genetic drift. Molecular genetic characterization also has revealed adaptive, host-beneficial traits such as amplification of genes underlying nutrient provision.