Buchnera aphidicola, a member of the Proteobacteria, is the primary endosymbiont of aphids, and has been studied in the pea aphid, "Acyrthosiphon pisum". "Buchnera" is believed to have had a free-living, Gram-negative ancestor similar to a modern Enterobacterales, such as "Escherichia coli". "Buchnera" is 3 µm in diameter and has some of the key characteristics of their Enterobacterales relatives, such as a Gram-negative cell wall. However, unlike most other Gram-negative bacteria, "Buchnera" lacks the genes to produce lipopolysaccharides for its outer membrane. The long association with aphids and the limitation of crossover events due to strictly vertical transmission has seen the deletion of genes required for anaerobic respiration, the synthesis of amino sugars, fatty acids, phospholipids, and complex carbohydrates. This has resulted not only in one of the smallest known genomes of any living organism, but also one of the most genetically stable.