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
Enterobacteriaceae, such as
Escherichia coli.
Buchnera is 3 µm in diameter and has some of the key characteristics of their Enterobacteriaceae 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.